What are the two types of carrier-mediated transport?

• A. Active transport and facilitated diffusion
• B. Passive diffusion and osmosis
• C. Endocytosis and exocytosis
• D. Bulk flow and filtration

Answer: (A)

Carrier-mediated transport relies on specific membrane proteins that bind and shuttle solutes across the membrane. There are two forms: facilitated diffusion and active transport.

In facilitated diffusion, the carrier helps move a solute down its concentration gradient without using cellular energy. The process is saturable—once all carrier proteins are occupied, increasing solute concentration doesn’t boost the rate—and it’s specific for certain substrates, which explains why some molecules use particular transporters (for example, glucose via GLUT transporters).

In active transport, energy is expended to move solutes against their gradient, often using ATP or a coupled ion gradient. This also depends on carrier proteins, is saturable, and is essential for maintaining gradients and moving nutrients into the cell when they’re scarce outside.

This carrier-mediated view contrasts with passive diffusion or osmosis, which don’t involve carriers, as well as with vesicular processes like endocytosis/exocytosis and with bulk flow or filtration, which are not carrier-driven.