and animal cells
a solution is separated from pure water by a partially permeable
membrance,the pressure which must be applied to prevent osmosisis
called the osmotic pressure. As this situation is a hypothetical
one, and as a solution does not actually exert any pressure in normal
circumstances, the term osmotic potential is preferred. As the osmotic
potential is the potential of a solution to full in water, its value
is always negative. The more concentrated a solution, the more negative
is its osmotic potential.
two solution have the same osmotic potential they are said to be
isotonic. Where one solution has a greater osmotic potential (i.e.
the less concentrated one) than another it is said to be hypertonic
to it. The one with the lower osmotic potential (i.e. the less concentrated
one)is said to be hypotonic.
for the support of animals
and plants that live in water rely on it for support. Animals such
as jellyfish do not have a skeleton but rely on water to support
their body organs. There are also large plants living in water.
Large land-living plant such as trees has to produce wood to support
their trunks. Some of the largest seaweeds are many hundreds of
yards in length and they get all the support they need from the
water. When they are washed up on to the shore they collapse as
they have lost their means of support.
animals rely on water for internal support. Worm, such as the marine
fireman, have a central cavity that contains water. The worm's muscles
push on the fluid cannot compress, so it is forced toward one end
of the worm. By controlling the pressure on the fluid and there
it is applied, the muscles can cause the worm's body to change shape.
By using one set of muscles, the front end of its body extents forwards,
while another set cause the back part of its body to contract and
move toward the front end. By repeating this process, the worm can
move its whole body forward.