WJEC AS Biology Unit 2 Topic 2 Gas Exchange - Basic Revision Notes

Adaptations for Gas Exchange

• Gas exchange happens over a respiratory surface.  

  • Total oxygen requirement is proportional to volume.  

  • Rate of absorption is proportional to surface area.

  • Surface area to volume ratio of an organism decreases as size of organism increases.

• The essential features of a gas exchange surface are

  • Permeable

  • Large surface area to volume ratio

  • Thin, short diffusion pathway

  • Mechanism to produce steep gradient

• Surface area to volume ratio affects the level of activity of an organism.  As organisms have increased over time they've developed ways to overcome this

• In small organisms, diffusion can happen across general body surface.  

• Multicellular organisms need a ventilation system to maintain concentration gradient, and must have respiratory surfaces inside as they are thin walled they will be fragile so need protection.

• Respiratory surfaces are adapted to the environment.  Fish gills are adapted to water, mammal lungs are adapted for terrestrial environments.

  • E.G. Lungs are adapted because water molecules pass through gas exchange surface keeping them moist and therefore likely to lose water.  So, they are inside the body to minimise loss of water and heat.

Adaptations for Gas Exchange in Insects

• Insects use a tracheal; system which is adapted to this gas exchange.

• They have an impermeable outside to reduce water loss by evaporation - their exoskeleton is  a thin waxy layer over a thin layer of chitin and protein.

• Adult insects have pairs of spiracles on thorax and abdomen.  They lead to branched air tubes called tracheae, which branch into smaller tracheoles.  

• Tracheoles enter cells and are the site of gas exchange.

• The spiracles open and close so gas exchange can take place and reduce water loss.  

• When they are resting, insects rely on diffusion through these for gas exchange.

• Whole body contractions aid ventilation by speeding up movement of air through spiracles.  These movements of the abdomen happen more during activity.  

• The tracheoles are fluid filled and close to muscle cells.  Oxygen dissolves into the fluid into muscle, carbon dioxide does the opposite.

• No respiratory pigment or blood circulation is needed.

Adaptations for Gas Exchange in Fish

• Gills are the respiratory surface in fish, they have…

  • One way current kept flowing by a ventilation system

  • Many folds making for a large surface area.  Extensions (gill lamellae and filaments) make large surface area.

  • Large surface area because water flowing through prevents the gills collapsing.  

  • Specialised surface rather than using the whole body.

  • Extensive network of blood capillaries.

• Cartilaginous Fish

  • Gills in five spaces on either side called gill pouches, which open to gill slits.

  • There ventilation system is less efficient than bony fish.

  • No mechanism to force water over gills so they must keep swimming.

  • Use a system of parallel flow - blood travels through capillaries in same direction water goes over gills.  Therefore diffusion can only happen to 50% maximum value.  

  • Gas exchange occurs over part of the gill lamellae until oxygen in blood = oxygen in water.

• Bony Fish

  • Gills are covered…