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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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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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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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Zooming into Chloroplasts: Light-Dependent and Light-Independent Reactions of Photosynthesis

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Within plant cells, chloroplasts are specialized organelles that serve as the sites of photosynthesis. The reactions that make up the process of photosynthesis can be divided into light-dependent reactions, which take place in the thylakoids, and light-independent reactions (also known as dark reactions or the Calvin cycle), which take place in the stroma.

1. Chloroplasts have a complex internal structure, and different reactions take place in different parts of the chloroplast.

Understanding what the inside of a chloroplast looks like is key to visualizing where the different reactions of photosynthesis occur.

Surrounding the chloroplast is a double membrane, consisting of an outer membrane and an inner membrane. This is similar in structure to the double membrane of mitochondria.

Interior to the chloroplast’s inner membrane and surrounding the thylakoids is a fluid called the stroma. The light-independent reactions of photosynthesis take place within the stroma. It contains enzymes that work with ATP and NADPH to “fix” carbon from carbon dioxide into molecules that can be used to build glucose. The chloroplast’s own genetic material (separate from that of the cell) is also stored in the stroma.

The interior of the chloroplast contains another membrane—the thylakoid membrane—which is folded to form numerous connected stacks of discs. Each disc is a thylakoid and each stack is a granum (pl. grana).

The light-dependent reactions of photosynthesis take place within the thylakoids. These reactions occur when the pigment chlorophyll, located within the thylakoid membranes, captures energy from the sun (photons) to initiate the breakdown of water molecules.

2. The light-dependent reactions convert light energy into chemical energy.

The goal of the light-dependent reactions of photosynthesis is to collect energy from the sun and break down water molecules to produce ATP and NADPH. These two energy-storing molecules are then used in the light-independent reactions.

Within chloroplasts, chlorophyll is the pigment that absorbs sunlight. It is stored in the thylakoid membranes in protein complexes called photosystem I and photosystem II. The series of light-dependent reactions begins when sunlight hits a molecule of chlorophyll, located in photosystem II. This excites an electron, which leaves the chlorophyll molecule and travels along the thylakoid membrane via a series of carrier proteins (known as the electron transport chain).

Then, something amazing happens—photosystem II splits a water molecule to restore this lost electron and fill the “energy vacuum” that has been created. This is a process humans haven’t been able to replicate exactly in a lab!

Each water molecule breaks down into two hydrogen (H) atoms and one oxygen (O) atom. The oxygen is released as a waste product—oxygen atoms from disassembled water molecules join up in pairs to form oxygen gas (O 2 ).

The hydrogen ions build up in high concentration in the lumen of the thylakoid. They pass through an enzyme called ATP synthase, and their movement provides the energy needed to add a third phosphate to ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). This energy-storing molecule powers many cellular processes. In fact, the glucose made during photosynthesis is broken down to produce more ATP later, during cellular respiration.

Meanwhile, the electron released from photosystem II arrives at photosystem I, which also contains chlorophyll. Energy from the sun excites the electron again, giving it enough energy to pass across the membrane and into the stroma, where it joins with a hydrogen ion and an NADP + to create the energy-carrying molecule NADPH.

ATP and NADPH move from the thylakoid into the stroma, where the energy they store is used to power the light-independent reactions.

3. The light-independent reactions (Calvin cycle) use stored chemical energy from the light-dependent reactions to “fix” CO 2 and create a product that can be converted into glucose.

The ultimate goal of the light-independent reactions (or Calvin cycle) is to assemble a molecule of glucose. This is the part of photosynthesis that requires the CO 2 the plant gets from the air.

Essentially, the plant needs the carbon from the CO 2 to create the building blocks for glucose. An enzyme in the stroma called ruBisCo combines a five-carbon molecule of RubP (ribulose biphosphate) with a molecule of carbon dioxide. This creates a six-carbon molecule that is broken down into two three-carbon molecules (3-phosphoglycerate). This part of the light-independent reactions is referred to as carbon fixation.

Then, the energy carriers from the light-dependent reactions make their contribution. ATP and NADPH give each 3-phosphoglycerate a hydrogen atom, creating two molecules of the simple sugar G3P (glyceraldehyde-3-phosphate). Ultimately, these two molecules of G3P are used to build one molecule of glucose. This part of the light-independent reactions is typically referred to as reduction (or reducing the sugar) because electrons are added.

It is important to note that the Calvin cycle typically uses six molecules of carbon dioxide at a time. This means that twelve molecules of G3P are generated. However, only two of them are used to produce a molecule of glucose—the rest are recycled back into RubP so that the cycle can keep running.

