Membrane transport 5
Active transport. Sodium-potassium pump and calcium pump - importance, role of ATP, molecular structure and model of action. Explain types of active transport and give examples.
Active transport
Active transport mechanisms use energy sources to "pump" ions against concentration gradients. It is estimated that cells expend about 25% of their ATP just on active transport. Three common active transport mechanisms are described below.
Ion pumps - Directly couple ATP hydrolysis to transport. A well-studied example is the sodium-potassium pump of the plasma membrane. Note that in one turn of the multistep cycle, two potassiums are pumped in, three sodiums are pumped out, and one ATP is cleaved. The pump can be blocked by ouabain which, in the heart, stimulates contraction because sodium concentration increases and stimulates the sodium-calcium pump to remove sodium and import calcium. Increasing calcium leads to stronger muscular contraction.
Cotransport Systems - The sodium-glucose cotransport system relies on the concentration gradient built up by the sodium-potassium pump to drive the import of glucose into cells. In this case, sodium outside the cell binds to the receptor and, upon binding of a glucose molecule, the sodium concentration gradient drives the sodium inward and glucose is carried with it.
Transport by Modification - This system relies upon covalently modifying a molecule during (or shortly after) passive or facilitated transport so that it can no longer pass back through the membrane. For example, the phosphotransferase system of E. coli uses ATP to phosphorylate sugars as they are transported into the cell. The phosphorylated sugars cannot pass back out.
A - Primary:
Energy source: ATP (chemical reaction). Na-K-pump, Ca- pump.
B – Secondary:
Energy source : Transmembrane gradients. Glucose-sodium symporter – uses the energy of Na+ gradient
Secondary active transport does not use the energy of ATP (or energy of chemical reactions) directly. It uses the energy of electrochemical gradients (i.e. concentration and electrical gradients). Animal’s cells use the energy of Na+ gradient while plant’s cells use the energy of H+ gradient. Fungus and bacteria use either the first or the second (or both).
The importance of Sodium-Potassium pump:
Sets-up & maintains [gradients] of Na+, K+ and ionic asymmetry of membrane (internal and external)
- [K+]i ~ (20 — 40) [K+]e
- [Na+]e ~ (10 — 20) [Na+]i
The importance of ionic asymmetry:
Generating of membrane potentials – e.g. generating and propagating of nerve impulses