Question

Discuss how active symporters and antiporters differ from facilitative symporters and antiporters.

Discuss why it would be wrong to categorize the Na+/K+ ATPase as a coupled active transporter instead of as a pump.

Explain how the movement of Na+ ions by Na+/K+ ATPase differs from the movement of Na+ ions by the Na+/glucose symporter.

Answer 1

Active transport and facilitative transport are two mechanisms of transport of molecules across the plasma membrane. As plasma membrane is selectively permeable so ions and polar molecules are unable to cross from it. Thus facilitative and active symportors and antiporters play a key role in the passage of these molecules across the plasma membrane. Facilitative symporters and antiporters works through concentration gradient i.e. molecules difuses from higher concentration to lower concentration whereas active symporters and antiporters works against concentration gradient i.e. diffusion occurs from lower concentration to higher concentration with the help ATP energy.

Na/K ATPase is a primary active transporter which requires ATP hydrolysis for energy which is further utilised inthe transport of Na and K ions across the membrane against their concentration For each ATP the pump breaks down, two potassium ions are transported into the cell and three sodium ions out of the cell. Though transportation of secondary molecules is involved but due to active role of ATP the sodium-potassium pump would not be considered a co-Cotransporters, on the other hand, move one substance down its concentration gradient in order to supply the energy to move another substance against its concentration gradient. They can be antiporters or symporters. Thus it cant be considered coupled active transporter but Na/K ATPase pump.

The movement of Na in Na/K pump is primary active transport whereas its transportion in Na/ Glucose symporters is secondary active transport. Na/K pump drives 3 Na ions out of the cell membrane and 2 K ions inside the membrane by using ATP hydrolysis as energy source. Whereas Na/ glucose symporters is a type of secondary active transporter. Sodium and glucose both bind to the symporter and co transported simultaneously into the cell membrane with the help of potential free energy stored in sodium electrochemical gradient established by Na/K pump.