Light-dependent reactions

Light-independent reactions.

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Article from Scitable that details the internal structure of chloroplasts.

A chapter on chloroplasts and photosynthesis from OpenOregon’s Principles of Biology.

A visual aid detailing the steps of the Calvin cycle from National Geographic.

A video about the Calvin cycle from TED-ed.

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How plants adjust their photosynthesis to changing light

Light supplies the energy plants need to build up biomass. A research team from Bergen, Bochum, Düsseldorf, Münster and Potsdam headed by Heinrich Heine University Düsseldorf (HHU) is researching how plants adapt their photosynthesis to changing light. In the scientific journal Nature Communications , they describe a key molecular mechanism that synchronises the processes involved.

Photosynthesis is the central process by which plants build up biomass using light, water and the carbon dioxide from the air. Gaining a detailed understanding of this process makes it possible to modify and thus optimise it -- for example with a view to increasing food production or stress tolerance.

The research group headed by Professor Dr Ute Armbruster from the Institute of Molecular Photosynthesis at HHU is examining this process from a range of perspectives. Together with an interdisciplinary research team, the group now presents its findings on the processes involved in plant reactions to different light conditions in a current publication in Nature Communications . The Max Planck Institute of Molecular Plant Physiology in Golm and research groups from the universities in Bergen (Norway), Bochum, Münster and Potsdam were involved in the work.

Photosynthesis comprises two steps or "modules." First of all, in the so-called light-driven reaction, light energy is converted into chemical energy that the plant can use in the form of the molecules ATP and NADPH. This energy is then used to fix carbon dioxide from the air into biomass by the "carbon-fixing reaction."

Plants live in often rapidly changing light conditions. To make optimum use of this light, the modules must be closely synchronised. There has been little scientific research into this synchronisation in particular to date.

If it is too bright, the plant cannot convert all the light energy; this is a potentially harmful situation. To ensure that no damage is caused by the excess light energy -- which can result in the formation of e.g. highly reactive oxygen species -- the plant activates a protective mechanism: The so-called energy-dependent quenching (for short: "qE") ensures that excess energy is discharged in the form of heat.

From earlier research, it is known that qE is switched off again more quickly by the "thylakoid K+-exchange antiporter 3" (KEA3) in the shade. However, the process is still so slow overall that usable light energy is lost when brightness decreases.

For the first time, the research team has now identified a molecular mechanism by means of which the two photosynthesis modules synchronise their activities via KEA3. To achieve this, the researchers used both computer simulations and various experimental approaches, including biosensors.

Firstly, the pH value of the medium surrounding the thylakoid membrane reacts highly dynamically to light changes. Secondly, the structure and thus the activity of KEA3 changes according to the pH value. However, this only occurs when KEA3 has also bound ATP and NADPH. In excess light, this leads to KEA3 being inactivated, thus allowing qE to be active. After a sudden transition to shade, KEA3 becomes activated, which upregulates the light-driven reactions of photosynthesis.

Professor Armbruster: "Through our work, we now understand for the first time how the two functional modules of photosynthesis communicate with each other via KEA3. It is important to know this with a view to developing strategies to improve photosynthesis in the field, in order to increase crop yields in the long term."

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Materials provided by Heinrich-Heine University Duesseldorf . Original written by Arne Claussen. Note: Content may be edited for style and length.

Journal Reference :

• Michał Uflewski, Tobias Rindfleisch, Kübra Korkmaz, Enrico Tietz, Sarah Mielke, Viviana Correa Galvis, Beatrix Dünschede, Marcin Luzarowski, Aleksandra Skirycz, Markus Schwarzländer, Deserah D. Strand, Alexander P. Hertle, Danja Schünemann, Dirk Walther, Anja Thalhammer, Martin Wolff, Ute Armbruster. The thylakoid proton antiporter KEA3 regulates photosynthesis in response to the chloroplast energy status . Nature Communications , 2024; 15 (1) DOI: 10.1038/s41467-024-47151-5

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• Light Reaction

We are all aware that the process of photosynthesis requires sunlight . But did you know chloroplast only absorb the blue and red light wavelengths from the sunlight? That is correct. Let us learn about Light Reaction and how it functions.

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Photosynthesis

Photosynthesis is the process by which autotrophic plants make their own food . Co2, water, chlorophyll, and sunlight are four important requirements for this process. Photosynthesis occurs in two steps: Light reaction and Dark Reaction.

• Light Reaction – It is a light dependent reaction
• Dark Reaction – It is a light-independent reaction. Learn about Dark Reaction here in more detail .

