Cell membrane is selective permeable. This means that it

Options

A. allows all materials to pass through
B. allows only water to pass through
C. allows only certain materials to pass through
D. allows only ions to pass through.

Explanation

### Step-by-Step Solution:

1. **Understanding Selective Permeability**: The term "selectively permeable" refers to the ability of the cell membrane to allow certain substances to pass through while restricting others. This is crucial for maintaining homeostasis within the cell.

2. **Examples of Selective Permeability**: The cell membrane does not allow all materials to pass through. For instance, it selectively permits water and certain ions (like sodium and potassium) to enter or exit the cell, while larger molecules or harmful substances are generally blocked.

3. **Mechanisms of Transport**: There are various mechanisms by which substances can cross the cell membrane:
- **Passive Transport**: This does not require energy and includes processes like diffusion and osmosis, where substances move from an area of higher concentration to an area of lower concentration.
- **Facilitated Diffusion**: This is a type of passive transport that requires specific carrier proteins to help certain molecules cross the membrane.
- **Active Transport**: This process requires energy (usually in the form of ATP) to move substances against their concentration gradient, meaning from lower to higher concentration.

4. **Conclusion**: Therefore, when we say that the cell membrane is selectively permeable, we mean that it allows only certain materials (like specific ions, water, or small molecules) to pass through while blocking others.

### Final Answer:
The cell membrane is selectively permeable, meaning it allows only certain materials to pass through it.

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