Photophosphorylation is the process of synthesis of ATP from ADP and Pi in the presence of light. It occurs during photosynthesis, specifically in the thylakoid membrane of chloroplasts in plants and algae.

Process of Photophosphorylation:

1. Light Absorption: The process of photophosphorylation begins with the absorption of light by chlorophyll molecules present in the thylakoid membrane. Chlorophyll absorbs light energy and becomes excited.

2. Electron Transport: The excited electrons from chlorophyll molecules are transferred to the electron carriers in the thylakoid membrane. These electron carriers form an electron transport chain, passing the electrons from one carrier to another.

3. Photosystem I and II: There are two photosystems involved in photophosphorylation - photosystem I (PSI) and photosystem II (PSII). The excited electrons from PSII are transferred to PSI.

4. Splitting of Water: In PSII, water molecules are split into oxygen, hydrogen ions (H+), and electrons. This process is known as photolysis. The released electrons replace the ones transferred to PSI.

5. Chemiosmosis: As the excited electrons are passed through the electron transport chain, they release energy that is used to pump hydrogen ions (H+) from the stroma into the thylakoid lumen, creating a concentration gradient.

6. ATP Synthesis: The concentration gradient of hydrogen ions created by chemiosmosis drives the synthesis of ATP. The hydrogen ions flow back into the stroma through ATP synthase, a protein complex embedded in the thylakoid membrane. This flow of hydrogen ions powers the synthesis of ATP from ADP and Pi (inorganic phosphate).

7. NADPH Production: In addition to ATP synthesis, the excited electrons from PSI are transferred to another electron carrier, NADP+, along with hydrogen ions. This produces NADPH, which is used in the Calvin cycle, the second stage of photosynthesis.

In summary, photophosphorylation is the process by which light energy is converted into chemical energy in the form of ATP. It involves light absorption, electron transport, splitting of water, chemiosmosis, and ATP synthesis. This process occurs in the thylakoid membrane of chloroplasts and is an essential step in photosynthesis.