All plants undergo the vital process of photosynthesis. We know it as the process by which plants manufacture food and the light energy gets transformed into chemical energy. Photosynthesis is the basis of life for green plants and all other organisms in the food chains and food webs. Chlorophyll, sunlight, and carbon dioxide are required for this process. Photosynthesis takes place in all green parts of the plant at the cellular level in the chloroplasts.
The process of photosynthesis takes place in a series of steps, as follows.
(a) Absorption of sunlight or light energy by chlorophyll.
(b) Conversion of light energy into chemical energy.
(c) Decomposition of water molecules into hydrogen and oxygen.
(d) Transformation of carbon dioxide to form carbohydrates.
These steps take place in all plants that make their own food but not necessarily in the same order. The entire process can be divided into two major phases, one which is light-dependent and the other, which is light-independent.
What is Photophosphorylation?
The light-dependent reaction phase of photosynthesis is known as photophosphorylation. This process involves producing ATP (three phosphate groups) molecules from the ADP (2 phosphate groups) in the plant cells' chloroplasts in the presence of light. These reactions generate the following two molecules needed for the next stage of photosynthesis by utilizing the light energy.
The energy storage molecule ATP
The reduced electron carrier NADPH
This process is carried out in the presence of photosystems. They are the functional units for photosynthesis. They consist of complex pigment organizations that absorb and transfer light energy and assist in transferring electrons. They are of 2 types: photosystem I and photosystem II.
Photophosphorylation is of the Following Two Types.
We will now study these two types of photophosphorylation, their features, and the difference between cyclic and non-cyclic photophosphorylation.
What is Cyclic Photophosphorylation?
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Cyclic photophosphorylation is the process in which organisms, especially prokaryotes, bring about the conversion of ADP to ATP to generate energy for the cells' immediate needs. It takes place in the lamellae of chloroplasts.
In the cyclic photophosphorylation, the electrons move in a circular pattern in photosystem I.
Here, the ATP synthesis is brought about by electron transport that is powered by Photosystem I only.
No oxygen or NADPH is produced in this phase.
The movement of electrons is from the primary acceptor to ferredoxin via cytochrome B6F found in mitochondria. The electron then passes to plastocyanin and finally returns to chlorophyll, completing a cycle.
What is Noncyclic Photophosphorylation?
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Noncyclic photophosphorylation is the other method of photophosphorylation. It results in the synthesis of ATP molecules from ADP using the energy from excited electrons by photosystem II. In this process, the movement of the electrons happens in a noncyclic manner. The excited electrons do not get back to the chlorophyll.
The noncyclic photophosphorylation occurs in two stages and involves two different chlorophyll photosystems, PS I and PS II.
It is a light reaction and occurs in the thylakoid membrane.
Noncyclic photophosphorylation produces ATP and NADPH.
The generation of ATP is along with a one-way flow of electrons from H₂O to NADP+.
The lost electrons by P680 of PS II are occupied by P700 of PS I.
They are not reverted to the energy center.
The complete movement of the electrons is in a non-cyclic manner or a unidirectional manner.
Now that we know about the two types of phosphorylation, let us study the difference between cyclic and noncyclic photophosphorylation.
Cyclic and Noncyclic Photophosphorylation Class 11
The table below explains the difference between cyclic and noncyclic phosphorylation.
Q1. What Happens During Photophosphorylation?
Answer: The process of photophosphorylation ensures the conversion of ADP to ATP using the sunlight by PS II activation. This process involves the splitting of the water molecule into oxygen and hydrogen protons (H+). This process is known as photolysis. It results in a continuous unidirectional flow of electrons from water to the PS I.
In this process, the electron movement is spontaneous from the donor to the acceptor through an electron transport chain. The ATP is made from the enzyme ATP synthase. This chain, which is responsible for electron transport, is made up of a series of redox reactions. The purpose of these reactions is to sequentially transfer electrons from a high-energy donor to a lower energy acceptor molecule.
During the function of the electron transport chain, the flow of electrons from the stroma to the thylakoid area results in the production of an electrochemical potential gradient across membranes. ATP is produced by phosphorylation.
Q2. What are Photosystems?
Answer: Photosystems are large complexes of proteins and light-absorbing molecules. They play a key role in the light reactions. Each photosystem has light-sensitive complexes made up of proteins, pigments, and chlorophylls. Photosystems are of two types-PS I and PS II.
When a pigment absorbs a photon, it gets high in energy. It then transfers energy to a neighbouring pigment through electromagnetic interactions in a process called resonance energy transfer. The central part of the photosystem, called the reaction center, thus gets all the energy. PS I consist of the P700, chlorophyll, and other pigments. It converts NADP+ to NADPH2 with light energy. It is located on the external surface of the thylakoid membrane. PS II consists of P680, chlorophyll, and multiple pigments. It is built up of a protein complex that absorbs light energy. It is located on the internal surface of the thylakoid membrane in plant cells. It transfers electrons from water and helps in the dissociation of water molecules. They produce H + protons and oxygen.