Facilitated diffusion primarily involves which of the following?

Facilitated diffusion is a process in cellular biology where molecules move across the cell membrane through specific proteins, without the expenditure of energy. This type of passive transport relies on the presence of protein channels and carriers that help larger or polar molecules that cannot easily cross the lipid bilayer of the membrane.

In facilitated diffusion, these protein channels or carrier proteins provide an alternative pathway for substances, allowing them to move from an area of higher concentration to one of lower concentration. This process is essential for the transport of glucose, amino acids, and ions into and out of cells, helping maintain cellular homeostasis.

While the other options involve transport mechanisms, they do not accurately describe facilitated diffusion. Active transport mechanisms require energy to move substances against their concentration gradients, which is not a characteristic of facilitated diffusion. Simple diffusion refers to the passive movement of small molecules directly through the membrane without assistance from proteins, and ion pumps are specialized proteins that actively move ions across the membrane, contrary to the passive nature of facilitated diffusion. Thus, the role of protein channels and carriers is crucial for facilitated diffusion, making that the correct answer.