structure of cell essay

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What is a cell?

What is cell theory, what do cell membranes do.

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A cell is a mass of cytoplasm that is bound externally by a cell membrane . Usually microscopic in size, cells are the smallest structural units of living matter and compose all living things. Most cells have one or more nuclei and other organelles that carry out a variety of tasks. Some single cells are complete organisms, such as a bacterium or yeast . Others are specialized building blocks of multicellular organisms , such as plants and animals .

Cell theory states that the cell is the fundamental structural and functional unit of living matter. In 1839 German physiologist  Theodor Schwann  and German botanist  Matthias Schleiden  promulgated that cells are the “elementary particles of organisms” in both plants and animals and recognized that some organisms are unicellular and others multicellular. This theory marked a great conceptual advance in biology and resulted in renewed attention to the living processes that go on in cells.

The cell membrane surrounds every living cell and delimits the cell from the surrounding environment. It serves as a barrier to keep the contents of the cell in and unwanted substances out. It also functions as a gate to both actively and passively move essential nutrients into the cell and waste products out of it. Certain proteins in the cell membrane are involved with cell-to-cell communication and help the cell to respond to changes in its environment.

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cell , in biology , the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast . Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals . Although cells are much larger than atoms , they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas ; some of these single-celled organisms are spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch), with a total mass of 10 −14 gram—equal to that of 8,000,000,000 hydrogen atoms. Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 μm across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 30,000,000,000,000 cells.

This article discusses the cell both as an individual unit and as a contributing part of a larger organism. As an individual unit, the cell is capable of metabolizing its own nutrients , synthesizing many types of molecules, providing its own energy, and replicating itself in order to produce succeeding generations. It can be viewed as an enclosed vessel, within which innumerable chemical reactions take place simultaneously. These reactions are under very precise control so that they contribute to the life and procreation of the cell. In a multicellular organism , cells become specialized to perform different functions through the process of differentiation. In order to do this, each cell keeps in constant communication with its neighbours. As it receives nutrients from and expels wastes into its surroundings, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells form tissues, and a cooperation between tissues in turn forms organs , which carry out the functions necessary to sustain the life of an organism.

Special emphasis is given in this article to animal cells, with some discussion of the energy-synthesizing processes and extracellular components peculiar to plants . (For detailed discussion of the biochemistry of plant cells, see photosynthesis . For a full treatment of the genetic events in the cell nucleus, see heredity .)

The nature and function of cells

A cell is enclosed by a plasma membrane , which forms a selective barrier that allows nutrients to enter and waste products to leave. The interior of the cell is organized into many specialized compartments, or organelles , each surrounded by a separate membrane. One major organelle , the nucleus , contains the genetic information necessary for cell growth and reproduction . Each cell contains only one nucleus, whereas other types of organelles are present in multiple copies in the cellular contents, or cytoplasm . Organelles include mitochondria , which are responsible for the energy transactions necessary for cell survival; lysosomes , which digest unwanted materials within the cell; and the endoplasmic reticulum and the Golgi apparatus , which play important roles in the internal organization of the cell by synthesizing selected molecules and then processing, sorting, and directing them to their proper locations. In addition, plant cells contain chloroplasts , which are responsible for photosynthesis, whereby the energy of sunlight is used to convert molecules of carbon dioxide (CO 2 ) and water (H 2 O) into carbohydrates . Between all these organelles is the space in the cytoplasm called the cytosol . The cytosol contains an organized framework of fibrous molecules that constitute the cytoskeleton , which gives a cell its shape, enables organelles to move within the cell, and provides a mechanism by which the cell itself can move. The cytosol also contains more than 10,000 different kinds of molecules that are involved in cellular biosynthesis , the process of making large biological molecules from small ones.

Specialized organelles are a characteristic of cells of organisms known as eukaryotes . In contrast, cells of organisms known as prokaryotes do not contain organelles and are generally smaller than eukaryotic cells. However, all cells share strong similarities in biochemical function.

The molecules of cells

Cells contain a special collection of molecules that are enclosed by a membrane. These molecules give cells the ability to grow and reproduce . The overall process of cellular reproduction occurs in two steps: cell growth and cell division . During cell growth, the cell ingests certain molecules from its surroundings by selectively carrying them through its cell membrane . Once inside the cell, these molecules are subjected to the action of highly specialized, large, elaborately folded molecules called enzymes . Enzymes act as catalysts by binding to ingested molecules and regulating the rate at which they are chemically altered. These chemical alterations make the molecules more useful to the cell. Unlike the ingested molecules, catalysts are not chemically altered themselves during the reaction, allowing one catalyst to regulate a specific chemical reaction in many molecules.

Biological catalysts create chains of reactions. In other words, a molecule chemically transformed by one catalyst serves as the starting material, or substrate, of a second catalyst and so on. In this way, catalysts use the small molecules brought into the cell from the outside environment to create increasingly complex reaction products. These products are used for cell growth and the replication of genetic material. Once the genetic material has been copied and there are sufficient molecules to support cell division, the cell divides to create two daughter cells. Through many such cycles of cell growth and division, each parent cell can give rise to millions of daughter cells, in the process converting large amounts of inanimate matter into biologically active molecules.

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Cell Structure

Reviewed by: BD Editors

Cells are the fundamental units of life from which all other living things are made. They contain all the molecules and structures needed for individual cell survival and the survival of the organism as a whole.

Different cells have different subcellular structures, but all eukaryotes contain the same three parts: the nucleus, the cell membrane, and the cytoplasm.

What is the Structure of Cells?

All eukaryotic cells consist of  three basic parts . These are the cell membrane, the nucleus, and the cytoplasm. The cell membrane surrounds the outside of the cell, the nucleus is found in the middle of the cell, and the cytoplasm fills the gap between the two. Buried in the cytoplasm are hundreds or thousands of subcellular structures called  organelles.   

The fluid inside cells is known as the  intracellular fluid (ICF),  while the environment outside of the cell is referred to as the  extracellular fluid (ECF).

The Cell Membrane

The cell membrane (AKA the plasma membrane) is a thin, flexible structure that surrounds the outside of the cell, creating a physical barrier between the cell interior and its external environment. It consists of a semipermeable lipid bilayer that regulates the passage of materials in and out of the cell.

The Cytoplasm

The cytoplasm is a jelly-like goo that fills the interior space of the cell. It cushions and protects the cell organelles and is also where many of the cell’s chemical reactions take place. The cytoplasm is mainly composed of water, but also contains salts and other organic molecules.

The Nucleus

The nucleus contains the cell’s DNA and genetic information. It is separated from the cytoplasm by a double membrane called the nuclear envelope and controls all cellular activities, including cell division, protein production, growth, and metabolism.

Cell Organelles

Organelles are the ‘tiny organs’ of a cell. They are distinct, specialized structures that are adapted to fulfill the essential life functions necessary for cell survival. Some organelles (for example, the mitochondria, ribosomes, and nucleus) are found in almost all cell types. Others (like the chloroplasts and cell wall) are only found in certain cell types, like plants and algae.

Animal Cell Organelles

Animal cell organelles are typically found in both animal and plant cells. They include mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, and lysosomes, as well as the nucleus, cytoplasm, and cell membrane.

Mitochondria

Mitochondria are the site of respiration in cells. They release the energy required to power all other cellular processes.

Ribosomes are where protein production takes place. These organelles ‘read’ the instructions contained in the genetic code and use them to assemble polypeptide chains from amino acids.

The Endoplasmic Reticulum

The ER is a  large, continuous, membrane-bound organelle  whose main functions are to process and transport newly-synthesized materials. The rough ER is studded with ribosomes and is used in the processing and transportation of proteins. The smooth ER has no ribosomes and is mainly involved in lipid and hormone synthesis.

Golgi Apparatus

Once materials leave the ER, they are sent to the Golgi apparatus where they are packaged and distributed to wherever they are needed. Some materials are incorporated into the plasma membrane, while others pass through the membrane and leave the cell.

Lysosomes are small, spherical organelles that use digestive enzymes to dispose of unwanted materials. They may be used to recycle old or damaged cell parts or invading pathogens, and also play a key role in  apoptosis  (programmed cell death).

Plant Cell Organelles

Some plant cell organelles are not found in animal cells. They include the chloroplasts, vacuole, and plant cell wall.

Chloroplasts

Chloroplasts are the site of photosynthesis, a process in which light energy from the sun is used to convert carbon dioxide and water into glucose. They are filled with a green pigment called chlorophyll, which harvests light energy and gives plants their green color.

The Vacuole

The vacuole is a large, sap-filled bubble that plant cells use to store water, proteins, and other molecules.

Another function of the vacuole is to maintain turgor pressure in plant cells, as this helps them to keep their shape and prevents wilting or bursting.

The Cell Wall

The cell wall surrounds plant cells, protecting and supporting the cell. This structure is mainly made of cellulose and is very strong.

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Introduction to cells

All living things are made from one or more cells. A cell is the simplest unit of life and they are responsible for keeping an organism alive and functioning. This introduction to cells is the starting point for the area of biology that studies the various types of cells and how they work.

There is a massive variety of different types of cells but they all have some common characteristics. Almost every different type of cell contains genetic material , a membrane and cytoplasm. Cells also have many other features such as organelles and ribosomes that perform specific functions.

Many different organisms on the tree of life contain only one cell and are known as single-celled or unicellular organisms. Their single cell performs all the necessary functions to keep the organism alive. All species of bacteria and archaea are single-celled organisms. On the other hand, large organisms like humans are made from many trillions of cells that work together to keep the organism alive.

The most basic categorisation of Earth’s organisms is determined by different types of cells. All cells can be divided into one of two classifications: prokaryotic cells and eukaryotic cells. Prokaryotic cells are found in bacteria and archaea. Eukaryotic cells are found in organisms from the domain Eukaryota which includes animals, plants, fungi and protists.

This introduction to cells will take you through the basic structure of cells, the difference between prokaryotic and eukaryotic cells and you will learn about organelles.

STRUCTURE OF A CELL

The genetic material of cells is found as molecules called DNA. The DNA of a cell holds all the information that a cell needs to keep itself alive. A DNA molecule contains a code that can be translated by a cell and tells it how to perform different tasks. A gene is a specific segment of a DNA molecule and each gene tells a cell how to perform one specific task.

The gel-like substance that the genetic material is found in is called the cytoplasm. The cytoplasm fills a cell and gives it it’s shape. The cytoplasm also allows for different materials to move around the cell. All cells have other structures in their cytoplasm that help the cell stay alive.

The cytoplasm of all cells is surrounded by a membrane called the plasma membrane. The plasma membrane separates the cell from the outside world and keeps the contents of the cell together. The plasma membrane provides a barrier that substances have to pass through before they can enter or exit a cell.

