Active transport uses specific transport proteins, called pumps, which use metabolic energy (ATP) to move ions or molecules against their concentration gradient. For example, in both vertebrates and invertebrates, the concentration of sodium ion is about 10 to 20 times higher in the blood than within the cell. The concentration of the potassium ion is the reverse, generally 20 to 40 times higher inside the cell. Such a low sodium concentration inside the cell is maintained by the sodium-potassium pump. There are different types of pumps for the different types of ions or molecules such as calcium pump, proton pump, etc. Examples of Active Transport
Na+- K+- ATPase is a tetramer of 2subunits. Na+ and K+ binding sites are present on both the subunits. It is an ion pump or cation exchange pump which is driven by energy expenditure of one ATP molecule to export three Na+ ions outside the cell in exchange of the import of two K+ ions inside the cell. Electrical organs of eels are found to be very rich in this enzyme or pump. N+- K+- ATPase is a transmembrane protein which is a dimer having two subunits, one smaller unit which is a glycoprotein of 50,000 Daltons M.W. having an unknown function; and another larger unit having 1, 20,000 Daltons M.W. The larger subunit of Na+- K+- ATPase performs the actual function of cation transport. It has three sites on its extra cytoplasmic surface: two sites for K+ ions and one site for the inhibitor ouabain that blocks the transport of Na+ from epithelial cells to intestinal lumen. On its cytosolic side, the larger subunit contains three sites for three Na+ ions and also has one catalytic site for an ATP molecule. It is believed that the hydrolysis of one ATP molecule drives conformational changes in the Na+- K+- ATPase that allows the pump to transport three Na+ ions out and two K+ ions inside the cell.
Calcium ATPase Calcium pump or Ca2+-ATPase pumps Ca2+-ions out of the cytosol, maintaining a low concentration of it inside the cytosol. In some types of cells such as erythrocytes, the calcium pumps are located in the plasma membrane and function to transport Ca2+ ions out of the cell. In contrast, in muscle cells, Ca2+-ion pumps are located in the membrane of ER or sarcoplasmic reticulum. The Ca2+-ATPase transports Ca2+ from the cytosol to the interior of the sarcoplasmic reticulum for causing the relaxation of the muscle cells. Release of Ca2+ ions from the sarcoplasmic reticulum into the cytosol of muscle cells causes contraction of the muscle cells. Sarcoplasmic reticulum tends to concentrate and store Ca2+ ions with the help of following two types of reservoir proteins.
- Calsequestrin (44,000 D M.W.; highly acidic protein) which tends to bind up to 43 Ca2+ ions with it.
- High affinity Ca2+- binding protein binds Ca2+ ions and reduces the concentration of free Ca2+ ions inside the sarcoplasmic reticulum vesicles and decreases the amount of energy needed to pump Ca2+ ions into it from the cytosol. A calcium pump is a 100,000 M.W., polypeptide, forming 80 per cent of integral membrane protein of sarcoplasmic reticulum. In it, hydrolysis of one ATP molecule transports two Ca2+ ions in the counter-transport of one Mg2+ ion.
Proton pump or H+ pump The lysosomal membrane contains the ATP-dependent proton pump that transports protons from the cytosol into the lumen of the organelle, keeping the interior of lysosomes very acidic (pH 4.5 to 5.0). The pH of the cytosol is about 7.0. Proton pumps also occur in mitochondria and chloroplasts where they participate in the generation of ATP from ADP.
ATP-driven pumps These are the pumps that use the energy released from the hydrolysis of ATP to ADP and phosphate to translocate solutes across the cell membrane.
In all the prokaryotes and eukaryotes, ATP driven pumps are divided into four main classes:
- P-type pumps use phosphate group from the ATP to phosphorylate themselves during the translocation of ions. These pumps help in maintaining the gradients of Na+, H+, K+ and Ca2+ ions across the membranes. These pumps are multipass transmembrane proteins which phosphorylate themselves during pumping and involve in ion transport e.g. sodium-potassium pump. This Na+-K+ pump translocates three sodium ions out of the cell and two potassium ions into the cell. Also, it contribute to the action potential over the membranes of the nerve cells.
- ATP binding cassettes (ABC) transporters These pumps help in pumping the small molecules across the cell membrane. Increased production of these pumps is linked to the resistance to chemotherapeutic drugs in cancer cells. These pumps are essential for many of the processes of the human body, both in prokaryotes and eukaryotes e.g. CFTR (Cystic fibrosis transmembrane conductance regulator) pump in the membrane of epithelial cells, if mutated can cause cystic fibrosis. This protein is involved in the transport of chloride ions across cell membranes or the gene that encodes this protein.
- F-type pumps are turbine like structures present in the bacterial membranes and the inner membrane of mitochondria and chloroplasts where they produce ATP by using the energy derived from the concentration gradient of H+ ions across the membrane.
- V-type pumps These pumps use ATP to drive translocation of H+ ions. They only transport protons but do not form a phosphoprotein intermediate like P-class proteins. These pumps maintain a low pH in vacuoles and lysosomes.
Uniport, symport and antiport those carrier proteins which simply transport a single solute from one side of the membrane to the other; are called uniports. Others function as coupled transporters, in which the transfer of one solute depends on the simultaneous transfer of a second solute, either in the same direction (symport) or in the opposite direction (antiport). Both symport and antiport collectively form the co-transport. Most animal cells, for example, must take up glucose from the extracellular fluid, where the concentration of the sugar is relatively high, by passive transport through the glucose carriers (such as D-hexose permease) that operate as the uniports. By contrast, intestinal and kidney cells must take up glucose from the lumen of the intestine and kidney tubules, respectively, where the concentration of the sugar is low. These cells actively transport glucose by symport with Na+ ions whose extracellular concentration is very high. The anion exchange permease of human erythrocytes operates as an antiport to the exchange of Cl- for HCO3.