……...is / are liable to saturation , respond to inhibitor and is under hormonal regulation and moves only along a concentration gradient .

Options

A. Simple diffusion
B. Facilitated transport
C. Active transport
D. Both facilitated and active transport

Answer

To solve the question, we need to identify which transport mechanism is described by the characteristics given: liable to saturation, responds to inhibitors, is under hormonal regulation, and moves only along a concentration gradient. Let's analyze each option step by step.

### Step 1: Analyze Simple Diffusion
- **Definition**: Simple diffusion is the process where molecules move from an area of higher concentration to an area of lower concentration without the need for energy or carrier proteins.
- **Characteristics**:
- Moves along the concentration gradient.
- Does not involve carrier proteins, hence does not saturate.
- Not sensitive to inhibitors.
- Energy is not required.

**Conclusion**: Simple diffusion does not satisfy the criteria of being liable to saturation or responding to inhibitors.

### Step 2: Analyze Facilitated Transport
- **Definition**: Facilitated transport (or facilitated diffusion) is the process where molecules move across a membrane via specific transport proteins from an area of higher concentration to an area of lower concentration.
- **Characteristics**:
- Moves along the concentration gradient.
- Requires special transport proteins.
- Sensitive to inhibitors (because it relies on carrier proteins).
- Saturation occurs when all carrier proteins are occupied.
- Under hormonal regulation (hormones can influence the activity of transport proteins).

**Conclusion**: Facilitated transport meets all the criteria provided in the question.

### Step 3: Analyze Active Transport
- **Definition**: Active transport is the process where molecules move against their concentration gradient (from lower concentration to higher concentration) using energy, typically in the form of ATP.
- **Characteristics**:
- Moves against the concentration gradient.
- Requires transport proteins.
- Sensitive to inhibitors.
- Saturation occurs when all carrier proteins are occupied.
- Also under hormonal regulation.

**Conclusion**: While active transport meets many of the criteria, it does not move along the concentration gradient, which disqualifies it from being the correct answer.

### Final Answer
Based on the analysis, the correct answer is **Facilitated Transport** as it is the only option that satisfies all the characteristics mentioned in the question.

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