EUKARYOTIC CELLS VS. PROKARYOTIC CELLS

The main difference between prokaryotic cells and eukaryotic cells is the presence of a nucleus and organelles. Prokaryotic cells do not have either a nucleus or organelles. The word prokaryotic can be translated to mean ‘before nucleus’.

Eukaryotic cells have both a nucleus and a range of different organelles. The nucleus is a structure found in eukaryotic cells that contains the cell’s DNA. Organelles are cellular ‘factories’ that perform important functions such as building different molecules of life , removing wastes and breaking down sugars.

Having organelles makes eukaryotic cells much more efficient at completing important cellular functions. Because they are more efficient, eukaryotic cells can grow much larger than prokaryotic cells.

For a cellular structure to be considered an organelle it must be surrounded by a membrane just as the nucleus is. Prokaryotic cells contain various structures that help with certain functions, such as ribosomes, but these structures are not encapsulated by membranes and are therefore not considered organelles.

Eukaryotic cells have evolved into multicellular organisms. By specializing into different types of cells, they are able to perform functions even more efficiently and are able to keep large, multicellular organisms alive.

Important organelles include the nucleus, mitochondria, chloroplasts, and the endoplasmic reticulum. Mitochondria are involved in the process of cellular respiration where sugar is broken down and converted into cellular energy.

Chloroplasts are found in the cells of plants and other photosynthetic organisms . Inside chloroplasts are where plant cells are able to use energy from the sun to create sugars from carbon dioxide and water.

The endoplasmic reticulum is a network of membranes that are attached to the membrane of the nucleus. The endoplasmic reticulum is involved with many important tasks such as producing proteins and breaking down fats and carbohydrates.

For more information on cells check out these pages on our website: Cells | Eukaryotic cells | Prokaryotic cells | Animal cells | Plant cells

Last edited: 30 August 2020

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Chapter 6. Cell Structure and Function

Ayda Basgul Martin

Unit Outline

Part 1. Characteristics of Life

Organization

Exchange of material, responsiveness, development, growth, and reproduction.

Part 2. Structural Organization of the Body

The Levels of Organization

The cellular level of organization.

Part 3. Cell Structure, Cellular Organelles, and Functions

• General Cell Structure: Plasma Membrane, Cytoplasm and Nucleus

Organelles of the Endomembrane System

• Organelles for Energy Pocessing

Part 4 . Cellular Processes Involved in Growth

Cell Division, Growth, and Differentiation

Cell specialization, learning objectives.

At the end of this unit, you should be able to:

I. Specify the characteristics associated with life and explain why the cell is the basic unit of life.

II. Describe the levels of structural organization in the body.

III. Describe the structure and the functions of the major components of a cell.

IV. Define metabolism and distinguish between anabolism and catabolism.

V. Describe the cellular processes involved in the growth of the human body from a fertilized egg to an adult.

VI. Describe the importance of cell differentiation to an organism.

VII. Describe the general characteristics of each of the following cell types and relate their characteristics to their functions: nerve cell, muscle cell, red blood cell (erythrocyte), and white blood cell (leukocyte).

Part 1: Characteristics of Life

The different organ systems each have different functions and therefore unique roles to perform in the body. These many functions can be summarized in terms of a few that we might consider definitive of human life: organization, metabolism , exchange of materials, responsiveness, movement, development, growth, and reproduction.

A human body consists of trillions of cells organized in a way that maintains distinct internal compartments. These compartments keep body cells separated from external environmental threats and keep the cells moist and nourished. They also separate internal body fluids from the countless microorganisms that grow on body surfaces, including the lining of certain tracts, or passageways. The intestinal tract, for example, is home to even more bacteria cells than the total of all human cells in the body, yet these bacteria are outside the body and cannot circulate freely inside the body.

Cells, for example, have a membrane (also referred to as the plasma membrane) that keeps the intracellular environment—the fluids and organelles —separate from the environment outside the cell (the extracellular environment). Blood vessels keep blood inside a closed system, and nerves and muscles are wrapped in tissue sheaths that separate them from surrounding structures. In the chest and abdomen, a variety of internal membranes keep major organs such as the lungs, heart, and kidneys protected and separate from others.

The body’s largest organ system is the integumentary system , which includes the skin and its associated structures, such as hair and nails. The surface tissue of the skin is a barrier that protects internal structures and fluids from potentially harmful microorganisms, toxins, and the external environment.

The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. Your basic function as an organism is to consume (ingest) energy and molecules in the foods you eat, convert some of it into fuel for movement, sustain your body functions, and build and maintain your body structures. There are two types of reactions that accomplish this: anabolism and catabolism .

• Anabolism is the process whereby smaller, simpler molecules are combined into larger, more complex substances. For example, amino acids can be combined together to make proteins. Your body can assemble, by utilizing energy, the complex chemicals it needs by combining small molecules derived from the foods you eat.
• Catabolism is the process by which larger, more complex substances are broken down into smaller, simpler molecules. For example, sugars are broken down into carbon dioxide and water. Catabolism releases energy. The complex molecules found in foods are broken down so the body can use their parts to assemble the structures and substances needed for life.

Taken together, these two processes are called metabolism. Metabolism is the sum of all anabolic and catabolic reactions that take place in the body. Both anabolism and catabolism occur simultaneously and continuously to keep you alive.

Every cell in your body makes use of a chemical compound, adenosine triphosphate (ATP) , to store and release energy. The cell stores energy in the molecule of ATP and then moves the ATP molecules to the location where energy is needed to fuel cellular activities. Then the ATP is broken down, and a controlled amount of energy is released, which is used by the cell to perform a particular job.

Organisms do not exist solely within their own boundaries but interact with the external environment that surrounds them. One of the ways in which they do this is by exchanging materials with their external environment: taking in materials from their external environment and expelling waste products out into their external environment. These materials and waste products may be anything from very small, relatively simple molecules (e.g., glucose, carbon dioxide) that must cross an individual cell’s plasma membrane to whole cells or foods that were ingested but not fully digested and/or absorbed and so must be excreted from the organism.

Responsiveness is the ability of an organism to adjust to changes in its internal and external environments. An example of responsiveness to external stimuli could include moving toward sources of food and water and away from perceived dangers. Changes in an organism’s internal environment, such as increased body temperature, can cause the responses of sweating and the dilation of blood vessels in the skin in order to decrease body temperature.

Human movement includes not only actions at the joints of the body but also the motion of individual organs and even individual cells. As you read these words, red and white blood cells are moving throughout your body, muscle cells are contracting and relaxing to maintain your posture and to focus your vision, and glands are secreting chemicals to regulate body functions. Your body is coordinating the action of entire muscle groups to enable you to move air into and out of your lungs, to push blood throughout your body, and to propel the food you have eaten through your digestive tract. Consciously, of course, you contract your skeletal muscles to move the bones of your skeleton to get from one place to another and to carry out all of the activities of your daily life.

• Development is all of the changes the body goes through in life. The development includes the process of cell differentiation , in which unspecialized cells become specialized in structure and function to perform certain tasks in the body. The development also includes the processes of growth and repair, both of which involve cell differentiation.
• Growth is the increase in body size. Humans, like all multicellular organisms, grow by increasing the number of existing cells, increasing the amount of non-cellular material around cells (such as mineral deposits in bone), and within very narrow limits, increasing the size of existing cells.
• Reproduction is the formation of a new organism from parent organisms. In humans, reproduction is carried out by the male and female reproductive systems. Because death will come to all complex organisms, without reproduction, the line of organisms would end.

Test Your Knowledge- Part 1: Characteristics of Life

• List, explain, and provide examples of each of the characteristics of life.
• In reference to your answer to question #1, above, explain in one sentence why the cell is the basic unit of life.

Part 2: Structural Organization of the Human Body

Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms , molecules , organelles , cells, tissues, organs, organ systems, and organisms (Figure 1).

To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms, and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles, such as the proton, electron, and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

A cell is the smallest independently functioning unit of a living organism. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. Even bacteria, which are extremely small single-celled, independently-living organisms, have a cellular structure.

A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based fluid, together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet the physiological needs of the body. Assigning organs to organ systems can be imprecise, since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.

The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.

You developed from a single fertilized egg cell into a complex organism containing trillions of cells that you see when you look in a mirror. Early during this developmental process, cells differentiate and become specialized in their structure and function. These different cell types form specialized tissues that work in concert to perform all of the functions necessary for the living organism. Cellular and developmental biologists study how the continued division of a single cell leads to such complexity.

Consider the difference between a cell in the skin and a nerve cell. A skin cell may be shaped like a flat plate (squamous) and live only for a short time before it is shed and replaced. Packed tightly into rows and sheets, the squamous skin cells provide a protective barrier for the cells and tissues that lie beneath. A nerve cell, on the other hand, may be shaped something like a star, sending out long processes up to a meter in length, and may live for the entire lifetime of the organism. With their long winding processes, nerve cells can communicate with one another and with other types of body cells and send rapid signals that inform the organism about its environment and allow it to interact with that environment. These differences illustrate one very important theme that is consistent at all organizational levels of biology: the form of a structure is optimally suited to perform particular functions assigned to that structure. Keep this theme in mind as you tour the inside of a cell and are introduced to the various types of cells in the body.

The concept of a cell started with microscopic observations of dead cork tissue by scientist Robert Hooke in 1665. Without realizing their function or importance, Hook coined the term “cell” based on the resemblance of the small subdivisions in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the first person to observe living and moving cells under a microscope. In the century that followed, the theory that cells represented the basic unit of life would develop. These tiny fluid-filled sacs house components responsible for the thousands of biochemical reactions necessary for an organism to grow and survive. In this chapter, you will learn about the major components and functions of a generalized cell and discover some of the different types of cells in the human body.

Test Your Knowledge- Part 2: Structural Organization of the Body

Describe the levels of structural organization in the body.

• Chemical level
• Cellular level
• Tissue level
• Organ level
• Organ system level
• Organismal level
• Write a clear description of the relationships between the chemical, cellular, tissue, organ, organ system, and organismal levels of organization in the body.

Part 3: Cell Structure, Cellular Organelles, and Functions

General cell structure: plasma membrane, cytoplasm, and nucleus.

The cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its external environment. This membrane provides a protective barrier around the cell and regulates which materials can pass in or out. It is primarily composed of phospholipids arranged in two layers but also contains cholesterol and a mosaic of different proteins. You will learn more about the structure and function of the plasma membrane in Unit 5. All living cells in multicellular organisms contain an internal cytoplasmic compartment, composed of cytosol and organelles. Cytosol , the jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions and is mostly composed of water. Eukaryotic cells, including all animal cells, also contain various cellular organelles. An organelle (“little organ”) is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function.

