Page 75 - Biology Form Two

Biology for Secondary Schools

In animal cells, osmosis is important time, it loses a lot of water, shrinks,
for reabsorption of water in the colon and shrivels. These effects of osmosis
and kidneys. This process helps to on animal cells can be observed in red
maintain water balance in the body. In blood cells. Under normal conditions,
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addition, the process of osmosis is used the osmotic pressure of red blood cells
to preserve food by salting. This causes is equal to that of the blood plasma
food such as meat to lose water and implying that they are isotonic. Thus,

become dry. Salting makes food to stay there is equal movement of water in and
longer without being contaminated by out of the cells. This helps to maintain
germs. the disc shape of these cells.

In lower organisms such as Amoeba, When red blood cells are placed in a
osmosis helps to remove waste and hypotonic solution, they absorb water,
excess water from their bodies. causing the cell volume to increase.
Excessive amounts of water cause
Effects of osmosis in living organisms haemolysis (bursting).

Osmosis is one of the most important
processes that help to keep the When red blood cells are placed in a
organisms body conditions stable for hypertonic solution, they lose water,
their survival. However, the process of leading to shrinking of the cell. This is
osmosis has various effects in the cells referred to as crenation (Figure 3.5).
of living things as described below.

Osmosis in animal cells: Animal cells
take in and lose water by the process of
osmosis. However, when an animal cell (a) Normal red (b) Haemolysed
is put in a hypotonic solution, it absorbs blood cell red blood cell

water. If it remains in the solution for a
long time, it absorbs excess amounts
of water. A cell that does not have a
mechanism for removing excess water (c) Crenated red blood cell
bursts due to its excessive internal Figure 3.5: Effects of osmosis on a red blood
pressure. cell

When an animal cell is placed in a Osmosis in plant cells: In an isotonic
hypertonic solution, it loses water. If solution, plant cells neither lose nor

it remains in the solution for a long gain water. In a hypotonic solution,

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