During ATP synthesis , electrons pass through

Options

A. `CO_2`
B. `O_2`
C. `H_2O`
D. Cytochromes

Answer

**Step-by-Step Solution:**

1. **Understanding ATP Synthesis**: ATP synthesis occurs through two main processes: oxidative phosphorylation and photophosphorylation. Oxidative phosphorylation occurs during aerobic respiration in animals, while photophosphorylation occurs in plants using light energy.

2. **Role of Proton Gradient**: Both processes require a proton gradient (also known as an electrochemical gradient) to synthesize ATP. This gradient is essential for the phosphorylation of ADP to ATP.

3. **Electron Transport Chain**: During ATP synthesis, electrons are generated from various substrates and pass through a series of protein complexes. These complexes are crucial for establishing the proton gradient.

4. **Identifying the Correct Pathway**: The question asks where the electrons pass through during ATP synthesis. The options provided are carbon dioxide, oxygen, water, and cytochrome.

5. **Analyzing the Options**:
- **Carbon Dioxide**: Incorrect, as it is a product in oxidative phosphorylation and a substrate in photophosphorylation, but not a pathway for electrons.
- **Oxygen**: Incorrect, as it is a product in oxidative phosphorylation and a substrate in photophosphorylation, but not a pathway for electrons.
- **Water**: Incorrect, as it is utilized in photophosphorylation to generate electrons and is formed in oxidative phosphorylation, but not a pathway for electrons.
- **Cytochrome**: Correct, as cytochromes are protein complexes through which electrons pass during both oxidative phosphorylation and photophosphorylation.

6. **Conclusion**: The correct answer is cytochrome, as it is the pathway through which electrons pass during ATP synthesis.

**Final Answer**: Cytochrome

---