CARRIERS
- Carriers do not catalyze ATP-hydrolysis.
- In other words, the transport process does not involve chemical modification of any of the compounds bound to the carrier.
- Rather, carriers catalyze the movement of inorganic ions and simple organic solutes across the membrane.
- One defining feature of the carrier is that they display saturability when the kinetics of transport are expressed to the substrate concentration.
- The array of ions and solutes translocated by the carrier is vast.
- The principal inorganic nutrients including NH4+/ NO3-,Pi,K+ and SO4- are all transported into the cell by plasma membrane carrier. Carriers are also responsible for taking up ions that play less roles in metabolism like Cl- ions.
- The organic solutes translocated into the cell by carrier are the fundamental building blocks of polymers: sugars, amino acids, purines and pyrimidines. .(Fig I).
- Active transport involves membrane carrier that moves substance against an electrochemical gradient ie. an energy requiring process.
- The energy releasing activity includes the hydrolysis of high energy molecules such as ATP, the absorption of photons of light, the transfer of electrons and the movement of other substances down their respective electrochemical gradients.
- The active transport of sodium and potassium ions is the classic example of an energy requiring transport system powered by the hydrolysis of ATP.
- In most cells the concentration of potassium is higher than the concentration of sodium.
- Maintaining this unequal distribution of sodium and potassium is important because potassium is needed inside the cell for the activity of certain enzymes and the sodium is required for the active transport of sugar and amino acids.
