7.6: Photosynthesis and the Importance of Light - Biology

7.6: Photosynthesis and the Importance of Light - Biology

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skills to develop

  • Describe the function and locations of photosynthetic pigments in eukaryotes and prokaryotes
  • Describe the major products of the light-dependent and light-independent reactions
  • Describe the reactions that produce glucose in a photosynthetic cell
  • Compare and contrast cyclic and noncyclic photophosphorylation

Heterotrophic organisms ranging from E. coli to humans rely on the chemical energy found mainly in carbohydrate molecules. Many of these carbohydrates are produced by photosynthesis, the biochemical process by which phototrophic organisms convert solar energy (sunlight) into chemical energy. Although photosynthesis is most commonly associated with plants, microbial photosynthesis is also a significant supplier of chemical energy, fueling many diverse ecosystems. In this section, we will focus on microbial photosynthesis.

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions (Figure (PageIndex{1})). In the light-dependent reactions, energy from sunlight is absorbed by pigment molecules in photosynthetic membranes and converted into stored chemical energy. In the light-independent reactions, the chemical ener gy produced by the light-dependent reactions is used to drive the assembly of sugar molecules using CO2; however, these reactions are still light dependent because the products of the light-dependent reactions necessary for driving them are short-lived. The light-dependent reactions produce ATP and either NADPH or NADH to temporarily store energy. These energy carriers are used in the light-independent reactions to drive the energetically unfavorable process of “fixing” inorganic CO2 in an organic form, sugar.

Figure (PageIndex{1}): The light-dependent reactions of photosynthesis (left) convert light energy into chemical energy, forming ATP and NADPH. These products are used by the light-independent reactions to fix CO2, producing organic carbon molecules.

Photosynthetic Structures in Eukaryotes and Prokaryotes

In all phototrophic eukaryotes, photosynthesis takes place inside a chloroplast, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see Unique Characteristics of Eukaryotic Cells). These chloroplasts are enclosed by a double membrane with inner and outer layers. Within the chloroplast is a third membrane that forms stacked, disc-shaped photosynthetic structures called thylakoids (Figure (PageIndex{2})). A stack of thylakoids is called a granum, and the space surrounding the granum within the chloroplast is called stroma.

Photosynthetic membranes in prokaryotes, by contrast, are not organized into distinct membrane-enclosed organelles; rather, they are infolded regions of the plasma membrane. In cyanobacteria, for example, these infolded regions are also referred to as thylakoids. In either case, embedded within the thylakoid membranes or other photosynthetic bacterial membranes are photosynthetic pigment molecules organized into one or more photosystems, where light energy is actually converted into chemical energy.

Photosynthetic pigments within the photosynthetic membranes are organized into photosystems, each of which is composed of a light-harvesting (antennae) complex and a reaction center. The light-harvesting complex consists of multiple proteins and associated pigments that each may absorb light energy and, thus, become excited. This energy is transferred from one pigment molecule to another until eventually (after about a millionth of a second) it is delivered to the reaction center. Up to this point, only energy—not electrons—has been transferred between molecules. The reaction center contains a pigment molecule that can undergo oxidation upon excitation, actually giving up an electron. It is at this step in photosynthesis that light energy is converted into an excited electron.

Different kinds of light-harvesting pigments absorb unique patterns of wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear the corresponding color. Examples of photosynthetic pigments (molecules used to absorb solar energy) are bacteriochlorophylls (green, purple, or red), carotenoids (orange, red, or yellow), chlorophylls (green), phycocyanins (blue), and phycoerythrins (red). By having mixtures of pigments, an organism can absorb energy from more wavelengths. Because photosynthetic bacteria commonly grow in competition for sunlight, each type of photosynthetic bacteria is optimized for harvesting the wavelengths of light to which it is commonly exposed, leading to stratification of microbial communities in aquatic and soil ecosystems by light quality and penetration.

Once the light harvesting complex transfers the energy to the reaction center, the reaction center delivers its high-energy electrons, one by one, to an electron carrier in an electron transport system, and electron transfer through the ETS is initiated. The ETS is similar to that used in cellular respiration and is embedded within the photosynthetic membrane. Ultimately, the electron is used to produce NADH or NADPH. The electrochemical gradient that forms across the photosynthetic membrane is used to generate ATP by chemiosmosis through the process of photophosphorylation, another example of oxidative phosphorylation (Figure (PageIndex{3})).

Figure (PageIndex{2}): (a) Photosynthesis in eukaryotes takes place in chloroplasts, which contain thylakoids stacked into grana. (b) A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. (credit: scale bar data from Matt Russell.)

Figure (PageIndex{3}): This figure summarizes how a photosystem works. Light harvesting (LH) pigments absorb light energy, converting it to chemical energy. The energy is passed from one LH pigment to another until it reaches a reaction center (RC) pigment, exciting an electron. This high-energy electron is lost from the RC pigment and passed through an electron transport system (ETS), ultimately producing NADH or NADPH and ATP. A reduced molecule (H2A) donates an electron, replacing electrons to the electron-deficient RC pigment.

