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Topic 2- Gas Exchange
`Surface Area: Volume' Ratio
The rate of exchange of substances depends of the organism's surface area.
The ability of an organism to meet it requirements depends on the surface area: volume ratio.
As organisms get bigger their surface area: volume ratio decreases.
This makes it more difficult for larger organisms to exchange substances with their surroundings.
This means individual cells cannot get much bigger than 100m.
Large organisms need to develop specialised exchange surfaces with large surface areas.
Small multicellular organisms adapted their body shape to increase their surface area without needing to develop
specialised exchange surfaces.
Gas Exchange in Single-celled Organisms
Exchange substances through the cell membrane.
Single celled organisms can't ventilate.
Large SA:V ratio due to being just one cell.
Concentration gradient is maintained by using up O2 and producing CO2 in respiration.
As the exchange surface is only the plasma membrane there is a short diffusion pathway.
Gas Exchange in Insects
Fairly small organisms but also very active so they need to respire quickly.
Covered in a cuticle impermeable to water and gases.
Openings in the cuticle called spiracles.
Trachea leading from the spiracles which branch off into smaller tracheoles.
Tracheoles branch all over body leading directly to cells.
There is no blood transport system for gases.
Oxygen and carbon dioxide diffuse between the tracheoles and the cells down their concentration gradients.
Trachea and larger tracheoles are supported by chitin which is impermeable so gas exchange can only happen in the ends
of the tracheoles.
Insects can't ventilate so they rely on a concentration gradient for their supply of
The more an insect respires, the bigger the concentration gradient is so the faster the
rate of diffusion.
Gas exchange in insects is a passive process.
During exercise increased respiration produces chemicals such as lactic acid are
produced which lower the water potential in the cells. This causes water to move
back into the cells by osmosis.
Less water in the tracheoles means that oxygen can diffuse into cells quicker. This
means active cells are able to respire more quickly.
The rate of gas exchange can be increased by abdominal pumping.
Pumping of muscles squeezes the airways so that diffusion rate is increased.
Water loss is a problem for terrestrial organisms.
Water diffuses from the cells into the tracheoles down its water potential gradient.
Water diffuses into the tracheoles until there is no water potential gradient between the cells and the tracheoles.
The water in the bottom of the tracheoles can be a problem because it slows down the rate of gas exchange.
Spiracles can be closed to prevent water loss, although they cannot be kept shut because the insects need to exchange
The bigger the insect, the more spiracles, and therefore the more water loss.
The warmer and more humid the climate the bigger the insect because increased temperature increases the rate of
diffusion. Increased water in the atmosphere means less water loss.
Gas Exchange in Fish
Water contains less oxygen than air so fish need a more efficient gas exchange system than terrestrial organisms.
Several gill arches each with many filaments which in turn have many lamellae on them which provides a large surface
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