The cell membrane is selectively permeable and is a barrier to the movement of ions and molecules, particularly polar molecules such as glucose and amino acids that are repelled by the non-polar, hydrophobic lipids of the membrane.
Facilitated diffusion is the movment of a polar, charged substance by transport proteins from a region of higher concentration to a region of low concentration (down a concentration gradient) across a membrane with no direct use of energy.
The role of transport proteins is to facilitate diffusion of substances that are insoluble in the phospholipid bilayer by:
1) providing hydrophillic channels in the form of transmembrane channel proteins
2) acting as carrier proteins that carry the substance across the membrane via conformational change
Diffusion can occur through the channel in either direction. Transport proteins are also highly specific.
Channel Proteins have a fixed shape and are transmembrane proteins. They provide a hydrophillic channel across the membrane that is selective for a particular solute. Eg. water channel proteins (aquaporins) found in the cells lining collecting ducts in kidney allow water molecules to flow very quickly from one side of the membrane to the other. Some channel proteins function as gated channels, where a chemical or electrical stimulus will cause them to open or close.
Carrier Proteins are proteins which exists in two alternate conformations. They undergo rapid changes in shape when the molecule being transported binds to it. Thus moving a solute across the membrane as the shape of the protein changes. Eg. entry of glucose molecules into red blood cells.
Factors affecting facilitated diffusion include:
1) Concentration of substances
Transport proteins can take up substances from both sides of the membrane, but direction of flow depends on the relative concentrations of the substrate across the membrane. It also depends on the chance collision between transport protein and substrate.
2) Number of carriers/channel proteins
Increaseing the number of carriers will result in an increased rate of facilitated diffusion.
3) Number of substrate binding sites on the carrer
Increasing the number of binding sites will increase the rate of facilitated diffusion.
Another way to transport substances across the membrane is by active transport. This is the energy (ATP)-consuming transport of molecules or ions across a membrane against a concentration gradient via carrier proteins.
Active transport is a major factor in the ability of a cell to maintain internal concentrations of small molecules that differ from concentrations in its external environment. Movement is usually in one direction only (unlike diffusion which is reversible). Energy is required because in the substance is moved against its natural tendency to diffuse in the opposite direction. The energy supplied is ATP (andosine tri-phosphate) which is manufactured by the process of respiration. Active transport is achieved by carrier proteins situated in the cell membrane which need a supply of energy (ATP) to keep changing shape.
One type of carrier protein in the sodium-potassium pump (as mentioned by the answerer above). Since the cell expends energy when transporting the ions, it is also referred to as an ion pump. This transport system pumps ions against steep concentration gradients. The pump ocscillates between two conformational states in a pumping cycle that translocates 3 Na+ out of the cell for every 2 K+ pumped into the cell. ATP powers the changes in conformation by phosphorylating the transport protein.
Other mechanisms that transport substances into or out of a cell include diffusion, osmosis and bulk transport (endocytosis and exocytosis).
