the are two stages of photosynthesis

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Two Stages of Photosynthesis

What Happens in the Light Reaction of Photosynthesis?

Photosynthesis represents the biological process by which plants convert light energy into sugar to fuel plant cells. Comprised of two stages, one stage converts the light energy into sugar, and then cellular respiration converts the sugar to Adenosine triphosphate, known as ATP, the fuel for all cellular life. The conversion of unusable sunlight makes plants green.

While the mechanisms of photosynthesis are complex, the overall reaction occurs as follows: carbon dioxide + sunlight + water ---> glucose (sugar) + molecular oxygen. Photosynthesis takes place through several steps which occur during two stages: the light phase and the dark phase.

Stage One: Light Reactions

In the light-dependent process, which takes place in the grana, the stacked membrane structure within chloroplasts, the direct energy of light helps the plant to make molecules that carry energy for utilization in the dark phase of photosynthesis. The plant uses light energy to generate the co-enzyme Nicotinamide adenine dinucleotide phosphate, or NADPH and ATP, the molecules that carry energy. The chemical bonds in these compounds store the energy and are used during the dark phase.

Stage Two: Dark Reactions

The dark phase, which takes place in the stroma and in the dark when the molecules that carry energy are present, is also known as the Calvin cycle or C 3 cycle. The dark phase uses the ATP and NADPH generated in the light phase to make C-C covalent bonds of carbohydrates from carbon dioxide and water, with the chemical ribulose biphosphate or RuBP, a 5-C chemical capturing the carbon dioxide. Six molecules of carbon dioxide enter the cycle, which in turn produces one molecule of glucose or sugar.

How Photosynthesis Works

A key component that drives photosynthesis is the molecule chlorophyll. Chlorophyll is a large molecule with a special structure that enables it to capture light energy and convert it to high energy electrons, which are used during the reactions of the two phases to ultimately produce the sugar or glucose.

In photosynthetic bacteria, the reaction takes place in the cell membrane and within the cell, but outside of the nucleus. In plants and photosynthetic protozoans -- protozoans are single-celled organisms belonging to the eukaryote domain, the same domain of life which includes plants, animals and fungus -- photosynthesis takes place within chloroplasts. Chloroplasts are a type of organelle or membrane-bound compartments, adapted for specific functions like creating the energy for plants.

Chloroplasts -- An Evolutionary Tale

While chloroplasts exist today within other cells, such as plant cells, they have their own DNA and genes. Analysis of the sequence of these genes has revealed that chloroplasts evolved from independently-living photosynthetic organisms related to a group of bacteria called cyanobacteria.

A similar process occurred when the ancestors of mitochondria, the organelles within cells where oxidative respiration, the chemical opposite of photosynthesis, takes place. According to the theory of endosymbiosis, a theory which was given a boost recently, because of a new study published in the journal Nature, both chloroplasts and mitochondria once lived as independent bacteria, but were engulfed within the ancestors of eukaryotes, leading ultimately to the emergence of plants and animals.

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• Oklahoma State University: Photosynthesis
• Georgia State University: NAD (Nicotinamide adenine dinucleotide)
• Georgia State University: The Calvin Cycle
• Nature Journal of Science: Gene Transfer to the Nucleus and the Evolution of Chloroplasts

About the Author

David Warmflash is an astrobiologist-writer, with a passion for communicating science to the general public. He serves as lead investigator for the Living Interplanetary Flight Experiment (LIFE), a Planetary Society-sponsored project, scheduled for launch in 2011 on the Russian Space Agency's Phobos-Grunt probe. Additionally, he is fascinated with ancient history.

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ENCYCLOPEDIC ENTRY

Photosynthesis.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

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Learning materials, instructional links.

• Photosynthesis (Google doc)

Most life on Earth depends on photosynthesis .The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O 2 ) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO 2 ) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts , which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent Reactions vs. Light-independent Reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH . The light-independent stage, also known as the Calvin cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 Photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water. The National Geographic Society is making this content available under a Creative Commons CC-BY-NC-SA license . The License excludes the National Geographic Logo (meaning the words National Geographic + the Yellow Border Logo) and any images that are included as part of each content piece. For clarity the Logo and images may not be removed, altered, or changed in any way.

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Development of the idea

Overall reaction of photosynthesis.

• Basic products of photosynthesis
• Evolution of the process
• Light intensity and temperature
• Carbon dioxide
• Internal factors
• Energy efficiency of photosynthesis
• Structural features
• Light absorption and energy transfer
• The pathway of electrons
• Evidence of two light reactions
• Photosystems I and II
• Quantum requirements
• The process of photosynthesis: the conversion of light energy to ATP
• Elucidation of the carbon pathway
• Carboxylation
• Isomerization/condensation/dismutation
• Phosphorylation
• Regulation of the cycle
• Products of carbon reduction
• Photorespiration
• Carbon fixation in C 4 plants
• Carbon fixation via crassulacean acid metabolism (CAM)
• Differences in carbon fixation pathways
• The molecular biology of photosynthesis

Why is photosynthesis important?

What is the basic formula for photosynthesis, which organisms can photosynthesize.

photosynthesis

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• Khan Academy - Photosynthesis
• Biology LibreTexts - Photosynthesis
• University of Florida - Institute of Food and Agricultural Sciences - Photosynthesis
• Milne Library - Inanimate Life - Photosynthesis
• National Center for Biotechnology Information - Chloroplasts and Photosynthesis
• Roger Williams University Pressbooks - Introduction to Molecular and Cell Biology - Photosynthesis
• BCcampus Open Publishing - Concepts of Biology – 1st Canadian Edition - Overview of Photosynthesis
• photosynthesis - Children's Encyclopedia (Ages 8-11)
• photosynthesis - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

Photosynthesis is critical for the existence of the vast majority of life on Earth. It is the way in which virtually all energy in the biosphere becomes available to living things. As primary producers, photosynthetic organisms form the base of Earth’s food webs and are consumed directly or indirectly by all higher life-forms. Additionally, almost all the oxygen in the atmosphere is due to the process of photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of gaseous oxygen.

The process of photosynthesis is commonly written as: 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 . This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products. The sugar is used by the organism, and the oxygen is released as a by-product.

The ability to photosynthesize is found in both eukaryotic and prokaryotic organisms. The most well-known examples are plants, as all but a very few parasitic or mycoheterotrophic species contain chlorophyll and produce their own food. Algae are the other dominant group of eukaryotic photosynthetic organisms. All algae, which include massive kelps and microscopic diatoms , are important primary producers.  Cyanobacteria and certain sulfur bacteria are photosynthetic prokaryotes, in whom photosynthesis evolved. No animals are thought to be independently capable of photosynthesis, though the emerald green sea slug can temporarily incorporate algae chloroplasts in its body for food production.

photosynthesis , the process by which green plants and certain other organisms transform light energy into chemical energy . During photosynthesis in green plants, light energy is captured and used to convert water , carbon dioxide , and minerals into oxygen and energy-rich organic compounds .

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth . If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria , which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal , oil , and gas ) that power industrial society . In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation , these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate .

Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to those needs—the so-called Green Revolution , begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical fertilizers , pest and plant- disease control, plant breeding , and mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas of the world despite rapid population growth , but it did not eliminate widespread malnutrition . Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant development of new plant varieties, also became problematic for farmers in many countries.

A second agricultural revolution , based on plant genetic engineering , was forecast to lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA ) with the aim of achieving improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug ( Elysia chlorotica ), for example, acquires genes and chloroplasts from Vaucheria litorea , an alga it consumes, giving it a limited ability to produce chlorophyll . When enough chloroplasts are assimilated , the slug may forgo the ingestion of food. The pea aphid ( Acyrthosiphon pisum ) can harness light to manufacture the energy-rich compound adenosine triphosphate (ATP); this ability has been linked to the aphid’s manufacture of carotenoid pigments.

