Question

1- What are the concentration gradients for Na+ and K+?
2- Explain why a positive Veq for a cation means an inward driving force and a negative value means an outward driving force?

Answer 1

1.) The concentration gradient of Na+ and K+ accross the plasma membrane is, intracelluar concentration of Na+ is high and outside is low while extracellular concentration of K+ is high while inside is low hence by selective movements of these ions along gradient create a difference in electric potential or voltage accross plasma membrane called membrane Potential.

In a normal potential, 3 Na+ moves out from the cell while 2K+ comes inside the cell.

2.) Vdf = Vm - Veq

Vdf = electrochemical driving force

Vm = membrane potential

Veq = equilibrium potential

The above equation describes the cause of inward and outward force of cation i.e. when two or more ions contribute to the membrane potential, it is likely the membrane potential would not be at equilibrium position for any of the contributing ions hence also disturbs equilibrium potential ( Vm is not equals to Veq). When ions is not at its equilibrium position an electrochemical driving force (Vdf) acts on the ions causing the net movement of ions across the membrane down its membrane potential. The driving force is quantified by above reaction. The sign if the driving force acting on an ion can be used to predict the direction of movement.

So, for cations positive Veq shows positive driving force (Vdf > 0) causing efflux i.e. Movement out of the cell

For cation negative Veq shows negative driving force (Vdf < 0) casing influx i.e. Ions movement into the cell.

If the membrane potential is at its equilibrium potential (Vm=Veq) from ions, the driving force acting on an ion will be zero hence in this no movement of ions takes place.