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+.

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.

3.0Difference between Cyclic and Non Cyclic Photophosphorylation