Osmosis is the passage of water from a region of
high water concentration through a semi-permeable membrane to a region of
low water concentration.
The definition contains three important
statements:
Osmosis is the passage of water
from a region of high water concentration through a
semi-permeable membrane to a region of low water
concentration.
Osmosis is the passage of water from a
region of high water concentration through a semi-permeable
membrane to a region of low water concentration.
Osmosis is the passage of water
from a region of high water concentration through a semi-permeable
membrane to a region of low water concentration.
It does not matter too much which order you put
these statements in. Nor does it matter if you write the definition as one
sentence or three sentences. All that matters in your exam is that you
make all three points when you explain what osmosis is.
Semi-permeable membranes are very
thin layers of material (cell membranes are semi-permeable) which
allow some things to pass through them but prevent other things from
passing through.
Cell membranes will allow small molecules like Oxygen, water, Carbon
Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell
membranes will not allow larger molecules like Sucrose, Starch,
protein, etc. to pass through.
A region of high concentration of water
is either a very dilute solution of something like sucrose or pure
water. In each case there is a lot of water: there is a high
concentration of water.
Some teachers use the definition which starts "Osmosis is the
passage of water from a dilute solution to a......" this means
exactly the same as the definition I have given.
A region of low concentration of water
is a concentrated solution of something like sucrose. In this case
there is much less water.
So you could use the definition "Osmosis is the passage of water from
a dilute solution through a semi-permeable membrane to a more
concentrated solution.
When you put an animal or plant cell into a
liquid containing water one of three things will happen.
If the medium surrounding the cell has a
higher water concentration than the cell (a very dilute solution) the
cell will gain water by osmosis.
Water molecules are free to pass across the cell membrane in both
directions, but more water will come into the cell than will leave.
The net (overall) result is that water enters the cell. The cell is
likely to swell up.
If the medium is exactly the same water
concentration as the cell there will be no net movement of water
across the cell membrane.
Water crosses the cell membrane in both directions, but the amount
going in is the same as the amount going out, so there is no overall
movement of water. The cell will stay the same size.
If the medium has a lower concentration of
water than the cell (a very concentrated solution) the cell will lose
water by osmosis.
Again, water crosses the cell membrane in both directions, but this
time more water leaves the cell than enters it. Therefore the cell
will shrink.
Plant cells always have a strong cell wall
surrounding them. When the take up water by osmosis they start to swell,
but the cell wall prevents them from bursting. Plant cells become
"turgid" when they are put in dilute solutions. Turgid means
swollen and hard. The pressure inside the cell rises, eventually the
internal pressure of the cell is so high that no more water can enter the
cell. This liquid or hydrostatic pressure works against osmosis. Turgidity
is very important to plants because this is what make the green parts of
the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar
solutions they lose water by osmosis and they become "flaccid";
this is the exact opposite of "turgid". If you put plant cells
into concentrated sugar solutions and look at them under a microscope you
would see that the contents of the cells have shrunk and pulled away from
the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution which
has exactly the same osmotic strength as the cells they are in a state
between turgidity and flaccidity. We call this incipient plasmolysis.
"Incipient" means "about to be". When I forget to
water the potted plants in my study you will see their leaves droop.
Although their cells are not plasmolsysed, they are not turgid and so they
do not hold the leaves up into the sunlight.
When animal cells are placed in sugar solutions
things may be rather different because animal cells do not have cell
walls. In very dilute solutions, animal cells swell up and burst: they do
not become turgid because there is no cell wall to support the cell
membrane. In concentrated solutions, water is sucked out of the cell by
osmosis and the cell shrinks. In either case there is a problem. So animal
cells must always be bathed in a solution having the same osmotic strength
as their cytoplasm. This is one of the reasons why we have kidneys. The
exact amount of water and salt removed from our blood by our kidneys is
under the control of a part of the brain called the hypothalamus. The
process of regulating the amounts of water and mineral salts in the blood
is called osmoregulation. My insulin page will tell you
more about other homeostatic mechanisms.
Animals which live on dry land must conserve
water; so must animals which live in the sea (the sea is very salty!), but
animals which live in freshwater have the opposite problem; they must get
rid of excess water as fast as it gets into their bodies by osmosis.
If you do a search of the
INTERNET for "osmosis" you will find lots of references to
osmosis in sailing boats. When yachtsmen and yachtswomen talk about
"osmosis" they mean that the the hull of their little boatie is
being damaged by water but this is nothing to do with osmosis as we
biologists use the term. Yacht people have stolen our word. So don't get
confused by what you may read out there on the INTERNET.
