ATP synthesis in cell requires

Options

A. `H^(+)` gradient across the membrane
B. `K^(+)` gradient across the membrane
C. `PO_(4)^(3-)` gradient across the membrane
D. `Ca^(2+)` gradient across the membrane.

Answer

To determine what is required for ATP synthesis in the cell, we can analyze the options provided and the process of ATP synthesis itself.

### Step-by-Step Solution:

1. **Understanding ATP Synthesis**: ATP synthesis occurs through a process called chemiosmosis, which is essential in both cellular respiration and photosynthesis.

2. **Identifying the Mechanism**: In cellular respiration, ATP is synthesized during oxidative phosphorylation, which occurs in the inner mitochondrial membrane. In photosynthesis, ATP is synthesized during the light reactions, known as photophosphorylation, which takes place in the thylakoid membranes of chloroplasts.

3. **Role of Proton Gradient**: Both processes rely on a proton (hydrogen ion) gradient across a membrane. This gradient is created by the movement of electrons through the electron transport chain, which pumps protons from one side of the membrane to the other.

4. **Function of ATP Synthase**: The proton gradient drives protons back across the membrane through ATP synthase, an enzyme that catalyzes the conversion of ADP and inorganic phosphate into ATP.

5. **Evaluating the Options**:
- **Hydrogen Gradient Across the Membrane**: This is essential for ATP synthesis as it provides the necessary energy for ATP production.
- **Potassium Gradient Across the Membrane**: Not directly involved in ATP synthesis.
- **Phosphate Gradient Across the Membrane**: Not required; ATP synthesis involves the availability of inorganic phosphate, but not a gradient.
- **Calcium Gradient Across the Membrane**: Not involved in ATP synthesis.

6. **Conclusion**: The correct answer is that ATP synthesis in the cell requires a hydrogen gradient across the membrane.

### Final Answer:
**ATP synthesis in the cell requires a hydrogen gradient across the membrane.**