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Osmosis and Diffusion Cells can not escape the laws of physics. In all systems, disequilibrium causes movement. This is true whether we are talking about a car rolling down a hill or substances moving in and out of the cell. Cells are greater than 70% water; the substances that leave and enter the cell are dissolved in water and are called solutes, the water is the solvent and together they form a solution. Concentration is the amount of solute dissolved in a particular solvent. The barrier between the external world and the interior of the cell is called the plasma membrane (a phospholipid bilayer with embedded proteins); it is selectively permeable, allowing some things through the membrane while inhibiting others. Diffusion is the movement of molecules from a region of high concentration to a region of low concentration, this process is driven by the random motion of the molecules themselves. Some solute can move through the plasma membrane by diffusion although others are impeded. Osmosis is a special form of diffusion and is defined as the movement of water across a membrane. A difference in concentration in two regions forms a concentration gradient; under these conditions molecules will move about until they hav