Photosynthesis reaction can be carried out through complex steps of reaction that occur in the presence and in the absence of sunlight. By photosynthetic reaction, plants and other photosynthetic organisms are capable of collecting solar energy. This is possible due to the presence of light-absorbing pigment molecules. These molecules are present in leaves. During the exposure to sunlight, photosynthetic organisms tend to absorb energy from the sunlight. Now, this photon energy is converted to the chemical energy by a series of chemical reactions that take place in photosynthetic organisms during photosynthesis. Hence, in this way, the plant also obeys the first law of thermodynamics. This chemical energy is stored in the form of energy molecule adenosine triphosphate (ATP).

During this reaction, photosynthetic pigments of plants absorb light that activates series of cellular process that ultimately converts light energy into chemical energy and stored in the bonds of the energy molecule ATP. The process of utilizing light energy and electron transport chain to make ATP is known as photophosphorylation. This reaction’s name itself suggests the process of gaining a phosphate molecule. ADP molecule gains this phosphate molecule and produces a molecule of ATP.

The photosynthesis reactions are split into two categories.

1. Light-dependent reaction

2. Light-independent reaction

First, the light-dependent reaction takes place which is followed by light-independent reaction. In the first step, i.e. light-dependent reaction, plants convert light energy into chemical energy. This reaction starts with the absorption of sunlight and followed by the transfer of light energy to reaction centers, to electron transport chain, and ultimately, this process leads to the synthesis of ATP and NADPH molecules. These molecules are of particular importance as they are utilized in the next stage of photosynthesis that is known as the Calvin cycle.

Light-dependent reactions can be defined as the first major set of processes in photosynthesis, in which light energy is converted in to chemical energy in the form of ATP and NADPH.

Location of Light - Dependent Photophosphorylation

The electron transport chains for photosynthesis is carried out in the thylakoid membranes of chloroplasts. This is mainly due to the availability of chlorophyll molecules and accessory pigments to absorb light energy. These are the must-required ingredients in order to produce ATP molecule while utilizing energy from the sunlight. Chlorophyll molecule acts as a reaction centers and the remaining molecules such as pigments within the membrane form an antenna complex.

Function of Reaction Centers and Antenna Complex

Antenna complex, as the name suggests, is responsible for the absorption of light energy (also known as photon molecule) and then, it transfers energy into the reaction centers. These reaction centers are key locations where the photon energy is transferred into the electron transport system.

Process of Light - Dependent Photophosphorylation

The electrons enter into an excited state i.e. higher energy state when the reaction center chlorophyll receives light energy. This step is causing them to the outer electron orbitals and then to attach to a protein in the electron transport chain. This is the step when the plant cell transfers light energy to chemical energy.

There are two types of photophosphorylation that occur in cells:

1. Noncyclic Photophosphorylation: It is also called as Z-scheme. In this type, an electron from the chlorophyll travels through the electron transport system and then reduces the NADP+ to form a molecule of NADPH. In this type, the electron does not travel complete the whole cycle and does not return to the chlorophyll as it is utilized in the reduction of NADP+. It is only one-way ride for an electron from water molecule to NADPH. Hence, it is called as noncyclic photophosphorylation.

2. Cyclic Photophosphorylation: In this type, when an electron gets excited, it leaves chlorophyll, then they travel through the electron transport circuit. Then, they return to chlorophyll again after the energy transfer process to ATP is completed. In this way, electron completes a whole cycle starting from electron activation by energy, leaving chlorophyll, enters into electron transport chain and again back to original position i.e. chlorophyll (a reaction centers). Hence, this type of photophosphorylation is called cyclic photophosphorylation.

The steps involved in light-dependent photophosphorylation are mentioned as follow:

1. Light photon energy is absorbed by antenna complex and followed by it transfer to chlorophyll (reaction centers).

2. Due to the gaining of light energy, the electron (from water molecule) present in reaction centers are excited and move to outer orbitals.

3. In this process, this electron enters into the electron transport chain. (Electron transport chain is collectively made up of membrane-embedded proteins and organic molecules. These are organized into four large complexes known as I to IV).

4. Proteins present in the electron transport chain tend to pull the electron from chlorophyll and pass them along the chain of proteins.

5. During this movement of the electron through different proteins of electron transport chain, chemiosmosis reaction takes place and as a result, ATP is formed.

