In ________, carrier molecules move a substance against (up) its concentration gradient.

A) simple diffusion
B) exocytosis
C) facilitated diffusion
D) active transport
E) endocytosis

Active Transport

The passage of molecules over a cell's membrane, moving from an area of decreased concentration to an area of increased concentration, an endeavor that utilizes energy.

Concentration Gradient

A difference in the concentration of a substance across a distance, which can drive diffusion of particles from an area of high concentration to an area of lower concentration.

Carrier Molecules

Proteins that assist in moving substances through a cell membrane by altering their form or attaching to the substance.