Question

Describe (mechanistically) how the Na/K pump and K channels create an electric potential across the cell membrane.

Answer 1

In neurons, K+ is present in higher concentrations inside the cell and Na+ are present at higher concentrations outside the cell. That is a stable concentration gradient is created across the membrane.as the ions are charged, they cannot pass directly through hydrophobic lipid region of the membrane. So they use specialized channels proteins as a tunnel across the membrane.

K+ channel is one such channel and is regulated in response to signal.in neurons; the resting potential depends on movement of potassium ions through K+ leak channels.

Because K+ is at higher concentration inside the cell than the outer fluid, if the K+ channels open, potassium ions will move down the concentration gradient out of the cell. And every time K+ ion leaves the cells loses its positive charge and becomes negative while the outer portion becomes positive charged. This set up a difference in electric potential across the membrane. At this point, the electrical force driving the k+ into the cells and chemical force driving the K+ out of the cell becomes equal and there is then no net movement of K+ in either direction. This system is considered to be in equilibrium.

But in case of neurons, resting potential is close but not identical to equilibrium. That is neuron resting potential are slightly less negative than K+ equilibrium potential. Another ion Na+ are present at higher concentration outside the cell than inside and hence it will move down the concentration gradient into the cell and will make the inside of cell positive to outside. It therefore forms sodium equilibrium potential.

As a result in resting neuron both K+ and Na+ are able to cross the membrane and so Na+ will drag the membrane towards positive and K+ will drag membrane potential towards negative. And so the real membrane potential will be between both the equilibrium potentials. Because resting membrane is more permeable to K+ than to Na +, the resting potential will be closer to potassium equilibrium potential. Na+ and K+ concentrations are maintained across the membrane of the cell by activity of protein called as Na+K+ ATPase, also called as sodium potassium pump. It is a membrane spanning proteins which transport ions against their electro chemical gradient. For every molecule of ATP broken down, 3 mol of Na+ are removed from cell to outside and 2K+ are moved from outer side to inside.