What describes carrier mediated diffusion?

Carrier mediated diffusion refers to the process where specific substances are transported across a cell membrane through specialized transmembrane protein carriers. These carriers are essential because they facilitate the movement of substances that cannot freely diffuse through the lipid bilayer due to their size, charge, or polarity. This mechanism is distinct because it relies on specific binding sites on the carrier proteins for the substrates being transported, ensuring that only appropriate substances are allowed through.

While it is true that carrier mediated diffusion generally occurs along a concentration gradient (from high to low concentration), the primary defining feature is the involvement of a transmembrane protein that specifically binds to the molecule being transported. This selective binding is what characterizes the process as "carrier-mediated."

The other choices do touch on aspects of diffusion; for instance, it does occur without the direct use of energy (making it passive) and is often influenced by concentration gradients. However, these aspects alone do not define carrier mediated diffusion as precisely as the presence of specific transmembrane protein carriers does. The carrier proteins ensure selectivity and facilitate the transport of various substances that would otherwise not be able to permeate the membrane effectively.