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As you start to think about what a cell membrane might do, you
should try to consider the situation in which it exists.  Here
is a little drawing of that situation. There's a cell, and it has some fluid inside
of it. It is also living in a fluid world. So there is a watery solution
inside of the cell and a watery solution outside of the cell. This is like the
situation I pointed out last week when you learned about phospholipids.
The only thing between the inside and outside worlds of the cell is
the membrane. Therefore, the membrane has to be able to maintain inside in, and
outside out (see the red arrows). But there's more than just that.
The membrane is also the only part of the cell that can interact
with anything in the world. So if the cell needs to notice something and respond to
it, it has to rely on its membrane (see the blue arrows). Also, the cell cannot
exist without getting material from its world to nourish it and without being able to dump
its wastes outside of itself. This getting specific things and releasing others is a
special function of membranes called selective permeability (see the green arrows).
It is for selective permeability that transport is used.
To recap these ideas and add a few others that we have talked about,
I have included a list of membrane functions below.
Plasma Membrane Function:
 | It must
the cell from its environment. In other words,
it must separate the cell from the environment around it and
all the intracellular contents within the cell. It has
to keep the stuff that is inside the cell inside the cell. |
| It must protect the cell. It isn't like armor or anything--
actually, it is pliable and moveable. But it has to keep stuff that is
supposed to be outside the cell outside the cell. It can't just let
anything in.
And it also has to bend so that it isn't rigid
and so that it won't break open. |
| It must allow for so that certain needed
materials from the environment can enter the cell and waste materials can leave the cell.
"Selective" because not ALL things should be able to cross the membrane
(that would be like poking a huge hole in the cell and it would die).
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| A recognition and response function. Because the outermost
part of the cell is the plasma membrane, it would be the first part of the cell to come
into contact with items in its environment. These items could be: toxins, food substances,
hormones, other organisms, etc. You should be able to see the importance of noticing such
items in your environment. For example, if a cell noticed food in its environment, it
should be able to then grab hold of the food and take it inside of the cell. Or if a
protist notices toxins (poisons) in its environment, it should be able to swim away from
that location into a safer one.
This can be considered analogous to a
communication function, and it is called signal transduction. |
| There are other functions of the plasma membrane that can be described... these are not
listed in your book, but some of them have already come up in our conversations and study:
 | Cell adhesion to other cells or to the world. This is how cells
stick to one another or how they stick to other materials in their world (which can also
be within a human body).
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| Attachment of the cytoskeleton to the membrane. This is the
problem in some forms of MD... You see, the cytoskeleton (which is inside the cell) has to
attach to the membrane to allow the entire cell to move. If the cytoskeleton were to
move without being attached to the membrane, the cell itself would not move anywhere or
change at all from the outside. Dystrophin is one of the proteins that attaches the
cytoskeleton to the membrane.
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| Provide an ID marker to the world. The membrane displays an
identity marker on its outside. This identity marker prevents our white blood cells
(that clean out the body of unwanted things) from eating up our own cells. This ID
marker is also what tells our white blood cells to chew up transplanted tissue. |
| Some cells can wrap their membranes around
other cells to help with an insulation function. This
can happen in the nervous system to insulate the cells that
transmit electrical signals. |
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