Exercise (PageIndex{1})

In a phototrophic eukaryote, where does photosynthesis take place?

Oxygenic and Anoxygenic Photosynthesis

For photosynthesis to continue, the electron lost from the reaction center pigment must be replaced. The source of this electron (H2A) differentiates the oxygenic photosynthesis of plants and cyanobacteria from anoxygenic photosynthesis carried out by other types of bacterial phototrophs (Figure (PageIndex{4})). In oxygenic photosynthesis, H2O is split and supplies the electron to the reaction center. Because oxygen is generated as a byproduct and is released, this type of photosynthesis is referred to as oxygenic photosynthesis. However, when other reduced compounds serve as the electron donor, oxygen is not generated; these types of photosynthesis are called anoxygenic photosynthesis. Hydrogen sulfide (H2S) or thiosulfate (S2O2−3)(S2O32−) can serve as the electron donor, generating elemental sulfur and sulfate (SO2−4)(SO42−)ions, respectively, as a result.

Photosystems have been classified into two types: photosystem I (PSI) and photosystem II (PSII) (Figure (PageIndex{5})). Cyanobacteria and plant chloroplasts have both photosystems, whereas anoxygenic photosynthetic bacteria use only one of the photosystems. Both photosystems are excited by light energy simultaneously. If the cell requires both ATP and NADPH for biosynthesis, then it will carry out noncyclic photophosphorylation. Upon passing of the PSII reaction center electron to the ETS that connects PSII and PSI, the lost electron from the PSII reaction center is replaced by the splitting of water. The excited PSI reaction center electron is used to reduce NADP+ to NADPH and is replaced by the electron exiting the ETS. The flow of electrons in this way is called the Z-scheme.

If a cell’s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power through cyclic photophosphorylation. Only PSI is used during cyclic photophosphorylation; the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.

Figure (PageIndex{4}): Eukaryotes and cyanobacteria carry out oxygenic photosynthesis, producing oxygen, whereas other bacteria carry out anoxygenic photosynthesis, which does not produce oxygen.

Figure (PageIndex{5}): (a) PSI and PSII are found on the thylakoid membrane. The high-energy electron from PSII is passed to an ETS, which generates a proton motive force for ATP synthesis by chemiosmosis, and ultimately replaces the electron lost by the PSI reaction center. The PSI reaction center electron is used to make NADPH. (b) When both ATP and NADPH are required, noncyclic photophosphorylation (in cyanobacteria and plants) provides both. The electron flow described here is referred to as the Z-scheme (shown in yellow in [a]). When the cell’s ATP needs outweigh those for NADPH, cyanobacteria and plants will use only PSI, and its reaction center electron is passed to the ETS to generate a proton motive force used for ATP synthesis.

Exercise (PageIndex{2})

Why would a photosynthetic bacterium have different pigments?

Light-Independent Reactions

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO2, the gas that is a waste product of cellular respiration.

The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathway used for fixation of CO2, is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration (see Appendix C for a detailed illustration of the Calvin cycle).

  • Fixation: The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyzes the addition of a CO2 to ribulose bisphosphate (RuBP). This results in the production of 3-phosphoglycerate (3-PGA).
  • Reduction: Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.
  • Regeneration: The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the system to continue CO2 fixation. Three more molecules of ATP are used in these regeneration reactions.

The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%–50% of the total soluble protein in plant chloroplasts.1 However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many nonphotosynthetic chemoautotrophs to fix CO2. Additionally, other bacteria and archaea use alternative systems for CO2 fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO2 fixation pathway.

Exercise (PageIndex{3})

Describe the three stages of the Calvin cycle.

Key Concepts and Summary

  • Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.
  • Different photosynthetic organisms use different mixtures of photosynthetic pigments, which increase the range of the wavelengths of light an organism can absorb.
  • Photosystems (PSI and PSII) each contain a light-harvesting complex, composed of multiple proteins and associated pigments that absorb light energy. The light-dependent reactions of photosynthesis convert solar energy into chemical energy, producing ATP and NADPH or NADH to temporarily store this energy.
  • In oxygenic photosynthesis, H2O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In anoxygenic photosynthesis, other reduced molecules like H2S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct.
  • Noncyclic photophosphorylation is used in oxygenic photosynthesis when there is a need for both ATP and NADPH production. If a cell’s needs for ATP outweigh its needs for NADPH, then it may carry out cyclic photophosphorylation instead, producing only ATP.
  • The light-independent reactions of photosynthesis use the ATP and NADPH from the light-dependent reactions to fix CO2 into organic sugar molecules.

Multiple Choice

During the light-dependent reactions, which molecule loses an electron?

A. a light-harvesting pigment molecule
B. a reaction center pigment molecule
D. 3-phosphoglycerate


In prokaryotes, in which direction are hydrogen ions pumped by the electron transport system of photosynthetic membranes?

