Light-dependent reaction

Light-dependent reaction
n., [laɪt dɪˈpɛndənt ɹiˈækʃən]
Definition: photosynthetic reaction requiring light

Many organisms, such as green plants, convert light energy into chemical energy through the mechanism of photosynthesis. In this process, green plants capture light energy and form organic compounds, which are rich in energy and oxygen (O) by using minerals, carbon dioxide (CO₂), and water (H₂O).

Light-dependent Reactions Definition

What is the light-dependent reaction? We can define light-dependent reaction as follows:

Those reactions which occur in the presence of light (photons) are known as light-dependent reactions.

The reaction starts when the sunlight or any other source of light causes the excitation of sensitive molecules. The molecules of chlorophyll actively take part in the reaction. They transfer sunlight (solar energy) into potential energy and store it as chemical energy in sugars. In biology, light-dependent reactions are very important as many green plants are carrying them out when light is available, such as during day time.

Photosynthesis is important as it plays an essential role in maintaining life on earth. If photosynthesis stops, the organic matter and the food on earth will come to an end. These organisms that rely on light energy for making food are called photoautotrophs. Conversely, organisms that produce food without needing light energy are referred to as chemoautotrophs. They don’t require light energy but rather utilize chemical sources to energize the process of making food.

To understand the difference between photoautotrophs and chemoautotrophs, read: Autotorophs Definition and Examples

How Can Light be Used to Make Food?

When we talk about light, the only thing which comes into our mind is that it helps living organisms to watch and see anything but light does more than that. Like other forms of energy, light is used to do work. It not only travels but also changes form.

In photosynthesis, light energy is used to drive a chemical reaction, which is utilized by the autotrophic organism to form molecules of carbohydrate. Although the sun gives a different level of electromagnetic radiation (solar energy), autotrophic organisms use a specific component of sunlight.

Visible light is only a small fraction of energy that is seen by humans. The traveling of solar energy can be measured and described in the form of waves. In a series of waves, the distance between two identical points like the crest to crest or trough to trough is known as wavelength. Scientists use wavelength to evaluate the amount of energy wave. There is an extensive array of wavelengths of radiations of the electromagnetic spectrum. Each wavelength carries a different level of energy and has different characteristics. The longer or stretched wavelength carries less energy but tight and short waves have more energy. Visible light is a type of electromagnetic radiation that comes from the light of the sun.

For photosynthesis, a plant’s pigment molecule absorbs visible light. Humans see visible light as white but in reality, it is in rainbow colors. The human eye can see the rainbow color when the white light passes from the water droplet or by the prism. In the rainbow, violet and blue have short wavelengths so they have high energy. Red has a longer wavelength so has low energy. Different types of pigments absorb the different wavelengths of visible light. The wavelengths that cannot be absorbed by the pigments will be reflected.

Plants are color green due to the presence of a photosynthetic pigment, which is a chlorophyll molecule. Chlorophyll absorbs wavelength from both ends (blue and red). Chlorophyll reflects the green and that’s why it appears green in color. The nature of pigment is recognized by the pattern of the wavelength, which is absorbed from the absorption spectrum of visible white light.

There is a combination of pigments in certain photosynthetic organisms. Such organisms can absorb an extensive series of visible light wavelengths. Each photosynthetic organism does not have a complete approach to solar light. For example, some photosynthetic organisms grow in the depth of water. Most of the sunlight will be absorbed by the water and a certain wavelength reaches the photosynthetic organism. Some organisms grow in a competitive environment like on the rainforest floor. In such an environment, tall and thick trees interrupt the sunlight for a short time and thus they do not get complete exposure to sunlight.

What is the Main Purpose of Light-dependent Reactions?

Light-dependent reactions are responsible for converting light energy into chemical energy in the Calvin cycle. Thus, the light reactions of photosynthesis supply the Calvin cycle with light energy. This energy is then converted into chemical energy. This chemical energy is utilized as fuel, for the formation of molecules of sugar.

Where do Light-dependent Reactions Occur?

The light-dependent reactions take place in the photosystem. A photosystem is a grouping of proteins and pigments. In membranes of thylakoids of chloroplast, photosystems are found. At a time, a molecule of pigment from the photosystems absorbs a photon-quantity or “packet” of light energy.

What is the Light-dependent Reaction of Photosynthesis?

