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Exchanging gases
Key understanding:
To discuss features of effective surfaces of
gaseous exchange and the mechanisms for gas
exchange in animals
Cellular Respiration
• What do we know?
C6H12O6 + 6O2
6CO2 + 6H2O + (36-38ATP)
• What are the inputs?
• What are the outputs?
• Carbon dioxide forms acid in solution so it must
be removed
• Oxygen needed in cellular respiration
• Some animals and plants can exchange gases
direct from cell→environment
• Others need a system
Mechanism for gas exchange in
Gas exchange
• Always takes place by diffusion across a moist
plasma membrane (extracellular fluid)
• Oxygen and carbon dioxide are uncharged (nonpolar) molecules so diffuse across membranes
• The rate of diffusion depends on: size and
maintenance of concentration gradient, and
properties of the membrane
Fick’s formula
Amount = Permeability x surface area x concentration gradient
Distance of diffusion
The following illnesses can effect gas exchange
because they affect one of the factors listed in
•Emphysema reduces the surface area of lungs
•Pneumonia increases distance of diffusion
•Anaemia reduces concentration gradient
Features of an efficient gas exchange
• Large surface area
• Thin barrier and made of material that allows
gas to pass through it easily
• Adequate supply of gas being transferred
• Efficient removal of substance after transfer
• Water flows in one direction: through the
mouth and pharynx, past the gills and out
under the operculum
• Countercurrent flow: blood flows in opposite
direction to the water so blood can extract up
to 90% of the oxygen in the water (mammals
can only extract 25%!)
• Air breathed in and passes into pharynx.
• From here it passes into the airways- the trachea,
paired bronchi and branching bronchioles.
• Terminal air sacs called alveoli.
• This is the site of gas exchange .
Features for efficient gas exchange
• Trachea and bronchi lined with ciliated cells
which produce mucus - traps dust and
bacteria and transports them back to the
pharynx, where they are swallowed
• Alveoli provide large surface area for gas
• Alveoli richly supplied with blood capillaries
• Diffusion barrier very thin
Respiratory pigments
• Increase the efficiency of gas transport in
• Consist of proteins complexed with iron and
• Haemoglobin is the most widely distributed
pigment found in all vertebrates and many
• Oxygen-carrying molecules
• Increasing the oxygen-carrying capacity of blood
reduces amount of energy needed to pump blood
• Mammalian respiratory pigment
• Increases oxygen-carrying capacity 100 fold
• In red blood cells
• Contains iron
• 4 oxygen molecules can combine with 1
haemoglobin molecule
In lungs
high O2
Hb + 4O2
Hb + 4O2
In tissue
low O2
•When haemoglobin combines with oxygen it is
called Oxyhaemoglobin
•Muscles contain type of haemoglobin called
Myoglobin. These carry a reserve store of oxygen.
What about the carbon dioxide?
• 7% dissolved in blood plasma
• 23% combines with haemoglobin molecules
forming carbaminohaemoglobin
• 70% converted to hydrogen carbonate ions in
red blood cells and transported to plasma
• Chapter 8 of Heinemann text
• Complete p145 -147 of Biozone. Use the
answer book for the questions.
• p148 Biozone – Respiratory pigments