Active transport

Sodium-Potassium pump, an example of Primary active transport

Active transport is the mediated process of moving particles across biological membrane against a concentration gradient. If the process uses chemical energy, such as from ATP, it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient.

Details

Specialized trans-membrane proteins recognize the substance to be transported and allow it (or, in the case of secondary transport, expend energy on forcing it) to cross the membrane when it otherwise wouldn't, either because it is one to which the lipid bilayer of the membrane is impermeable or because it is moved against the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, uses the chemical energy of, usually, ATP. The other cases, which usually derive their energy through exploitation of an electrochemical gradient, are known as secondary active transport and involve pore-forming proteins which form channels through the cell membrane.

Sometimes - but not always - one substance is transported in one direction at the same time as another substance is being cotransported in the other direction. Often, the mechanism is named for both substances, as in the sodium/potassium pump.

When particles are being moved from areas of low concentration to areas of high concentration (i.e. against the concentration gradient) then specific carrier proteins in the membrane are required to move these particles. The carrier proteins bind to specific molecules (eg. glucose) and transport them into the cell where they are released. Energy is required for this process so this is known as Active Transport. Examples: sodium is transported out of the cell and potassium into the cell by the sodium/potassium pump, a form of active transport. Active transport is a process which takes place in the internal lining of the small intestine.

Plants need to absorb mineral salts from the soil, but these salts are in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the concentration gradient.

Importance for living systems

Examples of plasma membranes actively transporting solutes include the following:

1. The absorption of digested food by the walls of the small intestines of vertebrates.
2. Selective reabsorption in the kidneys of animals with backbones.
3. The distrubtion of mineral salts from the soil by plant root hairs.

ABC pumps

ABC class pumps transport small molecules across membranes. They are also called the ABC superfamily. They consist of two transmembrane domains, and two ATP binding domains. ABC pumps are involved in the transport of small molecules, phospholipids, and lipophilic drugs in mammalian cells. In bacteria they transport amino acids, sugars, and peptides.^[1]

Examples

• Water, ethanol, and chloroform are simple molecules that do not require active transport to cross a membrane.
• Metal ions, such as Na⁺, K⁺, Mg⁺⁺, or Ca⁺⁺, require ion pumps or ion channels to cross membranes and distribute through the body
• In the epithelial cells of the stomach, gastric acid is produced by hydrogen potassium ATPase, a proton pump

See also

• Ion channel
• Protein sorting
• Translocon

References