The transport proteins integrated into the cell membrane are often highly selective about the chemicals they allow to cross. Some of these proteins can move materials across the membrane only when assisted by the concentration gradient, a type of carrier-assisted transport known as facilitated diffusion. Both diffusion and facilitated diffusion are driven by the potential energy differences of a concentration gradient. Glucose enters most cells by facilitated diffusion. There seem to be a limiting number of glucose-transporting proteins. The rapid breakdown of glucose in the cell (a process known as glycolysis) maintains the concentration gradient. When the external concentration of glucose increases, however, the glucose transport does not exceed a certain rate, suggesting the limitation on transport.
In the case of active transport, the proteins are having to move against the concentration gradient. For example the sodium-potassium pump in nerve cells. Na+ is maintained at low concentrations inside the cell and K+ is at higher concentrations. The reverse is the case on the outside of the cell. When a nerve message is propagated, the ions pass across the membrane, thus sending the message. After the message has passed, the ions must be actively transported back to their “starting positions” across the membrane. This is analogous to setting up 100 dominoes and then tipping over the first one. To reset them you must pick each one up, again at an energy cost. Up to one-third of the ATP used by a resting animal is used to reset the Na-K pump.
The Essay on Cellular Transport Cell Water Membrane
... through the cell membrane. In animal cells, active transport assists in glucose transport, while in plant cells, it regulates turgor. In both cells, it ... water concentration to a low water concentration through a semi-permeable cell membrane without the use of energy. This cell membrane is ... phosphate group), a phosphate group binds to the protein in a process called phosphorylation, and causes a ...
