Active transport is the process of moving molecules or ions against a concentration gradient, which requires energy to be expended. This process is important for many biological functions, such as the maintenance of ion gradients, the uptake of nutrients, and the removal of waste products. Active transport is facilitated by a number of specialized membrane proteins, including ion pumps, ion exchangers, and ATPases.
The plasma membrane, which separates the extracellular environment from the cytoplasm of the cell, is a selectively permeable barrier. The lipid bilayer of the membrane is relatively impermeable to most molecules, and thus the transport of substances across the membrane is primarily facilitated by membrane proteins. Active transport across the plasma membrane can be either primary or secondary.
Primary Active Transport:-
Primary active transport involves the direct use of ATP to move a substance across the membrane. The most well-known example of primary active transport is the Na+/K+ ATPase pump. This protein is responsible for maintaining the concentration gradients of sodium and potassium ions across the plasma membrane. The pump uses the energy of ATP hydrolysis to transport three Na+ ions out of the cell and two K+ ions into the cell.
The Na+/K+ ATPase pump consists of two subunits: the alpha subunit, which contains the ATP binding site, and the beta subunit, which anchors the pump to the membrane. The alpha subunit is composed of ten transmembrane helices and two large cytoplasmic domains. The transport cycle of the pump involves the binding of ATP to the cytoplasmic domains, the phosphorylation of the pump by ATP, the release of the phosphate group, and the release of the transported ions on the other side of the membrane.
Secondary Active Transport:-
Secondary active transport involves the use of a concentration gradient established by primary active transport to drive the movement of another molecule or ion across the membrane. This process is also known as co-transport or symport. The most well-known example of secondary active transport is the Na+/glucose symporter. This protein uses the Na+ gradient established by the Na+/K+ ATPase pump to transport glucose into the cell.
The Na+/glucose symporter consists of twelve transmembrane helices and a large cytoplasmic domain. The transport cycle of the symporter involves the binding of Na+ ions on the extracellular side of the membrane, the conformational change of the protein, the binding of glucose on the same side of the membrane, and the release of the transported molecules on the cytoplasmic side of the membrane.
