Year 11 / Exchange of nutrients and wastes
Across the boundary.
A selectively permeable membrane separates the cell from its surroundings. In diffusion, particles move down their concentration gradient. Osmosis describes net water movement across a selectively permeable membrane toward lower water potential.
As a cell grows, volume rises faster than surface area. A lower surface-area-to-volume ratio makes exchange less effective per unit of living material. Active transport can move substances against a gradient using cellular energy.
Membrane exchange
Explore: Change the concentration on either side and watch the net movement reverse.
Compare cube-shaped cells with sides 1 mm and 2 mm.
- Their SA:V ratios are 6:1 and 3:1 respectively.
- The smaller cell has more exchange surface per unit volume.
Assumed knowledge
Concentration and ratios.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Predict net diffusion and osmosis.
- Relate surface area to exchange capacity.
READY TO TRY IT?