Cell membrane battery resting potential
The Cell's Resting "Battery" Voltage
(or Membrane Resting Potential)
The cell's resting "battery" voltage represents
the health of the cell
Provides the "driving-force" for actively
transporting ingredients required for cellular energy production across the cell
membrane. This energy is then used to create a driving
force across its mitochondrial membranes for "oxidative-phosphorylation"-
the final, and most energy-yielding step, in the mitochondrion's energy production
process
Cellular respiration
Affects All Electrical activity of the cell.
The membrane resting potential
preparesthe "excitable"nerve and muscle cells for the propagation of
action potentials leading to nerve impulses and muscle contraction. E.g. the heart
muscle cells (myocardial cells) require a sufficient membrane potential for the
heart to beat; pain messages are passed via nerve impulses.
The membrane potential voltage controls the opening
and closing of the potassium and sodium gates. Most ion channels
are "gated"- stimulated
Unlike the "Resting" term implies, the cell
is actually very busy keeping the unequal distribution of ions on each side of the
cell membrane
This is to establish a healthy cell membrane potential difference
of about 70mV across the membrane, with the inside of the cell being more
negative than the outside
The concentration gradients of K, Na, Cl determine the overall
electrochemical gradient across the cell membrane
The concentration gradient of an ion is determined
by the difference in concentrations (shown in diagrams in mM, which is mol/m3 )
of the ions in solution on each side of the membrane.
Those concentrations have been converted here to their equivalent
electrical gradient in millivolts (mV) .
The largest net electrochemical gradient is 130mV, for
Na+ ions into the cell
There is a small gradient of 20mV for K+ ions out
of the cell
There is no gradient for Cl- ions
So therefore, this should mean more Na+
ions than K+ ions
But actually, this is not so, because we must also consider
the membrane permeability, which is about 50 times more permeable
to K+ than Na+
The membrane's permeability to an ion refers to the ability
of its membrane channels to conduct ions once they are open. This usually
depends on the size of the ions in solution compared to the size of
the channels.Most channels are large enough to pass the small hydrated
K+ ion, but few will carry the larger hydrated Na+
ion across the membrane.
How the Resting Membrane Potential is Produced
The cell's resting "battery" voltage is
established by the following 3 factors:
(1) The Preference
of the Cell Membrane for Potassium to travel through it -
More K+ ions diffuse out of the cell than Na+ ions diffuse into
the cell. Since the membrane is much more permeable to K+ ions,
there will be more K+ ions outside the membrane than can be compensated by the inflow
of Na+ ions.
(2) Negatively
Charged Molecules (Anions) are Trapped
inside the Cell - Large
negatively charged Molecules (here referred to as
A-
ATP , all
behaving as organic acids, giving off a hydrogen ion. When this positive ion is
incorporated with oxygen to form water, it leaves the negative ion inside the cell.Cumulatively,
these negative ions establish a negative charge trapped inside the cell.
The negative anions trapped inside the cell, which cannot follow the K+
ions across the membrane, together with the excess of K+ ions diffused out of the
cell over Na+ ions diffused into the cell, establish the resting potential.
The inside of the cell has a negative charge
(3) Sodium /
Potassium Pumps - Even though
the cell membrane is not nearly as permeable to sodium as potassium, over time the
slow diffusion of Na+ ions into the cell, together with the fast diffusion
of K+ ions out of the cell will cause the cell membrane to lose its potential voltage
by equalizing the charges on either side of the membrane.
Fortunately, the cell has a mechanism to bring back in
the K+ ions and move out the Na+ ions - This mechanism is provided
by the many energy-using Na/K pumps spanning the cell membrane, which continuously
pumpNa+ ions back out of and K+ ions back into the cell, against their concentration
gradients.The Na/K Pump uses
ATP energy
to simultaneously pump:
3 Na+ ions out of the cell and
2 K+ ions into the cell.
The Na/K Pump thus counters the potassium / sodium diffusion (leaking)
out of the cell, and increases the membrane potential by pumping out of the cell
one more sodium ion than it pumps potassium ions into the cell. The Na/K pump thus
provides a mechanism by which Na+ ions are driven out of the cell faster
than K+ ions are pulled in, keeping intracellular sodium levels low and
potassium levels high.
This accomplishes several vital functions:
-
It establishes an electrical potential difference (a "battery") across
the cell membrane - with the interior of the cell being negatively
charged relative to the exterior.
-
Maintains osmotic balance. Sodium ions accumulated
outside of the cell draw water out of the cell (otherwise, the cell would swell
and burst from the inward diffusion of water).
-
Provides energy for indirect pumps.
Harnessing the gradient of sodium ions. E.g.
to transport glucose into the cell.
Operating these continuously active Na/K pumps uses about 30%
of the total ATP energy produced by the cell
The Mitochondrial Cell Membrane "Battery"
The mitochondrial membrane uses positively charged
hydrogen ions (Protons or H+) to create
a strong membrane potential difference across the mitochondrial membrane -
Hydrogen ions are maintained in a high concentration on the
outside of the mitochondrial membrane by the action of the electron transport chain
(See Cellular
Respiration ), which creates a mitochondrial membrane potential
of about 40,000,000 volts/meter.
When this proton electricity flows back across
the inner mitochondrial membrane it is used to power a molecular, enzyme motor called
ATP syntase - which loads negatively charged phosphate anions
onto ADP thus creating
ATP.
