What type of transport uses specific proteins to aid the movement of substances across the cell membrane?

The correct answer is facilitated diffusion because this process specifically involves the use of transmembrane proteins to assist certain molecules in crossing the cell membrane. These proteins act as channels or carriers that facilitate the movement of substances that cannot easily pass through the lipid bilayer due to their size, polarity, or charge.

For instance, glucose and various ions are classic examples of substances that rely on facilitated diffusion for entry into cells. This method does not require energy (ATP) because it moves molecules along their concentration gradient, from areas of higher concentration to areas of lower concentration.

In contrast, active transport involves the movement of substances against their concentration gradient, requiring energy input. Simple diffusion, on the other hand, does not utilize any proteins and allows small or nonpolar molecules to move freely across the membrane. Phagocytosis refers to a form of endocytosis where the cell engulfs large particles or even other cells, which is a different mechanism altogether unrelated to the specific protein assistance seen in facilitated diffusion.