AQA GCSE Biology contents

Unit 1 · Cell Biology

Diffusion & Exchange Surfaces

3 min read · part of the full Exam Guide

Diffusion

  • Diffusion is the spreading out of particles of a substance in solution, or of a gas, giving a net movement from an area of higher concentration to an area of lower concentration
  • It is passive: no energy from the cell is needed
  • Substances that cross cell membranes by diffusion: oxygen into cells and carbon dioxide out during gas exchange, and the waste product urea from cells into the blood plasma, to be excreted by the kidneys
Factor Effect on rate of diffusion Why
Concentration gradient bigger difference, faster more particles move from the high side than come back
Temperature higher, faster particles have more energy and move faster
Surface area of the membrane larger, faster more space for particles to cross at once

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In the full topic

  • Diffusion
  • Surface area to volume ratio
  • Exchange surfaces
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