A. to the outside of the plasma membrane
B. to the inside (cytoplasm) of the cell
C. to the stroma
D. to the intermembrane space of the chloroplast


Which of the following does not occur during cyclic photophosphorylation in cyanobacteria?

A. electron transport through an ETS
B. photosystem I use
C. ATP synthesis
D. NADPH formation


Which are two products of the light-dependent reactions are ________.

A. glucose and NADPH
C. glyceraldehyde 3-phosphate and CO2
D. glucose and oxygen



Photosynthesis always results in the formation of oxygen.


Fill in the Blank

The enzyme responsible for CO2 fixation during the Calvin cycle is called ________.

ribulose bisphosphate carboxylase (RuBisCO)

The types of pigment molecules found in plants, algae, and cyanobacteria are ________ and ________.

chlorophylls and carotenoids

Short Answer

Why would an organism perform cyclic phosphorylation instead of noncyclic phosphorylation?

What is the function of photosynthetic pigments in the light-harvesting complex?

Critical Thinking

Is life dependent on the carbon fixation that occurs during the light-independent reactions of photosynthesis? Explain.


  1. 1 A. Dhingra et al. “Enhanced Translation of a Chloroplast-Expressed RbcS Gene Restores Small Subunit Levels and Photosynthesis in Nuclear RbcS Antisense Plants.” Proceedings of the National Academy of Sciences of the United States of America 101 no. 16 (2004):6315–6320.


  • Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at

Path of light in photosynthesis traced

Three billion years ago, light first zipped through chlorophyll within tiny reaction centers, the first step plants and photosynthetic bacteria take to convert light into food.

Heliobacteria, a type of bacteria that uses photosynthesis to generate energy, has reaction centers thought to be similar to those of the common ancestors for all photosynthetic organisms. Now, a University of Michigan team has determined the first steps in converting light into energy for this bacterium.

"Our study highlights the different ways in which nature has made use of the basic reaction center architecture that emerged over 3 billion years ago," said lead author and U-M physicist Jennifer Ogilvie. "We want to ultimately understand how energy moves through the system and ends up creating what we call the 'charge-separated state.' This state is the battery that drives the engine of photosynthesis."

laser 2.jpegPhotosynthetic organisms contain "antenna" proteins that are packed with pigment molecules to harvest photons. The collected energy is then directed to "reaction centers" that power the initial steps that convert light energy into food for the organism. These initial steps happen on incredibly fast timescales -- femtoseconds, or one millionth of one billionth of a second. During the blink of an eye, this conversion happens many quadrillions of times.

Researchers are interested in understanding how this transformation takes place. It gives us a better understanding of how plants and photosynthetic organisms convert light into nourishing energy. It also gives researchers a better understanding of how photovoltaics work -- and the basis for understanding how to build them better.

When light hits a photosynthetic organism, pigments within the antenna gather photons and direct the energy toward the reaction center. In the reaction center, the energy bumps an electron to a higher energy level, from which it moves to a new location, leaving behind a positive charge. This is called a charge separation. This process happens differently based on the structure of the reaction center in which it occurs.

In the reaction centers of plants and most photosynthetic organisms, the pigments that orchestrate charge separation absorb similar colors of light, making it difficult to visualize charge separation. Using the heliobacteria, the researchers identified which pigments initially donate the electron after they're excited by a photon, and which pigments accept the electron.

Heliobacteria is a good model to examine, Ogilvie said, because their reaction centers have a mixture of chlorophyll and bacteriochlorophyll, which means that these different pigments absorb different colors of lights. For example, she said, imagine trying to follow a person in a crowd -- but everyone is wearing blue jackets, you're watching from a distance and you can only take snapshots of the person moving through the crowd.

"But if the person you were watching was wearing a red jacket, you could follow them much more easily. This system is kind of like that: It has distinct markers," said Ogilvie, professor of physics, biophysics, and macromolecular science and engineering

Previously, heliobacteria were difficult to understand because its reaction center structure was unknown. The structure of membrane proteins like reaction centers are notoriously difficult to determine, but Ogilvie's co-author, Arizona State University biochemist Kevin Redding, developed a way to resolve the crystal structure of these reaction centers.

To probe reaction centers in heliobacteria, Ogilvie's team uses a type of ultrafast spectroscopy called multidimensional electronic spectroscopy, implemented in Ogilvie's lab by lead author and postdoctoral fellow Yin Song. The team aims a sequence of carefully timed, very short laser pulses at a sample of bacteria. The shorter the laser pulse, the broader light spectrum it can excite.

Each time the laser pulse hits the sample, the light excites the reaction centers within. The researchers vary the time delay between the pulses, and then record how each of those pulses interacts with the sample. When pulses hit the sample, its electrons are excited to a higher energy level. The pigments in the sample absorb specific wavelengths of light from the laser -- specific colors -- and the colors that are absorbed give the researchers information about the energy level structure of the system and how energy flows through it.

