Question

Define photophosphorylation and describe cyclic phosphorylation.

Answer 1

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.