Chemiosmotic theory of ATP synthesis in the chloroplasts and mitochondria is based on
(a) Proton gradient
(b) Accumulation of K+ ions
(c) Accumulation of Na+ ions
(d) Membrane potential

Options

A. Proton gradient
B. Accumulation of K ions
C. Accumalation of Na ions
D. Membrane potential

Answer

To solve the question regarding the chemiosmotic theory of ATP synthesis in chloroplasts and mitochondria, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Chemiosmotic Theory**: The chemiosmotic theory explains how ATP is synthesized in the chloroplasts and mitochondria. It was proposed by Peter Mitchell in 1961 and is crucial for understanding oxidative phosphorylation.

2. **Role of Proton Gradient**: The theory is based on the establishment of a proton gradient (H+ ions) across the inner membrane of the mitochondria and the thylakoid membrane of chloroplasts. This gradient is created during the electron transport chain, where electrons are transferred through a series of proteins.

3. **ATP Synthesis Mechanism**: The energy stored in the proton gradient is used by ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate (Pi). As protons flow back across the membrane through ATP synthase, the energy released drives the conversion of ADP to ATP.

4. **Evaluating the Options**:
- (a) **Proton gradient**: This is the correct answer as it directly relates to the chemiosmotic theory.
- (b) **Accumulation of K+ ions**: This is not relevant to ATP synthesis in this context.
- (c) **Accumulation of Na+ ions**: This is also not relevant to the chemiosmotic theory.
- (d) **Membrane potential**: While related, the specific basis of the chemiosmotic theory is the proton gradient, not just the membrane potential.

5. **Conclusion**: Based on the explanation of the chemiosmotic theory and the role of the proton gradient in ATP synthesis, the correct answer is (a) Proton gradient.

### Final Answer:
(a) Proton gradient