3.1. Diffusion

 

Diffusion is the net movement* of particles 

• from a region of their higher concentration 
• to a region of their lower concentration 
• down a concentration gradient, 
• as a result of their random movement

*net movement: 

= particles moved through - moved back

In organisms, gases and solutes diffuse through the cell membrane, which is a selectively permeable membrane ('selective' because it only allows certain molecules to pass through, not all molecules).

Diffusion helps living organisms to:

• provide essential gases and solutes 
• remove poisonous substances or waste
• gas exchange for respiration (O2 and CO2)

Other examples of diffusion: 

• CO2 uses by plants for photosynthesis is diffuses from the air into the leaves, through the stomata (pores at the surface of leaves). There is a lower concentration of CO2 inside the leaf, as the cells are using it up. O2 (waste product of photosynthesis diffuses out in the same way). 
• Flowering plants use diffusion to attract pollinators like bees. 
• Some of the products of digestion are absorbed from the ileum of mammals by diffusion.

Energy for diffusion comes from the kinetic energy of random movement of molecules and ions.

For the organism, this is a passive action, it doesn't give the process any energy for it to happen.

Factors that favor diffusion:

1. Distance (the shorter the better), e.g. thin walls of alveoli and capillaries. 
2. Concentration gradient (the bigger the better). This can be maintained by removing the substance as it passes across the diffusion surface. (Think about oxygenated blood being carried away from the surface of alveoli). 
3. Size of the molecules (the smaller the better). 
4. Surface area for diffusion (the larger the better). 
5. Temperature (molecules have more kinetic energy at higher temperature).

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