Just as the various bodily organs work together in harmony to perform all of a human’s functions, the many different cellular organelles work together to keep the cell healthy and perform all of its important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm. The nucleus is a cell’s central organelle, which contains the cell’s DNA (Figure 2).

Most membranous organelles found in a human cell together form a system within the cell called the endomembrane system . These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The components of the endomembrane system include the nuclear envelope, endoplasmic reticulum , Golgi apparatus , vesicles, and plasma membrane.

Endoplasmic Reticulum : The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus (see Part 7) and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions (Figure 3).

Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in different amounts depending on the type of cell. Rough ER (RER) is so called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy appearance. A ribosome is an organelle that serves as the site of protein synthesis, and it is composed of two subunits. Ribosomes can either be bound (attached to ER) or free (floating in the cytosol). Smooth ER (SER) lacks ribosomes.

One of the main functions of the smooth ER is in the synthesis of lipids . The smooth ER synthesizes phospholipids , the main component of biological membranes, as well as steroid hormones .

For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular calcium (Ca 2+ ), which is extremely important in cells of the nervous system where Ca 2+ is the trigger for neurotransmitter release. Additionally, the smooth ER, especially in the liver, performs a detoxification role, breaking down certain toxins.

In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle (a small fluid-filled sac) to the next stage in the packaging and shipping process: the Golgi apparatus .

The Golgi Apparatus: The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side (the cis face) of the apparatus receives products in vesicles . These products are sorted through the apparatus, and then they are released from the opposite side (the trans face) after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted (Figure 4).

Lysosomes : Some of the protein products from the Golgi include digestive enzymes that are meant to remain inside the cell for use in breaking down certain materials. These enzymes are packaged into vesicles called lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as a damaged organelle in a process called autophagy (“self-eating”).

Lysosomes are also important for breaking down foreign material. For example, when certain immune defense cells, like white blood cells, phagocytize (engulf) bacteria, the bacterial cell is transported to a lysosome and digested by the enzymes inside. Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. This “self-destruct” mechanism is called autolysis and makes the process of cell death controlled (a mechanism called “ apoptosis ”).

Organelles for Energy Processing

In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert to chemical energy that can be used to power biochemical reactions.

Mitochondrion : A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane (Figure 5). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. It is along this inner membrane that a series of proteins , enzymes , and other molecules perform the biochemical reactions of cellular respiration .

These reactions harvest the energy stored in nutrient molecules (such as glucose) to power the synthesis of ATP , which provides usable energy to the cell. Cells use ATP constantly, so the mitochondria are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly breathe them in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria.

Nerve cells also need large quantities of ATP to run their sodium-potassium pumps, which are used to generate an action potential. Therefore, an individual neuron will be loaded with over a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically active, might only have a couple of hundred mitochondria.

The Nucleus : The nucleus is the largest and most prominent of a cell’s organelles (Figure 6). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 7), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body.

Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of the cells that produce eggs and sperm) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that each new cell receives a full complement of DNA.

Organization of the Nucleus and Its DNA : Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope . This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA , and solutes between the nucleus and the cytoplasm .

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNAs necessary for the construction of ribosomes . Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 8). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. When a cell is in the process of division, the chromatin condenses into chromosomes so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.

Test Your Knowledge- Part 3: Cell Structure, Cellular Organelles, and Functions

Describe the structure and the functions of the major components of a cell.

• The cell membrane (plasma membrane)
• Endoplasmic reticulum
• Golgi apparatus (Golgi complex)
• Mitochondria
• Nuclear envelope
• Chromosomes
• Plasma membrane
• Smooth endoplasmic reticulum
• Rough endoplasmic reticulum
• Bound ribosomes
• Free ribosomes
• Golgi apparatus (or Golgi complex)
• Describe the structure (name all the components and describe their relationships to each other) and list the general functions of the “endomembrane system.”

Part 4: Cellular Processes Involved in Growth.

Cell Division: cells in the body must replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two new cells.

While there are a few cells in the body that do not undergo cell division (such as gametes , red blood cells, most neurons , and some muscle cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells ), are somatic cells.

Cell Growth: Once cells divide, they grow and increase in size. For example, nerve cells first appear as relatively small cells, but then they elongate to become extremely long cells. Similarly, muscle cells grow to become extremely long cells as muscles are formed.

Cell Differentiation : How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.

A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are unique in that they can also continually divide and regenerate new stem cells instead of further specializing. There are different stem cells present at different stages of a human’s life. They include the embryonic stem cells of the embryo , fetal stem cells of the fetus , and adult stem cells in the adult. One type of adult stem cell is the epithelial stem cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin.

When a cell differentiates, it becomes specialized; yet if all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the different cell types come to be so different? The answer is analogous to a movie script. The different actors in a movie all read from the same script; however, they are each only reading their own part of the script. Similarly, all cells contain the same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function. In biology, this is referred to as the unique genetic expression of each cell.

As cells specialize, they may undertake major changes in size, shape, metabolic activity, and overall function. The morphology (structure) of a mature cell is closely related to the function it is specialized to serve (Figure 9). Muscle fibers, for example, are far removed in structure and function from the zygote that they ultimately arose from: they are long, slender structures that are well-suited to contracting to produce macroscopic movements over relatively long distances. Some neurons (nerve cells) are exceptionally long and slender in shape, again to act over relatively long distances, although in this case, their function is to transmit information rather than move body structures directly. Erythrocytes (red blood cells) are used to transport oxygen in the blood; their tiny size and lack of a nucleus make them well-suited to squeezing through the smallest of capillaries, and their lack of mitochondria mean they do not themselves use up the oxygen they are supposed to be delivering to other cells. Leukocytes (white blood cells) on the other hand are noticeably larger than erythrocytes and do have mitochondria . The large size of macrophages, for example, means they are capable of physically engulfing relatively large particles or whole cells, such as bacteria by phagocytosis , and their mitochondria allow them access to the chemical energy required to move through body tissues toward invading pathogens.

Test Your Knowledge- Part 4: Cellular Processes Involved in Growth.

I. Define metabolism and distinguish between anabolism and catabolism.

• Define the term “metabolism.”
• Write a single sentence that clearly differentiates between “anabolism” and “catabolism.”

II. Describe the cellular processes involved in the growth of the human body from a fertilized egg to an adult.

• Distinguish between cell division, cell growth, and cell differentiation.
• Provide two examples of cell types in the human body that do not undergo cell division.
• Define the term “stem cell.”

For the image below, drag and drop the correct structures to the animal cell.

Image Descriptions

Figure 6.1. Levels of Structural Organization of the Human Body. The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. The tip of the pyramid represents the atoms and examples of atoms, the oxygen and hydrogen atoms are given, in the bracket, below shows water molecule to refer to atoms bonding to form molecules with dimensional structures. Next, complexity is shared at the cellular level with the example of a smooth muscle cell represented as an example of a variety of molecules combining to form the fluid and organelles of a body cell. The fourth complexity contains the illustration of smooth muscle tissue to refer to the tissue level of organization, which is defined as a community of similar cells from body tissue. One below in the pyramid organ level is presented to refer to two or more different tissues combined to form an organ, and a urinary bladder illustration is given as an example. The fifth level contains the urinary tract system to refer to the organ system level, which is defined as two or more organs working closely together to perform the functions of a body system. The last and most inclusive level is the organismal level, which is formed by many organ systems working harmoniously together to perform the functions of an independent organism. [Return to image.]

Figure 6.2. Typical Human Cell. While this image is not indicative of any one human cell, it is a typical example of an animal cell containing the primary organelles and internal structures. In the illustration, the plasma membrane is labeled as a border, in the cytoplasm; mitochondria, microtubule, centrosome, microfilament, microtubule, lysosome, smooth endoplasmic reticulum (has no ribosome attachment to their membrane), secretory vesicle, peroxisome, vacuole, cytoplasm, Golgi vesicle, Golgi apparatus, rough endoplasmic reticulum (ribosomes attached on their membrane), cytoplasmic ribosomes, which freely floats within the cytosol, intermediate filaments, and nucleus are drawn and labeled. Within the nucleus, chromatin and nucleolus are also drawn and labeled. [Return to image.]

Figure 6.4. Golgi Apparatus. (a) The illustration presents RER: transport vesicle carries substances from RER to the Golgi apparatus, and secretory vesicle carries substances from the Golgi apparatus toward the plasma membrane. The Golgi apparatus manipulates products from the rough ER. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell, and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new vesicles called lysosomes. The illustration shows a transport vesicle between the RER and Golgi apparatus and a secretory vesicle between the Golgi apparatus and the plasma membrane. Transport vesicles move materials within the cell, while secretory vesicles store and release materials into the cell or to the extracellular environment. (b) An electron micrograph of the Golgi apparatus is shown with labeled trans face and cis face. The cis face lies near the transitional region of the rough endoplasmic reticulum, while the trans face lies near the cell membrane. These two networks are responsible for the essential task of sorting proteins and lipids that are received (at the cis face) or released (at the trans face) by the organelle. [Return to image.]

Figure 6.5. Mitochondrion. The mitochondria are the energy-conversion factories of the cell. (a) Illustration of a mitochondrion shows labels for cristae and intermembrane space. The outer and inner membranes and intermembrane space are labeled within the illustration and the electron micrograph. A mitochondrion is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein. Cristae are folds in the inner mitochondrial membrane. Mitochondria are organelles in eukaryotic cells. The major function of cristae is to increase the surface area of the mitochondrial membrane. This allows membrane processes to produce more energy at a faster rate. (b) An electron micrograph of mitochondria. EM × 236,000.  (Micrograph provided by the Regents of University of Michigan Medical School © 2012). [Return to image].

Figure 6.6. The Nucleus. The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell. Illustration of a nucleus contains nucleolus, condensed chromatin, nuclear envelope, and nuclear pores. The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm. In eukaryotic cells, the nucleus is separated from the cytoplasm and surrounded by a nuclear envelope. This envelope safeguards the DNA contained in the nucleus. In spite of this barrier, there is still communication between the nucleus and the cytoplasm. This communication is regulated by the nuclear pores. Next to the nucleus, RER is illustrated, and cisternae of the RER is labeled. The RER is morphologically distinguishable by its series of convoluted, flattened-like membrane sheets (called cisternae) that arise near the nucleus and extend across the cytoplasm. [Return to image.]

Figure 6.9. Stem Cells. Stem cells have the remarkable potential to renew themselves. They can develop into many different cell types in the body during early life and growth. The capacity of stem cells to differentiate into specialized cells make them potentially valuable in therapeutic applications designed to replace damaged cells of different body tissues. There are several main categories: the “pluripotent” stem cells (embryonic stem cells and induced pluripotent stem cells) and nonembryonic or somatic stem cells (commonly called “adult” stem cells). The illustration is showing a totipotent embryonic stem cell can divide and differentiate into pluripotent embryonic stem cells such as the endoderm line, which differentiate later into multipotent stem cells to form the cells of the lung and pancreas. The pluripotent mesoderm line differentiates into multipotent stem cells to form the heart muscle and RBC. The pluripotent ectoderm line differentiates into multipotent stem cells to form the cells of the skin and neurons. [Return to image.]