Let’s us learn more about the light reaction of photosynthesis.

(Image Source: actforlibraries.com)

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

• Dark Reaction and Photorespiration
• Factors Affecting Photosynthesis
• Introduction To Photosynthesis

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:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2  + 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.

Solved Example for You

Q: Which colour light are chlorophylls most sensitive to?

Sol: Blue and Red are the colour which chlorophylls are most sensitive to.  Leaves are green and so they reflect the green wavelength of white light. The chlorophyll pigment most efficiently absorbs those wavelengths which lie in the red and blue light regions of white light.

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• 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

Photosynthesis definition states that the process exclusively takes place in the chloroplasts through photosynthetic pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. All green plants and a few other autotrophic organisms utilize photosynthesis to synthesize nutrients by using carbon dioxide, water and sunlight. The by-product of the photosynthesis process is oxygen.Let us have a detailed look at the process, reaction and importance 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.

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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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8.6: The Light-Dependent Reactions of Photosynthesis - Processes of the Light-Dependent Reactions

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Learning Objectives

• Describe how light energy is converted into ATP and NADPH.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions, the first stage of photosynthesis, is to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in light-independent reactions and fuel the assembly of sugar molecules. Protein complexes and pigment molecules work together to produce NADPH and ATP.

Producing Chemical Energy

Light energy is converted into chemical energy in a multiprotein complex called a photosystem. Two types of photosystems, photosystem I (PSI) and photosystem II (PSII), are found in the thylakoid membrane inside the chloroplast. Each photosystem consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules, as well as other pigments like carotenoids. Cytochrome b6f complex and ATP synthase are also major protein complexes in the thylakoid membrane that work with the photosystems to create ATP and NADPH.

The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. In PSII, energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms, and one oxygen atom. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, the electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred to another molecule creating a chain of redox reactions called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI; as electrons move between these two photosystems, they lose energy. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI. Therefore, another photon is absorbed by the PSI antenna. That energy is transmitted to the PSI reaction center. This reaction center, known as P700, is oxidized and sends a high-energy electron to reduce NADP+ to NADPH. This process illustrates oxygenic photosynthesis, wherein the first electron donor is water and oxygen is created as a waste product.

Cytochrome b6f and ATP synthase work together to create ATP. This process, called photophosphorylation, occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen (an area of high concentration) to the stroma (an area of low concentration). The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms ATP. This 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. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to maintain the right proportions of NADPH and ATP, which will carry out light-independent reactions later on.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: 2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

• Light energy splits water and extracts electrons in photosystem II (PSII); then electrons are moved from PSII to cytochrome b6f to photosystem I (PSI) and reduce in energy.
• Electrons are re-energized in PSI and those high energy electrons reduce NADP + to NADPH.
• In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen to the stroma; this energy allows ATP synthase to attach a third phosphate group to ADP, which forms ATP.
• In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH, maintaining the right proportions of NADPH and ATP.
• photosystem : Either of two biochemical systems, active in chloroplasts, that are part of photosynthesis.
• photophosphorylation : The addition of a phosphate (PO43-) group to a protein or other organic molecule by photosynthesis.
• chemiosmosis : The movement of ions across a selectively permeable membrane, down their electrochemical gradient.

Contributions and Attributions

• visible light. Provided by : Wiktionary. Located at : http://en.wiktionary.org/wiki/visible_light . License : CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...ol11448/latest . License : CC BY: Attribution
• Boundless. Provided by : Boundless Learning. Located at : www.boundless.com//biology/de...netic-spectrum . License : CC BY-SA: Attribution-ShareAlike
• wavelength. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/wavelength . License : CC BY-SA: Attribution-ShareAlike
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...e_08_02_03.jpg . License : CC BY: Attribution
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...e_08_02_02.jpg . License : CC BY: Attribution
• spectrophotometer. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/spectrophotometer . License : CC BY-SA: Attribution-ShareAlike
• carotenoid. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/carotenoid . License : CC BY-SA: Attribution-ShareAlike
• chlorophyll. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/chlorophyll . License : CC BY-SA: Attribution-ShareAlike
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...e_08_02_04.jpg . License : CC BY: Attribution
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest..._02_05abcd.jpg . License : CC BY: Attribution
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...e_08_02_06.jpg . License : CC BY: Attribution
• photosystem. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/photosystem . License : CC BY-SA: Attribution-ShareAlike
• Cell Biology/Energy supply/Light Dependent Reactions. Provided by : Wikibooks. Located at : en.wikibooks.org/wiki/Cell_Bi...dent_Reactions . License : CC BY-SA: Attribution-ShareAlike
• photophosphorylation. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/photophosphorylation . License : CC BY-SA: Attribution-ShareAlike
• chemiosmosis. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/chemiosmosis . License : CC BY-SA: Attribution-ShareAlike
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...e_08_02_08.jpg . License : CC BY: Attribution
• OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m44448/latest...08_02_07ab.png . License : CC BY: Attribution