General characteristics

The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist Joseph Priestley . Priestley had burned a candle in a closed container until the air within the container could no longer support combustion . He then placed a sprig of mint plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician Jan Ingenhousz expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.

In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots; the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.

This equation is merely a summary statement, for the process of photosynthesis actually involves numerous reactions catalyzed by enzymes (organic catalysts ). These reactions occur in two stages: the “light” stage, consisting of photochemical (i.e., light-capturing) reactions; and the “dark” stage, comprising chemical reactions controlled by enzymes . During the first stage, the energy of light is absorbed and used to drive a series of electron transfers, resulting in the synthesis of ATP and the electron-donor-reduced nicotine adenine dinucleotide phosphate (NADPH). During the dark stage, the ATP and NADPH formed in the light-capturing reactions are used to reduce carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.

Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO 2 to form CH 2 O).

By 1940 chemists were using heavy isotopes to follow the reactions of photosynthesis. Water marked with an isotope of oxygen ( 18 O) was used in early experiments. Plants that photosynthesized in the presence of water containing H 2 18 O produced oxygen gas containing 18 O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided definitive support for van Niel’s theory that the oxygen gas produced during photosynthesis is derived from water.

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Photosynthesis

What is photosynthesis.

It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word ‘photosynthesis’ is derived from the Greek word phōs, meaning ‘light’ and synthesis meaning ‘combining together.’

Jan Ingenhousz, the Dutch-born British physician and scientist, discovered the process of photosynthesis.

Where does Photosynthesis Occur

Photosynthesis takes place mainly in the leaves of green plants and also in the stems of herbaceous plants as they also contain chlorophyll. Sometimes it also occurs in roots that contain chlorophyll like in water chestnut and Heart-leaved moonseed. Apart from plants, photosynthesis is also found to occur in blue-green algae.

What Happens During Photosynthesis

It involves a chemical reaction where water, carbon dioxide, chlorophyll, and solar energy are utilized as raw materials (inputs) to produce glucose, oxygen, and water (outputs).

Stages of the Process

Photosynthesis occurs in two stages:

1) The Light-dependent Reaction

• Takes place in the thylakoid membranes of chloroplasts only during the day in the presence of sunlight
• High-energy phosphate molecules adenosine triphosphate ( ATP ) and the reducing agent NADPH are produced with the help of electron transport chain

2) The Light-independent or Dark Reaction ( Calvin cycle )

• Takes place in the stroma of chloroplast in the absence of light that helps to fix carbon
• ATP and NADPH produced in the light reaction are utilized along with carbon dioxide to produce sugar in the form of glucose

Factors Affecting the Rate of Photosynthesis

• Intensity of Light: The higher intensity of light increases the rate of photosynthesis
• Temperature:  Warmer the temperature, higher the rate of photosynthesis. The rate is highest between the temperatures of 25° to 35° C, after which it starts to decrease
• Concentration of Carbon dioxide: Higher concentration of carbon dioxide increases the rate of photosynthesis until it reaches a certain point, beyond which no further effects are found   

Although all the above factors together interact to affect the rate of photosynthesis, each of them individually is also capable of directly influencing the process without the other factors and thus called limiting factors.

Importance of Photosynthesis

It serves two main purposes that are essential to support life on earth:

• Producing food for organisms that depend on others for their nutrition such as humans along with all other animals
• Synthesizing oxygen by replacing carbon dioxide in the atmosphere

Ans. Photosynthesis is an endothermic reaction because it absorbs the heat of the sun to carry out the process.

Ans. The oxygen in photosynthesis comes from splitting the water molecules.

Ans. Chlorophyll is the main light-absorbing pigment in photosynthesis.

Ans. The role of water is to provide oxygen in the form of oxygen gas to the atmosphere.

Ans. Sunlight is the source of energy that drives photosynthesis.

Ans. The easiest way to measure the rate of photosynthesis is to quantify the carbon dioxide or oxygen levels using a data logger. The rate of photosynthesis can also be measured by determining the increase in the plant ’s biomass (weight).

Ans. Photosynthesis is an energy-requiring process occurring only in green plants, algae, and certain bacteria that utilizes carbon dioxide and water to produce food in the form of carbohydrates. In contrast, cellular respiration is an energy-releasing process found in all living organisms where oxygen and glucose are utilized to produce carbon dioxide and water.

Ans. Glucose produced in photosynthesis is used in cellular respiration to make ATP.

Article was last reviewed on Tuesday, April 21, 2020

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Photosynthesis

Reviewed by: BD Editors

Photosynthesis Definition

Photosynthesis is the biochemical pathway which converts the energy of light into the bonds of glucose molecules. The process of photosynthesis occurs in two steps. In the first step, energy from light is stored in the bonds of adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH). These two energy-storing cofactors are then used in the second step of photosynthesis to produce organic molecules by combining carbon molecules derived from carbon dioxide (CO 2 ). The second step of photosynthesis is known as the Calvin Cycle. These organic molecules can then be used by mitochondria to produce ATP, or they can be combined to form glucose, sucrose, and other carbohydrates. The chemical equation for the entire process can be seen below.

Photosynthesis Equation

Above is the overall reaction for photosynthesis. Using the energy from light and the hydrogens and electrons from water, the plant combines the carbons found in carbon dioxide into more complex molecules. While a 3-carbon molecule is the direct result of photosynthesis, glucose is simply two of these molecules combined and is often represented as the direct result of photosynthesis due to glucose being a foundational molecule in many cellular systems. You will also notice that 6 gaseous oxygen molecules are produced, as a by-produce. The plant can use this oxygen in its mitochondria during oxidative phosphorylation . While some of the oxygen is used for this purpose, a large portion is expelled into the atmosphere and allows us to breathe and undergo our own oxidative phosphorylation, on sugar molecules derived from plants. You will also notice that this equation shows water on both sides. That is because 12 water molecules are split during the light reactions, while 6 new molecules are produced during and after the Calvin cycle. While this is the general equation for the entire process, there are many individual reactions which contribute to this pathway.

Stages of Photosynthesis

The light reactions.

The light reactions happen in the thylakoid membranes of the chloroplasts of plant cells. The thylakoids have densely packed protein and enzyme clusters known as photosystems . There are two of these systems, which work in conjunction with each other to remove electrons and hydrogens from water and transfer them to the cofactors ADP and NADP + . These photosystems were named in the order of which they were discovered, which is opposite of how electrons flow through them. As seen in the image below, electrons excited by light energy flow first through photosystem II (PSII), and then through photosystem I (PSI) as they create NADPH. ATP is created by the protein ATP synthase , which uses the build-up of hydrogen atoms to drive the addition of phosphate groups to ADP.

The entire system works as follows. A photosystem is comprised of various proteins that surround and connect a series of pigment molecules . Pigments are molecules that absorb various photons, allowing their electrons to become excited. Chlorophyll a is the main pigment used in these systems, and collects the final energy transfer before releasing an electron. Photosystem II starts this process of electrons by using the light energy to split a water molecule, which releases the hydrogen while siphoning off the electrons. The electrons are then passed through plastoquinone, an enzyme complex that releases more hydrogens into the thylakoid space . The electrons then flow through a cytochrome complex and plastocyanin to reach photosystem I. These three complexes form an electron transport chain , much like the one seen in mitochondria. Photosystem I then uses these electrons to drive the reduction of NADP + to NADPH. The additional ATP made during the light reactions comes from ATP synthase, which uses the large gradient of hydrogen molecules to drive the formation of ATP.

