THE
CELL MEMBRANE
To see a picture of a real
cell membrane
History:
Up until now you knew that the function
of the cell membrane was to separate the inside of the cell from its outside.
In some pictures you have seen, the cell membrane may have looked like
this:
Scientists thought that this is what it looked like for a long time.
Then the electron microscope came along and they were able to see the cell
membrane. Below is a picture of how the cell membrane looked to them.
Scientists were
then able to break apart the cell membrane and find out what chemicals
were in it. The current best model of what a cell membrane looks
like is in the figure below. As you can see it is made up of two
parts. They are the phospholipids (magenta and green), the proteins
(orange and red), and the carbohydrates (black). Carbohydrates can
be attached to either the phospholipids or the proteins in the cell membrane.
The
cell membrane is
just like the other organelles of cells in that it serves the cell by having
its own specialized jobs. One of its jobs is to control what enters
and exits the cell and thereby to protect the cell. In the figure
above we see a magnified model of a cell membrane.
Location:
The cell membrane surrounds the cell.
Structure:
Cell membranes are made up of phospholipid molecules
(fats) with various large globular protein molecules suspended in them.
In the figure above, you can see the lipid bi-layer
in purple and the proteins in orange. The lipid bi-layer (two layers of
phospholipids) is formed because of the chemical structure of a phospholipid.
Since cells are constantly in water, the phospholipids form a double layer,
with the heads towards the water and the tails inside so that they can
stay away from the water. These bi-layers have proteins scattered
about in them. Sometimes carbohydrates (sugars) are attached to cell
membrane phospholipids and to cell membrane proteins.
A selectively permeable (sometimes called semi-permeable) membrane allows
some molecules across but not others.
JOB:
The cell membrane controls what enters
and exits the cell and thus protects the cell.
Click
HERE to find out more about:
Phospholipids
Osmosis
Diffusion
vs. Active Transport
Organelles
Q:
So how do things enter and exit the cell?
We did say that was one of the jobs of the cell membrane.
A:
Well,
molecules can only enter if they can go through the phospholipid bilayer
(fat) or they have a special protein in the cell membrane that they can
use.
So
now you know that there is much more to the cell membrane than meets the
eye.
1. Functionally
the cell membrane serves as both a gateway and a barrier for the cell.
~
2. The cell membrane
is composed of phospholipids and proteins.
~
3. The heads of
the phospholipids are polar so they like to be with water, which is
inside and outside of
the cell.
~
4. The tails of
the phospholipids are not polar.
~
5. The tails do
not like to be with water and thus a lipid bi-layer may be formed when phospholipids
are exposed to water.
To see a picture of a real
cell membrane
CLICK
HERE
Now that you know more about the cell membrane lets go on. Remember you
can learn about the organelles, diffusion, osmosis, phosopholipids, or take our
web page quiz.
Other Pages:
DEFINITIONS ~ LINKS
~ PAGE DIRECTORY
HOT TOPIC ~ EMAIL
~ GUEST BOOK ~ QUIZ
Meet the makers of this
page
This page was
created by Patrick Ireland (Patric@ireland.com).
It was last
updated January 20, 2000
