Light Dependent Reactions and Their Role in Photosynthesis

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.