Question

the phospholipid bilayer of the plasma membrane is least permeable to which substances?
a. starch molecules, due to their size
b. oxygen molecules, due to their solubility
c. cholesterol molecules, due to their polarity
d. carbon dioxide molecules, due to their charge

Explanation:

1. Analyze Option A: Starch is a large polysaccharide (polymer of glucose). The phospholipid bilayer has a hydrophobic interior and small pores. Large molecules like starch (too big to pass through the small spaces between phospholipids) have very low permeability.
2. Analyze Option B: Oxygen ($O_2$) is non - polar and small. Non - polar molecules can dissolve in the hydrophobic interior of the phospholipid bilayer and diffuse easily, so the reasoning about solubility here is wrong (oxygen is actually permeable due to its non - polar nature and small size).
3. Analyze Option C: Cholesterol is non - polar (not polar as stated in the option) and can integrate into the phospholipid bilayer, so it has relatively good permeability.
4. Analyze Option D: Carbon dioxide ($CO_2$) is non - polar and uncharged. It can diffuse through the phospholipid bilayer easily, and the statement about its charge is incorrect as $CO_2$ is uncharged.

Answer:

A. starch molecules, due to their size