In the presence of sunlight, the light reaction occurs. It is also known as photolysis. This reaction occurs in the grana of chloroplasts. Chlorophyll is the main pigment that carries out the process of photosynthesis (light reaction). Carotenoids act as an accessory pigment. In the thylakoid membrane of the chloroplast, chlorophyll is present which absorbs energy from the sun. This energy is utilized by the two-electron transport chains, which form ATP and NADPH (adenosine triphosphate and nicotinamide adenine dinucleotide phosphate hydrogen, respectively). In this process, water (H₂0) is consumed and oxygen (O₂) is released.

What Happens in the Light-dependent Reaction?

A photon travels till it reaches chlorophyll. The photon leads to the excitation in the electron of chlorophyll. The energy causes the electron to get separated freely from the atom of chlorophyll. This is the reason chlorophyll is known as a donor.

Water is split to replace the electron of chlorophyll. In thylakoid space, this splitting not only releases an electron but also leads to the formation of hydrogen ions (H⁺) and oxygen (O₂).

The splitting of water molecules gives a pair of electrons. So, it can substitute 2 donated electrons.
The chlorophyll responds to another photon due to the replacing of an electron. The oxygen which is a by-product of this reaction goes into the external environment. In the light-dependent reaction, H⁺ has an important part as a reminder to proceed with the reaction.

Two stages carry out the process of photosynthesis. These are:

1. Light-dependent reactions
2. Calvin cycle

Light-dependent Cycle happens in the thylakoid membrane of the chloroplast. In this reaction, by using water, chlorophyll absorbs the energy from the sun and transforms it into chemical energy. Water (H₂O) is split and oxygen (O₂) comes out as a by-product.

The Calvin cycle takes place in the stroma of the chloroplast. The chemical energy which is obtained from a light-dependent reaction carries out the formation of a sugar molecule and also the capture of carbon (C) in carbon dioxide (CO₂) molecules.

For transferring energy, these two reactions use carrier molecules. The carriers that bring energy from light-dependent reactions into the Calvin cycle reactions are termed as “full”. After releasing the energy which is now termed as “empty” energy goes back to light-dependent reactions to gain more energy. In cellular respiration, NADH and FADH₂ are the two energy carrier molecules. In photosynthesis, NADPH is used. NADPH is formed when a lower energy molecule NADP⁺ takes a high energy proton and electron. NADP⁺ formed again when NADPH releases an electron.

GENERATING ENERGY MOLECULE: ATP

In light-dependent reactions, the energy from the sunlight is saved in the form of ATP and NADPH. This energy is stored in a bond that holds the only atom to the molecule. In ATP and NADPH, phosphate and hydrogen atoms are present.

In the electron transport chain, from the citric acid cycle, NADH carries energy in the mitochondrion. They become lower energy particles ADP and NADP⁺ when releasing the energy into the Calvin cycle.
An electrochemical gradient is formed in thylakoid space due to hydrogen ions. This gradient is formed due to the difference between the proton’s concentration and the charge across the membrane. By chemiosmosis, the energy is stored in the form of ATP. Like mitochondria, there is an electrochemical gradient due to the movement of hydrogen ions.

ATP synthase (an embedded protein complex) causes the passage of hydrogen ions through the thylakoid membrane. In mitochondria, this protein generates ATP from ADP. In photophosphorylation, a molecule of ATP is formed. This molecule is formed by the energy of hydrogen ions which stream the ATP synthase to attach a third phosphate molecule to ADP.

What is chemiosmosis?  

Chemiosmosis is the flow of hydrogen ions (H⁺) by ATP synthase as the ions are moving from higher to lower concentrations through a semi-permeable structure.

GENERATING OTHER ENERGY CARRIERS: NADPH

NADPH, which is an energy carrier molecule, is also formed by the light-dependent reaction. When the electrons from the electron transport chain (ETC) reach the photosystem, the electron again gains energy with additional photons which are captured by the chlorophyll. This energy causes NADPH to form from NADP+. The energy from the light of the sun is stored in the form of energy carriers. This can be utilized for the formation of glucose molecules.

What are the 7 steps of light-dependent reactions? The seven steps are as follows:

1.  The energy of the sun is absorbed. (1st Time)
2. Split of the water molecule.
3. Across the thylakoid membrane, hydrogen ions (H⁺) are transported.
4. The energy of the sun is absorbed. (2nd Time)
5. NADP+ leads to the formation of NADPH.
6. Through protein channel, diffusion of Hydrogen ion (H⁺)
7. ADP transfers into ATP.

