4.1.2??Transport of molecules across cell membranes

? Transport proteins play a critical part in solute movement because membranes constitute a barrier to free diffusion of molecules. If membranes allowed rapid diffusion of both ions and water, gradients of ions would be slight and osmotic pressure could only be achieved with enormous energy consumption. Instead, some molecules, such as water and gases, move rapidly across membranes. Unrestricted movement of water relative to solutes is the basis of osmosis, and in plants the generation of P. Similarly, if the movement of CO2 and O2 to and from the sites of photosynthesis and respiration were substantially impeded, steep concentration gradients would reduce the ef?ciency of these vital processes. The principles of diffusion and selectivity, which are used to describe differential rates of molecular movement, provide a physical rationale for osmosis.