Question

Assume that the plasma membrane of a cell was suddenly permeable to the same degree to both Na+ and K+ and that both ions were present at a concentration gradient of the same magnitude. What would you expect to happen?

Answer 1

Plasma membrane is permeable to Na+ and K+ ions against their electrochemical gradient of the ions present inside and outside of the cell. The cell contains more amount of K+ ions within and less amount of Na+ inside whereas the Na+ ions are present at high concentrations outside the cell as compared to the K+ ions. This concentration gradient helps to maintain a membrane potential that stabilises the membrane and controls the in and out of the membrane components. For every 3Na+ ions that move out of the cell, 2K+ ions move into the cell. This is a phenomenon that is controlled by  ATP hydrolysis.

If the plasma membrane becomes permeable to both the ions in same concentration and that both the ions are present in same concentration, there won't be any controlling membrane potential that might allow the selective entry and exit of the components. Equal in and out would require more amount of ATP hydrolysis and that would cost the cell more of it's energy. This equilibrium would also affect the functioning of the neurons that are controlled by the resting membrane potential of the cell.