Gradients A. A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions. Molecules constantly collide and tend to move according to existing concentration gradients. Gradients in temperature, electric charge, and pressure, can influence movements. B. Passive Transport 1. In passive transport, solutes pass through the cell membrane with assistance from transport proteins in accordance with the concentration gradient.
Transport proteins change shape to move substances into and out of the cell.
C. Active Transport 1. The sodium-potassium pump is a major cotransport system in that it can set up concentration gradients that can in turn drive other transport activities. Movement of Water Across Membranes A. Osmosis 1. Bulk flow is the mass movement of one or more substances in response to pressure, gravity, of some other external force, like the flow of blood in the circulatory system. Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.