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April 8, 2024

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How plants adjust their photosynthesis to changing light

by Arne Claussen, Heinrich-Heine University Duesseldorf

Photosynthesis is the central process by which plants build up biomass using light, water, and carbon dioxide from the air. Gaining a detailed understanding of this process makes it possible to modify and thus optimize it—for example, with a view to increasing food production or stress tolerance.

The research group headed by Professor Dr. Ute Armbruster from the Institute of Molecular Photosynthesis at HHU is examining this process from a range of perspectives. Together with an interdisciplinary research team, the group now presents its findings on the processes involved in plant reactions to different light conditions in a current publication in Nature Communications .

The Max Planck Institute of Molecular Plant Physiology in Golm and research groups from the universities in Bergen (Norway), Bochum, Münster, and Potsdam were involved in the work.

Photosynthesis comprises two steps or "modules." First of all, in the so-called light-driven reaction, light energy is converted into chemical energy that the plant can use in the form of the molecules ATP and NADPH. This energy is then used to fix carbon dioxide from the air into biomass by the "carbon-fixing reaction."

Plants live in often rapidly changing light conditions. To make optimum use of this light, the modules must be closely synchronized. There has been little scientific research into this synchronization, in particular, to date.

If it is too bright, the plant cannot convert all the light energy; this is a potentially harmful situation. To ensure that no damage is caused by the excess light energy—which can result in the formation of, e.g., highly reactive oxygen species —the plant activates a protective mechanism : The so-called energy-dependent quenching (for short: "qE") ensures that excess energy is discharged in the form of heat.

From earlier research, it is known that qE is switched off again more quickly by the "thylakoid K+-exchange antiporter 3" (KEA3) in the shade. However, the process is still so slow overall that usable light energy is lost when brightness decreases.

For the first time, the research team has now identified a molecular mechanism by means of which the two photosynthesis modules synchronize their activities via KEA3. To achieve this, the researchers used both computer simulations and various experimental approaches, including biosensors.

Firstly, the pH value of the medium surrounding the thylakoid membrane reacts highly dynamically to light changes. Secondly, the structure and thus the activity of KEA3 changes according to the pH value. However, this only occurs when KEA3 has also bound ATP and NADPH. In excess light, this leads to KEA3 being inactivated, thus allowing qE to be active. After a sudden transition to shade, KEA3 becomes activated, which upregulates the light-driven reactions of photosynthesis.

Professor Armbruster said, "Through our work, we now understand for the first time how the two functional modules of photosynthesis communicate with each other via KEA3. It is important to know this with a view to developing strategies to improve photosynthesis in the field, in order to increase crop yields in the long term."

Journal information: Nature Communications

Provided by Heinrich-Heine University Duesseldorf

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Plants cleverly adapt their photosynthesis to changes in light

P lants use the process of photosynthesis to convert light into energy for growth. However, the amount of sunlight they receive can fluctuate throughout the day. Too much or too little sunlight can negatively impact their energy production.

Recent research sheds light on how plants manage these shifts, revealing a communication system that lets them adjust their photosynthesis in real-time.

Stages in photosynthesis

Photosynthesis can be broken down into two main interconnected stages:

Stage 1: The light-driven reaction

This stage takes place within the chloroplasts, specialized structures found in plant cells. Chloroplasts are packed with membranes called thylakoids, which is where the light-driven reactions occur.

• The power of chlorophyll : Chlorophyll, the green pigment that gives plants their color, is located within the thylakoid membranes. Its job is to capture light energy from the sun, acting like a tiny solar panel.
• Energy conversion : When sunlight hits chlorophyll, it excites electrons within the molecules. This newfound energy is passed through a series of proteins within the thylakoids. Think of this like a relay race where the energy gets passed along.
• Generating chemical energy : This electron transfer chain creates a sort of 'electrical current' within the chloroplast. The plant cell harnesses this energy flow to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Consider ATP and NADPH as energy-carrying molecules, like rechargeable batteries that power the next stage of photosynthesis.

Stage 2: The carbon-fixing reaction

This stage occurs in the stroma, the fluid-filled area of the chloroplast surrounding the thylakoids.

