chemiosmotic hypothesis

Options

A. Was explained by P. Mitchell
B. Proves that proton gradient is not required for ATP synthesis
C. Explains how ATP is synthesized in the chloroplast
D. Both (1) & (3)

Answer

### Step-by-Step Solution for the Chemiosmotic Hypothesis

1. **Definition of Chemiosmotic Hypothesis**: The chemiosmotic hypothesis, proposed by Peter Mitchell, explains how ATP is synthesized in chloroplasts and mitochondria through the creation of a proton gradient across a membrane.

**Hint**: Remember that the chemiosmotic hypothesis is fundamentally about the role of proton gradients in ATP synthesis.

2. **Role of Proton Gradient**: The hypothesis states that ATP synthesis is driven by a proton gradient, which is the difference in concentration of hydrogen ions (protons) across the thylakoid membrane in chloroplasts. This gradient is created when protons are pumped from the stroma into the thylakoid lumen.

**Hint**: Think of the proton gradient as a battery that stores potential energy, which is used to power ATP synthesis.

3. **Mechanism of ATP Synthesis**: As protons accumulate in the lumen, they create a higher concentration of protons there compared to the stroma. When the concentration gradient reaches a certain point, protons flow back into the stroma through ATP synthase, an enzyme located in the thylakoid membrane. This flow of protons provides the energy needed to convert ADP and inorganic phosphate (Pi) into ATP.

**Hint**: Visualize ATP synthase as a turbine that spins when protons flow through it, generating ATP in the process.

4. **Reduction of NADP+**: In addition to ATP synthesis, the protons that flow through ATP synthase can also be used to reduce NADP+ to NADPH, which is crucial for the Calvin cycle in photosynthesis.

**Hint**: Remember that NADPH is another important product of the light reactions of photosynthesis, alongside ATP.

5. **Conclusion**: The chemiosmotic hypothesis is validated by the fact that ATP synthesis in chloroplasts indeed relies on the proton gradient. Therefore, the correct answer to the question about the chemiosmotic hypothesis is that it was explained by Peter Mitchell and it explains how ATP synthesis occurs in chloroplasts.

**Hint**: When answering questions about hypotheses, always look for the key components that support or refute the main claims.

### Final Answer
The correct answer is: Both first and third options are correct.