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cyclic and non-cyclic photophosphorylation - Biology

During the process of photosynthesis, the phosphorylation of ADP into ATP by utilizing the energy of sunlight is known as photophosphorylation. Cyclic photophosphorylation takes place in both aerobic as well as anaerobic conditions. Two sources of energy are accessible to live organisms that are sunlight and reduction-oxidation (redox) reactions. Every organism produces ATP, which is recognized as the universal energy currency of all life forms. In photosynthesis, this process usually involves photolysis or photodissociation of water along with a continuous flow of electrons from water to photosystem II in one direction.

In photophosphorylation, light energy gets used to form a high-energy electron donor and a lower-energy electron acceptor. Electrons then move instantaneously from donor to acceptor through a chain of electron transport.

ATP and Reactions

An enzyme known as ATP synthase forms ATP. Both the structure of this enzyme along its associated gene is considerably alike in all recognizable aspects of life. The Calvin cycle is one of the most crucial parts of photosynthesis.

ATP synthase is strengthened by a transmembrane electrochemical potential gradient that is usually as a proton gradient. The function of the electron transport chain is to give a proton gradient. We need several redox reactions to give a transmembrane electrochemical potential gradient which is also known as proton motive force (pmf).

Redox reactions are a type of chemical reaction where electrons are transferred from one donor molecule to another acceptor molecule. The related force that drives these reactions is Gibbs's free energy of the reactants and products. The Gibbs free energy is the energy that is free to work. A reaction of any kind that decreases the entire Gibbs free energy of a system will proceed instantaneously (given that the system is isobaric as well as adiabatic) though the reaction might proceed gradually if it is kinetically inhibited.

The transfer of electrons from a high-energy donor molecule to a lower-energy acceptor molecule can be spatially distinguished into several intermediate redox reactions. This is known as an electron transport chain.

Light reaction

The light reaction occurs in the grana part of the chloroplast. Here, light energy gets transformed into chemical energy in the form of ATP and NADPH. here in the light reaction, the subsequent addition of phosphate in the presence of light or the synthesizing of ATP by cells is termed photophosphorylation.

Dark reaction

Whereas in the dark reaction, the energy produced formerly in the light reaction is used to fixate carbon dioxide molecules into carbohydrates. The location where this process takes place in the stroma of the chloroplasts.

Cyclic Photophosphorylation

The photophosphorylation process which results from the movement of the electrons cyclically for producing ATP molecules is known as cyclic photophosphorylation.

In this process, plant cells gain the ADP and convert it to ATP for instant energy for the cells. This process generally occurs in the thylakoid membrane and utilizes the two Photosystem I and the chlorophyll P700.

When cyclic photophosphorylation takes place, the electrons are sent back to P700 instead of moving inside the NADP from the electron acceptor. This movement that occurs downward of electrons from an acceptor to P700 results in the formation of ATP molecules.

Non-Cyclic Photophosphorylation

The photophosphorylation process which is a result of the movement of the electrons in a constant non-cyclic manner so that synthesizing ATP molecules can use the energy from excited electrons given by photosystem II is termed non-cyclic photophosphorylation.

This process is also known as non-cyclic photophosphorylation due to the loss of electrons by P680 of Photosystem II which gets occupied by P700 of Photosystem I and is not reversed back to P680. Here the entire movement of the electrons is given in a unidirectional or a non-cyclic manner.

During non-cyclic photophosphorylation, the electrons are freed by P700, get taken away by the primary acceptor, and are finally passed on to NADP. Here, the electrons mix with the protons – H+ which is later produced by dividing up the water molecule and reduces NADP to NADPH2.

Difference between Cyclic and Non-Cyclic Photophosphorylation

Cyclic Photophosphorylation Non-Cyclic Photophosphorylation
Only Photosystem I is needed. Both Photosystem I and II are required.
P700 is the active reaction center where it takes place. P680 is the active reaction center where it takes place.
Electrons move in a cyclic manner. Electrons move in a non – cyclic manner.
Electrons can reverse back to Photosystem I. Electrons from Photosystem I get accepted by NADP.

 

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