Sum of all catabolic and anabolic reactions that take place in the body.

Any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell.

Skin and its accessory structures.

Reactions that build smaller molecules into larger molecules.

Chemical reaction that breaks down more complex organic molecules.

Building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-chain.

Nucleotide containing ribose and an adenine base that is essential in energy transfer.

Becoming wider, larger, or more open.

Process by which unspecialized cells become more specialized in structure and function.

Consisting of more than one cell (as opposed to organisms such as bacteria, which are unicellular).

The smallest unit of an element that retains the unique properties of that element.

Two or more atoms covalently bonded together.

Group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function.

An anatomically distinct structure of the body composed of two or more tissue types.

Internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials.

Clear, semi-fluid medium of the cytoplasm, made up mostly of water.

One of two major divisions of living things (or their cells) that have membrane-bound nuclei and other organelles and can form large complex organisms (including all animals, plants, fungi). By contrast, bacteria are prokaryotic.

Cell’s central organelle; contains the cell’s DNA.

Set of cellular organelles that often work together to produce, package, and export certain products.

Cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively).

Cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport.

Cellular organelle that functions in protein synthesis.

Class of organic compounds that are composed of many amino acids linked together by peptide bonds.

Class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water.

An amphipathic lipid molecule containing a phosphate head (polar) and two fatty acid tails (non-polar). The major molecule comprising plasma membranes.

(Also, sterol) lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and molecules; not to be confused with anabolic steroids, a synthetic supplement

Secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body.

Chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell.

Membrane-bound structure that contains materials within or outside of the cell.

Membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes.

Cell process (a form of endocytosis) in which a cell engulfs and ingests another large particle or cell.

Programmed cell death.

One of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP).

Molecule (usually a protein) that catalyzes chemical reactions.

Production of ATP from glucose oxidation via glycolysis, the Krebs cycle, and oxidative phosphorylation.

Oxygen-carrying protein in erythrocytes (red blood cells).

Deoxyribose-containing nucleic acid that stores genetic information.

Membrane that surrounds the nucleus; consisting of a double lipid-bilayer.

One of the small, protein-lined openings found scattered throughout the nuclear envelope.

Ribose-containing nucleic acid that helps manifest the genetic code as protein.

Small region of the nucleus that functions in ribosome synthesis.

Substance consisting of DNA and associated proteins.

A long DNA molecule, combined with proteins that contains a number of genes. The normal chromosome complement is 23 pairs of homologous chromosomes, one each from mother and father.

Life cycle of a single cell, from its birth until its division into two new daughter cells.

Haploid reproductive cell (egg or sperm in humans) that contributes genetic material to form an offspring.

Excitable neural cell that transfer nerve impulses.

A body cell, excluding germ cells. Normally diploid, each cell containing a complete set of genes.

Cell that gives rise to a gamete.

Type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion.

Cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than becoming further specialized.

Developing human during weeks 3–8.

Developing human during the time from the end of the embryonic period (week 9) to birth.

Cell that produces keratin and is the most predominant type of cell found in the epidermis.

outermost tissue layer of the skin

Red blood cell.

White blood cell.

Human Anatomy and Physiology I Copyright © 2024 by Ayda Basgul Martin is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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What Is the Cell Theory? Why Is It Important?

General Education

If you’re studying biology, you’ll likely learn about the cell theory. The cell theory is one of the most important tenets of biology, and practically everything else you learn in science class relates back to it. But what is the cell theory? In this guide, we’ll give you a clear cell theory definition, explain key dates in the history of this theory, and explain why it’s so important to understand. After reading this guide, you’ll know everything you need to know about the cell theory!

Cell Theory Definition

What is the cell theory? It has three main parts:

1. All living things are made up of cells.

2. Cells are the basic building blocks of life.

3. All cells come from preexisting cells created through the process of cell division.

As science evolved, three more components were added to the theory. Some biology classes don’t require you to know these parts of the cell theory because they weren’t part of the original definition, but it’s still useful to be aware of them:

4. Energy flow occurs within cells.

5. Hereditary information is passed from cell to cell.

6. All cells have the same basic chemical composition.

So what does the cell theory actually mean? Let’s break it down. The first part of the cell theory states that all living things are made up of cells. Anything that’s alive, from bacteria to plants to humans, is composed of cells. And what are cells? The literal definition is a cell is a group of organelles surrounded by a thin membrane .

The cell theory definition states that cells are the building blocks of life. Cells both make up all living things and run the processes needed for life. Your hair, skin, organs, etc. are all made up of cells. In fact, each person is estimated to be made up of nearly 40 trillion cells! Each part of a cell has a different function, and your cells are responsible for taking in nutrients, turning nutrients into energy, removing waste, and more. Basically, everything your body does, it does because cells are directing the action!

The third part of the cell theory definition states that all cells come from preexisting cells. This means that cells don’t just appear out of thin air (known as “spontaneous generation”). New cells are always made from current cells. This means that all current life on the planet is descended from the very first cells, which first made an appearance on Earth roughly 3.5 billion years ago. Cells have been replicating themselves continuously ever since.

And what does the newer part of the cell theory state? Part four refers to the fact that, in all living cells, energy is continuously transformed from one type to another. Examples of these processes include photosynthesis (where plant cells convert light energy into chemical energy ) and cellular respiration (where both plant and animal cells convert glucose into energy). Part five refers to DNA and the fact it is passed from parent cell to child cell. Finally, part six of the cell theory tells us that all cells are made up of the same chemicals: water, inorganic ions, and organic molecules.

The History of the Cell Theory

The cell theory and ideas about cells and living things evolved over several centuries. Here are the key dates for the cell theory:

1665: Robert Hooke is the first person to observe cells when he looks at a slice of cork in a microscope.

1665: Francesco Redi disproves spontaneous generation by showing maggots will only grow on uncovered meat, not meat enclosed in a jar. His work later contributes to part three of the cell theory.

1670s: Antonie van Leeuwenhoek, a Dutch scientist, begins his work developing better microscopes that allow scientists to see cells and the organelles they contain more clearly.

1839: German scientists Matthias Schleiden and Theodor Schwann describe the first two parts of the cell theory. Schleiden stated that all plants are made up of cells, while Schwann stated all animals are made up of cells. Schleiden and Schwann are generally credited as the developers of cell theory.

1855: Rudolf Virchow, another German scientist, describes the third part of cell theory, that all cells come from existing cells.

Since then, microscopes have continued to become more and more refined, making it possible to study cells even more closely and allowing scientists to expand on the original cell theory.

How Is the Cell Theory Important for Biology?

You may be surprised by how obvious the cell theory seems. Anyone who’s taken a basic biology class already knows what cells are and that living things are made up of cells. However, that just goes to show how important the cell theory is. It’s one of the fundamental principles of biology, and it’s so important that it has become information many of us take for granted.

Knowing that all living things are made up of cells allows us to understand how organisms are created, grow, and die. That information helps us understand how new life is created, why organisms take the form they do, how cancer spreads, how diseases can be managed, and more. Cells even help us understand fundamental issues such as life and death: an organism whose cells are living is considered alive, while one whose cells are dead is considered dead.

Before the cell theory existed, people had a very different view of biology. Many believed in spontaneous generation, the idea that living organisms can arise from nonliving matter. An example of this would be a piece of rotten meat creating flies because flies often appear around rotten meat. Additionally, before cells and the cell theory were known, it wasn’t understood that humans, as well as all other living organisms, were made up of billions and trillions of tiny building blocks that controlled all our biological processes. Disease, how organisms grow, and death were much more of a mystery compared to what we know today. The cell theory fundamentally changed how we look at life.

Summary: What Is the Cell Theory?

The cell theory is one of the foundational theories of biology. It has three main components:

As our scientific knowledge has increased over time, additional parts have been added to the theory. Schleiden and Schwann, as well as Virchow, are generally seen as the founders of the cell theory, due to their pioneering scientific work in the 1800s. The cell theory is important because it affects nearly every aspect of biology, from our understanding of life and death, to how we manage diseases, and more.

What's Next?

Looking for more cell biology explanations? We have articles on everything from parts of the cell (like nucleotides and the endoplasmic reticulum ) to how mitosis works and how it's different from meiosis .

Are there other science topics you want to review? Then you're in luck!   Our guides will teach you loads of useful topics, including  how to convert Celsius to Fahrenheit  and what the density of water is .

What are the most important science classes to take in high school?  Check out our guide to learn all the high school classes you should be taking.

Are you learning about trig identities in your math classes? Learn all the trig identities that you must know by reading our guide!

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Christine graduated from Michigan State University with degrees in Environmental Biology and Geography and received her Master's from Duke University. In high school she scored in the 99th percentile on the SAT and was named a National Merit Finalist. She has taught English and biology in several countries.

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• Biology Article

Eukaryotic Cells

Eukaryotic cell definition.

“Eukaryotic cells are the cells that contain a membrane bound nucleus and organelles.”

Table of Contents

• Explanation
• Characteristics

What is a Eukaryotic Cell?

Eukaryotic cells have a nucleus enclosed within the nuclear membrane and form large and complex organisms. Protozoa, fungi, plants, and animals all have eukaryotic cells. They are classified under the kingdom Eukaryota.

They can maintain different environments in a single cell that allows them to carry out various metabolic reactions. This helps them grow many times larger than the prokaryotic cells.

Also refer:  Difference between Prokaryotic and Eukaryotic Cells

Characteristics of Eukaryotic Cells

The features of eukaryotic cells are as follows:

Eukaryotic cells have the nucleus enclosed within the nuclear membrane.

The cell has mitochondria.

Flagella and cilia are the locomotory organs in a eukaryotic cell.

A cell wall is the outermost layer of the eukaryotic cells.

The cells divide by a process called mitosis.

The eukaryotic cells contain a cytoskeletal structure.

The nucleus contains a single, linear DNA, which carries all the genetic information.

Structure Of Eukaryotic Cell

The eukaryotic cell structure comprises the following:

Plasma Membrane

• The plasma membrane separates the cell from the outside environment.
• It comprises specific embedded proteins, which help in the exchange of substances in and out of the cell.
• A cell wall is a rigid structure present outside the plant cell. It is, however, absent in animal cells.
• It provides shape to the cell and helps in cell-to-cell interaction.
• It is a protective layer that protects the cell from any injury or pathogen attacks.
• It is composed of cellulose, hemicellulose, pectins, proteins, etc.