The energy from the movement of electrons is used to transport hydrogen ions (H⁺) across the thylakoid membrane. Every single movement of electron transport is coupled with the movement of hydrogen ions. The energy associated with the movement of hydrogen ions is used to make ATP from ADP and inorganic phosphate. For this reaction to take place, enzyme ATP synthase is required.

6. After passing through the proteins of electron transport chain, this electron is accepted by the NADP+ molecule, and in turn, it is reduced and produces its reduced form i.e. NADPH. (NADP+ stands for nicotinamide adenine dinucleotide phosphate and NADPH is a reduced form of NADP+. NADP+ molecule acts as an electron carrier.)

The above-mentioned sixth step takes place only during non cyclic photophosphorylation. In cyclic photophosphorylation, the electron, after passing through the electron transport chain, instead of reacting with the NADP+, re-enter into the reaction center to repeat this cycle.

7. Some light energy is used to break water molecule (H₂O) by photolysis and produces protons (H+), electrons (e-), and oxygen gas (O₂). These electrons are now transferred to chlorophyll. This is particularly important in order to replace the lost electron. This step is also shown only in non cyclic photophosphorylation. The proton ions released by this reaction are released into the plant cell. The liberated oxygen by this reaction is released into the cell and ultimately, released in the atmosphere as a waste product of photosynthesis.

Note - Oxygen molecule (O₂) released as a part of photosynthesis does not come from carbon dioxide (CO2). As mentioned in the above step, it is produced when the water molecule is split to provide electron. In the above-mentioned seventh step, two molecules of water break down such that it produces oxygen molecule, not an oxygen atom.

The electron from the water molecule does not enter into the ATP molecule during the light reaction.

Details of Electron Transport Chain

Electron transport chain is collectively made up of a membrane-embedded proteins and organic molecules. The electronic transport chain components are found in the plasma membrane of prokaryotes, whereas in eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. The electron transport chain contains proteins such as Fd (ferredoxin), PQ (plastoquinone), Cyt C (cytochrome C), Q (ubiquinone), and PC (plastocyanin). The enzyme NADP reductase is also present. It is important in the reduction of an electron acceptor molecule and in generation of NADPH.

While travelling of electron through the chain, it enters into a lower energy level from a higher energy level. It means it moves from less electron-hungry molecules to more electron-hungry molecules. Hence, this type of transfer of electron is an example of downhill electron transfer. The above-mentioned different protein complexes use the released energy (released during electron transfer) and that turn out into pumping of the proton from mitochondrial matrix to the intermembrane space. This is particularly responsible for forming a proton gradient.

Difference Between Light - Dependent and Light - Independent Reaction:

Light-independent reaction is dependent on the products of the light-dependent reaction. However, vice versa is not true. In light-dependent reaction, the absorbed energy is converted into chemical energy in the form of ATP whereas in case of light-independent reaction, glucose molecule is produced by utilizing environmental CO₂ and the products of light-dependent reactions- ATP and NADPH. In this, ATP provides energy for glucose synthesis whereas NADPH is required for the reduction of CO₂ into glucose.

Summary of Light-dependent Reactions

1. In light reaction of photosynthesis, a plant converts energy from one form to another from solar energy to potential energy to chemical energy.

2. The location of light reaction is in the thylakoid membranes. It starts with absorbing energy from the sunlight, followed by a series of events and ultimately the generation of ATP molecules takes place.

3. A continual source of electrons to replenish the lost electrons from chlorophyll is required in light reactions. This electron comes from water molecule which breaks down and releases oxygen gas as a byproduct.

4. At the end of this process, the cell is full of high energy molecules like NADPH and ATP- those can be used in the Calvin cycle for the production of carbohydrates.

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FAQs on Light Dependent Reactions

1. What are the light-dependent reactions of photosynthesis?

The light-dependent reactions are the first stage of photosynthesis, where energy from sunlight is captured by pigment molecules and converted into chemical energy. This process occurs in the thylakoid membranes of chloroplasts and its primary purpose is to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are the energy-carrying molecules required for the next stage of photosynthesis (the Calvin cycle).

