Membrane Structure and Function - 8 Osmosis is the movement (diffusion) of water across a differentially permeable membrane in response to solute (dissolved substances) gradients that are maintained by the membrane. The "force" to move water through membranes is called osmotic pressure. It is comparable to physical pressure. Osmotic pressure may be resisted by the cell membrane (if it is strong enough) or the cell wall, in organisms that have cell walls. The wall or membrane exerts a mechanical pressure. The difference in the osmotic pressure and the wall or membrane pressure is known as water potential. Water potential is very important in a number of processes. For the process of osmosis: • The membrane is permeable to water. • The membrane is not permeable to the solute(s), and the solutes will be substances which can "bind" to water, affecting the free flow of water. • A water gradient exists, in part because dissolved substances always lower the concentration of water in a solution. (Pure water would have the highest
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