USA Biology Olympiad (USABO) Practice Exam

Which of the following molecules can readily cross the lipid bilayer of a cell?

1. Proteins

2. Starch

3. Steroid hormones

4. Nucleic acids

The correct answer is: Steroid hormones

The ability of a molecule to cross the lipid bilayer of a cell membrane depends largely on its size, polarity, and solubility in lipids. Lipid bilayers are primarily composed of phospholipids, which create a hydrophobic (water-repelling) environment in the interior of the membrane. Steroid hormones, which are derived from cholesterol, are lipid-soluble and nonpolar. This lipid solubility allows them to diffuse through the hydrophobic core of the lipid bilayer without the need for transport proteins. Once they cross the membrane, steroid hormones can bind to intracellular receptors and elicit various physiological responses. This property of steroid hormones makes them significantly more capable of crossing the cell membrane compared to larger or polar molecules. In contrast, proteins, starch, and nucleic acids are generally unable to cross the lipid bilayer easily due to their size and polarity. Proteins are large macromolecules that cannot diffuse through the lipid bilayer without assistance from specialized transport mechanisms. Starch is a complex carbohydrate that is too large to penetrate the hydrophobic layer of the membrane. Nucleic acids, being both large and highly polar, also require transport proteins to enter or exit the cell. Therefore, the molecule that can readily cross