The Calvin Cycle

With its electron carriers NADPH and ATP all loaded up with electrons, the plant is now ready to create storable energy. This happens during the Calvin Cycle , which is very similar to the citric acid cycle seen in mitochondria. However, the citric acid cycle creates ATP other electron carriers from 3-carbon molecules, while the Calvin cycle produces these products with the use of NADPH and ATP. The cycle has 3 phases, as seen in the graphic below.

During the first phase, a carbon is added to a 5-carbon sugar, creating an unstable 6-carbon sugar. In phase two, this sugar is reduced into two stable 3-carbon sugar molecules. Some of these molecules can be used in other metabolic pathways, and are exported. The rest remain to continue cycling through the Calvin cycle. During the third phase, the five-carbon sugar is regenerated to start the process over again. The Calvin cycle occurs in the stroma of a chloroplast. While not considered part of the Calvin cycle, these products can be used to create a variety of sugars and structural molecules.

Products of Photosynthesis

The direct products of the light reactions and the Calvin cycle are 3-phosphoglycerate and G3P, two different forms of a 3-carbon sugar molecule. Two of these molecules combined equals one glucose molecule, the product seen in the photosynthesis equation. While this is the main food source for plants and animals, these 3-carbon skeletons can be combined into many different forms. A structural form worth note is cellulose , and extremely strong fibrous material made essentially of strings of glucose. Besides sugars and sugar-based molecules, oxygen is the other main product of photosynthesis. Oxygen created from photosynthesis fuels every respiring organism on the planet.

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology 6th. ed . New York: W.H. Freeman and Company. Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry . New York: W.H. Freeman and Company.

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Photosynthetic Cells

Cells get nutrients from their environment, but where do those nutrients come from? Virtually all organic material on Earth has been produced by cells that convert energy from the Sun into energy-containing macromolecules. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

What Is Photosynthesis? Why Is it Important?

Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

What Cells and Organelles Are Involved in Photosynthesis?

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration.

What Are the Steps of Photosynthesis?

Photosynthesis consists of both light-dependent reactions and light-independent reactions . In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

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The 2 Stages of Photosynthesis (A-level Biology)

The 2 stages of photosynthesis, stages of photosynthesis.

Photosynthesis can be divided into two reactions, which will be outlined below:

Table of Contents

Light Dependent Stage of Photosynthesis

The light dependent reactions are the first phase of photosynthesis, and take place in the thylakoid membranes of the chloroplasts.

• Light energy is absorbed from the sun by the plant cells . This excites, and hence releases, the electrons in the chlorophyll, causing them to pass to an electron acceptor at the start of an electron transport chain, a process known as photo-ionisation .
• Redox reactions pass electrons down the chain from one electron carrier to the next . This helps to generate ATP from ADP and inorganic phosphate in a process known as photophosphorylation , which can either be cyclic or non-cyclic . NADP is also converted to reduced NADP
• The light energy splits water molecules into protons (H + ), electrons and oxygen (waste) . The electrons are used to replace those lost from the chlorophyll in the first step. The process of splitting water bonds is called photolysis . Protons get pumped across the membrane using the ATP created in the previous step in a process known as chemiosmosis , creating a chemical potential gradient .
• Reduced NADP gets generated . This is due to the electrons in the electric transport chain being transferred to NADP along with a proton (NADPH).
• ATP is synthesised . The proton passes back through the membrane with an ATP synthase enzyme , creating a molecule of ATP. It takes around 4 protons to make 1 ATP molecule.

Reduced NADP is a source of H+ for the light-independent stage, whilst ATP is a source of energy .

Light Independent Stage of Photosynthesis

The light independent reactions are the second and final phase of photosynthesis taking place in the stroma of the chloroplasts. The light-independent reactions are collectively known as the Calvin cycle and uses ATP and reduced NADP from the light-dependent stage to produce glucose.

• Carbon fixation occurs . 5-carbon ribulose bisphosphate ( 5C RuBP ) combines with carbon dioxide. The enzyme catalysing the reaction is known as RuBisCo , creating the process known as carbon fixation .
• GP molecules form . The RuBP gets converted into 2 glycerine 3-phosphate ( GP ) molecules.
• The GP molecules reduce to form other molecules . Reduced NADP and ATP molecules are used to reduce the GP molecules into triose phosphate ( TP ), also known as glyceraldehyde phosphate ( GALP ). During this reduction, the reduced NADP becomes oxidised.
• Glucose is produced . Some molecules of TP/GALP are used as a raw material to create glucose , which can then be converted into essential organic compounds. It takes 2 cycles to create 1 glucose molecule. Some of the organic compounds are mono/polysaccharides, lipids, amino acids and nucleic acids.
• RuBP is regenerated . ATP helps to turn the remaining TP/GALP molecules back into RuBP.

The light independent stage cannot function without the products of the light dependent stage (reduced NADP, ATP).

TP, GP and Organic Substances

Both TP/GALP and GP can be used to make organic substances :

• Glucose – 2 TP/GALP joined are together to make glucose and other hexose sugars .
• Starch, Cellulose, Sucrose – Many hexose sugars can be joined to make large carbohydrates
• Lipids – TP/GALP is used to make glycerol . GP is used to make fatty acids . Together they can make lipids .
• Amino acids – GP can be used to synthesise certain amino acids.

The two stages of photosynthesis are the light-dependent reactions and the light-independent reactions.

The light-dependent reactions take place in the thylakoid membranes of chloroplasts and involve the absorption of light energy by photosynthetic pigments, such as chlorophyll. This energy is then used to produce ATP and NADPH, which provide energy for the light-independent reactions.

The light-independent reactions take place in the stroma of the chloroplasts and involve the use of the energy from ATP and NADPH produced in the light-dependent reactions. Carbon dioxide is fixed into glucose through a process called the Calvin cycle.

Photosynthesis has two stages because it involves two distinct sets of reactions that occur in different parts of the chloroplasts in plant cells. The first stage of photosynthesis is called the light-dependent reactions, which occur in the thylakoid membranes of the chloroplasts. In these reactions, light energy is absorbed by chlorophyll and other pigments, and converted into chemical energy in the form of ATP and NADPH. Water molecules are also split in this stage, releasing oxygen gas as a byproduct. The second stage of photosynthesis is called the light-independent reactions, or the Calvin cycle. These reactions occur in the stroma of the chloroplasts and do not require light energy directly. Instead, the ATP and NADPH produced in the light-dependent reactions are used to power the synthesis of carbohydrates from carbon dioxide in the air. This process is also known as carbon fixation. By having 2 stages of photosynthesis, it can efficiently convert light energy into chemical energy and use that energy to power the synthesis of organic molecules from carbon dioxide. The light-dependent reactions capture energy from sunlight and convert it into chemical energy, while the light-independent reactions use that energy to build organic molecules that can be used for growth, repair, and energy storage in the plant.

The process of photosynthesis removes carbon dioxide from the atmosphere and produces oxygen as a by-product. This helps to regulate the levels of these gases in the atmosphere, which is important for maintaining a balance that supports life on Earth.

An increase in carbon dioxide levels can increase the rate of photosynthesis in plants. This is because the light-independent reactions of photosynthesis require carbon dioxide as a starting material, so an increase in its availability can lead to an increase in the rate of glucose production. However, too much carbon dioxide can also have negative effects on plants and the overall ecosystem.

