Which one of the following is correct for transmembrane proteins in lipid bilayer

Options

A. They are absent in animal cells.
B. They act as channel proteins
C. They are absent in plant cells.
D. They are only externally located.

Answer

To solve the question regarding the characteristics of transmembrane proteins in the lipid bilayer, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Transmembrane Proteins**:
- Transmembrane proteins, also known as intrinsic proteins, span across the lipid bilayer of the plasma membrane. They have regions that are exposed to both the extracellular environment and the cytoplasm.

**Hint**: Remember that transmembrane proteins are integral parts of the membrane, not just on one side.

2. **Identifying Their Functions**:
- These proteins often function as channel proteins, allowing specific ions and molecules to pass through the membrane. This is crucial for processes like active transport and cell signaling.

**Hint**: Think about how cells communicate and transport substances; channel proteins play a key role in this.

3. **Evaluating the Options**:
- **Option A**: They are absent in animal cells.
- This is incorrect because transmembrane proteins are present in both animal and plant cells.
- **Option B**: They act as channel proteins.
- This is correct, as transmembrane proteins facilitate the movement of substances across the membrane.
- **Option C**: They are absent in plant cells.
- This is incorrect; transmembrane proteins are found in plant cells as well.
- **Option D**: They are only externally located.
- This is incorrect because transmembrane proteins extend across the membrane, having parts both inside and outside the cell.

4. **Conclusion**:
- After evaluating all the options, the correct answer is **Option B**: They act as channel proteins.

### Final Answer:
The correct answer is **B: They act as channel proteins**.