Which of the following is true of the cell membrane?
a. It is freely permeable to all electrolytes.
b. It is freely permeable to water and all solutes.
c. It is freely permeable only to fats.
d. It is freely permeable to water but not to solutes.
The Correct Answer and Explanation is:
The correct answer is:
d. It is freely permeable to water but not to solutes.
Explanation:
The cell membrane, also called the plasma membrane, is a vital structure that surrounds the cell, providing a boundary between the intracellular environment (inside the cell) and the extracellular environment (outside the cell). Its primary role is to control what enters and leaves the cell, thus maintaining homeostasis.
Permeability refers to the ability of substances to cross the membrane. The cell membrane is selectively permeable, meaning it allows some substances to pass through freely while restricting others.
- Electrolytes and solutes: Electrolytes (ions such as sodium, potassium, calcium, chloride) and many other solutes cannot freely cross the membrane because the membrane is composed mainly of a phospholipid bilayer. The bilayer has a hydrophobic (water-repelling) interior that prevents charged particles and polar molecules from easily passing through. Instead, these molecules require specific transport proteins (channels, carriers, pumps) to enter or exit the cell, which is a controlled process.
- Water: Although water is a polar molecule, it is small enough to move through the membrane via osmosis. The cell membrane allows water to pass more freely, mainly through special protein channels called aquaporins. This free permeability to water helps regulate the cell’s internal fluid balance by balancing water movement based on solute concentration gradients.
- Fats (lipids): Fat-soluble substances (nonpolar molecules like oxygen, carbon dioxide, and steroid hormones) can diffuse through the lipid bilayer because they dissolve in the hydrophobic interior of the membrane. However, saying the membrane is freely permeable only to fats is inaccurate since water also passes freely.
- Not freely permeable to all solutes: The membrane restricts many solutes, especially charged ions and large molecules, preventing uncontrolled influx or loss that would disrupt cellular function.
Summary:
- The cell membrane is freely permeable to water but not to all solutes.
- It selectively allows substances based on size, polarity, and charge.
- Transport proteins facilitate the movement of ions and larger molecules.
- This selective permeability is essential for maintaining cellular homeostasis.
Thus, option d correctly describes the permeability characteristics of the cell membrane.
