What is the relative distribution of the critical ions across the
membrane and why are they so distributed?
Hello cinderga7,
I asked for clarification a few hours ago on what type of cell you
were talking about, but because of the subject of the question, I am
going to assume you are looking mostly for this information in nerve
cell. Ion concentration is also important with other cells and I will
go over that as well, but because the ion distribution is especially
important with nerve cells, I will focus more on that aspect.
First, a quick overview of what an ion is can be found at this page:
http://www.schoolscience.co.uk/questions/4/physics/physsm/2.html
Basically, “an ion is an atom that has gained an electron or lost an
electron giving it a positive or negative charge”.
Ion distribution in neurons is extremely important to vertebrates
because the electrical charge is used to transmit messages throughout
the nervous system. Cell membranes have something called the
“Sodium-Potassium Pump” which pump potassium into the cell, and pumps
sodium out using active transport. Active transport is required
because the sodium potassium pump is pumping AGAINST the concentration
gradient, from an area of high concentration to an area of low
concentration.
As a result of this, there is an electrical charge difference between
the inside of the cell membrane and the outside of the cell membrane
which is called the membrane potential. Electrophysiologists can
measure this potential as a voltage by using microelectrodes connected
to a voltmeter. The typical charge difference is about -50 to -100 mV
(millivolts). A neuron at its resting stage (not transmitting a
signal) is usually at about -70mV.
According to Campbell, the approximate distributions of ions for a
mammalian cell in milimoles per liter are as follows:
Outside the membrane:
5mM potassium (K+)
150mM sodium (Na+)
120mM chloride (Cl-)
======================== <--- this is the cell membrane
Inside the membrane of a cell:
150mM potassium (K+)
15mM sodium (Na+)
10mM Chloride (Cl-)
100mM Anions (A-)
The rest of the answer will focus on why this distribution is
important:
Potassium diffuses outside of the cell down its concentration gradient
(through leaky spots in the membrane) but the anions cannot follow so
the interior of the cell develops a net negative charge. Another cause
of the charge is that although sodium and potassium leak through the
“leaky channels” of the membrane, they do not leak equally. The
membrane is generally more leaky to potassium ions than to sodium
ions, so more potassium leaks out than sodium leaks in.
The Sodium Potassium pump plays an important role in preventing
cellular swelling and water retention in normal cells. The most
important aspect of it however is the role it plays in the nervous
system and in transmitting signals in neurons/muscle cells.
At resting stage, we have a relative charge of -70mV inside the cell
membrane. This is relative because although the outside of the
membrane is also negative, the inside is MORE negative than the
outside (thus relatively negative inside). Imagine the cell working
hard to maintain this -70mV charge on the inside of the membrane using
the Sodium Potassium pump (expending energy) to keep this charge and
although sodium and potassium are leaking, the cell is working harder
so the charge stays relatively constant.
While all cells have a membrane potential, only neurons and muscle
cells have the ability to use these charges to transmit signals.
For example, a neuron cell has channels (not to be confused with the
leaky spots) through which potassium and sodium can flood in/out
quickly. Remember that sodium and potassium are always trying to reach
an equilibrium on both sides of the membrane because as solutes, they
try to equally space themselves in the solvent that is directly inside
and outside the cell membrane. At the resting stage, these channels
are closed and the cell is still working to maintain the -70mV resting
potential.
The gated ion channels can be opened by a stimulus which may be light
(in the case of photoreceptors in the eye) or vibrations in the air
(in the case of receptors in the ear). There are two types of
channels, chemical channels which open and close in response to a
specific chemical stimuli, and voltage gated channels which open and
close in response to a change in the membrane potential. We can
further break this up into sodium chemical/voltage channels and
potassium chemical/voltage channels, which only let through their
specific type of ion.
Suppose a certain stimulus opens a few Na+ channels. If the opening of
these channels is enough to reach the THRESHOLD Potential, further
voltage Na+ voltage channels suddenly will open (in response to the
change in voltage) and Na+ will flood into the cell membrane. The K+
channels remain closed, but the sodiums are rushing into the cell and
the interior of the cell becomes more positive reaching a peak of
about 40mV. At this point, the sodium channels close and the potassium
channels open. Potassium ions leave the cell and there is a quick loss
of positive charge as the cell becomes more negative than the outside.
We now have what is called Undershoot, where the membrane is more
negative on the inside (about -85mV) than the normal -70mV. This
happens because it takes the gates a bit longer to close, and since
the Na+ ions cannot get in, and the K+ ions are still moving out,
there is a slightly more negative charge than during resting stage.
Why is this sequence of events important? Well, imagine a long chain
of neurons at resting stage. Suddenly, one neuron is stimulated and as
a result, the neuron voltage changes to such that the next one is
stimulated and eventually the entire chain is stimulated. This is how
neurons transmit messages! In nerve cells, there is a what is called
an axon, which is the main length of the cell and moves the signal
along (also known as a transmitter). The chain of voltage changes
along the axon is noot actually the action potential traveling down
the axon, but the action potential is being regenerated again and
again because the voltage ion channels are being stimulated as a
result of the voltage change in the cell behind in the chain. It kind
of works like a row of dominoes, once the first one is tipped over,
they will all fall over.
Another interesting note is that because it takes the membrane a few
milliseconds to get back to the “resting stage”, the signal does not
get recycled back and forth. If you were to hurt your finger, the
message would be one way to your brain instead of bouncing back and
forth endlessly because of the “Undershoot” stage where the membrane
is in the process of getting back to the Resting stage.
I hope this is the type of answer you were looking for. If you need
any clarification, please let me know and I will do my best to further
assist you.
tisme-ga
Sources Used:
http://faculty.washington.edu/chudler/ap.html
http://www.naturalhealthnotebook.com/Biochemistry/Sodium-Potassium_Pump.htm
http://www.abe.msstate.edu/classes/abe4323/2002/neurons_2/neurons_II_ques.html
http://bio.bio.rpi.edu/HB/Universal%20Files/Lectures/L39MembranePot/Memb%20Pot/MembPot.html
http://human.physiol.arizona.edu/SCHED/CV/Wright/14membra.htm
http://lessons.harveyproject.org/development/nervous_system/cell_neuro/memb_potl/K_electrode/Nernst.html
Also used my cell biology notes from university extensively and
referred to the sixth edition of “Biology” by Campbell & Reece.
Search Strategy:
"what is an ion"
sodium potassium pump
"measuring membrane potential"
membrane potential
membrane potential ions |