"That's an important role of spectroscopy: When we just look at the structure of something, it's not always obvious how it works. Spectroscopy allows us to follow a structure as it's functioning, as the energy is being absorbed and making its way through those first energy conversion steps," Ogilvie said. "Because the energies are quite distinct in this type of reaction center, we can really get an unambiguous look at where the energy is going."

Getting a clearer picture of this energy transport and charge separation allows the researchers to develop more accurate theories about how the process works in other reaction centers.

"In plants and bacteria, it's thought that the charge separation mechanism is different," Ogilvie said. "The dream is to be able to take a structure and, if our theories are good enough, we should be able to predict how it works and what will happen in other structures -- and rule out mechanisms that are incorrect."

Photonic multilayer structure of Begonia chloroplasts enhances photosynthetic efficiency

Enhanced light harvesting is an area of interest for optimizing both natural photosynthesis and artificial solar energy capture 1,2 . Iridescence has been shown to exist widely and in diverse forms in plants and other photosynthetic organisms and symbioses 3,4 , but there has yet to be any direct link demonstrated between iridescence and photosynthesis. Here we show that epidermal chloroplasts, also known as iridoplasts, in shade-dwelling species of Begonia 5 , notable for their brilliant blue iridescence, have a photonic crystal structure formed from a periodic arrangement of the light-absorbing thylakoid tissue itself. This structure enhances photosynthesis in two ways: by increasing light capture at the predominantly green wavelengths available in shade conditions, and by directly enhancing quantum yield by 5–10% under low-light conditions. These findings together imply that the iridoplast is a highly modified chloroplast structure adapted to make best use of the extremely low-light conditions in the tropical forest understorey in which it is found 5,6 . A phylogenetically diverse range of shade-dwelling plant species has been found to produce similarly structured chloroplasts 7–9 , suggesting that the ability to produce chloroplasts whose membranes are organized as a multilayer with photonic properties may be widespread. In fact, given the well-established diversity and plasticity of chloroplasts 10,11 , our results imply that photonic effects may be important even in plants that do not show any obvious signs of iridescence to the naked eye but where a highly ordered chloroplast structure may present a clear blue reflectance at the microscale. Chloroplasts are generally thought of as purely photochemical we suggest that one should also think of them as a photonic structure with a complex interplay between control of light propagation, light capture and photochemistry.

Photonic crystals are periodic nanoscale structures that interact with light, resulting in a number of optical phenomena. In artificial systems, these photonic properties have been investigated for their light-harvesting properties as they can strongly enhance the performance of devices for solar energy production, through either light-trapping 1 or slow light mechanisms 2 . However, one group of photonic crystals that has repeatedly been associated with light-harvesting roles, including optimized light distribution within photosynthetic tissues 3 , but which has yet to be directly linked to enhanced light harvesting, is natural biological photonic crystals. Photonic structures are widespread in nature, where they are typically associated with structural colour 4,12,13 . Although more extensively researched in animals, photonic effects have further implications in plants given the importance of light manipulation for photosynthesis 4 . A striking example of structural colour in plants is presented by the iridescent blue leaves observed in a diverse range of tropical plant species adapted to deep forest shade conditions however, little direct evidence of the function of this structural colour has been presented 14,15 .

7.6: Photosynthesis and the Importance of Light - Biology

Why is Carbon Dioxide Important for Photosynthesis

Most of us know that humans and animals inhale oxygen and exhale carbon dioxide, which is absorbed by plants, but do we know why do plants need carbon dioxide?

What is the food source for plants?

There do they get the energy to grow?

There is a lot of talk on why carbon emissions are dangerous for the climate, but do we why it is dangerous for the environment?

Carbon dioxide is an important element in photosynthesis, which is a process that converts energy from sunlight to chemical energy stored in glucose.

The by-product of photosynthesis is oxygen, which is an essential element of life.

Carbon dioxide is important for photosynthesis because it provides the carbon required for the plant to produce glucose, which is used to complete cellular processes in the plant.

These cellular processes enable the plants to develop seeds, grow, make fruit, and form flowers.

What is photosynthesis?

During photosynthesis, plants gather carbon dioxide and water from air and soil. The water goes through oxidation, while the carbon dioxide goes through reduction.

This process converts carbon dioxide into glucose and the water into oxygen. Inside in the plant cell, there is a light-absorbing pigment called chlorophyll, which has the role of absorbing blue and red light waves from sunlight and reflect green light waves.

This gives the plants their green color. Chlorophyll allows us to absorb the energy needed to complete the photosynthesis process.

The process of photosynthesis can be divided into two main types of reactions. There are light-dependent reactions and light-independent reactions.

The light-dependent reactions convert chemical energy in the form of NADPH and ATP. The light-dependent reactions include the Calvin Cycle, during which energy is used to gather glucose from carbon dioxide.

Carbon molecules are converted from carbon dioxide to stored fuel in the form of carbohydrates. These carbohydrates are used as a source of food or energy for the plant.

The process of photosynthesis is often written as the following chemical formula:

This means that six molecules of carbon dioxide (CO2) react with six molecules of water (H2O) to form glucose molecules and oxygen.

