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Proteins that transport molecules and change their shape during the process are called carrier proteins. Carrier proteins are involved in both passive (moving molecules from high concentration to low concentration) and active transport (moving molecules from low concentration to high concentration).
Since active transport moves molecules against the concentration gradient, it requires some form of energy to function. This energy can come in different forms: 1. A carrier molecule can be used in which two molecules are moved at the same time, one with its concentration gradient (providing the energy), and one against its concentration gradient. 2. ATP can be used to provide energy to change the shape of the carrier protein. 3. Light can also power carrier proteins, but this is usually only seen in bacterial cells.
Another type of active transport involves the use of transporting vesicles filled with larger molecules that cannot go through the cell membrane or transport proteins. If these molecules are moving out of the cell it is called exocytosis and involves a membrane vesicle moving to the membrane, fusing with the membrane, and releasing its contents to the outside of the cell. If these molecules are moving into the cell it is called endocytosis and involves a membrane vesicle forming around the molecules and bringing them into the cell. The transport of the vesicles inside the cell occurs from motor proteins that attach themselves to the vesicles and then 'walk' along the microtubules that are part of the cell's cytoskeleton. Each 'step' the motor protein takes requires ATP, thus making this a type of active transport.
A carrier protein is used in the process of active transport.
In active transport, energy is needed by a carrier protein to carry a specific substance against a concentration gradient. These carrier proteins are embedded into the phospholipid bilayer of biological membranes, and substances are effectively pumped across its channel(s) through the membrane, to an area of higher substance concentration.
An example of a carrier protein that facilitates active transport is the sodium-potassium pump, located in cell membranes. The concentration of sodium outside of body cells is (typically) higher than the concentration inside the cell. However, sodium concentration must be kept at a low, steady level to maintain a healthy cell. Hence, the sodium-potassium pump (the carrier protein in this example), with an input of energy from ATP being reduced to ADP +Pi, actively pumps sodium against the concentration gradient and through the pump, to the outside of the cell; from low concentration to high concentration.
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