Active Transport

Active transport is defined by two things happening together: molecules move against (uphill on) their concentration gradient, and this uphill movement is paid for with cellular energy from ATP made during respiration. This is a fundamentally different energy source from passive transport, which simply harnesses the existing kinetic energy of molecules moving down a gradient on their own. The practical value of active transport is control - purely passive diffusion has no way of stopping harmful substances from entering a cell along a favourable gradient, whereas the selective permeability that active transport provides lets the cell decide precisely which molecules and ions to admit or expel, and in what quantity. The same channel and carrier proteins already introduced under facilitated diffusion can, when supplied with ATP (or in some systems, light energy), be run in this …