Cyclic Photophosphorylation

Cyclic Photophosphorylation is a process to generate Adenosine triphosphate (ATP) without producing NADPH. This process generally takes place in the thylakoid membrane of chloroplasts. In the context of photosynthesis, Nicotinamide adenine dinucleotide phosphate (NADPH) is a reducing agent that works as the carrier of high-energy electrons.

These electrons can reduce the amount of carbon dioxide (CO₂) in carbohydrates during light-independent reactions. In this article today, we will discuss What is cyclic photophosphorylation, the types of photophosphorylation, Steps of the cyclic photophosphorylation, and the difference between the Cyclic and Non-Cyclic Photophosphorylation process in detail.

What is Cyclic Photophosphorylation?

Cyclic photophosphorylation occurs in the thylakoid membrane of the chloroplasts during the light-dependent phase of photosynthesis. Since the term 'cyclic' is used, it means the electrons return to the reaction center of Photosystem I. The main aim of cyclic photophosphorylation is the production of ATP. Non-cyclic photophosphorylation is responsible for the production of NADPH.

Light and Dark Reaction

Photosynthesis occurs in two phases- light-dependent reaction and light-independent reaction. The light reaction occurs during daytime in the grana and its end products are ATP and NADPH. The dark reaction occurs during night in the stroma of the chloroplast. The end product is glucose that is utilized by the plant.

Also Read: Difference between Dark And Light Reaction

Types of Photophosphorylation

Photophosphorylation is the process of converting ADP to ATP in the presence of light energy. In simple words, photophosphorylation converts light energy into chemical energy by producing ATP in the grana of the chloroplast. There are two types of photophosphorylation, as mentioned below:

Cyclic Photophosphorylation

Cyclic photophosphorylation is a cyclic movement of electrons during the process of ATP synthesis with the help of Photosynthesis. In simple words, it is the process of producing carbohydrates by using water and carbon dioxide in the presence of sunlight. It implies that cyclic photophosphorylation is a fully light-dependent process. Prokaryotes depend on this process to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP) to generate immediate energy for the cells.

Non-Cyclic Photophosphorylation

Non-cyclic Photophosphorylation is a non-cyclic movement of electrons during ATP molecules synthesis. In this, ATP molecules use the energy of excited electrons in Photosystem II. This type of photophosphorylation occurs in the granal thylakoids of chloroplasts.

Steps of Cyclic Photophosphorylation

Cyclic photophosphorylation is primarily responsible for generating ATP. It generally addresses the energy needs of the cell, specifically when there is a comparatively low demand for reducing power (NADPH). It is totally a light-dependent process that has some major components, as mentioned below:

Components Involved In Cyclic Photophosphorylation
Photosystem I (PS I)	It is a complex structure of proteins and pigments generally found in the thylakoid membrane. The major role of Photosystem I is to absorb sunlight (wavelength 700 nm). Photosystem I's reaction center primarily contains chlorophyll molecules, which capture the excited electrons for the process.
Electron Transport Chain	The electrons present in Photosystem I pass through the Electron transport chain and return to the same place, i.e., Photosystem I, after the process.
Cytochrome b6f Complex	The Cytochrome b6f complex transfers the electrons to plastocyanin. This Cytochrome b6f Complex is also responsible for transferring the electrons back to Photosystem I.
Plastocyanin	This is a carrier protein that extracts the electrons from the cytochrome b6f complex.
ATP Synthase	While the excited electrons pass through the electron transport chain, they usually generate protons all over the thylakoid membrane. These protons are mainly used to synthesize ATP and inorganic phosphate (Pi).

Now let's see how cyclic photophosphorylation really happens in the thylakoid membrane of chloroplasts during photosynthesis.

1. Photon Absorption by Photosystem I (PS I)- Chlorophyll pigments absorb energy from light in Photosystem I (PS I)
2. Energy Transfer to the P700 Reaction Center - The absorbed photon energy is transferred to the P700 reaction center.
3. Electron Excitation and Release - In the P700 reaction center, an elevated-energy electron becomes unstable and moves to a higher-energy orbital. This energy is used to release a single electron from the P700 reaction center.
4. Electron Transfer to Ferredoxin (Fd)- The released electron is captured by chlorophyll A, which moves through Fe-S (iron-sulfur) cluster to reach ferredoxin (Fd).
5. Electron Transport to Plastoquinone (PQ)- The electron then gets transferred to Plastoquinone (PQ) which is an electron carrier.
6. Plastoquinone to Cytochrome b6f (Cyt b6f) - Plastoquinone carries the electron to the Cytochrome b6f (Cyt b6f) complex in the thylakoid membrane. During this transferring process, two hydrogen ions (H⁺) are moved from the stromal side to the lumen side.
7. Electron Transfer from Cytochrome b6f to Phycocyanin and Back to P700 - Cytochrome b6f transfers the electron to the protein phycocyanin. From the protein Phycocyanin, the electron travels through the lumen and returns to Photosystem I (PSI), specifically to the P700 reaction center.
8. Compensation for Electron Deficits- Phycocyanin compensates for the electron deficits in the P700 reaction center to restore its ability to accept another photon and initiate the cyclic process again.
9. ATP Synthesis- Electrons move through a chain from Photosystem I to Ferredoxin to Plastoquinone to Cytochrome b6f and back to Photosystem I, thus creating a proton difference in the thylakoid membrane. ATP synthase uses this difference to make ATP from ADP and phosphate.
10. Completion of the Cycle- The process continues in a cycle. It allows the continuous synthesis of ATP without the production of NADPH.

Conditions Leading to Cyclic Photophosphorylation

Cyclic photophosphorylation is a process that occurs during light-dependent reactions, known as photosynthesis. Here are the conditions leading to cyclic photophosphorylation:

1. Low NADP⁺ levels- In the cyclic process, NADP⁺ is not reduced to NADPH. If the levels of NADP⁺ are low or there is a limited demand for NADPH, in that case, cyclic photophosphorylation happens.
2. High Proton Gradient - The movement of electrons through the electron transport chain in photosystem I creates a proton gradient across the thylakoid membrane. This proton gradient is crucial for ATP synthesis.
3. High ATP Demand - Cyclic photophosphorylation is preferred when there is a high demand for ATP within the chloroplast and the cell. This happens when the cell needs more ATP for processes other than the Calvin cycle.

Difference between Cyclic and Non-Cyclic Photophosphorylation

Here are some major differences between Cyclic and Non-Cyclic Photophosphorylation as mentioned below:

Cyclic Photophosphorylation	Non-Cyclic Photophosphorylation
Occurs in photosystem I (PS I).	Occurs in both photosystem I (PS I) and photosystem II (PS II).
Electrons follow in a cyclic pathway and return to PS I.	Electrons flow linearly from PS II to PS I. After that it flows to NADP+.
Only ATP is produced .	Both ATP and NADPH produced.
No NADPH production.	NADPH is produced.
No role in the Calvin cycle .	Supplies NADPH for the Calvin cycle.
Primarily generates ATP for energy when NADPH demand is low.	Provides both ATP and NADPH for the synthesis of organic molecules.

Also Read,

• C4 Cycle of Photosynthesis
• Calvin Cycle
• Chemiosmotic Hypothesis