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CIE 1 Cell structure

Roles of atp (a-level biology), atp as an energy source (a-level biology), the synthesis and hydrolysis of atp (a-level biology), the structure of atp (a-level biology), magnification and resolution (a-level biology), calculating cell size (a-level biology), studying cells: confocal microscopes (a-level biology), studying cells: electron microscopes (a-level biology), studying cells: light microscopes (a-level biology), life cycle and replication of viruses (a-level biology), cie 10 infectious disease, bacteria, antibiotics, and other medicines (a-level biology), pathogens and infectious diseases (a-level biology), cie 11 immunity, types of immunity and vaccinations (a-level biology), structure and function of antibodies (a-level biology), the adaptive immune response (a-level biology), introduction to the immune system (a-level biology), primary defences against pathogens (a-level biology), cie 12 energy and respiration, anaerobic respiration in mammals, plants and fungi (a-level biology), anaerobic respiration (a-level biology), oxidative phosphorylation and chemiosmosis (a-level biology), oxidative phosphorylation and the electron transport chain (a-level biology), the krebs cycle (a-level biology), the link reaction (a-level biology), the stages and products of glycolysis (a-level biology), glycolysis (a-level biology), the structure of mitochondria (a-level biology), the need for cellular respiration (a-level biology), cie 13 photosynthesis, limiting factors of photosynthesis (a-level biology), cyclic and non-cyclic phosphorylation (a-level biology), photosystems and photosynthetic pigments (a-level biology), site of photosynthesis, overview of photosynthesis (a-level biology), cie 14 homeostasis, ectotherms and endotherms (a-level biology), thermoregulation (a-level biology), plant responses to changes in the environment (a-level biology), cie 15 control and co-ordination, the nervous system (a-level biology), sources of atp during contraction (a-level biology), the ultrastructure of the sarcomere during contraction (a-level biology), the role of troponin and tropomyosin (a-level biology), the structure of myofibrils (a-level biology), slow and fast twitch muscles (a-level biology), the structure of mammalian muscles (a-level biology), how muscles allow movement (a-level biology), the neuromuscular junction (a-level biology), features of synapses (a-level biology), cie 16 inherited change, calculating genetic diversity (a-level biology), how meiosis produces variation (a-level biology), cell division by meiosis (a-level biology), importance of meiosis (a-level biology), cie 17 selection and evolution, types of selection (a-level biology), mechanism of natural selection (a-level biology), types of variation (a-level biology), cie 18 biodiversity, classification and conservation, biodiversity and gene technology (a-level biology), factors affecting biodiversity (a-level biology), biodiversity calculations (a-level biology), introducing biodiversity (a-level biology), the three domain system (a-level biology), phylogeny and classification (a-level biology), classifying organisms (a-level biology), cie 19 genetic technology, cie 2 biological molecules, properties of water (a-level biology), structure of water (a-level biology), test for lipids and proteins (a-level biology), tests for carbohydrates (a-level biology), protein structures: globular and fibrous proteins (a-level biology), protein structures: tertiary and quaternary structures (a-level biology), protein structures: primary and secondary structures (a-level biology), protein formation (a-level biology), proteins and amino acids: an introduction (a-level biology), phospholipid bilayer (a-level biology), cie 3 enzymes, enzymes: inhibitors (a-level biology), enzymes: rates of reaction (a-level biology), enzymes: intracellular and extracellular forms (a-level biology), enzymes: mechanism of action (a-level biology), enzymes: key concepts (a-level biology), enzymes: introduction (a-level biology), cie 4 cell membranes and transport, transport across membranes: active transport (a-level biology), investigating transport across membranes (a-level biology), transport across membranes: osmosis (a-level biology), transport across membranes: diffusion (a-level biology), signalling across cell membranes (a-level biology), function of cell membrane (a-level biology), factors affecting cell membrane structure (a-level biology), structure of cell membranes (a-level biology), cie 5 the mitotic cell cycle, chromosome mutations (a-level biology), cell division: checkpoints and mutations (a-level biology), cell division: phases of mitosis (a-level biology), cell division: the cell cycle (a-level biology), cell division: chromosomes (a-level biology), cie 6 nucleic acids and protein synthesis, transfer rna (a-level biology), transcription (a-level biology), messenger rna (a-level biology), introducing the genetic code (a-level biology), genes and protein synthesis (a-level biology), synthesising proteins from dna (a-level biology), structure of rna (a-level biology), dna replication (a-level biology), dna structure and the double helix (a-level biology), polynucleotides (a-level biology), cie 7 transport in plants, translocation and evidence of the mass flow hypothesis (a-level biology), the phloem (a-level biology), importance of and evidence for transpiration (a-level biology), introduction to transpiration (a-level biology), the pathway and movement of water into the roots and xylem (a-level biology), the xylem (a-level biology), cie 8 transport in mammals, controlling heart rate (a-level biology), structure of the heart (a-level biology), transport of carbon dioxide (a-level biology), transport of oxygen (a-level biology), exchange in capillaries (a-level biology), structure and function of blood vessels (a-level biology), cie 9 gas exchange and smoking, lung disease (a-level biology), pulmonary ventilation rate (a-level biology), ventilation (a-level biology), structure of the lungs (a-level biology), general features of exchange surfaces (a-level biology), understanding surface area to volume ratio (a-level biology), the need for exchange surfaces (a-level biology), edexcel a 1: lifestyle, health and risk, phospholipids – introduction (a-level biology), edexcel a 2: genes and health, features of the genetic code (a-level biology), gas exchange in plants (a-level biology), gas exchange in insects (a-level biology), edexcel a 3: voice of the genome, edexcel a 4: biodiversity and natural resources, edexcel a 5: on the wild side, reducing biomass loss (a-level biology), sources of biomass loss (a-level biology), transfer of biomass (a-level biology), measuring biomass (a-level biology), net primary production (a-level biology), gross primary production (a-level biology), trophic levels (a-level biology), edexcel a 6: immunity, infection & forensics, microbial techniques (a-level biology), the innate immune response (a-level biology), edexcel a 7: run for your life, edexcel a 8: grey matter, inhibitory synapses (a-level biology), synaptic transmission (a-level biology), the structure of the synapse (a-level biology), factors affecting the speed of transmission (a-level biology), myelination (a-level biology), the refractory period (a-level biology), all or nothing principle (a-level biology), edexcel b 1: biological molecules, inorganic ions (a-level biology), edexcel b 10: ecosystems, nitrogen cycle: nitrification and denitrification (a-level biology), the phosphorus cycle (a-level biology), nitrogen cycle: fixation and ammonification (a-level biology), introduction to nutrient cycles (a-level biology), edexcel b 2: cells, viruses, reproduction, edexcel b 3: classification & biodiversity, edexcel b 4: exchange and transport, edexcel b 5: energy for biological processes, edexcel b 6: microbiology and pathogens, edexcel b 7: modern genetics, edexcel b 8: origins of genetic variation, edexcel b 9: control systems, ocr 2.1.1 cell structure, structure of prokaryotic cells (a-level biology), eukaryotic cells: comparing plant and animal cells (a-level biology), eukaryotic cells: plant cell organelles (a-level biology), eukaryotic cells: the endoplasmic reticulum (a-level biology), eukaryotic cells: the golgi apparatus and lysosomes (a-level biology), ocr 2.1.2 biological molecules, introduction to eukaryotic cells and organelles (a-level biology), ocr 2.1.3 nucleotides and nucleic acids, ocr 2.1.4 enzymes, ocr 2.1.5 biological membranes, ocr 2.1.6 cell division, diversity & organisation, ocr 3.1.1 exchange surfaces, ocr 3.1.2 transport in animals, ocr 3.1.3 transport in plants, examples of xerophytes (a-level biology), introduction to xerophytes (a-level biology), ocr 4.1.1 communicable diseases, structure of viruses (a-level biology), ocr 4.2.1 biodiversity, ocr 4.2.2 classification and evolution, ocr 5.1.1 communication and homeostasis, the resting potential (a-level biology), ocr 5.1.2 excretion, ocr 5.1.3 neuronal communication, hyperpolarisation and transmission of the action potential (a-level biology), depolarisation and repolarisation in the action potential (a-level biology), ocr 5.1.4 hormonal communication, ocr 5.1.5 plant and animal responses, ocr 5.2.1 photosynthesis, ocr 5.2.2 respiration, ocr 6.1.1 cellular control, ocr 6.1.2 patterns of inheritance, ocr 6.1.3 manipulating genomes, ocr 6.2.1 cloning and biotechnology, ocr 6.3.1 ecosystems, ocr 6.3.2 populations and sustainability, related links.

