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

Options

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

Answer

To determine the correct statement regarding transmembrane proteins in the lipid bilayer of the plasma membrane, we can analyze the options provided. Here’s a step-by-step solution:

### Step 1: Understand the Structure of the Plasma Membrane
The plasma membrane consists of a lipid bilayer with embedded proteins. The lipid bilayer has polar heads facing the aqueous environment and non-polar tails facing inward, creating a hydrophobic core.

**Hint:** Remember that the plasma membrane is made up of lipids and proteins, with the proteins either embedded partially or fully.

### Step 2: Identify the Role of Transmembrane Proteins
Transmembrane proteins span the entire lipid bilayer and can function as channels or transporters for ions and molecules, facilitating their movement across the membrane.

**Hint:** Focus on how transmembrane proteins interact with the lipid bilayer and their function in transport.

### Step 3: Evaluate the Options
1. **They are absent in animal cells.**
- This statement is incorrect because transmembrane proteins are present in animal cells.

2. **They act as channel proteins.**
- This statement is correct. Transmembrane proteins can function as channel proteins, allowing specific ions and molecules to pass through the membrane.

3. **They are absent in plant cells.**
- This statement is incorrect. Transmembrane proteins are also present in plant cells.

4. **They are only externally located.**
- This statement is incorrect. Transmembrane proteins are embedded within the membrane, not just located externally.

### Step 4: Conclusion
The correct answer is that transmembrane proteins act as channel proteins, which corresponds to option 2.

**Final Answer:** The correct statement regarding transmembrane proteins in the lipid bilayer of the plasma membrane is that they act as channel proteins.