Also refer: Cell Wall 

Cytoskeleton

The cytoskeleton is present inside the cytoplasm, which consists of microfilaments, microtubules, and fibres to provide perfect shape to the cell, anchor the organelles, and stimulate the cell movement.

Endoplasmic Reticulum

It is a network of small, tubular structures that divides the cell surface into two parts: luminal and extraluminal.

Endoplasmic Reticulum is of two types:

Rough Endoplasmic Reticulum contains ribosomes.

Smooth Endoplasmic Reticulum that lacks ribosomes and is therefore smooth.

• The nucleoplasm enclosed within the nucleus contains DNA and proteins.
• The nuclear envelop consists of two layers- the outer membrane and the inner membrane. Both the membranes are permeable to ions, molecules, and RNA material.
• Ribosome production also takes place inside the nucleus.

Golgi Apparatus

• It is made up of flat disc-shaped structures called cisternae.
• It is absent in red blood cells of humans and sieve cells of plants.
• They are arranged parallel and concentrically near the nucleus.
• It is an important site for the formation of glycoproteins and glycolipids.

Also read: Golgi Apparatus

These are the main site for protein synthesis and are composed of proteins and ribonucleic acids.

Mitochondria

• These are also known as “powerhouse of cells” because they produce energy.
• It consists of an outer membrane and an inner membrane. The inner membrane is divided into folds called cristae.
• They help in the regulation of cell metabolism.

They are known as “suicidal bags” because they possess hydrolytic enzymes to digest protein, lipids, carbohydrates, and nucleic acids.

These are double-membraned structures and are found only in plant cells . These are of three types:

Chloroplast that contains chlorophyll and is involved in photosynthesis.

Chromoplast that contains a pigment called carotene that provides the plants yellow, red, or orange colours.

Leucoplasts that are colourless and store oil, fats, carbohydrates, or proteins.

Eukaryotic Cell Diagram

Eukaryotic cell diagram mentioned below depicts different cell organelles present in eukaryotic cells. The nucleus, endoplasmic reticulum, cytoplasm, mitochondria, ribosomes, lysosomes are clearly mentioned in the diagram.

Explore more about  Cell organelles

Eukaryotic Cell Diagram illustrated above shows the presence of a true nucleus.

Eukaryotic Cell Cycle

The eukaryotic cells divide during the cell cycle. The cell passes through different stages during the cycle. There are various checkpoints between each stage.

Quiescence (G0)

This is known as the resting phase, and the cell does not divide during this stage. The cell cycle starts at this stage. The cells of the liver, kidney, neurons, and stomach all reach this stage and can remain there for longer periods. Many cells do not enter this stage and divide indefinitely throughout their lives.

In this stage, the cells grow and take in nutrients to prepare them for the division. It consists of three

checkpoints:

Gap 1 (G1) – Here the cell enlarges. The proteins also increase.

Synthesis (S) – DNA replication takes place in this phase.

Gap 2 (G2) – Ther cells enlarge further to undergo mitotic division.

Mitosis involves the following stages:

Prometaphase

• Cytokinesis

On division, each daughter cell is an exact replica of the original cell.

Examples of Eukaryotic Cells

Eukaryotic cells are exclusively found in plants, animals, fungi, protozoa, and other complex organisms. The examples of eukaryotic cells are mentioned below:

Plant Cells

The cell wall is made up of cellulose, which provides support to the plant. It has a large vacuole which maintains the turgor pressure. The plant cell contains chloroplast, which aids in the process of photosynthesis.

Fungal Cells

The cell wall is made of chitin. Some fungi have holes known as septa which allow the organelles and cytoplasm to pass through them.

Animal Cells

These do not have cell walls. Instead, they have a cell membrane. That is why animals have varied shapes. They have the ability to perform phagocytosis and pinocytosis.

Protozoans are unicellular organisms. Some protozoa have cilia for locomotion. A thin layer called pellicle provides supports to the cell.

For more information on Eukaryotic Cells, its definition, characteristics, structure, and examples, keep visiting BYJU’S website or download BYJU’S app for further reference.

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Frequently Asked Questions

Are eukaryotic cells unicellular or multicellular.

Eukaryotic cells may be unicellular or multicellular. Paramecium, Euglena, Trypanosoma, Dinoflagellates are unicellular eukaryotes. Plants and animals are multicellular eukaryotes.

What is the most important characteristic of eukaryotic cells that distinguishes it from prokaryotic cells?

Eukaryotic cells have a membrane-bound nucleus. On the contrary, prokaryotic cells lack a true nucleus, i.e., they have no nuclear membrane. Unlike eukaryotic cells, the prokaryotic cells do not have mitochondria, chloroplast and endoplasmic reticulum.

Are viruses eukaryotes?

Viruses are neither eukaryotes nor prokaryotes. Since viruses are a link between living and non-living they are not considered in either category.

What are the salient features of a eukaryotic cell?

A eukaryotic cell has the following important features:

• A eukaryotic cell has a nuclear membrane.
• It has mitochondria, Golgi bodies, cell wall.
• It also contains locomotory organs such as cilia and flagella.
• The nucleus has a DNA that carries all the genetic information.

How does a eukaryotic cell divide?

A eukaryotic cell divides by the process of mitosis. It undergoes the following stages during cell division:

When did the first eukaryotic cell evolve?

The first eukaryotic cells evolved about 2 billion years ago. This is explained by the endosymbiotic theory that explains the origin of eukaryotic cells by the prokaryotic organisms. Mitochondria and chloroplasts are believed to have evolved from symbiotic bacteria.

What is the evidence for endosymbiotic theory?

The first evidence in support of the endosymbiotic theory is that mitochondria and chloroplast have their own DNA and this DNA is similar to the bacterial DNA. The organelles use their DNA to produce several proteins and enzymes to carry out certain activities.

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Cell Membrane: Structure, Composition, and Functions

What is a cell membrane.

The cell membrane, also called the plasma membrane, is a thin layer that surrounds the cytoplasm of all prokaryotic and eukaryotic cells, including plant and animal cells. It is a selectively permeable cell organelle,allowing certain substances inside the cell while preventing others to pass through and thus is analogous to a barrier or gatekeeper in their function. It also serves as the site of attachment for the cytoskeleton that helps to provide shape and support to the cell.

The most widely accepted model of the cell membrane was given by S.J. Singer and Garth L. Nicolson in 1972, popularly known as the fluid mosaic model .

Structure and Composition: What is the Cell Membrane Made Of

The main components that make up all cell membranes are lipids, proteins, and carbohydrates. Their proportions vary between different types of eukaryotic cells, but their basic characteristics remain the same. For a typical human cell, proteins account for about 50 percent of the composition by mass, lipids account for about 40 percent, and the remaining 10 percent comes from carbohydrate molecules. The arrangement of different proteins and lipids in the cell membrane looks like the arrangement found in a mosaic floor.

Described below are the three major parts along with their detailed make up:

It is one of the main components of the cell membrane that makes up the cell’s structural framework. Membrane lipids are of the following types:

• Phospholipid : Major component of the cell membrane forming a bilayer structure. The hydrophilic (water-loving) head of phospholipids points towards the inner cytoplasmic side and outer extracellular fluid. While the hydrophobic (water-hating) tail faces away from them. Thislipid bilayer is semi-permeable, meaning that it allows only certain molecules to diffuse across the membrane.
• Cholesterol : Another lipid component of animal cell membranes that are selectively dispersed between phospholipid molecules. Cholesterol works by preventing phospholipids from being too closely packed thus preventing the cell membranefrom becoming stiff

2. Proteins

It is the second major part of the cell membrane. The two main categories of membrane proteins are:

• Integral Membrane Proteins : Also called intrinsic proteins, they are permanently embedded within the cell membrane. Structurally, the integral proteins are hydrophobic in nature that penetrates the phospholipid bilayer, thus anchoring the protein to the membrane.
• Peripheral Membrane Proteins : Also called extrinsic proteins, they are only temporarily associated with the membrane. Most peripheral membrane proteins are hydrophilic, so they are usually attached to integral membrane proteins or are loosely bound to the phospholipid head group. They help in cell signaling and are often associated with ion channels and transmembrane receptors.

3. Carbohydrates

It is the least abundant component of the cell membrane. Carbohydrates are found on the outside surface of cells that exists in either of the following two forms:

• Glycoproteins : Proteins having carbohydrate chains attached to them . They are embedded within the cell membrane and are important in cell-to-cell communications and transport of substances across the membrane.
• Glycolipids : Lipids having carbohydrate chains attached to them . They are located on the surface of the cell membrane, extending from the phospholipid bilayer into the extracellular environment. Glycolipids help to maintain membrane stability and to facilitate cellular recognition and cell-to-cell communication.

Functions: What Does the Cell Membrane Do 

Primary role.

Selectively Permeable : Creates a potential gradient across the membrane that allows small uncharged molecules such as oxygen, carbon dioxide, and water as well as hydrophobic substances such as lipids to get through the membrane passively inside the cell without any loss of metabolic energy. While charged ions such as sodium, potassium, and calcium as well as large molecules like amino acid and carbohydrates cannot pass through. This is important for the cell to preserve its internal milieu irrespective of any environmental changes and thus is the main function of the cell membrane .

Other Purposes

• Protection and Cell Defense : Insulates the interior of the cell and provides mechanical support from outside shock and harmful agents
• Maintaining Homeostasis : Determines the internal milieu of the cell, the physiological conditions such as temperature and osmotic pressure by maintaining the salt balance
• Maintaining Concentration Gradient : Maintains the differences in concentration of substances inside and outside the cell thus helping in their transport
• Signal Transduction : Receives and processesthe extracellular signals by receptor molecules present in the cell membrane and relay them inside the cell for necessary actions
• Catalysis of Chemical Reactions : Stimulates chemical reactions that help in the growth and metabolism of the cell using enzymes
• Cell Communication : Allows exchange (receiving and sending) of messages between adjacent cellsthus helping them to function in a coordinated fashion
• Adaptation and Response : Helping to sense the extracellular environment and thus regulating the fluidity of the cell membranes by altering the lipid of the cell
• Maintaining Cell Shape and Morphology : Acting as the base of attachment for the cytoskeleton that helps in cell movement

Ans . All living cells, including bacterial cells, have a cell membrane that helps in selective permeability of substances.

Ans . Viruses being non-cellular in origin do not have a cell membrane. In enveloped viruses an analogous structure called the envelope serves similar functions to a cell membrane.

• Cell Membrane – Biologydictionary.net
• Cell Membrane – Britannica.com
• Cell Membrane Function and Structure – Thoughtco.com
• Structure of the Plasma Membrane – Khanacademy.org
• Structure and Function of Plasma Membranes – Courses.lumenlearning.com
• Cell (Plasma) Membrane- Structure, Composition, Functions – Microbenotes.com

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Cell: essay on cells in human body.

