When does a concentration gradient exist?

Options

A. When solute concentrations are low
B. When membranes rupture
C. When solute concentration is high
D. When solute concentration differ-on either side of a membrane

Answer

### Step-by-Step Solution:

1. **Understanding Concentration Gradient**: A concentration gradient refers to the difference in the concentration of a substance (solute) between two areas, such as inside and outside a cell. It is a key concept in biology, particularly in the context of transport mechanisms.

2. **Identifying Conditions for Concentration Gradient**: A concentration gradient exists when there is a difference in solute concentration across a membrane. This can lead to movement of substances from an area of higher concentration to an area of lower concentration.

3. **Evaluating the Options**:
- **Option 1: When solute concentration is low**: This does not define a concentration gradient, as it does not indicate a difference between two areas.
- **Option 2: When membrane ruptures**: This scenario refers to cell lysis, not a concentration gradient.
- **Option 3: When solute concentration is high**: Similar to option 1, this alone does not indicate a gradient without a comparison to another area.
- **Option 4: When solute concentration differs on either side of a membrane**: This is the correct definition of a concentration gradient, as it indicates a difference that can drive movement.

4. **Conclusion**: A concentration gradient exists specifically when there is a difference in solute concentration on either side of a membrane, allowing for the movement of substances.

### Final Answer:
A concentration gradient exists when solute concentration differs on either side of a membrane.

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