It is not just plants that can photosynthesize, but other eukaryotic and prokaryotic organisms also able to harvest their energy from photosynthesis.

The importance of carbon dioxide for plants

Photosynthesis is critical for the existence of life on earth. In a way, how the energy in the biosphere reaches living things on earth. The organisms that use photosynthesis form the primary producers of oxygen in the world.

Almost all of the oxygen on earth comes from photosynthesis. If this process was to stop, the world is left with no oxygen. The importance of photosynthesis extends to many life forms on earth, including plants.

Only organisms that could exist without oxygen are certain bacteria. Everything else is dependent on photosynthesis to produce oxygen.

It has been established that the process of photosynthesis cannot be completed without carbon dioxide. People often used they are feeding the plant by watering them or providing fertilizers, but the needs of the plant are not complete without carbon dioxide and sun.

Photosynthesis is one of the most important processes on earth. Not only is it used by plants but also other microorganisms and algae. Just as humans and animals need respiration to stay alive, plants need photosynthesis.

The entire process of photosynthesis can be summarized as a way to transfer energy from the sun to the plant.

Different types of plants have evolved to require a different amount of water, sunlight, and carbon dioxide. Plants in the desert, such as a cactus plant, are naturally designed to require less water, whereas plants in a pond as an abundance of water. Similarly, different plants require different levels of carbon dioxide.

What effect does increasing levels of carbon dioxide have on photosynthesis?

The amount of carbon dioxide in the world is increasing. According to, carbon dioxide levels in the world are its highest level since 800,000 years.

There are several reasons for the rise in carbon dioxide levels. The primary reason is the increase in the burning of fossil fuels such as oil and coal. These fossil fuels contain carbon, which has been absorbed through hundreds of years of photosynthesis.

You might think that if carbon dioxide is so important to plants, an increasing level of carbon dioxide should be immensely useful to plants, including food crops.

The level of food production in the world should increase, and the entire talk of fossil fuels being bad for the environment should be rubbished. It is true that in complete isolation, increasing levels of carbon dioxide will increase photosynthesis.

However, it is important to note that plants require more than just carbon dioxide to function. They need water, nitrogen, and other nutrients to function.

Any limitations of these essential elements will not allow the plants to grow. This is why plants need fertilizers that contain nitrogen. Any positive effect of increasing carbon dioxide in the world is negated by the increase in global temperature caused by global warming.

The increase in temperature has a devastating effect on plant life.

Future of photosynthesis

The increasing world population and the use of fossil fuels are putting an intense strain on the natural resources of the world. Food security is a critical issue for the survival of life on earth.

If the productivity of crops can be increased, it will have a significant positive impact on the sustainability of life.

Photosynthesis is a magnificent process of nature. However, there is a flaw in the process, or maybe nature intended it to be that way.

The oxygen and carbon dioxide molecules are similar in size and shape. An enzyme by the name of RuBisCO in plants is used to harvest carbon dioxide.

At times, this enzyme harvests an oxygen molecule mistaking it for a carbon dioxide molecule. The harvesting of oxygen molecules instead of carbon dioxide is putting a strain on the energy and resources of plants.

With global warming, the temperature of the earth is increasing. With the increase in temperature, the RubisCO is getting more prone to errors. Water also evaporates faster in increased temperatures.

This is straining the ecosystem of plants around the globe. As the RubisCO enzyme gets limited carbon dioxide, it depletes the energy of the plant is trying to harvest oxygen.

Some plants have evolved to handle this issue by pushing extra carbon dioxide to the RubisCO enzyme. This is like a turbocharged version of photosynthesis.

Plants that can do this are categorized as C4 plants. These plants can be highly effective in hot and dry weather, but as the global climate gets hotter, more of such plants will be seen everywhere.

At the moment, only 3% of the world’s flowering plants take the C4 route to photosynthesis. However, this 3% of the plants account for 24% of the world’s plant primary productivity in the world.

The type of plants mostly using the C4 pathway includes corn and sorghum. If somehow, other productive crops such as rice use the C4 pathway to photosynthesis, it can have an immense impact on the economics and food security of the world.

Links and Resources

Photosynthesis *suitable for home teaching*

This resource, from the Royal Society of Chemistry (RSC) &lsquoChallenging Plants&rsquo resource pack, provides background information on the process of photosynthesis, including details of the leaf structure, the role of chlorophyll and light dependent reactions.

The importance of photosynthesis is clearly explained.

Students could make use of this two page summary in a variety of ways: as a revision sheet, they could work in pairs read through the resource and then ask each other 5 questions that should be answered without using the resource

Activity Procedure

1. Choose one of the leaves that has been deprived of carbon dioxide.

2. Submerge this leaf in boiling water for about 3 minutes.

3. Remove the leaf from the water and insert it into a test tube containing ethanol and plug the test tube with a piece of cotton wool. Note: the test tube should be less than half full of ethanol.

