Which type of molecules is likely to diffuse rapidly across a plasma membrane made only of phospholipids?

The correct answer is oxygen, as it is a small, nonpolar molecule that can easily diffuse through the lipid bilayer of the plasma membrane. Phospholipids create a hydrophobic core in the membrane, which primarily allows nonpolar substances, such as oxygen, to pass through without the need for transport proteins.

Oxygen's small size and nonpolarity enable it to move quickly and freely across the membrane, facilitating efficient gas exchange necessary for cellular respiration. In contrast, molecules like glucose, which is larger and polar, require specific transport mechanisms to cross the membrane, while water, despite being a small polar molecule, also utilizes specialized channels known as aquaporins for rapid transport. Calcium ions, being charged, cannot diffuse through the hydrophobic core and require ion channels to enter or leave the cell. Thus, oxygen stands out as the molecule most capable of rapid diffusion in this context.