Diffusion vs. Active Transport: molecules move across cell membranes by two major processes diffusion or active transport.  Diffusion is the movement from a high concentration of molecules to a low concentration of molecules.  Molecules can diffuse across membranes through the phospholipid bilayer or using a special protein.  Either kind of diffusion does not need energy from the cell.  Some special proteins move certain molecules across cell membranes only with the help of cell energy (either directly or indirectly).  Moving molecules with cell energy is called active transport.  The cell energy is the ATP made in the mitochondria.

Created March 22, 2013
Center for Teaching and Learning