2. Where in the chloroplast do the light-dependent reactions take place?

The light-dependent reactions are specifically located in the thylakoid membranes within the chloroplasts. Thylakoids are flattened, sac-like structures which are often arranged in stacks called grana. This membrane-bound location is vital because it allows for the establishment of a proton gradient, which is essential for synthesising ATP.

3. What are the key products generated during the light-dependent reactions?

The light-dependent reactions yield three crucial products that are either used in the next phase of photosynthesis or released from the plant. These are:

ATP: The main energy currency of the cell, used to power the synthesis of sugar.
NADPH: A molecule that carries high-energy electrons, providing the reducing power for the Calvin cycle.
Oxygen (O₂): A byproduct created from the splitting of water molecules (photolysis), which is then released into the atmosphere.

4. What are the main events that occur during the light-dependent reactions?

The light-dependent reactions consist of a series of coordinated events:

Light Absorption: Pigments in Photosystem II (PS II) and Photosystem I (PS I) absorb photons of light, exciting electrons to a higher energy level.
Water Splitting (Photolysis): To replace the electron lost from PS II, a water molecule is split, releasing electrons, protons (H+), and oxygen.
Electron Transport: The high-energy electrons travel along an electron transport chain, moving from PS II to PS I.
ATP and NADPH Synthesis: The energy from the electron transport chain is used to create a proton gradient that drives ATP synthesis (photophosphorylation). The re-energised electrons at PS I are then used to reduce NADP+ to NADPH.

5. How do light-dependent reactions differ from light-independent reactions?

The primary differences lie in their function, location, and requirements:

Requirement for Light: Light-dependent reactions are directly driven by light energy. Light-independent reactions (Calvin cycle) do not directly use light but rely on the chemical energy (ATP and NADPH) produced by the light reactions.
Location: Light-dependent reactions occur in the thylakoid membranes, while light-independent reactions take place in the stroma of the chloroplast.
Function: The function of light-dependent reactions is to capture and convert light energy. The function of light-independent reactions is to use that chemical energy to fix CO₂ and synthesise glucose.

6. Why are two photosystems, PS I and PS II, required for non-cyclic photophosphorylation?

Two photosystems are essential because they work in tandem to generate enough energy to produce both ATP and NADPH. PS II (P680) absorbs light to energise electrons taken from water, but these electrons lose energy as they travel down the electron transport chain to produce ATP. PS I (P700) then absorbs another photon of light to re-energise these electrons to a much higher level, giving them sufficient energy to reduce NADP+ to NADPH. This two-step process, known as the Z-scheme, is necessary because a single photosystem cannot generate enough power to both split water and create the strong reducing agent NADPH.

7. What is the specific role of water in the light-dependent reactions, and why is it so important?

The role of water is critically important; it serves as the initial electron donor through a process called photolysis. When the reaction centre of Photosystem II loses an electron after absorbing light, it becomes highly oxidising and needs a replacement. It gets this electron by splitting a water molecule (H₂O) into electrons (e⁻), protons (H⁺), and an oxygen atom. This replenishes the photosystem, allowing the electron transport chain to continue. Without water, the entire process would halt, and the oxygen released during photosynthesis is a direct result of this essential step.

8. What is the difference between cyclic and non-cyclic photophosphorylation?

The key difference lies in the electron pathway and the products formed:

Non-Cyclic Photophosphorylation: This is the standard 'Z-scheme' pathway involving both PS II and PS I. Electrons flow from water to NADP⁺. It produces ATP, NADPH, and O₂.
Cyclic Photophosphorylation: This process involves only PS I. Excited electrons from PS I are passed down the electron transport chain and cycled back to PS I, rather than moving to NADP⁺. This pathway generates only ATP and does not produce NADPH or O₂. It is thought to occur when the cell's demand for ATP is higher than its demand for NADPH.

9. How is a proton gradient created across the thylakoid membrane, and why is it essential?

The proton gradient (a high concentration of H⁺ ions inside the thylakoid lumen) is generated by two key processes: the splitting of water, which releases protons into the lumen, and the transport of protons from the stroma into the lumen by the cytochrome b6f complex as electrons pass through it. This gradient is a form of stored potential energy. Its essential purpose is to drive the synthesis of ATP through a process called chemiosmosis. As protons flow down their concentration gradient back into the stroma via the ATP synthase enzyme, the energy released is used to convert ADP to ATP.