Active transport of substances across the plasma membrane
This form of transport is divided into two types, primary active transport, and secondary active transport according to energy.
Active transport is a form of transport that expends ATP energy to move substances against their concentration gradient.
This form of transport is accomplished through the role of specific transmembrane proteins that act as ATP-operated pumps to propel ions such as Na+, K+, H+, Ca2+, I-, Cl- or molecules as small as amino acids, monosaccharides go against their concentration gradient.
This form of transport is divided into two categories (1) primary active transport and (2) secondary active transport depending on whether ATP energy is used directly or indirectly in the transport of substances.
Primary active transport
Primary active transport is a form of transport in which energy from ATP is used directly to "pump" a substance across the membrane in the opposite direction of the concentration gradient.
The cell will use this energy to change the shape of the transport proteins on the plasma membrane to carry out the transport. About 40% of the cell's ATP serves this purpose.
Figure: Sodium-potassium pump operation.
The sodium pump (Figure 6) is a good example of a primary form of transport:
Through the action of the sodium pump, sodium ions (Na+) will be "pumped" out of the cell (where there is a higher concentration of sodium ions) and potassium ions (K+) will be "pumped" into the cell (where there is a higher concentration of sodium ions). higher potassium ion concentration).
In this way, the sodium pump maintains a steady concentration of sodium and potassium ions inside and outside the cell, which is important for cell survival.
All cells have sodium pumps, there are hundreds of such pumps per micro square meter of the plasma membrane and they must operate continuously to maintain the stability of Na+ and K+ ions due to Na+ and K+ ions. Continuous diffusion across the membrane through channels disrupts the steady-state of these ions.
The sodium pump is also sometimes called the Na+/K+ ATPase pump because the protein carrying out the transport acts as an enzyme that separates energy from ATP. In the structure of the ATPase molecule, there are 4 subunits (2 a and 2 b units). The subunit has enzymatic activity that converts ATP to energy-releasing ADP and has ionic binding sites inside and outside the cell. Inside the cell there are sites for binding 3 Na+ ions and ATP, outside the cell, there are sites for binding 2 K+ ions.
Pump operation can be divided into two stages:
(1) When three ions Na+ and ATP are attached to the inner side of the pump, a phosphate group is transferred from the ATP molecule to the aspartic acid moiety of subunit a. The presence of an energetic phosphate group will change the structure of the pump, which will move 3 Na ions out of the cell.
(2) When two K+ ions are attached to the outer surface of the cell, the bond between the phosphate group and aspartic acid is hydrolyzed. The energy released from this DE phosphoryl (dephosphorylate) process will change the structure of the second pump causing 2 K+ ions to be introduced into the cell.
Inhibition of Pump Operation: The pump will not work if the concentrations of Na+, K+ and ATP ions are too low. The action of digitalis, a drug used in the treatment of heart failure, is based on its ability to bind to subunit a on the cell surface and thereby interfere with the DE phosphoryl of the pump, inhibiting its activity. of the pump.
In addition to the Na+/K+ pump, primary active transport is also seen in the action of the K+/H+ pump on the gastric mucosal cell membrane, which controls the secretion of H+ ions into the stomach during digestion. Ca2+ pump is present on the endoplasmic reticulum of muscle cells to maintain the Ca2+ ion concentration in the cell always below 0.1 (mol/L.
secondary active transport
In this form of transport, the energy stored due to the difference in the concentration gradient of Na+ ions is used to transport substances against their concentration gradient across the membrane.
The sodium pump maintains a large difference in the concentration of Na+ ions on either side of the plasma membrane, if there is a pathway through which Na+ ions can travel from high concentration to low concentration, energy remains. stored due to the difference in concentration of Na+ will be converted into kinetic energy to help transport another substance against the concentration gradient of that substance.
Since the concentration difference of Na+ ions is established through the primary active transporter, which requires ATP directly, it can be assumed that the second mode of transport uses ATP indirectly to carry out its transport. active transport across membranes.
Many types of ions and nutrients are transported in this form:
The transport of glucose, galactose and amino acids along with Na+ ions across the membranes of the small intestine and the cells of the renal tubules takes place in this way, through which nutrients in food are absorbed smoothly. completely in the small intestine and reabsorbed by the renal tubules to return to the bloodstream.
Transport of Ca2+ ions out of the cytoplasm by ventricular cells and other types of muscle cells (this transport of Ca2+ ions in combination with Ca2+ pumping action on the endoplasmic reticulum of muscle cells causes muscle relaxation)
H+ ions formed during cellular metabolism are pumped out of the cell by this form of transport. This mechanism is important for maintaining a constant pH in the cells and in the lumen of the proximal tubule of the kidney (which helps in bicarbonate reabsorption).
Figure: Synergistic and antagonistic phenomena.
a: synergistic phenomenon; b: antagonistic phenomenon
1: extracellular fluid; 2: cytoplasmic membrane; 3: cytoplasm; 4: protein agonist; 5: amino acids; 6: Sodium ions; 7: calcium ions; 8: antagonist protein; 9: passive diffusion along the concentration gradient; 10: secondary active transport.
The stored energy due to the electrochemical gradient of Na+ ions will change the conformation of the transport protein.
When Na+ ions bind to a carrier protein, it increases the affinity of this protein for the carrier.
When both the Na+ ion and the carrier are attached to the transport protein, it changes the structure of this protein, allowing the Na+ ion and the carrier to be transported across the membrane.
When two substances are transported in the same direction across the membrane, this process is called synergy, as the transport of glucose and amino acids across the intestinal mucosa and renal tubules.
When two substances are transported in different directions across the membrane, this process is known as anti-potation as the active transport of Ca2+ ions across the membrane.
The greater the difference in the concentration of Na+ ions on either side of the membrane, the faster the secondary active transport occurs.