CONCLUSION

There are two types of transport across a cell's plasma membrane: active and passive. Any time energy is required for movement across the plasma membrane, active transport has occurred. When such energy does not need to be expended, passive transport is in operation. Passive transport can include either simple diffusion or facilitated diffusion. Simple diffusion is diffusion that does not require a protein channel. Facilitated diffusion is diffusion that requires a protein channel. In facilitated diffusion, transport proteins act as channels for hydrophilic substances that, due to their size and electrical charge, cannot diffuse through the plasma membrane. In active transport, molecules again move through a transport protein, but now energy must be expended to move them against their concentration gradient. The cell's solution to moving solutes against their concentration gradients is pumps. Many kinds of pumps are in operation in active transport, but each is specific for one or perhaps two substances. The energy source for such transport is ATP (adenosine triphosphate).