The rate of diffusion
The rate of diffusionThe movement of molecules from an area of higher concentration to an area of lower concentration. can be affected by several factors:
| Factor | How the factor affects the rate of diffusion |
| Concentration gradient | The greater the difference in concentration, the quicker the rate of diffusion |
| Temperature | The 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 different regions | The greater the surface area, the faster the rate of diffusion |
| Factor | Concentration gradient |
|---|---|
| How the factor affects the rate of diffusion | The greater the difference in concentration, the quicker the rate of diffusion |
| Factor | Temperature |
|---|---|
| How the factor affects the rate of diffusion | The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly |
| Factor | Surface area of the cell membrane separating different regions |
|---|---|
| How the factor affects the rate of diffusion | The greater the surface area, the faster the rate of diffusion |
The rate of diffusion is calculated by Fick’s law which is:
\(\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}}\)
Diffusion, surface area and volume
For 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 simple multicellular organisms, such as small plants such as 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 osmosisThe movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration..
Simple organismLiving entity, eg animals, plants or microorganisms. therefore take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. For example, the surface area to volume ratio of a puppy is several times greater than that of an adult dog.
Question
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.