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Module 6: Metabolic Pathways

Photosynthesis, identify the basic components and steps of photosynthesis.

The processes in all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating, that is, by ingesting other organisms. But where does the stored energy in food originate? All of this energy can be traced back to photosynthesis.

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis around 300 million years ago.

Figure 1. Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria. (credit a: modification of work by Steve Hillebrand, U.S. Fish and Wildlife Service; credit b: modification of work by “eutrophication&hypoxia”/Flickr; credit c: modification of work by NASA; credit d: University of Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research Center, UAF, NOAA)

Figure 2. The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator that eats these deer receives a portion of the energy that originated in the photosynthetic vegetation that the deer consumed. (credit: modification of work by Steve VanRiper, U.S. Fish and Wildlife Service)

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (Figure 1). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs .

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (Figure 2), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

Learning Objectives

• Identify the reactants and products of photosynthesis
• Describe the visible and electromagnetic spectrums of light as they applies to photosynthesis
• Describe the light-dependent reactions that take place during photosynthesis
• Identify the light-independent reactions in photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 3). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

Figure 3. Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis.

The following is the chemical equation for photosynthesis (Figure 4):

Figure 4. The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two three-carbon GA3Ps.

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the  mesophyll . The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a  chloroplast . For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids . Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen . As shown in Figure 5, a stack of thylakoids is called a granum , and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Practice Question

Figure 5. Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

On a hot, dry day, plants close their stomata to conserve water. What impact will this have on photosynthesis?

The Two Parts of Photosynthesis

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the  light-dependent reactions , energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions , the chemical energy harvested during the light-dependent reactions drive the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 6 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Figure 6. Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 .

Photosynthesis at the Grocery Store

Figure 7. Foods that humans consume originate from photosynthesis. (credit: Associação Brasileira de Supermercados)

Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle (Figure 7) contains hundreds, if not thousands, of different products for customers to buy and consume.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: for instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) can be derived from algae or from oil, the fossilized remains of photosynthetic organisms. Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

Spectrums of Light

How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules. However, autotrophs only use a few specific components of sunlight.

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure 8).

Figure 8. The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 9). The difference between wavelengths relates to the amount of energy carried by them.

Figure 9. The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure 9) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.

Absorption of Light

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically active radiation.

The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 10).

Figure 10. The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA)

Understanding Pigments

Different kinds of pigments exist, and each has evolved to absorb only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules. There are five major chlorophylls:  a , b , c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the  absorption spectrum . The graph in Figure 11 shows the absorption spectra for chlorophyll  a , chlorophyll b , and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

Figure 11. (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum.

Figure 12. Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger)

Many photosynthetic organisms have a mixture of pigments; using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure 12).

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a  spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

Light-Dependent Reactions

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure 13. Protein complexes and pigment molecules work together to produce NADPH and ATP.

Figure 13. A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a  photosystem , two types of which are found embedded in the thylakoid membrane, photosystem II (PSII) and photosystem I (PSI) (Figure 14). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll  a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Figure 14. The photosystem II (PSII) reaction center and the photosystem I (PSI).

In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP + to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP. What is the initial source of electrons for the chloroplast electron transport chain?

• carbon dioxide

The reaction center contains a pair of chlorophyll  a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact . It is at this step in the reaction center, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate. PSII and PSI are two major components of the photosynthetic electron transport chain , which also includes the cytochrome complex . The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII (called  P680 ) delivers its high-energy electrons, one at the time, to the primary electron acceptor , and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water; thus, water is split and PSII is re-reduced after every photoact. Splitting one H 2 O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O 2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called  P700 ). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Generating an Energy Carrier: ATP

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 14). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.

Light-Independent Reactions

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

In plants, carbon dioxide (CO 2 ) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them. Another term, the Calvin cycle , is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is dark reactions, because light is not directly required (Figure 15). However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.

Figure 15. Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place.

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

Stage 1: Fixation

In the stroma, in addition to CO 2 , two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in Figure 16. RuBP has five atoms of carbon, flanked by two phosphates.

Figure 16. The Calvin cycle has three stages.

In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

Which of the following statements is true?

• In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. GA3P and water are products.
• In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. GA3P and oxygen are products.
• In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are products.
• In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen are products.

RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP, two molecules of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO 2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called  carbon fixation , because CO 2 is “fixed” from an inorganic form into organic molecules.

Stage 2: Reduction

ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a  reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.

Stage 3: Regeneration

Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

Evolution of Photosynthesis

Figure 17. The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: Piotr Wojtkowski)

During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy.

Photosynthesis in desert plants has evolved adaptations that conserve water. In the harsh dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO 2 , water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO 2 allows plants to adapt to living with less water. Some plants such as cacti (Figure 17) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, a mechanism to face extremely dry periods.

Now that we’ve learned about the different pieces of photosynthesis, let’s put it all together. This video walks you through the process of photosynthesis as a whole:

In Summary: An Overview of Photosynthesis

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesis evolved to allow living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.

Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.

The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll  a to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does  not  count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

• Authored by : Shelli Carter and Lumen Learning. Provided by : Lumen Learning. License : CC BY: Attribution
• Biology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]
• Photosynthesis: Crash Course Biology #8. Authored by : CrashCourse. Located at : https://youtu.be/sQK3Yr4Sc_k . License : All Rights Reserved . License Terms : Standard YouTube License

Where Does Photosynthesis Takes Place and What are Two Stages of Photosynthesis?

Virtually all animals and plants need water, light, air, and nutrition to grow and survive. Green plants get their nutrition through a complex chemical process known as photosynthesis.

So, what is photosynthesis ?

Photosynthesis is the process by which green plants and certain microorganisms use the energy from the sun to produce sugar. Water and Carbon dioxide are the primary raw materials of the process. Oxygen is normally released as a by-product during the photosynthetic process.

From the energy generated, other living organisms, including animals and plants, get the fuel to live. The organisms depend on the energy for the metabolic and physiological processes that take place in their cells.

While the process of photosynthesis is complex, the overall reactions can be summarized as follows:

Sunlight + Water + Carbon dioxide = glucose (carbohydrate) + molecular oxygen.

The overall reaction can be denoted with the following equation:

6H2O + 6CO2 = C6H12O6+ 6O2

Now that you know what photosynthesis, let’s see where it occurs.

Where Does Photosynthesis Takes Place?

In plants, photosynthesis normally occurs in leaves. You should be aware that a typical leaf has several layers of cells. So, the photosynthesis process takes place in a middle layer known as the mesophyll.

Leaves have regulated openings known as stomata on their underside. These openings allow the entry and exit of carbon dioxide and oxygen, respectively. The stomata also regulate the water balance in the leaf. In fact, they are located under the leaf mainly to minimize water loss.

Every stoma features guard cells, which swell or shrink in reaction to osmotic change, resulting in the opening and closing of the stomata.

Within every mesophyll cell, there are organelles called chloroplasts. Various chemical reactions take place in various parts of chloroplasts. Photosynthesis is one of these reactions.

