All the chemicals and structures that
make up a cell are contained within the
cell membrane. This is good!
The membrane keeps all the elements that
the cell needs together, and forms a
boundary between the living cell and the
mixture of substances in the cell's
environment. The membrane forms a
protective barrier that keeps
unwanted substances out of the cell.
However, a cell is alive, and it cannot live in a sealed
container. It needs to be able to
have water, oxygen, and carbon dioxide
come to the inside of the cell and move
out again. The cell membrane
is porous with tiny openings. The
cell needs to get materials from the
environment and it needs to get rid of
Think about this for a minute.
Look at the back of your hand. Can
you see the little pores in the
skin? What purpose do they
serve? If you look at the
underside of a leaf you will see that it
has little pores too -- pores called
stomata. Plants breathe through
their stomata. Look carefully at
an egg and you will see tiny openings, so that a developing chick will
have access to oxygen. Cells are
very tiny, and they they also need tiny
openings in their membranes to
We say that the cell membrane is permeable.
This means that tiny atoms and
molecules can go
through the holes in the membrane. Actually, a cell
is so tiny that its membrane is really
only semi-permeable. It can let
small molecules in through the membrane,
such as molecules of oxygen, water,
carbon dioxide, and glucose, a very
simple sugar. However, big
molecules cannot get through the tiny
The movement of water or carbon dioxide
or oxygen through the cell
membrane is called diffusion.
Tiny molecules float through the cell
membrane with no problems. If the cell
uses up oxygen inside itself, more
oxygen just moves in. In a cell
has too much carbon dioxide, some of the
carbon dioxide molecules float out
through the membrane.
tries to stay in harmonious balance with
sometimes big molecules create problems
for the cell because only small
molecules can diffuse through the cell
wall. What happens then is called osmosis.