• The word potential means presence of different charges on the sides of membrane (inside and out side).
• In cell membrane there is negative charge inside and positive out side.
• The charge that present in side the cell (negative) is what we use to describe the membrane potential .
• This feature of the cells (presence of membrane potential) is what makes cells a life, as to say in death there is no such potential.
• Let’s see following figure:
Side “A” contain equal (10) charges negative and (10) charges positive and side “B” the same.
Let’s assume that (5) positive charges from A transported to “B”, then side B will have more positive [15+ / 10-], and by this, a potential is created on tow side, we can use this model to simplify what happen across cell membrane.
Review of compositions of Ions:
• K+ is abundant in side cell about (142) m mol/L then outside (4) m mol/L, as to say, the inside K+ is 35 time the outside.
• Na+ outside is more abundant than inside, about 140-150 m mol/L out and 14-15 m mol/L in, as to say, Na+ is (10) time more outside than inside.
• Also we have Cl- which is abundant outside, and (Po4, proteins) abundant inside.
• These ions participate in membrane potential, but, practically Na+ and K+ are the main.
What is the importance of membrane potential?
• Presence of membrane potential is the base to generate action potential (which is the electrical signal) in nerves.
How membrane potential created?
1. Let’s assume that K+ inside the cell (abundant) is allowed to permeate through membrane to outside by a (Leak channels) and by doing this, there will be negative charge inside and positive outside.
2. This process will stop after few (K+) go out side why?
Because there is an opposing force (which is electricity) is created that oppose transport of further negative charge from inside to outside, as to say, opposing the force of concentration difference of K+ from in to out, we call this balance between electricity and concentration difference as (Equilibrium potential).
3. They found that the electricity generated which balance concentration difference for K+ equal to (- 94) and we mean by -94, that the inside cell is negative.
4. In a summary, a few K+ leaked (by diffusion) from in to out, made a potential of (-94) [regarding inside], so this diffusion eventually create a potential, so we call this diffusion of K+ as [diffusion potential].
5. Same of above is applied to Na+, as to say, Na+ is allowed to permeate through leak channels from outside to inside by concentration difference until the electricity generated from this diffusion balance the force of concentration difference. They found that at this equilibrium potential, the cell is (+60) or (+61) [as to say inside cell is positive].
6. So if we imagine that Na+ and K+ do above process in the same time, what will be the membrane potential of the cell?…… it is (-90), but, why it is near to the value of K+ (which is -94) and far away from the value of Na+ (+60), and why it is not in the middle of both?
• The answer is very important; because K+ has permeability through leak channels which is [50-100] time that of Na+ in resting membrane potential
7. Because of above, we say that K+ has large contribution (المشارك الأكبر) in creating membrane potential (at rest).
Note: all above events occur at rest (we mean that the cell is not in action potential state).
8. if the events above occurred, that means Na+ with time will enter the inside and finally there will no concentration difference and the same is for K+ and if this occurred it is a big problem…… so …. What maintain the concentration difference? In other word what maintain the membrane potential? The answer is ……….. The famous ( Na+ – K+ )pump……
In a summary: what are the factors that contribute for membrane potential?
1. Distribution of ions between inside and outside [Na+ out and K+ in].
2. Permeability of membrane for ions through leak channels [which is very large for K+ than Na+].
3. Na+ – K+ pump (which maintain membrane potential).
4. Presences of proteins inside the cell (which are negatively charged) participate in membrane potential (but for small share)
Equilibrium potential for an ion is the same of diffusion potential .
How they (scientists) found the Equilibrium potential for ions …… e.g., Equilibrium potential for Na+ [ENa+].
Answer is …….. By Nernst equation which is:
• Nernst equation is used to calculate the potential created from signal ion ….. Then how they (scientist) calculate the potential for several ions in the same time?
Answer …… by Goldman – Hodgkin – Katz equation (GHK) (this equation got the Nobel Prize) it is:
EMF (for several ions) = C Na+ in + C K+ in + Ccl- out + …….. (other ions)
C Na+ out + C K+ out + C cl- in + …….. (other ions)
C = concentration
In = inside cell, out = outside cell
And by this equation they found that membrane potential is (-86) ….. But, we said previously that membrane potential is (-90) ….? Why this difference?
Answer: because Na+ – K+ pump (as we know) continuously pumping one positive charge to outside (3Na+ out and 2 K+ in) and by this …… increase negativity of inside from (-86) to (-90)
Note: membrane potential may be different from one cell to other in the body …. And for simplicity we assume that (-90) is the membrane potential for cells but; it may be less than this number for example (-70)