An external source of electrons is not required in

Options

A. Cyclic photophosphorylation
B. Non-cyclic photophosporyation
C. Z-scheme of flow of electrons
D. All of these

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis consists of two main processes: light-dependent reactions and light-independent reactions (Calvin cycle). The light-dependent reactions occur in the thylakoid membranes of chloroplasts and involve the conversion of light energy into chemical energy.

2. **Identifying Electron Sources**: During photosynthesis, electrons are essential for the formation of ATP and NADPH. The source of these electrons can vary depending on the type of photophosphorylation.

3. **Cyclic Photophosphorylation**: In cyclic photophosphorylation, electrons are excited from chlorophyll a molecules and are cycled back to the same chlorophyll molecule after passing through the electron transport chain. This process does not require an external source of electrons, as the electrons are recycled.

4. **Non-Cyclic Photophosphorylation (Z-scheme)**: In contrast, non-cyclic photophosphorylation (also known as the Z-scheme) involves the splitting of water molecules (H2O) by the oxygen-evolving complex (OEC). This process releases electrons, protons, and oxygen. Here, an external source of electrons (from water) is required.

5. **Conclusion**: Since cyclic photophosphorylation does not require an external source of electrons, the correct answer to the question is that an external source of electrons is not required in cyclic photophosphorylation.

**Final Answer**: An external source of electrons is not required in cyclic photophosphorylation.

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