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Read this Essay on Cells in Human Body !

The body of any living organism is made up of cells. Cells are very minute in size and extremely complicated in structure. Human is no exception. Each cell is basically a unit of protoplasm, which is said to be the “material basis of life”. The protoplasm is the combination of cytoplasm and nucleus.

The cytoplasm is semi-fluid, jelly-like, hyaline substance; its outer surface is much more thick to form the boundary of the cell. This specialized boundary of the cell is known as cell membrane or plasma membrane. The cell membrane controls the in-and-out of various chemical substances. Various living and non-living bodies are found in the cytoplasm.

Of the living bodies the mitochondria and the Golgi body are very important. The mitochondria are the minute, semi-solid body enclosed in a membrane with a complex internal structure. Although the main constituents of mitochondria are protein and fat, it also contains several enzymes, notably oxidative enzyme systems. The Golgi body is the local clump of material present in the cytoplasm.

Its functions are not clearly known, but seem to be associated with the formation of secretions. The vacuole is important as the fluid-filled space within the cytoplasm of a cell, which controls the cell sap isotonic with cytoplasm by expelling or entering the water from the environment, following the process of osmosis. The centrosome is the region of differentiated cytoplasm containing centriole. The centriole is the minute granule present in many resting cells, just outside the nuclear membrane.

The nucleus is denser than the cytoplasm and it is regarded as the dynamic centre of life. Its shape is absolutely spherical. A membrane, known as nuclear membrane surrounds it to keep the core protected. The nuclear cavity remains filled up with a dense jelly-like substance, known as nucleoplasm. Some delicate thread-like structures that suspend in the nucleoplasm are called nuclear reticulum or chromatin network.

The threads are easily stainable by basic dyes; they are made up of a substance, called chromatin. Apart from these, one or two small spherical bodies are found in the nucleoplasm, known as nucleolus. The nucleus exerts a direct influence on different activities of the cell and plays a great role in transmitting hereditary characters from the parents to offspring.

The cell can be divided into two types – the somatic cell or body cell and the germ cell or reproductive cell. The somatic cell helps in construction and maintenance of different bodily structures. It can also be sub-divided into nerve cells, muscle cells, etc. in accordance with their nature and function.

The germ cell is useful in reproducing new species. It can be sub-divided into sperms or male sex cells and ova or female sex cells. The zygote is also a cell, which is formed by the fusion of an egg or ovum, and a sperm. The human ovum is spherical in shape, and about 1/7th of a millimeter or about 1/175th of an inch in diameter.

Though it is the largest of all cells in human body, still not visible in naked eye. In premature stage, a sperm cell is similar to a somatic cell consisting of a nucleus and a mass of cytoplasm. But a mature sperm cell is long and thread-like with an enlarged head formed by the nucleus. It also possesses a long slender tail and a conical middle piece. The tail is used in movement.

A new life starts when a sperm fertilizes an ovum. After the union of the sperm and the ovum, the fertilization takes place when the head and the middle piece of the sperm sink into the egg. The tail of the sperm is left outside. However, the head of the sperm starts functioning as a normal nucleus by absorbing fluid from the cytoplasm of the ovum.

Fertilization is thus a process of the nuclear fusion. The sperm loses its identity and the fertilized ovum or egg is called a zygote, which begins to divide into two, four, eight and so on, by the process of cell division. From the early stage, some of these cells are set apart to form the germ cells while others go to form various body parts, known as somatic cells.

The cell division is not a simple process; complicated changes have been noted in the substances of the nucleus. In this regard it is necessary to know about the chromosomes. The chromosomes are the slender, rope-like bodies that usually occur in pairs and found in the nucleus. The number of chromosomes remains constant in each species, which generally ranges between 2 to 200.

The size and form of chromosomes also vary from species to species. In fact, differences of chromosome constitution mark off one species from another.In case of man, the usual number of chromosomes is 46. There are altogether 23 pairs of chromosome; 22 pairs act as autosomes and the other pair is known as sex-determining chromosome or sex chromosome.

Chromosomes are composed of two kinds of nucleic acids and two main types of proteins. Chromatin is the most important nucleo-protein in chromosomes. The main nucleic acid of the chromosome is Deoxyribonucleic acid or DNA. In 1951, a renowned biologist James Watson with his chemist friend Francis Crick proposed the structure of DNA.

The other kind of nucleic acid in the chromosomes is the Ribonucleic acid or RNA, which exists in cytoplasm. DNA samples from different tissues of the same species are all identical in composition. RNA not only differs from DNA; the amount of ribonucleic acid in the chromosomes varies from tissue to tissue.

It is found in large quantities in the cytoplasmic particles of the cell. In some cases, the nucleolus also contains large quantities of RNA. It is believed by the scholars that RNA plays an essential role in protein synthesis and carries instruction to the cytoplasm of a cell. In general, the DNA content of an egg is greater than that of a sperm. A variety of suggestions have been made regarding the relationship between the DNA and protein components of the chromosomes but no steady conclusion has yet been achieved.

As a matter of fact the nucleus and the cytoplasm exist in a state of symbiosis. The nucleus depends on the cytoplasm for its energy supply and also for the supply of materials out of which new nucleic acid molecules and chromosomal proteins can be synthesized. On the other hand, the cytoplasm is fully dependent on the nucleus for the maintenance of its essential biosynthetic mechanisms.

Related Articles:

• Fixed Cells: Useful notes on the Structure of Fixed Cells
• Prokaryotic Cells: 7 Most Important Characteristics of Prokaryotic Cells

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Critical Thinking Questions

Why is it advantageous for the cell membrane to be fluid in nature?

Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane?

How can a cell use an extracellular peripheral protein as the receptor to transmit a signal into the cell?

Which explanation identifies how the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled?

Why does water move through a membrane?

Both of the regular intravenous solutions administered in medicine, normal saline and lactated Ringer’s solution, are isotonic. Why is this important?

Describe two ways that decreasing temperature would affect the rate of diffusion of molecules across a cell’s plasma membrane.

A cell develops a mutation in its potassium channels that prevents the ions from leaving the cell. If the cell’s aquaporins are still active, what will happen to the cell? Be sure to describe the tonicity and osmolarity of the cell.

Where does the cell get energy for active transport processes?

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

Glucose from digested food enters intestinal epithelial cells by active transport. Why would intestinal cells use active transport when most body cells use facilitated diffusion?

The sodium/calcium exchanger (NCX) transports sodium into and calcium out of cardiac muscle cells. Describe why this transporter is classified as secondary active transport.

Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?

Why do ions have a difficult time getting through plasma membranes despite their small size?

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Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Access for free at https://openstax.org/books/biology-2e/pages/1-introduction

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Essay on Nucleus: Structure, Position and Functions

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In this essay we will discuss about:- 1. Definition of Nucleus 2. Number of Nucleus 3. Position 4. Shape 5. Biochemical Analysis 6. Structure 7. Functions.

• Essay on the Functions of Nucleus

Essay # 1. Definition of Nucleus:

Nucleus (L. nucleus- kernel) is a specialized double membrane bound protoplasmic body which contains all the genetic information for controlling cellular metabolism and transmission to the posterity.

A nucleus in the non-dividing or metabolic phase is called interphase nucleus. Like other cellular structures, living unstained nucleus does not show much internal differentiation. For detailed study of nucleus, the cells must be properly killed, fixed and stained.

Nucleus is the largest cell organelle. Though first observed by Leeuwenhoek in red blood cor­puscles of fish, nucleus was first studied in orchid root cells by Robert Brown in 1831.

A nucleus is present in all living eukaryotic cells with the excep­tion of mature sieve cells of vascular plants and red blood corpuscles of mammals. Even here a nucleus is present during the early stages of their development. Presence of hereditary information in the nucleus was proved by the work of Joachim Hammerling (1953) on single celled alga Acetabularia (Fig. 8.51).

Essay # 2. Number of Nucleus:

Commonly cells are uninucleate, that is, they possess a single nucleus. The protistan Paramecium caudatum has two nuclei (bi- nucleate), macronucleus for controlling metabolic activities of the organism and micronucleus pos­sessing hereditary information.

Multinucleate or polynucleate condition is found in some cells of bone marrow, striated muscles, latex vessels, sev­eral fungi and algae. Multinucleate animal or protistan cells are called syncytial cells (e.g., epider­mis of Ascaris) while in plants and fungi they are called coenocytic cells (e.g., Rhizopus, Vaucheria). Acellular slime moulds have a multinucleate proto­plasmic body called Plasmodium.

Essay # 3. Position of Nucleus:

Nucleus is usually found in the re­gion of maximum metabolic activity in the cyto­plasm. Commonly it is situated in the geometric centre of the cell. In plant cells it is pushed to peripheral position on one side due to the develop­ment of a large central vacuole. Nucleus is periph­eral in fat-storing cells or adipocytes, and basal in glandular cells. It is suspended in central vacuole by cytoplasmic strands in Spirogyra.

Essay # 4. Shape of Nucleus:

The nuclei are generally rounded in outline. They appear oval or elliptical in plant cells having large central vacuoles. Disc-shaped nuclei occur in the cells of squamous epithelium, lobed in white blood corpuscles and irregularly branched in silk spinning cells of insects.

Essay # 5. Biochemical Analysis of Nucleus:

DNA- 9-12%. RNA- 5%. Lipids- 3%. Basic Proteins- 15%. Acid proteins, neutral proteins and enzymes- 65%. Traces of minerals like Calcium, Mag­nesium, Potassium and Sodium (Phosphorus is a constituent of DNA, RNA and acid pro­teins).

Essay # 6. Structure of Nucleus:

A typical interphase nucleus is 5-25 pm in diameter. It is differentiated into five parts— nuclear envelope, nucleoplasm, nuclear matrix, chromatin and nucleolus (Fig. 8.53).

a. Nuclear Envelope (= Karyotheca):

It bounds the nucleus on the outside. The nuclear envelope separates the nucleus from the cytoplasm. It is made up of two lipoprotein and trilaminar membranes, each of which is 60-90A thick. The inner membrane is smooth.

The outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the rough endoplasmic reticulum. The two membranes of the nuclear envelope are separated by an electron transparent perinuclear space. The space is 100—500 A in width. The outer membrane is often connected to endoplasmic reticulum.

Nuclear envelope contains a large number of pores or perforations (Fig. 8.52). In some cases 10% of the envelope is occupied by pores. The two membranes of the envelope become continuous in the region of pores.

Nuclear pores have complex structure. They may have diaphragm, septum, plug of electron dense material or nucleoplasmin, blebs or annuli. Annuli are circular structures around the pores. The pores and their annuli form a pore complex called annulated pore.

