Select the correct statement from the following regarding cell membrane.

Options

A. Lipids are arranged in bilayer with polar heads towards the inner part
B. Fluid mosaic model of cell membrane was proposed by singer and Nicolson
C. `Na^(+) and K^(+)` ions move across cell membrane by passive transport
D. Proteins make up 60 to 70% of the cell membrane

Answer

**Step-by-Step Solution:**

1. **Understanding the Structure of the Cell Membrane:**
- The cell membrane is primarily composed of a lipid bilayer, which consists of phospholipids. Each phospholipid has a hydrophilic (polar) head and two hydrophobic (non-polar) tails.
- In a lipid bilayer, the polar heads face outward towards the aqueous environment, while the non-polar tails face inward, away from the water.

**Hint:** Remember the orientation of the lipid bilayer: heads out, tails in.

2. **Evaluating the Statements:**
- **Statement 1:** "Lipids are arranged in a bilayer with polar heads towards the inner part."
- This statement is incorrect because the polar heads face outward, not inward.

- **Statement 2:** "The fluid mosaic model of the cell membrane was proposed by Singer and Nicholson."
- This statement is correct. The fluid mosaic model describes the cell membrane as a mosaic of various proteins floating in or on the fluid lipid bilayer. This model was proposed by Singer and Nicholson in 1972.

**Hint:** The fluid mosaic model is key to understanding membrane dynamics.

3. **Analyzing Other Statements:**
- **Statement 3:** "Sodium and potassium move across the cell membrane by passive transport."
- This statement is incorrect. Sodium and potassium ions are charged and typically move via facilitated diffusion through specific carrier proteins, not by passive transport.

- **Statement 4:** "Proteins make up 60-70% of the cell membrane."
- This statement is also incorrect. Proteins make up approximately 52% of the cell membrane, while lipids account for about 40%.

**Hint:** Check the percentages of proteins and lipids in the cell membrane.

4. **Conclusion:**
- After evaluating all the statements, the only correct statement regarding the cell membrane is Statement 2, which pertains to the fluid mosaic model proposed by Singer and Nicholson.

**Final Answer:** The correct statement is that the fluid mosaic model of the cell membrane was proposed by Singer and Nicholson.