Difference Between Cyclic and Non-Cyclic Photophosphorylation

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 carbon dioxide and water to glucose and oxygen.

Photosynthetic processes can be divided into two categories: oxygenic and anoxygenic. Both work on the same principles, although plants, algae, and cyanobacteria use oxygenic photosynthesis the most.

Light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to make carbohydrates during oxygenic photosynthesis. The CO2 is "reduced," or gains electrons, while the water is "oxidised," or loses electrons, in this process. Along with carbs, oxygen is generated.

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

"Anoxygenic Photosynthetic Bacteria," anoxygenic photosynthesis uses electron donors that aren't water and don’t produce oxygen. Green sulfur bacteria and phototrophic purple bacteria are among the microorganisms that undergo this activity.

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

However, this entire process of photosynthesis occurs in two different processes:

light reaction and dark reaction.

• Light Reaction

The light reaction of the photosynthesis occurs in the chloroplast inside the grana. In this reaction, the light energy is converted to 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. Carotenoids make up the accessory pigments. The chlorophyll in the thylakoid membrane of chloroplasts absorbs the energy from the sun. Two-electron transport chains generate ATP and NADPH, which are then transferred to ATP and NADPH. During the process, both water and oxygen are utilized.

• Dark Reaction

In the dark reaction of photosynthesis, the energy which is produced in the light reaction is used for converting carbon dioxide into carbohydrates. This reaction happens in the stroma of the chloroplasts. The nighttime reactions of photosynthesis are propelled by the energy provided by ATP (made during the light reactions). The phrase "dark reactions" does not imply that the reactions take place at night or that darkness is required. It means that the reactions can continue regardless of how much light is present. The phrase is solely used to differentiate between dark and light reactions, both of which require light.

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What is Photophosphorylation?

Otto Kandler published the first experimental evidence for photophosphorylation in vivo in 1950, utilising intact Chlorella cells and interpreting his findings as light-dependent ATP production. With the use of P32, Daniel I. Arnon identified photophosphorylation in isolated chloroplasts in vitro in 1954. In 1956, he released his first review of early photophosphorylation studies.

Photophosphorylation is the process in which 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:

1. Cyclic photophosphorylation
   

2. Non-cyclic photophosphorylation

• Cyclic Photophosphorylation

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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 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 downward movement of the electrons to the P700 from the acceptor.

This electron transport chain generates a proton-motive force, which pumps H+ ions across the membrane and creates a concentration gradient that can be used to activate ATP synthase during chemiosmosis. Cyclic photophosphorylation is the name given to the entire process. It does not create oxygen or NADPH.

Two ATP molecules are created during cyclic photophosphorylation

• Non-Cyclic Photophosphorylation

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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 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. Non-cyclic photophosphorylation produces 1 ATP and 2 NADPH2 molecules. The electrons here get combined with the protons H⁺ that is produced when the water molecules split up and reduce NADP into NADPH. The reaction is shown below.

NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺

Difference Between Cyclic and Noncyclic Photophosphorylation

The electron emitted from the P700 molecule (PSI) is cycled back in this electron transport mechanism. It produces ATP for the dark reaction, which isn't created enough in non-cyclic processes. Cyclic transport is uncommon and only happens when NADPH accumulates in the chloroplast under specific conditions.