An annulated nuclear pore may possess 9 cylinders, one central and eight peripheral. Instead, there may be a network of granules and filaments. The nuclear pores control the passage of substances to the inside or outside of the nucleus, e.g., RNAs, ribosomes, proteins.

b. Nucleoplasm (Nuclear Sap, Karyolymph, Strasburger, 1882):

It is a transparent, semifluid and colloidal substance which fills the nucleus. It contains nucleosides and a number of enzymes (e.g., DNA polymerase, RNA polymerase, nucleoside phosphorylase) which are required for the synthesis and functioning of DNA, RNA, nucleoproteins, etc. Some of the proteins present in nucleoplasm are essential for spindle formation.

c. Nuclear Matrix:

It is a network of fine fibrils of acid proteins that function as scaffold for chromatin. On the periphery, below the nuclear envelope, nuclear matrix forms a dense fibrous layer called nuclear lamina. Terminal ends of chromatin fibres or telomeres are embedded in nuclear or fibrous lamina. Nuclear matrix consists of two types of inter­mediate filaments, lamin A and lamin B.

Nuclear matrix and nuclear lamina form:

(i) Scaffold for chromatin,

(ii) Attachment sites to telomeric parts,

(iii) Mechanical strength to nuclear envelope, and

(iv) Components of nuclear pore complex.

d. Chromatin:

It is hereditary DNA-protein fibrillar complex which is named so be­cause of its ability to get stained with certain basic dyes. Chromatin occurs in the form of fine overlapping and coiled fibres which appear to produce a network called chromatin reticulum.

Chromatin fibres are distributed throughout the nucleoplasm. They are differentiated into two regions— euchromatin and heterochro­matin, Heitz (1928). Euchromatin is narrow (10-30nm thick) lightly stained and diffused fibrous part which forms the bulk of chromatin.

Heterochromatin is wider (100 nm thick), darkly stained and condensed granular part which is attached here and there on the euchro­matin. Depending upon the size of granules formed by heterochromatin they are called chromocentres, karyosomes or false nucleoli.

The whole of chromatin is not functional. Generally only a portion of euchromatin which is associated with acid proteins takes part in transcription or formation of RNAs. During prophase of nuclear division, the chromatin fibres condense to form a definite number of thread-like structures called chromosomes.

e. Nucleolus (plural-nucleoli):

It was first discovered by Fontana in 1781, described by Wagner in 1840 and provided with its present name by Bowman in 1840. Nucleolus is a naked, round or slightly irregular structure which is attached to the chromatin at a specific region called nucleolar organizer region (NOR).

Commonly 1-4 nucleoli are found in a nucleus. Up to 1600 nucleoli are reported in the oocytes of Xenopus. A covering membrane is absent around nucleolus. Calcium seems to be essential for maintaining its configuration. Nucleolus has four components— amorphous matrix, granular part, fibrillar portion and chromatin (Fig. 8.54).

(i) Amorphous Matrix:

It is the homoge­neous ground substance of the nucleolus. Matrix is formed of protein.

(ii) Granular Portion:

It consists of gran­ules of the size of 150-200 A which lie scattered in the amorphous matrix. The granules are formed of protein and RNA in the ratio of 2:1. They are believed to be precursors of ribosomes.

(iii) Fibrillar Portion (Nucleolonema):

It is formed of a large number of small fibrils that are 50—so A long. The fibrils are made up of both protein and RNA and are believed to be precursors of granules.

(iv) Chromatin Portion:

It is that part of chromatin which is associated with nucleolus. Depending upon its position nucleolar chromatin is of two types— perinucleolar and intra-nucleolar. The perinucleolar chromatin lies around the periphery of the nucleolus. It gives rise to ingrowths or trabeculae which produce the intra-nucleolar chromatin.

(i) Nucleolus is the principal site for the development of ribosomal RNAs.

(ii) It is the centre for the formation of ribosome components,

(iii) Nucleolus stores nucleoproteins. The same are synthesised in the cytoplasm (over the ribosomes) and transferred to nucleolus,

(iv) It is essential for spindle formation during nuclear division.

Essay # 7. Functions of Nucleus:

Nucleus is an essential and integral part of the eukaryote cell. It stores genetic information in its DNA molecules which can be passed on to daughter cells. It also controls cellular activities.

i. Chromatin:

Nucleus contains hereditary material called chromatin. Chromatin is DNA- protein complex. It is made of a number of fine fibres that condense to form chromosomes. Number of chromosomes is fixed for a species. They bear genes.

ii. Genetic Information:

Chromatin part of nucleus possesses all the genetic information that is required for growth and development of the organism, its reproduction, metabolism and behaviour.

iii. Cellular Activities:

Nucleus controls cell metabolism and other activities through the formation of RNAs (mRNA, rRNA, tRNA) which control synthesis of particular type of enzymes.

iv. Ribosomes:

Ribosomes are formed in nucleolus part of the nucleus.

v. Variations:

All variations are caused by changes in genetic material present in the nucleus.

vi. Cell Growth and Maintenance:

With the help of RNAs, nucleus directs the synthesis of some structural proteins and chemicals required for cell growth and maintenance.

vii. Cell Differentiation:

It directs cell differentiation by allowing certain particular sets of genes to operate.

viii. Cell Replication:

Replication of nucleus is essential for cell replication.

Related Articles:

• Structure of Nucleus (With Diagram) | Botany
• Nucleus of the Controlling Centre of Cell (With Diagram)

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The structure of cell: a research on the bricks of the human body!

Aakanksha M.

Checked : T. M. H. , Grayson N.

Latest Update 20 Jan, 2024

Table of content

Hypothesis on the origin of the cell

The structure of a eukaryotic cell, there are two types of cells, cell membrane, mitochondria, endoplasmatic reticle, dna and the genetic code, dna structure, the genetic code.

We are made up of 100,000 billion cells. Understanding the basic composition and functioning of the cell can help us understand what happens at more complex levels of organs. The first phase is called chemical evolution, from very simple molecules such as hydrogen, ammonia, methane, monoxide, and carbon dioxide, by the action of ultraviolet radiation (sun), heat, and electric discharges (lightning). They are very reactive intermediate molecules formed such as acetaldehyde, hydrogen cyanide, formaldehyde, and perhaps even some simple fatty acids such as acetic acid and some simple amino acids such as aniline.

A second phase followed, called   biological evolution , in which more complex molecules of acids, amino acids, proteins, and especially nucleic acids were formed.

• The first prokaryotic cell (plant cell) appeared about 3.5-3.0 billion years ago.
• The first eukaryotic cell (animal cell) appeared much later, around 0.9 billion years ago.
• Homo sapiens made his appearance 400,000 years ago.

According to Jacques Monod and other authors, the gradually increasing order structures formed in the cell are determined by the genetic information contained in the DNA from the primary sequence of a polypeptide chain. There are gradually the secondary and tertiary structures of the proteins, then the formation of compounds oligomeric, and finally, by the interaction between proteins, lipids, and nucleic acids, the formation of the first simple plant cells.

Probably, only with the appearance of the genetic code was it possible to determine the formation of the first heterotrophic prokaryotic cell (which feeds on organic molecules) and anaerobic (which lives in the absence of oxygen). Subsequently, autotrophic prokaryotes appeared (using the food synthesized inside), obviously provided with photosynthetic pigments.

With the appearance and accumulation of oxygen, the first autotrophic and aerobic prokaryotic cells are born. In turn, this type of cell opens the way for the formation of the first eukaryotic cell, which appears after 2.6-3.1 billion years after that of the prokaryotic cell.

The cells have uniform morphological characters only in the simplest organisms; in others, the cells differ in shape, size, and tasks.

• Eukaryotic cell (animals and humans), which contains a cell nucleus delimited by a nuclear membrane and separate organelles;
• Prokaryotic (plant) cells: These cells are instead free of the nuclear membrane and intracellular organelles, with the exception of ribosomes.

In cells, we can distinguish:

The typical structure of the cell membrane consists of a double phospholipidic layer between two protein layers located at the level of the separation surfaces between the internal and external phases of the cell. Water enters and exits easily, while for many other substances, this passage is not so easy.

This ability of the membrane to allow or not the passage of substances is called “selective capacity.”

It constitutes the most important part of the cell by controlling all its activities. For example, the nucleus brings hereditary information to the cell for the construction of a "unique" type of organism. It directs the cell's activities, ensuring that the complex molecules that the cell requires for the realization of the various cellular activities and for the formation of organelles and other structures. They are of the necessary number and type; decides when the right time for cellular reproduction is and controls all phases.

The nucleus is wrapped in the nuclear membrane that separates it from the cytoplasm. The nuclear membrane is sprinkled with numerous small holes called nuclear pores, due to which various substances pass from the cytoplasm into the nucleus.

The nucleus contains DNA combined with proteins (chromatin). Chromosomal DNA performs two types of activity, i.e., auto synthetic and all-synthetic; in the first case, the DNA molecule is replicated through a semi-conservative process; in the second case, it synthesizes the three types of RNA. When the cell divides, chromosomes form from the chromatin. The cell genome is inherent in the chromosomes. Inside the nucleus, there is a very dense body, the nucleolus, also formed, like chromosomes, by DNA and proteins; a particular type of RNA, ribosomal RNA, is also formed in the nucleolus.

It is the viscous substance between the cell membrane and the nuclear envelope. The majority of cellular activities take place inside the cytoplasm, and the energy necessary for the life of the cell is also produced. In it, there are organelles used for the various functions that the cell must perform. They are:

Microscopic power plants, where the formation of ATP (energy necessary for cells) takes place through biochemical reactions

They are of fundamental importance because protein synthesis occurs in them. Ribosomes are the most numerous cell organelles; they consist of two subunits, a major and a minor, which dissociate at the end of each protein synthesis cycle. The RNA sequence is translated into the corresponding sequence of amino acids assembled to form the protein. The ribosome is, therefore, an apparatus for synthesizing proteins, capable of bringing together in the appropriate arrangement the molecules necessary for the synthesis reaction.

Intracellular vesicular organelles delimited by a single membrane, containing various   enzymes , and localizable around the nucleus It can be considered the digestive system of the eukaryotic cell, for the action, carried out by the various enzymes with degradation. For example, of glycoproteins and glycolipids by means of lysosomal hydrolases, and degradation of senescent components of the same cell, as well as recycling and transport of degraded molecules to the sectors where they are necessary for the synthesis of new products, with consequent energy savings.

Dynamic structure, which increases or decreases according to cellular activity. In cells, we have the rough endoplasmic reticulum and the smooth endoplasmic reticulum. The first one is ribosomes, which are attached and involved in the synthesis and the transport of proteins out of the cell. The second, which physically is a portion of the same rough endoplasmic reticulum, but is free of ribosomes, is important in the synthesis of lipids.

