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. Active transport/ Active Uptake can be:
Primary: Uses the chemical energy from ATP or other sources.
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 intake of mineral salts from the soil by plant root hairs.