Photosynthesis is referred to as the process to convert the light energy of the Sun into chemical energy. During this process, the light energy gets captured and is then used to convert the carbon dioxide and water to glucose and oxygen. However, this entire process of photosynthesis occurs as two different processes: light reaction and dark reaction.
The light reaction of the photosynthesis occurs in the chloroplast inside the grana. In this reaction, the light energy is converted to the chemical energy in the form of ATP and NADPH. In this reaction, when phosphate is added in the presence of sunlight or by the process of ATP synthesis by cells, it is referred to as photophosphorylation.
In the dark reaction of photosynthesis, the energy which is produced in the light reaction is used for converting the carbon dioxide into carbohydrates. This reaction happens in the stroma of the chloroplasts.
Photophosphorylation is the process in which the light energy is used from photosynthesis to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP). It is the process in which the energy-rich ATP molecules are synthesized by the transfer of the phosphate group to the ADP molecule during the presence of sunlight.
Photophosphorylation is of two different types:
The cyclic photophosphorylation diagram is given below.
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Cyclic photophosphorylation is a process that results in the movement of the electrons in a cyclic way to synthesize the ATP molecules. In this process, the plant cells convert ADP to ATP to gain immediate energy for their cells. The process of cyclic photophosphorylation generally occurs in the thylakoid membrane and makes use of the Photosystem I and Chlorophyll P700.
During this process of cyclic photophosphorylation, the electrons get transferred back to P700 from the electron acceptor and they do not move to the NADP. ATP molecules get formed as a result of this type of a downward movement of the electrons to the P700 from the acceptor.
The non-cyclic photophosphorylation diagram is given below.
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Non-cyclic photophosphorylation is a process that results in the movement of the electrons in a non-cyclic way to synthesize the ATP molecules by using the energy from the excited electrons that are provided by Photosystem II.
This process is called non-cyclic photophosphorylation since the electrons lost by P680 of Photosystem II get occupied by P700 of Photosystem I and do not revert to P680. In this process, the complete movement of these electrons happens in a non-cyclic or a unidirectional manner.
During the process of non-cyclic photophosphorylation, the electrons that are released by the P700 get carried by the primary acceptor and then get passed on to the NADP. The electrons here get combined with the protons H⁺ that are produced when the water molecules split up and reduce NADP into NADPH. The reaction is shown below.
NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺
Q1. Explain the Process of Non-cyclic Photophosphorylation in Detail.
Ans. Phosphorylation refers to the process in which ATP gets formed from ADP in the light reaction of the process of photosynthesis. This process is carried out in two different ways, namely, cyclic photophosphorylation and non-cyclic photophosphorylation.
The non-cyclic photophosphorylation refers to the process in which the electrons that are expelled from the excited photocentre do not return. This process happens when both the photosystems I and II are involved. The photolysis of water leads to the release of electrons and hence, a constant water supply is needed. In this process, both NADPH and ATP get formed.
Q2. Differentiate Between Cyclic and Noncyclic Photophosphorylation.
Ans. The differences between cyclic and noncyclic photophosphorylation include
Cyclic photophosphorylation happens only in the photosystem I but non-cyclic photophosphorylation occurs in both the photosystems I and II.
In the cyclic photophosphorylation, only ATP is produced, whereas, in the non-cyclic photophosphorylation both NADPH and ATP are produced.
In cyclic photophosphorylation, the electrons get expelled by photosystem I and they return to the system. On the other hand, in non-cyclic photophosphorylation, the electrons that are expelled by the photosystems do not return.
Photolysis of water does not occur in cyclic photophosphorylation, but it occurs in non-cyclic photophosphorylation.
Oxygen does not get released in cyclic photophosphorylation, but it gets released in the case of non-cyclic photophosphorylation.
Water does not get consumed in cyclic photophosphorylation, but it gets consumed in non-cyclic photophosphorylation.