Chloroplasts have special features that enable them to to accomplish the photosynthesis reactions. Each chloroplast contains disc-like structures called thylakoids. These thylakoids are stacked like pancakes in piles known collectively as grana.

The space around the grana is filled with fluid and it is known as the stroma while the space between one thylakoid and the other is called the thylakoid space.

Embedded in the membrane of each thylakoid is a green-colored pigment called chlorophyll. Chlorophyll gives plants their green color and helps to capture the sunlight that is needed for the photosynthesis process.

Now you know where photosynthesis occurs. Let’s explore the two stages of this process.

What are Two Stages of Photosynthesis?

The two successive stages in which photosynthesis takes place are:

• The Light-dependent reactions
• The Calvin Cycle, or the light-independent reactions

Let’s have a detailed look at these stages:

The Light-dependent Reactions

The first phase of photosynthesis is the light-dependent reactions. This phase requires sunlight. Chlorophyll absorbs energy from sunlight and converts it into chemical energy.

This chemical energy is stored in two forms:

• Nicotinamide Adenine Dinucleotide Phosphate (NADPH) an electron carrier molecule
• Adenosine Triphosphate (ATP), an energy carrier molecule

The light-dependent reactions occur on the thylakoid membrane within the chloroplast. The conversion of light energy into chemical energy occurs in a multi-protein known as photosystem. There are two types of photosystems:

• Photosystem I (PSI)
• Photosystem II (PSII)

These photosystems are found in the thylakoid membrane and each one helps to capture the energy from sunlight by activating electrons. Energy carrier molecules, which drive the light-independent reactions, then transport these excited electrons.

Photosystems are composed of a reaction center and a light-harvesting center. The light-harvesting complex features pigments that convey light energy to two special chlorophyll molecules:

• P700 molecules – These are PSI chlorophyll molecules and they absorb light with a 700nm peak wavelength
• P680 molecules – These are PSII chlorophyll molecules and they absorb light with a 680nm peak wavelength.

The light-dependent reactions begin in PSII, and here’s the breakdown of the process:

• A P680 chlorophyll molecule absorbs a light photon. This occurs in the light harvesting center of PSII
• The energy that is produced from the light is conveyed from one P680 molecule to another until it gets to PSII’s reaction center (RC).
• The RC has a pair of P680 chlorophyll molecules. High energy levels in the molecules excite an electron, making it unstable and hence gets released.
• The light harvesting complex captures more photons of light, more energy is transferred to the RC, and more electrons are released and the cycle continues.
• These released electrons are transported via electron transport chain (ETC). ETC encompasses a series of protein complexes and mobile carriers.
• Once released from PSII, the electrons are replaced by splitting water into electrons, hydrogen ions, and oxygen. The process is called Photolysis as light is used to split the water.
• The oxygen and hydrogen ions produced during photolysis are released into the thylakoid lumen before the oxygen is eventually released into the atmosphere as a photosynthesis by-product.
• As the electrons are conveyed through the ETC, hydrogen ions from the stroma are transported and released into the thylakoid lumen. Consequently, the lumen will have a higher concentration of hydrogen ions, otherwise referred to as a proton
• The proton gradient in the lumen results in the hydrogen ions being transferred to ATP synthase and deliver the energy that is used to combine ADP and Pi to generate ADP.
• Through the ETC, electrons are transferred to Cytochrome b6f, then to Plastocyanin, and eventually gets to PSI.

Here’s what happens aftwards:

• At PSI, the electrons get energy from light absorbed by P700 chlorophyll molecules before they are conveyed to the mobile carrier, ferredoxin.
• From ferredoxin, they are moved to ferredoxin NADP reductase (FNR). FNR is the final electron acceptor and where NAPDH is generated by combining the electrons and hydrogen ion with NADP.
• Electrons from PSII replace the electrons lost from PSI through the ETC.

Light-independent Reactions (The Calvin Cycle)

The Calvin Cycle is the second phase of the photosynthesis process and takes place in the stroma of the chloroplast. In light-independent reactions, carbon dioxide is transformed into glucose and other products using the electrons from NADPH and energy from ATP.

Here’s a breakdown of the process:

• A carbon dioxide molecule is combined with a Ribulose Bisphosphate (RuBP) molecule, which is a 5-carbon
• This combination results in an unstable 6-carbon intermediate, which disintegrates quickly, resulting into two 3-carbon molecules called 3-phosphoglycerate (PGA).
• The two PGA molecules obtain energy from ATP and generate two 1,3-bisphosphoglycerate (BPGA) molecules.
• Each BPGA molecule combines with an electron from NADPH, producing two Glyceraldehyde 3-phosphate (G3P) molecules as a result.
• These two G3P molecules can make only one glucose molecule. That means there is a need to regenerate more RuBP in order to produce more glucose molecules. To achieve this, 12 molecules of G3P will be required.

At this point, you should realize that the photosynthesis process requires 6 molecules of carbon dioxide. You can see that in the photosynthesis equation (6CO2).

These 6 carbon dioxide molecules will have to be utilized to generate the 12 molecules of G3P. That means the steps used in generating the first two molecules of G3P will have to be repeated another five times to generate ten more G3P molecules.

While the two G3P molecules are used to generate glucose, the ten additional molecules are utilized in regenerating RuBP.

• https://sciencing.com/two-stages-photosynthesis-5421327.html
• https://www.ck12.org/na/pho-where-does-photosynthesis-take-place-1/lesson/Where-does-Photosynthesis-take-Place-xi-biology/

I am Sonia Madaan, a mother with a passion for science, computing, and environmental issues. Motivated by my passion and education, I started a website to spread awareness about climate change and its causes, like rising greenhouse gas levels. You can read more about me here .

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8.1 Overview of Photosynthesis

Learning objectives.

By the end of this section, you will be able to do the following:

• Explain the significance of photosynthesis to other living organisms
• Describe the main structures involved in photosynthesis
• Identify the substrates and products of photosynthesis

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates from sunlight and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. It is also a source of oxygen necessary for many living organisms. In brief, the energy of sunlight is “captured” to energize electrons, whose energy is then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis 350 to 200 million years ago during the Carboniferous Period.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis ( Figure 8.2 ). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds. For this reason, they are referred to as chemoautotrophs .

The importance of photosynthesis is not just that it can capture sunlight’s energy. After all, a lizard sunning itself on a cold day can use the sun’s energy to warm up in a process called behavioral thermoregulation . In contrast, photosynthesis is vital because it evolved as a way to store the energy from solar radiation (the “photo-” part) to energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer ( Figure 8.3 ), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to visible light, to photosynthesis, to vegetation, to deer, and finally to the wolf.

Main Structures and Summary of Photosynthesis

Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide (which is low in energy), and water as substrates ( Figure 8.4 ). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (G3P), as well as simple carbohydrate molecules (high in energy) that can then be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

The following is the chemical equation for photosynthesis ( Figure 8.5 ):

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

Basic Photosynthetic Structures

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll . The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss due to high temperatures on the upper surface of the leaf. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast . For plants, chloroplast-containing cells exist mostly in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane), and are ancestrally derived from ancient free-living cyanobacteria. Within the chloroplast are stacked, disc-shaped structures called thylakoids . Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen . As shown in Figure 8.6 , a stack of thylakoids is called a granum , and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Visual Connection

On a hot, dry day, the guard cells of plants close their stomata to conserve water. What impact will this have on photosynthesis?

The Two Parts of Photosynthesis

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions. In the light-dependent reactions , energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions , the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, however, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers . The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 8.7 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Link to Learning

Click the link to learn more about photosynthesis.

Everyday Connection

Photosynthesis at the grocery store.

Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle ( Figure 8.8 ) contains hundreds, if not thousands, of different products for customers to buy and consume.

Although there is a large variety, each item ultimately can be linked back to photosynthesis. Meats and dairy link, because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: For instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) are derived from “algae” (unicellular plant-like organisms, and cyanobacteria). Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

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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

• Authors: Mary Ann Clark, Matthew Douglas, Jung Choi
• Publisher/website: OpenStax
• Book title: Biology 2e
• Publication date: Mar 28, 2018
• Location: Houston, Texas
• Book URL: https://openstax.org/books/biology-2e/pages/1-introduction
• Section URL: https://openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis

© Jul 10, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

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Photosynthesis 2: The Two Phases of Photosynthesis (Interactive Tutorial)

Looking for a student learning guide? It’s on the main menu for your course. Use the “Courses” menu above.

Page Outline

• Introduction
• Chloroplasts
• Photosynthesis Occurs in Two Phases
• Quiz: The Two  Phases of Photosynthesis

1. Introduction

In the previous tutorial, we described photosynthesis as an endergonic oxidation-reduction reaction that occurs in the chloroplasts of plant cells. If we view photosynthesis in the context of Earth’s history, however, that last part is only partially correct. From about 3.5 billion years ago until the origin of eukaryotic cells about 1.5 billion years ago, photosynthesis was carried out by cyanobacteria . These photosynthetic bacteria (also known as blue-green bacteria)  are the organisms that put oxygen into our atmosphere. A modern example of cyanobacteria is shown on the left. Note that while the cells are combined into linear filaments, each cell is a completely autonomous individual: there’s no multicellularity in these prokaryotic organisms. 

Through endosymbiosis, an ancient cyanobacterium took up residence inside another cell and evolved into the chloroplasts found today. You can find chloroplasts in unicellular protists (like the Euglena on the right, which you might have observed swimming in a drop of water under a microscope), algae (like seaweed), or plants. Let’s continue by putting chloroplasts into the context where terrestrial primates like us usually find them: in leaves.

A leaf’s structure involves tradeoffs between maximizing surface area for absorption of light while minimizing water loss. To prevent water loss, a waxy cuticle (“1”) covers the upper (“2”) and lower (“5”) epidermis. The main function of the epidermis is protective, and much of the way that protection occurs is by secreting the waxy layer at “1.”

Within the leaf, most of the photosynthesis occurs in the middle, a section called the mesophyll (at “8”). The cells that make up the mesophyll are packed with chloroplasts (“3”).

Water comes into the leaf through bundles of vascular tissue (“4”), which we commonly refer to as veins. The veins also allow sugars to leave the leaf and move to other parts of the plant where they may, in a plant like a potato, get converted into polysaccharides like starch for long-term energy storage.

Carbon dioxide enters the plant through pores on the lower epidermis. These pores are called stomata (“7”), and they are formed by guard cells (“6”), which can change shape to adjust the size of the stomatal opening, even to the point of closing up altogether when the plant is experiencing a lack of water.

3. Chloroplasts

Chloroplasts, like mitochondria, have their own circular, bacteria-like chromosome (at “3”). Number 4 shows another vestige of the chloroplast’s once-independent existence. These are ribosomes, which have a bacterial (as opposed to a eukaryotic) molecular structure.

Chloroplasts are filled with membrane-bound sacs called thylakoids (“5”), which are organized into stacks (like a stack of pancakes). Outside of the thylakoid is a fluid called the stroma (“6”), which is analogous to the cytosol of a eukaryotic cell, or the matrix of a mitochondrion.

4. Within Chloroplasts, Photosynthesis Occurs in Two Phases

In the last tutorial, we looked at the overall equation for photosynthesis:

6CO 2 + 6H 2 O + light energy –> C 6 H 12 O 6 + 6O 2

Within a chloroplast, photosynthesis occurs in two distinct phases, shown in roman numerals (“I” and “II”) in the diagrams below. Because of the complexity of the diagram, I’m providing you with both labeled and unlabeled versions.

Phase I (“I,” in the diagram below) makes up the light reactions . Driven by light energy (“1”), water gets oxidized: it’s split apart into high-energy electrons and protons (neither of which are shown below). These electrons and protons are used to reduce the oxidized electron carrier NADP + (shown at “8”) into its reduced, high-energy form, NADPH (shown at “5”). Energy harvested during the light reactions is also used to convert low-energy ADP and inorganic phosphate (“9” and “10,” respectively) into high-energy ATP (“4”). The waste product (but one essential for aerobic organisms like us) is oxygen, shown at “3”. All of this occurs in the thylakoid sacs, with the outputs of light reactions (NADPH and ATP) moving into the stroma to support photosynthesis’ second phase.

Phase II of photosynthesis uses the products of Phase 1 as inputs (excepting oxygen, which bubbles into the atmosphere). Phase 2 is known as the Calvin cycle , after Berkeley Biochemist Melvin Calvin, who received the Nobel Prize for his co-discovery of the cycle in 1961 (also the year Mr. W was born). Because the Calvin cycle doesn’t depend directly on light, you’ll also see it referred to as the light-independent reactions (or even as the dark reactions) .

During the Calvin Cycle, the key external input is carbon dioxide (“6”). Using energy from ATP (“4”), and electrons and hydrogen from the energy-rich, reduced electron carrier NADPH (“5”), carbon dioxide is reduced to become the three-carbon sugar glyceraldehyde phosphate , or G3P (“7”), which can be converted into glucose (C 6 H 12 O 6  ) or anything else the plant needs.

5. Quiz: The Two Phases of Photosynthesis

In the next tutorials, we’ll dive into the details of the light reactions and the Calvin cycle. But let’s consolidate what we’ve learned above through the following quiz.

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-PSN: The Two Phases of Photosynthesis”]

[h]The Two Phases of Photosynthesis

[q question_number=”1″ dataset_id=”SMV_PSN_Two_Phases_of_PSN|71727174b15e4″] The first organisms to perform photosynthesis were

[c]IGFsZ2FlLCBsaWtlIG1vZGVybiBzZWF3ZWVkcy4=[Qq]

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[c]IGN5YW5vYm FjdGVyaWEu[Qq]

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[c]IHNpbXBsZSBwbGFudHMsIGxpa2UgbW9zc2VzIGFuZCBmZXJucy4=[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|716da41c2b1e4″ question_number=”2″] In the diagram below, which number shows the cells that secrete the upper waxy layer that helps prevent water loss?

[textentry single_char=”true”]

[c]ID I=[Qq]

[f]IEV4Y2VsbGVudC4gVGhlIHVwcGVyIGVwaWRlcm1pcyBzZWNyZXRlcyB0aGUgd2F4eSBsYXllciB0aGF0IGhlbHBzIG1pbmltaXplIHdhdGVyIGxvc3Mu[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IE5vLCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgdXBwZXIgd2F4eSBsYXllciBpcyByaWdodCBuZXh0IHRvIHRoZSBjZWxscyB0aGF0IHNlY3JldGUgdGhhdCBsYXllci4gV2hpY2ggY2VsbHMgYXJlIHJpZ2h0IG5leHQgdG8gdGhlIHVwcGVyIHdheHkgbGF5ZXI/

[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71692145215e4″ question_number=”3″] In the diagram below, which number shows the organelle that’s performing photosynthesis?

[c]ID M=[Qq]

[f]IFRlcnJpZmljLiAmIzgyMjA7MyYjODIyMTsgaW5kaWNhdGVzIHRoZSBjaGxvcm9wbGFzdHMsIHRoZSBvcmdhbmVsbGVzIHRoYXQgYXJlIHBlcmZvcm1pbmcgcGhvdG9zeW50aGVzaXMu[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7164792d595e4″ question_number=”4″] Photosynthesis requires carbon dioxide as an input. Which part shows where carbon dioxide would enter the leaf?