4. Submerge the test tube in the boiling water and leave it to boil until the leaf loses all of its colour.

5. Once the leaf has lost its colour, remove it from the ethanol solution and dip it brie y into the boiling water to remove the ethanol and soften it.

6. Spread the decolourized leaf on a white tile and add iodine solution drop wise until the whole leaf is covered.

7. Record your observations and draw a picture showing the colour pattern of the leaf. The leaves should test negative for starch.

Explainer: How photosynthesis works

Green plants take in light from the sun and turn water and carbon dioxide into the oxygen we breathe and the sugars we eat.

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October 28, 2020 at 6:30 am

Take a deep breath. Then thank a plant. If you eat fruit, vegetables, grains or potatoes, thank a plant too. Plants and algae provide us with the oxygen we need to survive, as well as the carbohydrates we use for energy. They do it all through photosynthesis.

Photosynthesis is the process of creating sugar and oxygen from carbon dioxide, water and sunlight. It happens through a long series of chemical reactions. But it can be summarized like this: Carbon dioxide, water and light go in. Glucose, water and oxygen come out. (Glucose is a simple sugar.)

Photosynthesis can be split into two processes. The “photo” part refers to reactions triggered by light. “Synthesis” — the making of the sugar — is a separate process called the Calvin cycle.

Both processes happen inside a chloroplast. This is a specialized structure, or organelle, in a plant cell. The structure contains stacks of membranes called thylakoid membranes. That’s where the light reaction begins.

Chloroplasts are found in plant cells. This is where photosynthesis takes place. The chlorophyll molecules that take in energy from sunlight are located in the stacks called thylakoid membranes. blueringmedia/iStock/Getty Images Plus

Let the light shine in

When light hits a plant’s leaves, it shines on chloroplasts and into their thylakoid membranes. Those membranes are filled with chlorophyll, a green pigment. This pigment absorbs light energy. Light travels as electromagnetic waves. The wavelength — distance between waves — determines energy level. Some of those wavelengths are visible to us as the colors we see. If a molecule, such as chlorophyll, has the right shape, it can absorb the energy from some wavelengths of light.

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Chlorophyll can absorb light we see as blue and red. That’s why we see plants as green. Green is the wavelength plants reflect, not the color they absorb.

While light travels as a wave, it also can be a particle called a photon. Photons have no mass. They do, however, have a small amount of light energy.

When a photon of light from the sun bounces into a leaf, its energy excites a chlorophyll molecule. That photon starts a process that splits a molecule of water. The oxygen atom that splits off from the water instantly bonds with another, creating a molecule of oxygen, or O2. The chemical reaction also produces a molecule called ATP and another molecule called NADPH. Both of these allow a cell to store energy. The ATP and NADPH also will take part in the synthesis part of photosynthesis.

Notice that the light reaction makes no sugar. Instead, it supplies energy — stored in the ATP and NADPH — that gets plugged into the Calvin cycle. This is where sugar is made.

But the light reaction does produce something we use: oxygen. All the oxygen we breathe is the result of this step in photosynthesis, carried out by plants and algae (which are not plants) the world over.

Give me some sugar

The next step takes the energy from the light reaction and applies it to a process called the Calvin cycle. The cycle is named for Melvin Calvin, the man who discovered it.

The Calvin cycle is sometimes also called the dark reaction because none of its steps require light. But it still happens during the day. That’s because it needs the energy produced by the light reaction that comes before it.

While the light reaction takes place in the thylakoid membranes, the ATP and NADPH it produces end up in the stroma. This is the space inside the chloroplast but outside the thylakoid membranes.

The Calvin cycle has four major steps:

  1. carbon fixation: Here, the plant brings in CO2 and attaches it to another carbon molecule, using rubisco. This is an enzyme, or chemical that makes reactions move faster. This step is so important that rubisco is the most common protein in a chloroplast — and on Earth. Rubisco attaches the carbon in CO2 to a five-carbon molecule called ribulose 1,5-bisphosphate (or RuBP). This creates a six-carbon molecule, which immediately splits into two chemicals, each with three carbons.
  2. reduction: The ATP and NADPH from the light reaction pop in and transform the two three-carbon molecules into two small sugar molecules. The sugar molecules are called G3P. That’s short for glyceraldehyde 3-phosphate (GLIH- sur-AAL-duh-hide 3-FOS-fayt).
  3. carbohydrate formation: Some of that G3P leaves the cycle to be converted into bigger sugars such as glucose (C6H12O6).
  4. regeneration: With more ATP from the continuing light reaction, leftover G3P picks up two more carbons to become RuBP. This RuBP pairs up with rubisco again. They are now ready to start the Calvin cycle again when the next molecule of CO2 arrives.

At the end of photosynthesis, a plant ends up with glucose (C6H12O6), oxygen (O2) and water (H2O). The glucose molecule goes on to bigger things. It can become part of a long-chain molecule, such as cellulose that’s the chemical that makes up cell walls. Plants also can store the energy packed in a glucose molecule within larger starch molecules. They can even put the glucose into other sugars — such as fructose — to make a plant’s fruit sweet.

