A student builds three artificial exchange surfaces to investigate glucose diffusion across the wall of the small intestine. The student uses this relationship.
rate of diffusion∝surface area×concentration differencethickness of membrane \text{rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}} rate of diffusion∝thickness of membranesurface area×concentration differenceThe measurements are shown in the table.
| Model exchange surface | Surface area (cm²) | Concentration difference (mmol dm⁻³) | Membrane thickness (μm) |
|---|---|---|---|
| Normal | 180 | 8.0 | 20 |
| A | 135 | 12.0 | 30 |
| B | 150 | 8.0 | 25 |
The student needs a model with a rate of diffusion of at least 70% of the rate across the normal surface.
Use calculations to show which of models A and B meets this requirement.
The concentration difference across model B is increased from 8.0 mmol dm−38.0\,\mathrm{mmol\,dm^{-3}}8.0mmoldm−3 to 12.0 mmol dm−312.0\,\mathrm{mmol\,dm^{-3}}12.0mmoldm−3. No other measurement changes.
Explain which of models A and B would then have the higher rate of diffusion.
102 exam-style questions on Edexcel GCSE Biology 8.1 Exchange surfaces and diffusion, covering 8.1.1 Introduction to exchange and transport, 8.1.2 Alveoli adapted for gas exchange, 8.1.3 Factors affecting rate of diffusion, and 8.1.4 Rate of diffusion using Fick's law. Each one has a worked solution and a mark scheme showing where the marks go.