Page 11 - 02 Cells Biology Notes

Importance of gaseous and solute diffusion

Diffusion helps living organisms to:

• obtain many of their requirements
• get rid of many of their waste products
• gas exchange for respiration

Examples

• 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.

Site of diffusion Substance Description
Alveoli of lungs O2 Alveoli à Blood capillaries
CO2 Blood capillaries à Alveoli
Stomata of leaf O2 Air spaces of leaf à Atmosphere

Factors favoring diffusion

• Distance (the shorter the better), e.g. thin walls of alveoli and
capillaries.
• 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).
• Size of the molecules (the smaller the better).
• Surface area for diffusion (the larger the better).
• Temperature (molecules have more kinetic energy at higher
temperature).

Importance of water as a solvent

• Most cells contain about 75% of wate.
• Many substances move around a cell dissolved in water.
• Many important reactions take place in water.