The following concepts serve to deepen how the DNA of a cell, for example, establishes which proteins are needed by our body and when they are needed.

DNA is present in all cells capable of reproducing. Even if it does not take part directly in protein synthesis, DNA contains the code to build all the proteins that are needed starting from the 20 amino acids, most of which are also synthesized by cells, except for some amino acids which must necessarily be introduced with the diet, and which therefore are called essential Deoxyribonucleic or deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic information necessary for the biosynthesis of RNA and proteins.

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DNA is an organic polymer made up of monomers called nucleotides.

All nucleotides are made up of three basic components:

• a phosphate group;
• deoxyribose (pentose sugar);
• A nitrogenous base that binds to deoxyribose with an N-glycosidic bond.

Four nitrogen bases can be used in the formation of nucleotides and incorporated in the DNA molecule, i.e., adenine (A), guanine (G), cytosine (C), and thymine (T).

To preserve and exchange genetic information, in over 3 billion years, nature has built the genetic code. The DNA contains adenine, thymine, guanine, and cytosine. Three base pairs form a codon, which identifies either a specific amino acid to be used for protein synthesis, or a signal to stop the synthesis itself. Almost all living things use the same genetic code, called the standard genetic code.

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• How to structure an essay: Templates and tips

How to Structure an Essay | Tips & Templates

Published on September 18, 2020 by Jack Caulfield . Revised on July 23, 2023.

The basic structure of an essay always consists of an introduction , a body , and a conclusion . But for many students, the most difficult part of structuring an essay is deciding how to organize information within the body.

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Table of contents

The basics of essay structure, chronological structure, compare-and-contrast structure, problems-methods-solutions structure, signposting to clarify your structure, other interesting articles, frequently asked questions about essay structure.

There are two main things to keep in mind when working on your essay structure: making sure to include the right information in each part, and deciding how you’ll organize the information within the body.

Parts of an essay

The three parts that make up all essays are described in the table below.

Order of information

You’ll also have to consider how to present information within the body. There are a few general principles that can guide you here.

The first is that your argument should move from the simplest claim to the most complex . The body of a good argumentative essay often begins with simple and widely accepted claims, and then moves towards more complex and contentious ones.

For example, you might begin by describing a generally accepted philosophical concept, and then apply it to a new topic. The grounding in the general concept will allow the reader to understand your unique application of it.

The second principle is that background information should appear towards the beginning of your essay . General background is presented in the introduction. If you have additional background to present, this information will usually come at the start of the body.

The third principle is that everything in your essay should be relevant to the thesis . Ask yourself whether each piece of information advances your argument or provides necessary background. And make sure that the text clearly expresses each piece of information’s relevance.

The sections below present several organizational templates for essays: the chronological approach, the compare-and-contrast approach, and the problems-methods-solutions approach.

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The chronological approach (sometimes called the cause-and-effect approach) is probably the simplest way to structure an essay. It just means discussing events in the order in which they occurred, discussing how they are related (i.e. the cause and effect involved) as you go.

A chronological approach can be useful when your essay is about a series of events. Don’t rule out other approaches, though—even when the chronological approach is the obvious one, you might be able to bring out more with a different structure.

Explore the tabs below to see a general template and a specific example outline from an essay on the invention of the printing press.

• Thesis statement
• Discussion of event/period
• Consequences
• Importance of topic
• Strong closing statement
• Claim that the printing press marks the end of the Middle Ages
• Background on the low levels of literacy before the printing press
• Thesis statement: The invention of the printing press increased circulation of information in Europe, paving the way for the Reformation
• High levels of illiteracy in medieval Europe
• Literacy and thus knowledge and education were mainly the domain of religious and political elites
• Consequence: this discouraged political and religious change
• Invention of the printing press in 1440 by Johannes Gutenberg
• Implications of the new technology for book production
• Consequence: Rapid spread of the technology and the printing of the Gutenberg Bible
• Trend for translating the Bible into vernacular languages during the years following the printing press’s invention
• Luther’s own translation of the Bible during the Reformation
• Consequence: The large-scale effects the Reformation would have on religion and politics
• Summarize the history described
• Stress the significance of the printing press to the events of this period

Essays with two or more main subjects are often structured around comparing and contrasting . For example, a literary analysis essay might compare two different texts, and an argumentative essay might compare the strengths of different arguments.

There are two main ways of structuring a compare-and-contrast essay: the alternating method, and the block method.

Alternating

In the alternating method, each paragraph compares your subjects in terms of a specific point of comparison. These points of comparison are therefore what defines each paragraph.

The tabs below show a general template for this structure, and a specific example for an essay comparing and contrasting distance learning with traditional classroom learning.

• Synthesis of arguments
• Topical relevance of distance learning in lockdown
• Increasing prevalence of distance learning over the last decade
• Thesis statement: While distance learning has certain advantages, it introduces multiple new accessibility issues that must be addressed for it to be as effective as classroom learning
• Classroom learning: Ease of identifying difficulties and privately discussing them
• Distance learning: Difficulty of noticing and unobtrusively helping
• Classroom learning: Difficulties accessing the classroom (disability, distance travelled from home)
• Distance learning: Difficulties with online work (lack of tech literacy, unreliable connection, distractions)
• Classroom learning: Tends to encourage personal engagement among students and with teacher, more relaxed social environment
• Distance learning: Greater ability to reach out to teacher privately
• Sum up, emphasize that distance learning introduces more difficulties than it solves
• Stress the importance of addressing issues with distance learning as it becomes increasingly common
• Distance learning may prove to be the future, but it still has a long way to go

In the block method, each subject is covered all in one go, potentially across multiple paragraphs. For example, you might write two paragraphs about your first subject and then two about your second subject, making comparisons back to the first.

The tabs again show a general template, followed by another essay on distance learning, this time with the body structured in blocks.

• Point 1 (compare)
• Point 2 (compare)
• Point 3 (compare)
• Point 4 (compare)
• Advantages: Flexibility, accessibility
• Disadvantages: Discomfort, challenges for those with poor internet or tech literacy
• Advantages: Potential for teacher to discuss issues with a student in a separate private call
• Disadvantages: Difficulty of identifying struggling students and aiding them unobtrusively, lack of personal interaction among students
• Advantages: More accessible to those with low tech literacy, equality of all sharing one learning environment
• Disadvantages: Students must live close enough to attend, commutes may vary, classrooms not always accessible for disabled students
• Advantages: Ease of picking up on signs a student is struggling, more personal interaction among students
• Disadvantages: May be harder for students to approach teacher privately in person to raise issues

An essay that concerns a specific problem (practical or theoretical) may be structured according to the problems-methods-solutions approach.

This is just what it sounds like: You define the problem, characterize a method or theory that may solve it, and finally analyze the problem, using this method or theory to arrive at a solution. If the problem is theoretical, the solution might be the analysis you present in the essay itself; otherwise, you might just present a proposed solution.

The tabs below show a template for this structure and an example outline for an essay about the problem of fake news.

• Introduce the problem
• Provide background
• Describe your approach to solving it
• Define the problem precisely
• Describe why it’s important
• Indicate previous approaches to the problem
• Present your new approach, and why it’s better
• Apply the new method or theory to the problem
• Indicate the solution you arrive at by doing so
• Assess (potential or actual) effectiveness of solution
• Describe the implications
• Problem: The growth of “fake news” online
• Prevalence of polarized/conspiracy-focused news sources online
• Thesis statement: Rather than attempting to stamp out online fake news through social media moderation, an effective approach to combating it must work with educational institutions to improve media literacy
• Definition: Deliberate disinformation designed to spread virally online
• Popularization of the term, growth of the phenomenon
• Previous approaches: Labeling and moderation on social media platforms
• Critique: This approach feeds conspiracies; the real solution is to improve media literacy so users can better identify fake news
• Greater emphasis should be placed on media literacy education in schools
• This allows people to assess news sources independently, rather than just being told which ones to trust
• This is a long-term solution but could be highly effective
• It would require significant organization and investment, but would equip people to judge news sources more effectively
• Rather than trying to contain the spread of fake news, we must teach the next generation not to fall for it

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Signposting means guiding the reader through your essay with language that describes or hints at the structure of what follows.  It can help you clarify your structure for yourself as well as helping your reader follow your ideas.

The essay overview

In longer essays whose body is split into multiple named sections, the introduction often ends with an overview of the rest of the essay. This gives a brief description of the main idea or argument of each section.

The overview allows the reader to immediately understand what will be covered in the essay and in what order. Though it describes what  comes later in the text, it is generally written in the present tense . The following example is from a literary analysis essay on Mary Shelley’s Frankenstein .

Transitions

Transition words and phrases are used throughout all good essays to link together different ideas. They help guide the reader through your text, and an essay that uses them effectively will be much easier to follow.

Various different relationships can be expressed by transition words, as shown in this example.

Because Hitler failed to respond to the British ultimatum, France and the UK declared war on Germany. Although it was an outcome the Allies had hoped to avoid, they were prepared to back up their ultimatum in order to combat the existential threat posed by the Third Reich.

Transition sentences may be included to transition between different paragraphs or sections of an essay. A good transition sentence moves the reader on to the next topic while indicating how it relates to the previous one.

… Distance learning, then, seems to improve accessibility in some ways while representing a step backwards in others.

However , considering the issue of personal interaction among students presents a different picture.

If you want to know more about AI tools , college essays , or fallacies make sure to check out some of our other articles with explanations and examples or go directly to our tools!

• Ad hominem fallacy
• Post hoc fallacy
• Appeal to authority fallacy
• False cause fallacy
• Sunk cost fallacy

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The structure of an essay is divided into an introduction that presents your topic and thesis statement , a body containing your in-depth analysis and arguments, and a conclusion wrapping up your ideas.

The structure of the body is flexible, but you should always spend some time thinking about how you can organize your essay to best serve your ideas.

An essay isn’t just a loose collection of facts and ideas. Instead, it should be centered on an overarching argument (summarized in your thesis statement ) that every part of the essay relates to.

The way you structure your essay is crucial to presenting your argument coherently. A well-structured essay helps your reader follow the logic of your ideas and understand your overall point.

Comparisons in essays are generally structured in one of two ways:

• The alternating method, where you compare your subjects side by side according to one specific aspect at a time.
• The block method, where you cover each subject separately in its entirety.

It’s also possible to combine both methods, for example by writing a full paragraph on each of your topics and then a final paragraph contrasting the two according to a specific metric.

You should try to follow your outline as you write your essay . However, if your ideas change or it becomes clear that your structure could be better, it’s okay to depart from your essay outline . Just make sure you know why you’re doing so.

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