Osmosis is the movement of water (and only water) through diffusion from a high to low level of concentration.

Osmosis in Animals

If a cell uses up water in its chemical reactions, the cytoplasm becomes more concentrated and more water will immediately be drawn in by means of osmosis from outside the cell membrane. Similarly if the cytoplasm becomes too dilute because water is produced during the chemical reactions taking place, water will leave the cell by osmosis restoring the water balance.

However osmosis can also cause some very serious problems in animal cells. If the solution outside the cell is more dilute than the cells contents, then water will move into the cell by osmosis, causing the cell to swell and in some serious cases even burst.

On the other hand, if the solution outside the cell is more concentrated than the cell contents, then water will move out of the cell by osmosis. The cytoplasm will become too concentrated and the cell will shrivle up. Once you understand homeostasis and maintaining constant internal conditions becomes very clear.

Osmosis in Plants

Plants rely on well-regulated osmosis to support their stems and leaves. Water moves into plant cells by osmosis, makeing the cytoplasm swell and press against the plant cells walls. The pressure builds up until no more water can physically enter the cell, this makes the cell hard and rigid.

This swollen state keeps the leaves of and stems of the plant rigid and firm. So for plants there isn’t too much of a problem when the water supply is abundant because the plant will also only draw the water from the ground that it needs. The problem comes when there isn’t enough water for the plant.

When the water supply is very sparse water is drawn out of the the plant cells to support other processes in the plant for which water is vital. As this happens the cells of the stems ond leaves become flaccid and the plant begins to droop and wilt. If the water supply does not return then the plant will wilt and die.