[c]ID c=[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|715ff6564f9e4″ question_number=”5″] Photosynthesis requires water as an input. Which part shows where water would enter the leaf?

[c]ID Q=[Qq]

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[q multiple_choice=”true” dataset_id=”SMV_PSN_Two_Phases_of_PSN|715b4e3e879e4″ question_number=”6″] The similarities between cyanobacteria and chloroplasts is evidence that chloroplasts arose through

[c]IG9zbW9zaXMu[Qq]

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[c]IGV4b2N5dG9zaXM=[Qq]

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[c]IGVuZG9zeW 1iaW9zaXM=[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7156cb677dde4″ question_number=”7″] The light reactions occur in which part?

[c]NQ ==[Qq]

[f]QXdlc29tZSEgJiM4MjIwOzUmIzgyMjE7IHNob3dzIHRoZSB0aHlsYWtvaWQgc2Fjcywgd2hpY2ggaXMgd2hlcmUgdGhlIGxpZ2h0IHJlYWN0aW9ucyBvY2N1ci4=[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7152234fb5de4″ question_number=”8″] The Calvin Cycle occurs in which part of the chloroplast?

[c]Ng ==[Qq]

[f]QXdlc29tZSEgJiM4MjIwOzYmIzgyMjE7IHNob3dzIHRoZSBzdHJvbWEsIHdoaWNoIGlzIHdoZXJlIHRoZSBDYWx2aW4gQ3ljbGUgb2NjdXJzLg==[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgQ2FsdmluIEN5Y2xlIG9jY3VycyBpbiB0aGUgZXF1aXZhbGVudCBvZiB0aGUgY3l0b3BsYXNtIG9mIHRoZSBjaGxvcm9wbGFzdC4gV2hpY2ggcGFydCBpcyBtb3N0IGFraW4gdG8gdGhlIGN5dG9wbGFzbT8=

[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|714d7b37edde4″ question_number=”9″]One of the strongest pieces of evidence for the endosymbiotic origins of chloroplasts is the fact that they have DNA, which is shown at

[c]Mw ==[Qq]

[f]QXdlc29tZSEgJiM4MjIwOzMmIzgyMjE7IHNob3dzIHRoZSBjaGxvcm9wbGFzdCYjODIxNztzIGNpcmN1bGFyLCBiYWN0ZXJpYS1saWtlIGNocm9tb3NvbWUuIFRoZSBleGlzdGVuY2Ugb2YgdGhpcyBjaHJvbW9zb21lIGlzIG9uZSBwaWVjZSBvZiBldmlkZW5jZSBmb3IgdGhlIGlkZWEgb2YgZW5kb3N5bWJpb3NpczogdGhhdCBjaGxvcm9wbGFzdHMgd2VyZSBvbmNlIGZyZWUtbGl2aW5nLCBpbmRlcGVuZGVudCBvcmdhbmlzbXMgdGhhdCBhdCBvbmUgcG9pbnQsIHRvb2sgdXAgcmVzaWRlbmNlIHdpdGhpbiBhbm90aGVyIGNlbGwu[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7148f860e41e4″ question_number=”10″]If number 4 represents ATP, then which number would represent NADP + ?

[c]OA ==[Qq]

[f]QXdlc29tZSEgJiM4MjIwOzgmIzgyMjE7IGlzIE5BRFA= Kw== LCB3aGljaCBnZXRzIHJlZHVjZWQgdG8gTkFEUEg=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBJZiAmIzgyMjA7NCYjODIyMTsgaXMgQVRQLCB0aGVuIDkgYW5kIDEwIHdvdWxkIGJlIEFEUCBhbmQgUC4gQmVzaWRlcyB3YXRlciwgd2hhdCYjODIxNztzIHRoZSBvdGhlciBpbnB1dCBmb3IgdGhlIGxpZ2h0IHJlYWN0aW9ucz8=

[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71447589da5e4″ question_number=”11″]What letter or number represents the stroma?

[c]Yg ==[Qq]

[f]QXdlc29tZSEgTGV0dGVyICYjODIyMDtiJiM4MjIxOyByZXByZXNlbnRzIHRoZSBzdHJvbWEu[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBJZiBhIGNobG9yb3BsYXN0IGhhZCBjeXRvcGxhc20sIHRoaXMgd291bGQgYmUgaXQu

[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|713fcd72125e4″ question_number=”12″]If 9 is ADP, then what letter or number represents the reduced molecule that’s an output of the light reactions and an input for the Calvin cycle?

[f]QXdlc29tZSEgTGV0dGVyICYjODIyMDs1JiM4MjIxOyByZXByZXNlbnRzIE5BRFBILCBhIGNoZW1pY2FsbHkgcmVkdWNlZCBvdXRwdXQgb2YgdGhlIGxpZ2h0IHJlYWN0aW9ucyBhbmQgYW4gaW5wdXQgZm9yIHRoZSBDYWx2aW4gY3ljbGUu[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|713b255a4a5e4″ question_number=”13″]Which number or letter represents water?

[c]Mg ==[Qq]

[f]VGVycmlmaWMhIExldHRlciAmIzgyMjA7MiYjODIyMTsgcmVwcmVzZW50cyB3YXRlciwgd2hpY2ggcHJvdmlkZXMgdGhlIGVsZWN0cm9ucyAoYW5kIGh5ZHJvZ2VucykgZm9yIHRoZSByZWR1Y3Rpb25zIHRoYXQgYXJlIGEga2V5IHBhcnQgb2YgcGhvdG9zeW50aGVzaXM=[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71367d42825e4″ question_number=”14″]Which number or letter represents oxygen?

[f]VGVycmlmaWMhIExldHRlciAmIzgyMjA7MyYjODIyMTsgcmVwcmVzZW50cyBveHlnZW4sIHdoaWNoIGlzIHdoYXQmIzgyMTc7cyBsZWZ0IG9mIHdhdGVyICgmIzgyMjA7MiYjODIyMTspIGFmdGVyIGl0cyBlbGVjdHJvbnMgYW5kIHByb3RvbnMgYXJlIHN0cmlwcGVkIGF3YXku[Qq]

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71314027c15e4″ question_number=”15″]Which number or letter represents carbon dioxide?

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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|712afe47cc9e4″ question_number=”16″]During photosynthesis, carbon dioxide gets reduced. What number or letter represents what carbon dioxide gets reduced into?

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• Photosynthesis 3: Light and Pigments
• Photosynthesis Main Menu

• Biology Article

Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

• What is Photosynthesis?
• Site of photosynthesis

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words  phōs  (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις   means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

• Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae.  All green parts of a plant, including the green stems, green leaves,  and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read:  Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f,  which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the  plant cell   and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a  and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 ,  chlorophyll- c2 ,  chlorophyll- d and chlorophyll- f .

Also Read:   Biological Pigments

Process Of Photosynthesis

At the cellular level,  the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for  growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
• Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
• Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

• Dark reaction is also called carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
• In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read:  Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

• Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
• Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

Frequently Asked Questions

1. what is photosynthesis explain the process of photosynthesis., 2. what is the significance of photosynthesis, 3. list out the factors influencing photosynthesis., 4. what are the different stages of photosynthesis, 5. what is the calvin cycle, 6. write down the photosynthesis equation..

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Please What Is Meant By 300-400 PPM

PPM stands for Parts-Per-Million. It corresponds to saying that 300 PPM of carbon dioxide indicates that if one million gas molecules are counted, 300 out of them would be carbon dioxide. The remaining nine hundred ninety-nine thousand seven hundred are other gas molecules.

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