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NEET Biology
Photophosphorylation

Photophosphorylation

Photophosphorylation is a process in which phosphorylation of adenosine diphosphate (ADP) occurs to form adenosine triphosphate (ATP), which is a molecule that cells use as a source of energy. This process occurs during photosynthesis, where light energy is converted into chemical energy, specifically into ATP and NADPH, which are then used to convert carbon dioxide into organic compounds like glucose.

Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin Cycle). These stages work together to convert light energy into chemical energy, which is then used to synthesize organic compounds from carbon dioxide (CO2). There are two pathways through which light-dependent reactions can occur: cyclic and non-cyclic photophosphorylation.

1.0Cyclic Photo-phosphorylation

Cyclic photophosphorylation primarily takes place within the stroma lamellae. This process hinges on the movement of high-energy electrons originating from P700, a specific pigment within photosystem I. Following their release, these electrons embark on a circular journey, beginning at photosystem I. They are first transferred to ferredoxin via FRS (Ferredoxin reducing system), and from there, the electrons move to the cytochrome b6f complex, akin to a similar complex found in mitochondria, and then to plastocyanin, before ultimately circling back to photosystem I.

This electron flow facilitates the creation of a proton-motive force by actively transporting H+ ions across the membrane, thereby generating a concentration gradient. This gradient is then harnessed by ATP synthase during chemiosmosis to produce ATP. Notably, this process, termed cyclic photophosphorylation, neither yields oxygen (O2) nor produces NADPH, diverging from non-cyclic photophosphorylation, where electrons are not recycled back to the cytochrome b6f complex but rather utilized to reduce NADP+.

Photo-Phosphorylation

2.0Non Cyclic Photo-phosphorylation

This process involves the sequential action of Photosystem I (PSI) and Photosystem II (PSII), initiated when PSI's chlorophyll-a molecule, known as P700, absorbs a photon. This absorption ejects an electron from P700, which is captured by the Ferredoxin Reducing Substance (FRS). The electron is then transferred to ferredoxin (FD), and from ferredoxin, it moves to NADP+ through an intermediate protein, ferredoxin-NADP reductase (FNR). This transfer reduces NADP+ to NADPH + H+.

When Photosystem II's chlorophyll-a molecule, referred to as P680, absorbs a photon, it becomes excited, and an electron is ejected, leaving P680 in a deficit of electrons. The ejected electron is initially transferred to pheophytin, leaving behind a positively charged P680+ (P680 plus). 

P680+ uses energy in two steps to split water into 2H+ + 1/2 O2 + 2e- (a process known as photolysis or light-splitting). 

An electron from the split water molecule replenishes P680, restoring it to its neutral state, while the hydrogen ions and oxygen are released. The electron from pheophytin moves to plastoquinone (PQ), which, in two steps, acquires 2e- from pheophytin and two H+ ions from the stroma, becoming PQH2. This plastoquinol then reverts to PQ, releasing the 2e- to the cytochrome b6f complex and releasing two H+ ions into the thylakoid lumen. 

The electrons continue through cytochrome b6 and cytochrome f to plastocyanin, with energy from PSI pumping hydrogen ions into the thylakoid space, creating a gradient that allows H+ ions to flow back into the stroma, providing the energy needed to regenerate ATP. Since electrons from water replace those lost by PSII, they don't return to PSII as they do in cyclic electron flow. Instead, they move to PSI, where a second solar photon raises their energy level.  

These high-energy electrons are passed to a series of acceptors, ultimately reaching the enzyme ferredoxin-NADP+ reductase, which catalyzes their use in reducing NADP+ + 2H+ + 2e- to NADPH + H+. This step utilizes the H+ ions produced by water splitting, resulting in a net output of 1/2O2, ATP, and NADPH + H+, all generated from the consumption of solar photons and water. The concentration of NADPH within the chloroplast can influence the path electrons take during light reactions. When the chloroplast needs more ATP for the Calvin cycle and NADPH accumulates, the plant may switch from non-cyclic to cyclic electron flow, adjusting its energy production needs accordingly.

Non cyclic photo-phosphorylation

3.0Difference between Cyclic and Non Cyclic Photophosphorylation

Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation

1. PS I only involved

1. PS I and PS II involved

2. Reaction centre is P700

2. Reaction centre is P680

3. Electrons released are cycled back

3. Electrons released are not cycled back

4. Photolysis of water does not take place

4. Photolysis of water takes place

5. Only ATP synthesized

5. ATP and NADPH + H⁺ are synthesized

6. Phosphorylation takes place at two places

6. Phosphorylation takes place at only one place

7. It does not require an external electron donor

7. Requires external electron donor like H₂O or H₂S

8. It is not sensitive to dichlorodimethylurea (DCMI)

8. It is sensitive to DCMI and inhibits electron flow

Table of Contents


  • 1.0Cyclic Photo-phosphorylation
  • 2.0Non Cyclic Photo-phosphorylation
  • 3.0Difference between Cyclic and Non Cyclic Photophosphorylation

Frequently Asked Questions

Photophosphorylation is the process by which ATP (adenosine triphosphate) is synthesized from ADP (adenosine diphosphate) and inorganic phosphate (Pi) using the energy from light during photosynthesis light is essential for this reaction thus it is also called light-dependent reaction of photosynthesis. It is a crucial step in converting light energy into chemical energy.

Cyclic photophosphorylation is a type of photophosphorylation where electrons released from the reaction center (like P700 in Photosystem I) are passed through a series of carriers and eventually return to the same reaction center. This process generates ATP but does not produce NADPH or oxygen. It is called "cyclic" because the electrons follow a circular path.

Non-cyclic photophosphorylation involves two photosystems (Photosystem I and Photosystem II) and produces both ATP and NADPH. In this process, electrons do not return to their original state but are transferred from water to NADP+, forming NADPH. This process also results in the photolysis of water, releasing oxygen as a byproduct.

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