IGCSE NOTES : Biology - Osmosis

A partially permeable membrane is porous but allows water to pass through more rapidly than dissolved substances.

Since a dilute solution contains, in effect, more water molecules than a concentrated solution, there is a diffusion gradient which favours the passage of water from the dilute solution to the concentrated solution. In living cells, the cell membrane is partially permeable and the cytoplasm and vacuole (in plant cells) contain dissolved substances. As a consequence, water tends to diffuse into cells by osmosis if they are surrounded by a weak solution, e.g. fresh water. If the cells are surrounded by a stronger solution, e.g. sea water, the cells may lose water by osmosis. These effects are described more fully later.

A partially permeable membrane is porous but allows water to pass through more rapidly than dissolved substances. Since a dilute solution contains, in effect, more water molecules than a concentrated solution, there is a diffusion gradient which favours the passage of water from the dilute solution to the concentrated solution. In living cells, the cell membrane is partially permeable and the cytoplasm and vacuole (in plant cells) contain dissolved substances. As a consequence, water tends to diffuse into cells by osmosis if they are surrounded by a weak solution, e.g. fresh water. If the cells are surrounded by a stronger solution, e.g. sea water, the cells may lose water by osmosis. These effects are described more fully later.

Animal cells

The diagram of an animal cell can be shown very simply. The coloured circles represent molecules in the cytoplasm.
They may be sugar, salt or protein molecules. The blue circles represent water molecules. The cell is shown surrounded by pure water. Nothing is dissolved in the water; it has 100% concentration of water molecules. So the concentration of free water molecules outside the cell is greater than that inside and, therefore, water will diffuse into the cell by osmosis. The membrane allows water to go through either way. So in our example, water can move into or out of the cell. The cell membrane is partially permeable to most of the substances dissolved in the cytoplasm. So although the concentration of these substances inside may be high, they cannot diffuse freely out of the cell.

The water molecules move into and out of the cell, but because there are more of them on the outside, they will move in faster than they move out. The liquid outside the cell does not have to be 100% pure water. As long as the concentration of water outside is higher than that inside, water will diffuse in by osmosis.

Water entering the cell will make it swell up and, unless the extra water is expelled in some way, the cell will burst. Conversely, if the cells are surrounded by a solution which is more concentrated than the cytoplasm, water will pass out of the cell by osmosis and the cell will shrink. Excessive uptake or loss of water by osmosis may damage cells.

For this reason, it is very important that the cells in an animal’s body are surrounded by a liquid which has the same concentration as the liquid inside the cells. The liquid outside the cells is called tissue fluid and its concentration depends on the concentration of the blood. In vertebrates, the concentration of the blood is monitored by the brain and adjusted by the kidneys.

By keeping the blood concentration within narrow limits, the concentration of the tissue fluid remains more or less constant and the cells are not bloated by taking in too much water or dehydrated by losing too much.

Plant cells

The cytoplasm of a plant cell and the cell sap in its vacuole contain salts, sugars and proteins which effectively reduce the concentration of free water molecules inside the cell. The cell wall is freely permeable to water and dissolved substances but the cell membrane of the cytoplasm is partially permeable. If a plant cell is surrounded by water or a solution more dilute than its contents, water will pass into the vacuole by osmosis. The vacuole will expand and press outwards on the cytoplasm and cell wall. The cell wall of a mature plant cell cannot be stretched, so there comes a time when the inflow of water is resisted by the inelastic cell wall.

If enough air is pumped in, it pushes the inner tube against the tyre and makes the tyre hard. When plant cells have absorbed a maximum amount of water by osmosis, they become very rigid, due to the pressure of water pressing outwards on the cell wall. The end result is that the stems and leaves are supported. If the cells lose water there is no longer any water pressure pressing outwards against the cell walls and the stems and leaves are no longer supported. At this point, the plant becomes limp and wilts.

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) through a partially permeable membrane.

How osmosis works

When a substance such as sugar dissolves in water, the sugar molecules attract some of the water molecules and stop them moving freely. This, in effect, reduces the concentration of water molecules.

Water potential

The water potential of a solution is a measure of whether it is likely to lose or gain water molecules from another solution. A dilute solution, with its high proportion of free water molecules, is said to have a higher water potential than a concentrated solution, because water will flow from the dilute to the concentrated solution (from a high potential to a low potential). Pure water has the highest possible water potential because water molecules will flow from it to any other aqueous solution, no matter how dilute. When adjacent cells contain sap with different water potentials, a water potential gradient is created. Water will move from a cell with a higher water potential (a more dilute solution) to a cell with a lower water potential (a more concentrated solution). This is thought to be one way in which water moves from root hair cells through to the xylem of a plant root.

The importance of water potential and osmosis in the uptake of water by plants A plant cell with the vacuole pushing out on the cell wall is said to be turgid and the vacuole is exerting turgor pressure on the inelastic cell wall. If all the cells in a leaf and stem are turgid, the stem will be firm and upright and the leaves held out
straight. If the vacuoles lose water for any reason, thecells will lose their turgor and become flaccid.

If a plant has flaccid cells, the leaves will be limp and the stem will droop. A plant which loses water to this extent is said to be ‘wilting’. Root hair cells are in contact with water trapped between soil particles. When the water potential of the cell sap is lower than that of the soil water, the water will enter the cells by osmosis providing the plant with the water it needs.

When a farmer applies chemical fertilisers to the soil, the fertilisers dissolve in the soil water. Too much fertiliser can lower the osmotic potential of the soil water. This can draw water out of the plant root hair cells by osmosis, leading to wilting and death of crop plants.

Irrigation of crops can have a similar effect. Irrigation which provides just enough water for the plant can lead to a build-up of salts in the soil. The salts will eventually cause the soil water to have a lower water potential than the plant root cells. Crops can then no longer be grown on the land, because they wilt and die because of water loss by osmosis. Much agricultural land in hot countries has become unusable due to the side-effects of irrigation.