The process of diffusion and its importance in living organisms.
 Outline process of diffusion. ‘Diffusion = process by which a substance moves from a region of high
concentration of that substance to a region of lower concentration of the same substance.
 Briefly describe what affects diffusion :
- Concentration gradient.
- Distance over which diffusion occurs.
- Area over which diffusion occurs.
- Nature of structure over which diffusion
occurs.
- Size/nature of diffusing molecule.
Factors are summarised in Fick’s Law:
Diffusion S.A. x Difference in concentration.
Thickness of exchange surface.
 Different types of diffusion: - Osmosis. Gradient in water potential.
- Facilitated diffusion. Role of proteins in membrane.
 Plants:
Photosynthesis - CO2 diffuses into plant and Os diffuses out through
stomata. Respiration.
Root hairs - water enters root system by osmosis.
Transpiration - water diffuses out of plant.
 Gas exchange in animals - simple animals have large SA:volume and so do not need to have a
specialised gas exchange surface and/or transport system as the O2 demands of the animal are met by
simple diffusion. Eg. Cnidaria and platyhelminthes (no special gas exchange surface or transport),
annelids (no special gas exchange surface, but have transport system), arthropods (e.g. insect tracheal
system, no transport system for gases). Larger animals with small SA:volume need specialised gas
exchange surface and transport system.
 Gas exchange in mammals:
- Diffusion of O2 and CO2 in blood into/from alveoli in the lungs. Lungs = highly adapted
exchange surface. Ventilation movements help to maintain concentration gradients. Transport
system with haemoglobin.
Placenta - Diffusion from mothers blood into baby and from baby’s blood
to mother’s.
Water = transferred by osmosis. Ions = transferred by active transport.
Glucose = transferred by facilitated diffusion.
Excretory products (urea and CO2 also diffuse across).
 Gas exchange in fish:
- Diffusion of O2 from H2O into fish’s blood via the gills. CO2
diffuses out of blood in the same way.
- Fish = very good example of exchange surface well adapted
for diffusion. Counter-current exchange mechanism maintain conc. gradients.
 The Kidney:
- Water is reabsorbed from the nephron by osmosis.
- In the descending limb of the loop of Henle water moves out by
osmosis. In the ascending limb of the loop of Henle sodium and chloride ions
are pumped out by active transport.
- Water moves out of collecting duct by osmosis. Permeability affected by ADH.
 Absorption in the mammalian digestive system:
- Amino acids and glucose absorbed by diffusion.
- Water absorbed by osmosis in the small and large intestine.
- Fatty acids and glycerol combine with bile salts to form micelles. The
micelles dissolve in cell surface membrane and diffuse into cytoplasm
of the epithelial cells.
 Synapses:
- Calcium ions diffuse into the neurone after calcium channel proteins in the
presynaptic membrane have opened.
- Acetylcholine diffuses across synaptic cleft and binds to receptors on postsynaptic
membrane.