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^(2+)` ions
(d) Membrane potential

Options

A. Proton gradient
B. Accumulation of `K^(+)` ions
C. Accumulation of `Na^(2+)` ions
D. Membrane potential

Answer

To solve the question regarding the chemiosmotic theory of ATP synthesis in chloroplasts and mitochondria, we will analyze the options provided and identify the correct answer step by step.

### Step-by-Step Solution:

1. **Understanding Chemiosmotic Theory**:
- The chemiosmotic theory explains how ATP is synthesized in both chloroplasts and mitochondria. It involves the movement of protons (H⁺ ions) across a membrane, creating a gradient that drives ATP synthesis.

2. **Identifying the Key Component**:
- The core concept of the chemiosmotic theory is the generation of a proton gradient. This gradient is established by the accumulation of protons in specific compartments of the organelles.

3. **Proton Gradient in Chloroplasts**:
- In chloroplasts, protons are accumulated in the thylakoid lumen during the light reactions of photosynthesis. This creates a high concentration of protons inside the lumen compared to the stroma.

4. **Proton Gradient in Mitochondria**:
- In mitochondria, protons are pumped from the mitochondrial matrix into the intermembrane space during the electron transport chain, creating a proton gradient across the inner mitochondrial membrane.

5. **ATP Synthesis Mechanism**:
- The proton gradient created in both chloroplasts and mitochondria is utilized by ATP synthase (F₀F₁ ATP synthase) to synthesize ATP as protons flow back into the matrix (in mitochondria) or stroma (in chloroplasts).

6. **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 the chemiosmotic theory.
- (c) Accumulation of Na²⁺ ions - This is also not relevant.
- (d) Membrane potential - While related, it is not the primary basis of the chemiosmotic theory.

7. **Conclusion**:
- The correct answer to the question is (a) Proton gradient.

### Final Answer:
**(a) Proton gradient**