1. Osmosis
• Osmosis is the net movement of water moleculesfrom the region of
higher water potentialto a region of lower water potential througha
partially permeable membrane.
• A partially permeable membrane is a membrane that only allows
certain substances to pass through but not the other.
• It restricts the movement of substances based on their size.
Larger size substancescannot pass through it.
• The partially permeablemembrane is divided into 2 groups:
- Natural: Cellmembrane found in plant cells and animal cells
- Artificial: Visking tubing and cellophane bag
• A diluted solution has a higher water potential thana
concentrated solution.
Eg: The concentrationof water molecules is higher in a diluted
solution. Therefore, water molecules move from the dilute solution into
the concentrated solution.
• Osmosis affects the size and the mass of the living cells.
• Some examples of osmosis in living things:
2. - Uptake of water molecules from the soil by the root hair cells
- Absorption of water molecules in human large intestine
Plant cells
• The cell wall in plant cells is permeableto all substances.
• When a plant cell is placed in distilledwater:
1. The concentrationof water molecules is higher in the
surroundings than in the cytoplasm.
2. As a result, more water molecules enter the cell by osmosis
and create a pressure on the cell wall known as turgor pressure.
3. The turgorpressure makes the plant cell become firm or
turgid.
• When a plant cell is placed in a concentratedsolution:
1. The concentrationof water molecules is lower in the
surroundings than in the cytoplasm.
2. As a result, water molecules from the cell start to move out
form the cytoplasm into the concentrated solution by osmosis.
3. The cytoplasmpeels away from the cell wall and the cell
membrane becomes visible under the microscope.
4. The cell loses its turgidity. It is said to be plasmolysed.
3. Animal cell
• Animal cells do not have cell walls. Therefore, the effect of osmosis
will cause the cell to shrink or to burst.
• When an animal cell is placed in distilledwater:
1. The concentrationof water molecules is higher in the
surroundings than in the cytoplasm.
2. As a result,more water moleculesenter the cell by osmosis
and cause the cell to swell.
3. Due to the lack of a cell wall, the animal cell will burst eventually.
• When an animal cell is placed in a concentratedsolution:
1. The concentratedof water molecules is lower in the
surroundings than in the cytoplasm.
2. As a result, water molecules from the cell start to move out
form the cytoplasm into the concentrated solution by osmosis.