All of these molecules are carbohydrates — chemicals containing carbon, oxygen and hydrogen. (CarbOHydrate makes it easy to remember.) The plant uses the bonds in these chemicals to store energy. But we use the these chemicals too. Carbohydrates are an important part of the foods we eat, particularly grains, potatoes, fruits and vegetables.

Power Words

algae: Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

ATP: Short for adenosine triphosphate. Cells make this molecule to power almost all of their activities. Cells use oxygen and simple sugars to create this molecule, the main source of their energy. The small structures in cells that carry out this energy-storing process are known as mitochondria. Like a battery, ATP stores a bit of usable energy. Once the cell uses it up, mitochondria must recharge the cell by making more ATP using energy harvested from the cell’s nutrients.

bond: (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

Calvin cycle: Named for Melvin Calvin, the man who discovered it, this cycle is the process by which plants and algae turn carbon dioxide into basic carbohydrates.

carbohydrates: Any of a large group of compounds occurring in foods and living tissues, including sugars, starch and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down in an animal’s body to release energy.

carbon dioxide: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

cell: The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cellulose: A type of fiber found in plant cell walls. It is formed by chains of glucose molecules.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction: A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chlorophyll: Any of several green pigments found in plants that perform photosynthesis — creating sugars (foods) from carbon dioxide and water.

chloroplast: A tiny structure in the cells of green algae and green plants that contain chlorophyll and creates glucose through photosynthesis.

electromagnetic: An adjective referring to light radiation, to magnetism or to both.

excite: (in chemistry and physics) To transfer energy to one or more outer electrons in an atom. They remain in this higher energy state until they shed the extra energy through the emission of some type of radiation, such as light.

fructose: A simple sugar. Along with glucose, fructose makes up half of each molecule of sucrose (also known as table sugar).

glucose: A simple sugar that is an important energy source in living organisms. As an energy source moving through the bloodstream, it is known as “blood sugar.” It is half of the molecule that makes up table sugar (also known as sucrose).

glyceraldehyde 3-phosphate or G3P: A molecule created as part of the chemical steps that form simple carbohydrates. Two molecules of G3P are made in plants and algae as part of the Calvin cycle of photosynthesis. Animals and bacteria also create G3P as part of their own steps of making carbohydrate.

hydrogen: The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

membrane: A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

NADPH or NADP+: Short for nicotinamide adenine dinucleotide phosphate (but no one calls it that). This molecule is a way for cells to store and transfer energy. When the NADP+ molecule is transformed to NADPH, it contains energy, which it can then use to power other reactions in a cell.

organelle: Specialized structures, such as mitochondria, found within a cell.

oxygen: A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

particle: A minute amount of something.

photon: A particle representing the smallest possible amount of light or other type of electromagnetic radiation.

photosynthesis: (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

reduction: (v. reduce) A chemical reaction that adds one or more electrons. It’s also viewed as the opposite of oxidation. As rust oxidizes iron, the process reduces those nearby oxygen atoms. That means that they gain electrons, which have a negative charge.

ribulose 1,5-bisphosphate: (RuBP) A molecule that completes the first and last steps of the Calvin cycle, which creates sugar out of carbon dioxide. This molecule contains five carbons and binds to the enzyme rubisco. Rubisco hooks up RuBP with carbon dioxide from the air, the first step in making a carbohydrate.

rubisco: This acronym stands for ribulose bisphosphate carboxylase/oxygenase. It is the most common protein on Earth. In its role as an enzyme, it plays a pivotal role in photosynthesis.

starch: A soft white chemical made by all green plants. It’s a relatively long molecule made from linking together a lot of smaller, identical building blocks — all of them glucose, a simple sugar. Plants and animals use glucose as an energy source. Plants store that glucose, in the form of starch, as a reserve supply of energy. Animals that consume starch can break down the starch into glucose molecules to extract the useful energy.

stroma: (in botany) The colorless fluid inside a chloroplast, where the Calvin cycle portion of photosynthesis takes place. (in anatomy) The supportive structure surrounding an organ. It includes the connective tissue that holds the organ in place and the blood vessels that bring the organ oxygen and sugars.

synthesis: (v. synthesize) The production of a substance by the combining of simpler chemical building blocks.

thylakoid membranes: An internal system of connected membranes inside a chloroplast. The membranes contain not only the green pigment chlorophyll, but also proteins. These membranes are where the light reaction portion of photosynthesis takes place, producing oxygen and energy to power the sugar-making portions of photosynthesis.

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.


Book:​ ​​N.A. Campbell, J.B. Reese and L.G. Mitchell. Biology, 5 th Edition. Benjamin-Cummings Publication Co., 1999.

About Bethany Brookshire

Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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Cyclic Electron Transport in Photosynthesis

Photophosphorylation refers to the use of light energy to ultimately provide the energy to convert ADP to ATP, thus replenishing the universal energy currency in living things. In the simplest systems in prokaryotes, photosynthesis is used just for the production of energy, and not for the building of any biological molecules. In these systems there is a process called cyclic photophosphorylation, which just accomplishes the ADP to ATP process for immediate energy for these cells. This process uses only Photosystem I and the chlorophyll P700.

The above sketch of the cyclic process is patterned after a visualization in Moore, et al. Two photons from either the red or blue end of the spectrum fit the sensitive response of the pigments. They are captured by the antenna complex and transferred to the Photosystem I reaction center, which contributes two high energy electrons to the primary electron receptor. They are passed to ferrodoxin (Fd), an iron containing protein which acts as an electron carrier. A second electron carrier plastoquinone (Pq) carries the electrons to a complex of two cytochromes. In the process, energy is provided to produce a proton gradient across the membrane which can be used for the ADP to ATP conversion. The electrons are returned by plastocyanin (Pc) to the P700 pigment in the reaction center to complete the cycle.

This sketch follows the lead of Karp to place the events relative to the membrane. This makes it clearer that the process of producing the ATP is driven by the proton gradient. Karp points out that this cyclic photophosphorylation also takes place in isolated chloroplasts and can provide additional ATP to aid the carbohydrate synthesis taking place as result of the non-cyclic electron transport.


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Photosynthesis is the process in which plants, using sunlight, create sugars by combining carbon dioxide with water. It is important that students of middle school Biology understand photosynthesis and this quiz will help them to revise. Simply put, photosynthesis is the process by which plants feed. Since they are at the start of all food chains and food webs, you could say that they make all of the food for the planet!

Photosynthesis is the making of sugars from carbon dioxide and water. It requires energy in the form of light, hence the name of the process (photo means relating to light and in terms of chemicals, synthesis means building a more complex compound from simpler substances). The waste product is oxygen which is needed for aerobic respiration in living organisms. Until green plants had built up sufficient oxygen in the Earth's atmosphere to create an ozone layer, animal life was restricted to living under water, where it was protected from the UV radiation from the Sun.

Light energy is absorbed by the chlorophyll in green plants and algae. This energy is used to convert carbon dioxide from the air and water from the soil into glucose. This glucose can be stored as starch, converted into cellulose for making cell walls, converted into proteins or used in respiration. Plant cells respire at the same time as photosynthesis takes place but during the day, photosynthesis produces more oxygen than is used up in respiration.

Most plants have evolved leaves to make the process of photosynthesis more effective. Leaves have a large surface area and are very thin. This means that gasses can get in and out more easily and it also maximizes the amount of light energy reaching the chlorophyll. Chlorophyll is found in most plant cells and is contained within structures named chloroplasts. Chlorophyll absorbs the red and blue wavelengths of light from the sun but reflects the green, which is why plants appear green. There are types of chlorophyll that absorb other wavelengths, making certain plants and algae appear more red. It is possible to grow plants under artificial lighting but sunlight is the best source of light for photosynthesis.

Light Reaction

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The light reaction of light dependent reaction occurs in the chloroplast of the mesophyll cells of the leaves. The chloroplasts are double-membraned cell organelles that are comprised of stacked disc-like structures known as thylakoids. The pigment, chlorophyll, which is required for the process is present on the membrane of these thylakoids and this is where the light reaction occurs.

Browse more Topics under Photosynthesis In Higher Plants

The Steps Involved in the Light Reaction

The main purpose of the light reaction is to generate organic energy molecules such as ATP and NADPH which are needed for the subsequent dark reaction.

  • Chlorophyll absorbs the red and blue segment of the white light and photosynthesis occurs most efficiently at these wavelengths.
  • When the light falls on the plant, the chlorophyll pigment absorbs this light and electrons in it gets excited.
  • This process occurs in a complex protein system which is collectively called as a photosystem. There are two closely linked photosystems known as PSI and PSII.
  • The chlorophyll pigments which are excited give up their electrons and to compensate for the loss of electrons, water is split to release four H+ ions and four electrons and O2. The electrons that are lost from the PSII enter into an electron transfer chain or ETC.
  • The electrons finally reach the reaction centre where they combine with NADP+ and reduce it to NADPH
  • While the electrons are taken care of, the built up of H+ ions inside the thylakoid lumen is of equal importance.
  • The hydrogen ions building up inside the lumen creates a positive gradient and in the presence of the enzyme ATP synthetase, these H+ ions combine with the ADP in the nearby region to form ATP.
  • The oxygen that is a waste product is released by the plant into the atmosphere and some of it is used in photorespiration if the plant needs to.

To summarise the light reaction, we can write it down as the following reaction:

2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

For any plant performing photosynthesis, four factors influence this process. CO2, water, light, and chlorophyll are the raw materials for photosynthesis. But, in case of light dependent reaction or light reaction of photosynthesis, it is most influenced by presence or absence of light. The other three factors do not play a critical role in it.

Watch the video: Τα TOP 5 Πιο Επικίνδυνα Και ΘΑΝΑΤΗΦΟΡΑ ΦΥΤΑ Στον Κόσμο. TopTenGR (August 2022).