Revision:Gcse biology - diffusion


Diffusion is relatively slow, however, the higher the high concentration the faster it goes. Also, heat and some movement can speed it up (e.g. stirring a beaker or a gentle breeze). Also bigger objects take longer to diffuse through because the volume to surface area ratio is too small. This is why mammals have a respiratory system to carry oxygen to the furthest tissues, and why amoebas can get all the oxygen they need from diffusion.

Diffusion in plants

  • Diffusion – The passive movement of particles from an area of high concentration to an area of low concentration
  • For photosynthesis to occur carbon dioxide must diffuse into the leaf (via the stomata) to the cells (especially the palisade cells). However, the oxygen and water vapour created by photosynthesis diffuses out of the leaf the same way, and this is called transpiration.
  • Transpiration happens fastest in hot dry windy conditions, and slowest in humid cool not windy conditions (like mist).

Diffusion in animals

  • Diffusion over a cell membrane only allows glucose or amino acids through, blocking proteins and starch. *This happens from a high concentration to a low one.
  • In the gut food is absorbed through villi. They have a large surface area, a moist wall, a good blood supply, thin walls, and the villi themselves are covered in micro-villi. When the villi absorb glucose and amino acids these substances are carried away by the blood, meaning that the concentration of glucose and amino acids in the blood is lower than in the gut so diffusion can occur successfully. However, if the level of useful substances is higher in the blood than in the gut then the body must use active transport to get the substances. This stops us
  • Active transport is the movement of a substance against the concentration gradient with the help of energy. This is shown best in root hairs, where there are more minerals in the root hair than in the soil, but minerals are still absorbed by the root hairs. This also happens in the gut (as shown above), and in the coiled tubules in the kidneys where glucose is re-absorbed.
  • The lungs contain millions of air sacs called alveoli which maximize the diffusion of carbon dioxide and oxygen.


The Lungs

The lungs have

  • Big surface area
  • Moist lining (moisture dissolves gasses)
  • Thin walls (alveoli have only 1 cell thick walls)
  • Copious blood supply
  • The capillaries have only 1 cell thick walls

Within the body’s normal cells, the carbon dioxide levels are higher than in the blood. However, this means that the carbon dioxide diffuse out of the cells into the blood which then heads back to the lungs for re-oxidation.


Osmosis is the same as diffusion but with water, and ONLY water, no other liquid, just water.

Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

Partial permeable membrane – a membrane with holes so small only water can get through nothing else. Visking tubing is an example of this, and is used in kidney dialysis machines (dialysis tubing). The water molecules pass both ways in a two way traffic system, but because there will be more on one side there is a steady net flow into the region where there is fewer molecules. This causes the side with the lower concentration of water to increase in volume and become diluted. Plant cells absorb water by osmosis, but the cell wall stops them exploding, so they become turgid and are useful is making a plant stand up. Animal cells will absorb water (this is called endosmosis). They swell up like a balloon and burst. However, in a salt solution the cell will give out water (exosmosises) and shrivel up.

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: