Movement down a concentration gradient that requires transport proteins is known as what?

Movement down a concentration gradient that requires transport proteins is referred to as facilitated diffusion. This process allows specific molecules or ions to cross the cell membrane with the help of transport proteins, without requiring ATP (energy). Unlike simple diffusion, which allows small, nonpolar molecules to pass through the lipid bilayer freely, facilitated diffusion is necessary for larger or polar substances that cannot directly diffuse through the membrane.

Facilitated diffusion is characterized by its reliance on the concentration gradient: substances move from areas of higher concentration to lower concentration to reach equilibrium. This movement is selective and can be regulated by various factors, including the availability of the transport proteins and the specific binding affinity for the transported substance, ensuring that essential molecules such as glucose and ions are efficiently taken up by cells.

In contrast, active transport relies on energy input to move substances against their concentration gradient. Simple diffusion occurs without assistance from proteins and is mainly for small, nonpolar molecules. Osmosis specifically refers to the movement of water across a semipermeable membrane, also following its concentration gradient, but only for water molecules. Thus, facilitated diffusion is the precise term for this transport mechanism involving transport proteins moving substances down their concentration gradients.