Questions: Which of the following is correct about biological membranes? Choose one: A. Molecules freely pass through the membrane. B. Biological membranes are made up of a lipid monolayer. C. Phospholipid head groups are directed toward the aqueous environment. D. Phospholipid tails are directed toward the aqueous environment.

The answer is C: Phospholipid head groups are directed toward the aqueous environment.

Explanation for each option:

A. Molecules freely pass through the membrane.

• This is incorrect. Biological membranes are selectively permeable, meaning that not all molecules can freely pass through. Small nonpolar molecules can pass through easily, but larger or charged molecules require specific transport mechanisms.

B. Biological membranes are made up of a lipid monolayer.

• This is incorrect. Biological membranes are composed of a lipid bilayer, not a monolayer. The bilayer structure is crucial for the membrane's function and integrity.

C. Phospholipid head groups are directed toward the aqueous environment.

• This is correct. In a biological membrane, the hydrophilic (water-attracting) phospholipid head groups face the aqueous environments both inside and outside the cell, while the hydrophobic (water-repelling) tails face inward, away from the water.

D. Phospholipid tails are directed toward the aqueous environment.

• This is incorrect. The hydrophobic tails of phospholipids are oriented away from the aqueous environment, facing each other inside the bilayer, which helps to form a stable barrier between the cell and its surroundings.

In summary, the correct statement about biological membranes is that phospholipid head groups are directed toward the aqueous environment.