Photophosphorylation refers to the process of synthesizing ATP from ADP (adenosine diphosphate) using the energy from light. It occurs during photosynthesis in plants, algae, and some bacteria. This process takes place in the thylakoid membranes of chloroplasts.

The process of Photophosphorylation can be divided into two types: non-cyclic and cyclic photophosphorylation.

1. Non-cyclic Photophosphorylation:
Non-cyclic photophosphorylation is the primary mode of ATP synthesis during photosynthesis. It involves two photosystems, known as photosystem I (PSI) and photosystem II (PSII), working together to transfer electrons and create a proton gradient.

- Photosystem II (PSII): PSII absorbs light energy, exciting an electron in a chlorophyll molecule. This electron is then passed through an electron transport chain, releasing energy that is used to pump protons (H+) across the thylakoid membrane from the stroma to the thylakoid lumen. This creates a proton gradient.

- Photosystem I (PSI): PSI absorbs another photon of light energy, exciting an electron in a chlorophyll molecule. This electron is then passed through a second electron transport chain, which uses the energy to produce NADPH. The electron from PSI is replaced by an electron from PSII.

- ATP Synthesis: The proton gradient created by the electron transport chain in PSII is utilized by ATP synthase, an enzyme located in the thylakoid membrane, to synthesize ATP from ADP and inorganic phosphate (Pi). The flow of protons through ATP synthase drives the phosphorylation of ADP to ATP.

2. Cyclic Photophosphorylation:
Cyclic photophosphorylation occurs in some bacteria and plants, particularly when there is a high demand for ATP and low demand for NADPH. It involves only PSI and does not produce NADPH or release oxygen.

- During cyclic photophosphorylation, the excited electron from PSI is passed back to the electron transport chain, rather than being transferred to NADP+.

- The electron is then used to pump protons across the thylakoid membrane, creating a proton gradient.

- The ATP synthase enzyme utilizes this proton gradient to produce ATP from ADP and inorganic phosphate (Pi).

In conclusion, Photophosphorylation is the process of synthesizing ATP from ADP using the energy from light. It occurs during photosynthesis and involves the function of photosystems and electron transport chains in the thylakoid membranes of chloroplasts. The non-cyclic photophosphorylation is the primary mode of ATP synthesis, while cyclic photophosphorylation occurs in specific conditions.