Answer
ATP supplies the energy necessary for operation of the sodium-potassium pump. This pump which comprises certain intramembranous proteins, carries out two important functions; 1. It expels sodium ions(Na+)from the cytosol. 2. It brings potassium ions (K+) from the extracellular fluid(ECF) into the cytosol.
Both these processes are part of a primary active transport mechanism. Therefore, they require energy. This energy is made available by the hydrolysis of the molecule ATP.
Work Step by Step
All plasma membranes have many intramembranous molecules that serve as carriers. The carriers of the sodium-potassium pump are very important because they maintain the concentration gradients of sodium and potassium ions between the cytosol, and the extracellular fluid (ECF).The maintenance of a low intracellular concentration of sodium(Na+) ions is vital for cell viability. Indeed, Therefore, it is essential for the life of the human organism.In humans, the optimal concentration is close to 12mM.
A brief description of the working of the pump will explain the role of ATP:
1. When sodium concentration is too high in the cytosol, three sodium ions bind to each pump
carrier molecule.This causes part of the carrier molecule to act like an ATPase enzyme.
2. The ATPase naturally hydrolyses available ATP molecules to give ADP+P(Phosphate)
3. The phosphate group attaches to the carrier molecules that have the bound Na ions.
4, This changes the shape of the carrier protein molecule, and it releases the bound sodium ions to
the extracellular fluid. The changed shape of the pump protein also enables it to bind two
potassium ions per carrier molecule . It picks these up from the ECF.
5. The binding of the potassium ions to the carrier causes it to release the phosphate group(P),
which was picked up during ATP hydrolysis.
6. The loss of the phosphate group causes the carrier molecule to resume its original conformation.
.7. It releases the bound K+ ions to the cytosol.
8. The pump is now ready to pick up three more Na ions per carrier molecule. But this requires energy which has to be supplied by ATP hydrolysis.
Relevant concentration gradients:
[Na +]
Cytosol 12mM
ECF
145 mM
[K+]
cytosol 139 mM
ECF 4 mM
The carrier proteins are located in the plasma membrane; therefore, they have access to both cytosol and ECF. Forty percent of cellular ATP is used in this form of primary active transport.
