Cyclic Photophosphorylation happens with the help of photosynthesis, a process of producing carbohydrates by green plants using carbon dioxide and water in the presence of sunlight. The two-step process comprises of:
The light reaction which takes place in the grana of the chloroplast where light energy gets converted to chemical energy as ATP and NADPH. In this very light reaction, the addition of phosphate in the presence of light or the synthesizing of ATP by cells is known as photophosphorylation.
While in the dark reaction, the energy produced previously in the light reaction is utilized to fix carbon dioxide to carbohydrates. The location where this happens is the stroma of the chloroplast.
When plants use light energy from photosynthesis to convert ADP to ATP, the process is known as Photophosphorylation.
There are two types of photophosphorylation –
1) Cyclic Photophosphorylation
In prokaryotes, the process of photosynthesis is used for the production of energy and not for the formation of biological molecules. Cyclic Photophosphorylation is the process, in which systems (like prokaryotes), just accomplishes the ADP to ATP for immediate energy for the cells.
This type of photophosphorylation usually occurs in the thylakoid membrane. The electron begins in a pigment complex called photosystem I in cyclic electron flow. It then further passes from the primary acceptor to ferredoxin and eventually to cytochrome b6f. Cytochrome b6f is similar to what is found in mitochondria. The electron then passes to plastocyanin before returning to chlorophyll.
A proton-motive force is produced throughout this electron transport chain which pumps H+ ions across the membrane and produces a concentration gradient that can be used to power ATP synthase during chemiosmosis. This entire pathway is known as cyclic photophosphorylation. It neither produces O2 nor NADPH.
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