Which substance moves more easily across a phospholipid membrane?

Fatty acids move more easily across a phospholipid membrane due to their hydrophobic nature. The structure of phospholipid membranes consists of a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. Since fatty acids have long hydrocarbon chains that are non-polar, they can readily dissolve in the lipid bilayer of the membrane. This characteristic allows them to diffuse through the membrane without the need for transport proteins.

In contrast, substances like glucose, which is polar and fairly large, require specific transport mechanisms, such as facilitated diffusion, to cross the membrane. Calcium ions are charged particles and also need channels or transporters to assist their movement across the membrane due to their inability to pass through the lipid bilayer freely. Water, while it is a small molecule, also utilizes specialized channels called aquaporins to facilitate its transport, implying that it does not easily diffuse across the membrane compared to fatty acids.