Diffusion, osmosis and active transport
Diffusion
Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated.
No energy is needed. Oxygen and carbon dioxide in gas exchange and urea leaving cells all move this way.
It is faster with:
- a bigger difference in concentration (a steeper gradient)
- a higher temperature
- a bigger surface area of membrane
- a shorter distance (a thinner membrane)
Small organisms have a big surface area to volume ratio, so diffusion through their surface is enough. Big ones need exchange surfaces such as lungs and gills.
- Cube of side 2 cm: surface area 6 × 2 × 2 = 24 cm²
- volume 2 × 2 × 2 = 8 cm³
- ratio 24 ÷ 8 = 3 : 1
Osmosis and active transport
Osmosis is the movement of water through a partially permeable membrane from a dilute solution to a more concentrated one.
In pure water an animal cell swells and may burst; a plant cell swells but its wall stops it bursting (it becomes turgid).
Active transport moves substances against the concentration gradient, from dilute to concentrated, using energy from respiration. Root hairs take in mineral ions this way, and the gut absorbs sugar into the blood.
Required practical: potato and osmosis
Cut equal cylinders, blot and weigh them, leave each in a different sugar concentration for the same time, blot and weigh again.
The cylinders start at different masses, so compare percentage change.
percentage change = (final mass − starting mass) ÷ starting mass × 100
- Start 2.50 g, end 2.35 g
- (2.35 − 2.50) ÷ 2.50 × 100 = −6.0%
Plot percentage change against concentration. Where the line of best fit crosses 0% there is no net movement of water: that concentration matches the cell sap.
A loss in mass is negative. Divide by the starting mass, not the final one.
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