• Building with carbon dioxide : The carbon-fixing reaction, also known as the Calvin Cycle, uses the ATP and NADPH generated in the light-driven reaction to capture carbon dioxide (CO2) from the air.
• Sugar production : Through a series of enzyme-driven steps, CO2 is combined with other molecules and rearranged, using the energy stored in ATP and NADPH. The ultimate output of this process is sugar, primarily in the form of glucose.
• Fuel for the plant : These glucose molecules form the building blocks for the plant's growth and development. They can be used immediately for energy, combined into more complex carbohydrates like starches for storage, or even used to build the plant's physical structure in the form of cellulose.

Fluctuating light and photosynthesis

Light availability for plants is in a state of near-constant change. Several factors contribute to this:

• Clouds : Cloud cover can dramatically reduce the amount of sunlight reaching a plant, often switching between bright sun and shade within minutes.
• Shade : Plants growing beneath trees, within dense foliage, or in the shadows of buildings experience less direct sunlight and may need to adapt accordingly.
• Day and night : The natural cycle of day and night presents the most predictable change in light conditions. Plants often adjust their internal processes in preparation for darkness.

Hence, plants require a delicate balance within their photosynthetic machinery to ensure survival and efficient growth. In low-light conditions, plants need to focus on capturing as much sunlight as possible to produce the chemical energy that fuels their growth. They need to ramp up their light-harvesting processes to make the most of the limited light.

Too much sunlight can be harmful. The intense energy can overload the plant's photosynthetic machinery, leading to the production of damaging molecules called reactive oxygen species. To counter this, plants need mechanisms to quickly dissipate excess energy and protect themselves.

Due to dynamic light conditions, plants have evolved sophisticated ways to fine-tune their photosynthesis on a moment-to-moment basis, ensuring they either make the most of available sunlight or protect themselves from its excess.

"qE": Plant's protection mechanism

Plants activate a protective mechanism called "energy-dependent quenching" (qE) when there's varying sunlight. This mechanism gets triggered when sensors inside the plant's chloroplasts signal an energy overload.

Once active, qE acts like a safety valve – excess light energy is released as heat, preventing damage to the plant's photosynthetic machinery. While qE is crucial for survival in bright light, it reduces the plant's ability to harvest energy, especially when light levels change quickly.

Understanding this balance between protection and energy production is an important area of research for scientists aiming to improve plant growth and agricultural yields.

KEA3: The master switch of qE

Scientists have discovered that a protein called KEA3 acts as a control switch for photosynthesis.

"The research team has decoded the molecular mechanism used by the plant to synchronize the two sub-processes of photosynthesis with each other," explained Professor Dr. Ute Armbruster from the Institute of Molecular Photosynthesis , the study's lead author.

KEA3 functions by monitoring conditions inside the chloroplast:

• Sensor 1 (pH levels) : The acidity (or pH level) of the area around chloroplasts changes in response to light – it becomes more acidic in bright light and less acidic in the shade.
• Sensor 2 (Energy availability) : KEA3 also gauges the plant's energy supply by directly sensing the levels of the ATP and NADPH molecules.

KEA3 uses the information from these two sensors to regulate photosynthesis . As the light-driven reactions ramp up, they pump protons (positively charged hydrogen ions) into the interior of the thylakoid membranes. This leads to a rapid drop in pH within the chloroplast, making it more acidic.

KEA3 role in photosynthesis during varying light

Increased light also means a surge in the production of the energy-carrying molecules ATP and NADPH. The chloroplast becomes flush with chemical fuel.

KEA3, the master switch protein, directly senses both the pH change and the high levels of ATP and NADPH. This combination of signals causes a change in the protein's structure that renders it inactive.

With KEA3 inactive, the protective qE mechanism is allowed to fully engage. Excess energy is safely released as heat, preventing damage caused by the overload of sunlight. Lots of sunlight leads to a rapid drop in pH and ample fuel supply. This inactivates KEA3, allowing qE to engage (protecting the plant).

When shifting to shade, the pH rises again, and fuel supplies decrease. This activates KEA3, which then signals to other parts of the plant to boost the light-harvesting reactions and maximize energy production under lower light conditions.

Understanding photosynthesis in changing light

"Through our work, we now understand for the first time how the two functional modules of photosynthesis communicate with each other via KEA3," said Professor Armbruster.

"It is important to know this with a view to developing strategies to improve photosynthesis in the field, in order to increase crop yields in the long term."

The study is published in the journal Nature Communications .

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