The process of light reaction is given below:

• In a light reaction, chlorophyll absorbs the solar energy and converts it into chemical energy. The energy is saved or preserved in the form of electron charge carriers like ATP and NADPH.
• In the thylakoid membranes of the chloroplast, Photosystems I and II use light energy.
• By using the chemical energy, which is gathered during the reactions, carbohydrates can be gained from carbon dioxide.
• The light energy breaks down into water molecules and picks up the electron from Photosystem II. The electron travels from Photosystem II to b6f (cytochrome) and then to Photosystem I. By this it reduces in the form of energy.
• Again, the electrons gain energy in Photosystem I. Due to this NADPH forms from NADP+.
• In non-cyclic photophosphorylation, to pump the hydrogen ions in the lumen of the stroma, the cytochrome utilizes the energy from Photosystem II. This energy causes the binding of another phosphate group which leads to the formation of ATP from ADP.
• In cyclic photophosphorylation, for increasing the formation of ATP and reducing the formation of NADPH, cytochrome b6f (cytochrome) utilizes the electron energy from Photosystem 1 and 2. In this way, the balanced quantity of ATP and NADPH is maintained.

Products of Light-dependent Reactions

ATP & NADPH are two major products of the light-dependent reaction. In the Calvin cycle, these reactions help in the manufacturing of glucose in photosynthesis. In cyclic photophosphorylation, which is another form of these reactions, the path of the electron is different and circular and at the end of the reaction, only ATP is produced. There is no production of NADPH. In many green plants, photosynthesis produces carbohydrates, which is an important organic product. The production of glucose and carbohydrates are simply explained by the chemical equation which is mentioned below:

In plants, a small amount of glucose is produced. Many glucose molecules are joined together and form starch. Sometimes glucose is joined with fructose and leads to the formation of sucrose.
During photosynthesis, not only the carbohydrates but also lipids, fats, amino acids, and proteins are synthesized.

To manufacture these compounds essential nutrients provide the necessary and essential elements, such as Phosphorus (P), Nitrogen (N), and sulfur( S). Chemical bonds between carbon (C), nitrogen (N), oxygen (O), sulfur (S), and hydrogen(H) from the plant nutrients are broken. There is the formation of new bonds between the organic compound and gaseous oxygen (O₂).

A huge amount of energy is released during bond breakage between the oxygen (O₂) and other elements like nitrate, sulfate, and water whereas a small amount of energy is required to make new bonds.

Inorganic products, a large amount of light energy is stored in the form of chemical energy. The residual amount of energy is stored for the formation of large and complex molecules from simple ones.

Light-dependent Reactions Equations

The light-dependent reaction of photosynthesis is the light energized oxidation-reduction (redox) mechanism. Oxidation is the loss of an electron while gaining of an electron is referred to as reduction. In photosynthesis, Hydrogen ions (H⁺), electrons, and oxygen (O₂) are produced by water oxidation. Carbon dioxide (CO₂) is reduced to organic products due to the transfer of removed hydrogen ions and electrons. In amino acids, other hydrogen ions and electrons are utilized in the reduction of sulfate and nitrate to sulfhydryl and amino groups. These are the necessary as well as essential building blocks of proteins. The major organic product of photosynthesis is starch and sugar sucrose. The overall reaction is shown below:

The actual photosynthesis is carried out by various enzymes, which involve several reactions. The reaction is completed in two stages: 1st is the “light” stage and 2nd is the “dark” stage.

The light stage comprises photochemical reactions but in the dark stage, chemical reactions are preceded by enzymes.

What is a Photosystem?

Photosystem is defined as the photosynthetic pigments (like carotenoids, chlorophyll a & b), which are structurally aligned in complexes with proteins. In the photosystem different pigments, protein, and about 300-400 chlorophylls are embedded in light-harvesting complexes. These pigments help to absorb a photon where in this way their electron absorbs energy and moves from lower energy orbital to higher energy orbit.

In the photosystem, the most common process is resonance energy transfer, in which mostly pigment works as an energy conduit., They transfer the energy into the main reaction area where one of the pigments absorbs the light and transfers its energy by direct electromagnetic interaction to the adjacent or nearby pigments. The neighboring pigment transfers its energy to its close or nearby pigment and in this way, the process repeats many times.

