Page 76 - Biology Form Two

Biology for Secondary Schools

plant cells absorb water, causing the Vacuole Vacuole Vacuole
cell membrane to push against the cell
wall. The cell is then said to be turgid.
That is, it does not burst because the cell
wall resists pressure exerted from the
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cell membrane and restricts additional
intake of water. Turgidity helps plant (a) Normal (b) Plasmolysed (c) Turgid
cell
cell

cell
cells to maintain their shape.
Figure 3.6: Effect of osmosis on a plant cell
In a hypertonic solution, plant cells
lose water. This causes the vacuole to Osmosis in unicellular organisms:
shrink and the cell surface membrane Unicellular organisms that live in fresh
to pull away from the cell wall, making water such as Amoeba and Euglena,
the cell flaccid. The flaccid cell is said are hypertonic to their surroundings.
to be plasmolysed, and the process is Hence, water enters into the organisms
called plasmolysis. If a plasmolysed by osmosis. These organisms have a

cell is placed in a hypotonic solution, contractile vacuole, which collects
it absorbs water and becomes turgid the excess water and removes it
(Figure 3.6). from the cell as shown in Figure 3.7.
This prevents the cell from bursting.
Osmosis is also used for regulating
their body mineral contents.

Contractile vacuole

Nucleus Nucleus
removed excess water

(a) Excess water collected by (b) Excess water removed by
contractile vacuole contractile vacuole

Figure 3.7: Water balance in amoeba through contractile vacuole

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