Mixing particles

If a crystal of a coloured soluble chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles.

When the potassium manganate(VII) has dissolved it becomes the solute.

The water is the solvent. The mixture that results is the solution.

The particles have diffused from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

The rate of diffusion

The rate of diffusion can be affected by several factors:

FactorEffect on diffusion
Concentration gradientThe greater the difference in concentration, the quicker the rate of diffusion.
TemperatureThe higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.
Surface area of the cell membrane separating the different regionsThe greater the surface area, the faster the rate of diffusion.

Diffusion, surface area and volume

In a bacterium, substances diffuse into and out of the bacterial cell across its surface. Once inside, because of the bacterium’s size, substances will need to diffuse 1 μm or less to where they are needed, for instance oxygen for aerobic respiration.

For simple multicellularorganisms such as small plants like mosses, substances diffuse into the leaves and simple roots over their surface. Again, once inside the plant, they don’t need to move far.

Substances move into and around the moss plants by diffusion and osmosis.

Simple organisms therefore take in substances all over their body surface. The amount of each substance they need is determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. This sets a size limit on organisms that can only use their body surface for taking in substances. Large organisms have a large volume but relatively small surface area. They have a small surface area:volume ratio and so need to evolve additional exchange surfaces to take in the materials they need.

The surface area to volume ratio of a puppy is several times greater than that of an adult dog.


Suggest why puppies are more at risk of losing body heat than adult dogs.

Dogs lose heat over their body surface. Puppies have a larger surface area to volume ratio than adult dogs, so will lose heat more readily.


Osmosis is the diffusion of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane.

A dilute solution contains a high concentration of water molecules while a concentrated solution contains a low concentration of water molecules.

Osmosis refers to the movement of water molecules only.

The image shows an example of osmosis showing the direction of movement of water between two different concentrations of sugar solutions:

Diffusion of sugar and water molecules in water

Water molecules move from solution one to solution two.

When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. There will be no net movement of water molecules. In theory, the level of solution two will rise, but this will be opposed by gravity and will be dependent on the width of the container.

Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar.

Osmosis across living cells

Cells contain dilute solutions of ions, sugars and amino acids.

The cell membrane is partially permeable.

Water will move into and out of cells by osmosis.

Plant cells

Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist, root hair cells will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.

Plant cells have a strong cellulosecell wall outside the cell membrane. The cell wall is fully permeable to all molecules and supports the cell and stops it bursting when it gains water by osmosis.

If plant cells are placed in solutions of increasing solute concentration:

Pure water

In pure water, the cell contents - the cytoplasm and vacuole – push against the cell wall and the cell becomes turgid.

Fully turgid cells support the leaves and the stems of non-woody plants.

Concentrated solutions

In a more concentrated solution, the cell contents lose water by osmosis. They shrink and pull away from the cell wall. The cell becomes flaccid. It is becoming plasmolysed.

Highly concentrated solution

In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.

Diagram showing the effects of osmosis in animals

Plants would be exposed to higher concentrations of solutes if there was less water in the soil - for instance, if plants were not watered, or plants in drought conditions. Plant cells would then lose water by osmosis.

Aquatic, freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also lose water by osmosis.

Animal cells

Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.

For example, red blood cells:

Red blood cells in different solutionsRed blood cells lose water and shrink in a concentrated solution. They swell and burst in a solution that is too dilute.

In animals, the concentration of body fluids - blood plasma and tissue fluid - must be kept within strict limits. If cells lose or gain too much water by osmosis, they do not function efficiently.