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Diffusion & Osmosis (Chapter 13)
*Note: As part of this section, you must conduct an experiment to demonstrate osmosis.
Membranes surrounding the cell and within the cell are selectively permeable.
What does selectively permeable mean?
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A selectively permeable membrane allows some substances, but not all substances to pass
through it.
Examples: Cell membrane, mitochondrial membrane, nuclear membrane, chloroplast membrane
These membranes allow molecules such as water, oxygen and carbon dioxide to pass through freely,
but do not allow substances such as sugars, proteins and salts to pass through easily.
Diffusion
Definition: Diffusion is the spreading out of molecules from a region of high concentration to a
region of low concentration.
(Diffusion is said to be passive, as it doesn’t require external energy)
Examples: perfume spreading through a room, milk spreading in a cup of tea
Osmosis
Definition: Osmosis is the movement of water molecules across a semi-permeable membrane from
a region of high water concentration to a region of low water concentration.
Examples of osmosis: movement of water across cell membranes, movement of water through
visking tubing
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If an animal cell in put in a solution that is the same water concentration as the cytoplasm,
water will move in and out of the membrane at the same rate.
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If it is put into a solution with a higher water concentration than the cell, water will move into
the cell by osmosis, causing it to expand and possibly burst.
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In a solution of lower water concentration, water will move out of the cell, causing it to shrivel.
Plant Cells and Turgidity
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The cell wall of a plant cell is fully permeable to everything. The cell wall gives strength to the
plant cell.
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If the plant cell is placed in a solution of higher water concentration than its cytoplasm, water
will move into the cytoplasm and vacuole of the cell by osmosis.
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The cell begins to swell, but the rigid cell wall prevents it from swelling too much, so the plant
cell doesn’t burst.
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The plant cell is now said to be turgid.
What is turgor?
Turgor, or turgor pressure, is the pressure of the cytoplasm and vacuole against the cell wall of a
plant cell.
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Likewise, if a plant cell is placed in a solution of lower water concentration than its cytoplasm,
it loses water by osmosis.
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The loss of water from the cell and the movement of the cell membrane away from the cell wall
is known as plasmolysis.
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When all the cells of a plant are plasmolysed, the plant wilts.
Application of Osmosis in Food Preservation
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Bacteria and fungi are the most common causes of food spoilage.
Both these cells are enclosed by a membrane.
If a food is placed in a sugary or salty solution, then any bacteria or fungi present will lose the
water in their cells to the more concentrated solution outside and die.
Fish and Meat (bacon) may be stored in a salty solution
Jams, marmalades and tinned fruits are stored in a sugary solution
By the end of this section, you should be able to:
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understand that membranes in and surrounding the cell are selectively permeable
define diffusion and osmosis and give examples of both
define turgor and give a simple explanation of turgidity in plants
describe the applications of high sugar/salt concentration in food preservation
*describe an experiment to demonstrate osmosis