What is meant by the term 'passive movement' in the context of facilitated diffusion?

In the context of facilitated diffusion, the term 'passive movement' refers to the process by which substances move across a cell membrane without requiring energy input from the cell. This type of movement occurs along a concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration, which is a spontaneous process.

Facilitated diffusion specifically involves the use of membrane proteins, such as channel or carrier proteins, to help transport larger or polar molecules that cannot freely diffuse through the lipid bilayer of the membrane. Despite the involvement of these proteins, the movement remains passive because it does not necessitate energy from adenosine triphosphate (ATP) or other energy sources. Instead, it relies on the inherent kinetic energy of the molecules being transported.

Movement that requires energy input or occurs against a concentration gradient would not be considered passive and instead classifies as active transport. Furthermore, while facilitated diffusion does involve membrane proteins, the requirement for such proteins alone does not define the nature of the movement as passive or active.