Which of the following substances can diffuse through cell membranes?

The ability of a substance to diffuse through cell membranes is largely dependent on its size and polarity. Glucose is a small, polar molecule that can pass through cell membranes with the help of specific transport proteins, as it is not small enough to freely diffuse through the lipid bilayer. While it is a sugar and does require assistance, it is correctly identified here as a substance that can traverse cell membranes via facilitated diffusion.

In contrast, starch is a large polysaccharide and does not efficiently pass through membranes because of its size. Proteins, even smaller peptides, are typically too large to cross cell membranes without specific transport mechanisms. Cellulose, a structural polysaccharide found in plant cell walls, is also too large to enter cells by passive diffusion. Therefore, glucose stands out as the substance that can move through cell membranes, albeit through specific transport channels rather than by simple diffusion.