1. Definition
Active transport is a transport process at the biomembrane that takes place under energy consumption. Contrary to the concentration or electrochemical gradient, molecules and ions can reach the inside or outside of the cell.
2. Classification
There are three different types of active transport:
2.1. Primary active transport
In primary active transport, chemical binding energy is used to transport substances through the biomembrane. This kind of transport only functions when an ATP molecule binds, which then splits, to form ADP (adenosindiphosphate) and inorganic phosphate. The transmembrane proteins using this kind of transport are called „ATPases“.
A typical example of primarily active transport is the sodium-potassium pump.
2.2. Secondary active transport (Cotransport)
Secondary active transport uses the energy of an existing concentration gradient for its transport process. It was previously built up by a primary active transport, for example by the sodium-potassium pump. It is not directly ATP-dependent. The energy is ultimately generated by the breakdown of the concentration or charge gradient.
A typical example of secondary active transport is the glucose-symporter SGLT1.
2.3. Tertiary active transport
Tertiary active transport also uses energy that is generated by the breakdown of a concentration or charge gradient. The difference is that the transport is based on the resulting concentration gradient of the secondary active transport and not on that of the primary active transport.
A typical example of tertiary active transport is the peptide transporter 1 of the intestinal enterocytes for the absorption of di- and tripeptides.
3. Directions of Transport
There are three different groups of transport directions:
3.1. Uniport
Proteins that have a uniport can bind a maximum of one molecule at a time. Uniport only allows transport in one direction.
Example: GLUT1
3.2. Symport
Proteins that have a symport can bind a maximum of two molecules at a time. Symport also only allows transport in one direction.
Example: SGLT1
3.3. Antiport
Proteins that have a antiport can bind a maximum of two molecules at a time. Antiport only allows transport in opposite directions.
Example: Sodium-Calcium-Antiport