Cumulative Clicker Practice Test 2026 - Free Practice Questions and Study Guide

The molecule with the highest permeability across the cell membrane is carbon dioxide. This is primarily due to its small size and nonpolar nature. The cell membrane is composed of a phospholipid bilayer, which selectively allows certain molecules to pass through while restricting others. Nonpolar molecules, such as carbon dioxide, can easily diffuse through the lipid bilayer because they do not interact strongly with the polar heads of the phospholipids, allowing for ease of movement across the membrane.

In the case of valine, it is an amino acid and is polar, which makes it less permeable compared to nonpolar molecules. K+ ions are charged particles and require specific ion channels to cross the membrane, significantly restricting their permeability. Ethanol, while it is a small molecule and somewhat polar, does not surpass the permeability of carbon dioxide due to its partial polarity and the presence of a hydroxyl group that interacts with the polar regions of membrane lipids.

Thus, when evaluating these various molecules, carbon dioxide stands out as the most permeable due to its optimal characteristics that facilitate rapid diffusion across the cell membrane.