In this process, the receiver requires less energy than the donor because it absorbs light of a longer wavelength. Accumulatively, all stored energy of different pigments transfers to the core part which is known as the reaction center of the photosystem.

Photosystem I vs. Photosystem II

Photosystem II (PSII) & Photosystem I (PSI) are two categories of the photosystem. In the path of electron flow, PSII comes first. It was named second because of its discovery later to PSI.

The major difference between the two systems are mentioned below:

Photosystem II

In Photosystem II, P680 becomes excited after absorbing energy. As the excited state of P680 is a good donor for electrons so it transfers an electron to the pheophytin, which is a primary electron acceptor. In a series of electron transfer and redox reactions, the electron is passed to the first leg of the photosynthetic electron transport chain. It gets a positive charge and also needs an electron after releasing an electron pair. The Manganese center of PSII splits the water molecule and provides electrons. The P680 has a positive charge and it strongly attracts the electron from water. A molecule of oxygen (O), four electrons, and hydrogen ions (H⁺) release when the water splits at the manganese center. To carry out oxidative phosphorylation, 10% oxygen is used by mitochondria present in leaves. The remaining goes into the external environment to carry out the process of respiration by organic organisms.

Electron transport chains and Photosystem I

After leaving the photosystem-II, the electron finally reached plastocyanin (copper-containing protein) while passing through plastoquinone (tiny organic protein) and cytochrome complex. Energy always leaves when electrons travel from higher energy to lower energy levels. Most of the energy is used to promote the hydrogen ion (H⁺) (proton) from the outer to the interior thylakoid membrane.
The transfer of hydrogen ion (H⁺) ions from water splitting creates a gradient that helps to make adenosine triphosphate (ATP).

Electron reaches the PSI, once it moves towards the first limb of the electron transport chain, then it joins the distinct pair of chlorophyll called P700. The reason is that the electron lost its whole energy to the influx at PSI, it is compulsory for the electron to re-energize by absorbing another photon.
P700 has a marvelous ability to donate an electron when it’s exciting, and send its electron to the electron transport chain, where a sequence of reactions can happen. In the first step, the electron passed towards the ferredoxin (Fd) and then traveled towards NADP⁺ reductase (which is an enzyme). Electrons travel from NADP⁺ reductase to electron carrier NADP⁺ to form NADPH, which will transport to the Calvin Cycle. In which the electrons help to construct the sugars from carbon dioxide.

ATP is another desirable major ingredient required to the Calvin cycle, which is supplied by light reactions. In the previous discussion, we saw that H⁺ ions make a concentration gradient inside the interior of thylakoid and these H⁺ ions (protons) want to diffuse into the concentration gradient, in the stroma. The specific path of the route is ATP synthase (enzyme). Phosphate (Pi) and adenosine diphosphate (ADP) are converted into adenosine triphosphate (ATP) by the connected flow of electrons. Chemiosmosis is the process in which energy is stored by ATP in the form of a chemical gradient.

Some Electrons Flow Cyclically

In linear photophosphorylation, electrons move in a series from water through PSII and PSI to NADPH. Sometimes electrons do not follow this pattern. They start to loop back to the electron transport chain. Now the electrons repeat in a cycling manner through PSII and do not end in NADPH. This is known as cyclic photophosphorylation. After leaving PSI, electrons move back to plastoquinone (Pq) or cytochrome complex (Cyt) in the electron transport chain. These electrons then initiate the production of ATP and proton pumping through PSI. NADPH does not form from the cyclic electron flow because the electrons go away from NADP+ to reductase.

Try to answer the quiz below to check what you have learned so far about light-dependent action.

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Further Reading

• Photosynthesis – Photolysis and Carbon Fixation (Biology Tutorial)
• Light on Growth (Biology Tutorial)

References

• Biology.arizona. (2018). Photosynthesis Problem Set 1. Retrieved from http://www.biology.arizona.edu/biochemistry/problem_sets/photosynthesis_1/03t.html
• Britannica. (2019). Electromagnetic spectrum. Retrieved June 20, 2021, from https://www.britannica.com/science/electromagnetic-spectrum
• Cikgurozaini. (2019). Photosynthesis (Higher Level). Retrieved June 30, 2021, from http://cikgurozaini.blogspot.com/2010/08/photosynthesis-higher-level.html
• Hyperphysics. (2019). Chlorophyll in Leaves. Retrieved June 30, 2021, from http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/chlorleaf.html

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