Which compound is capable of passively crossing a cell membrane?

Water is indeed capable of passively crossing a cell membrane, primarily due to its small size and polar nature. Cell membranes are composed of a lipid bilayer that serves as a barrier to most substances. However, water molecules can move through the phospholipid bilayer via two primary methods: simple diffusion and facilitated diffusion through specialized protein channels known as aquaporins.

Because of the concentration gradient, water tends to move from areas of higher concentration to areas of lower concentration, assisting in maintaining osmotic balance within and outside the cell. This passive transport does not require energy, making water uniquely capable of traversing the membrane compared to larger, polar molecules like glucose, or charged molecules like amino acids, which generally require transport proteins to assist their movement across the lipid bilayer. Thus, the ability of water to cross the cell membrane without energy expenditure underscores its vital role in cellular functions.