Question

Define photophosphorylation and describe cyclic phosphorylation.

Answer 1

Hint: In the word, photophosphorylation - Photo means light and phosphorylation is the addition of a phosphoryl group to a molecule. Cyclic phosphorylation is termed due to the movement of electrons in a cyclic manner.

Complete answer:
As understood from the etymology, photophosphorylation is a process in which plants convert adenosine diphosphate ($$ADP$$) to adenosine triphosphate ($$ATP$$) through phosphorylation using sunlight as an energy source. The $$ATP$$ produced is the source for energy for the cells. This phosphorylation is of two types: cyclic and non-cyclic photophosphorylation.
Cyclic photophosphorylation, occurring on the stroma lamellae or fret channels, is initiated by the high-energy electron produced by chlorophyll A in $$p700$$ of photosystem $$1$$ (PS $$1$$) through sunlight. The electron produced activates a series of events starting from the primary acceptor to ferredoxin to plastoquinone then to cytochrome $$b6f$$ to plastocyanin and finally goes back to PS $$1$$. During this excitation, two $$ATP$$ molecules are released. The proton motive force generates a concentration gradient which can be used during chemiosmosis.

Note: Cyclic and non-cyclic photophosphorylation are entirely different. In cyclic photophosphorylation, only PS $$1$$ is involved whereas in non-cyclic both PS $$1$$ and PS $$2$$ are involved. Neither oxygen nor $$NADPH$$ is released during cyclic photophosphorylation but both are released during non-cyclic photophosphorylation.
Similarly, cyclic photophosphorylation also occurs in photosynthetic bacteria to meet their energy needs. Bacteria have only one photosystem $$1$$ whose chlorophyll absorbs wavelengths of $$800-1000nm$$, unlike plants which absorb $$650-750nm$$. However, cyanobacteria possesses two photosystems and performs similar to plants. In plants $$H2O$$ donates electrons to photophosphorylation. But in bacteria $$H2S$$and other chemical compounds donate electrons to generate energy.