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Detailed notes following from the Biology B Edexcel textbook with the specification points and notes underneath - A level course
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TOPIC 4Gas Exchange
i Understand how insects, fish and mammals are adapted for gas exchange.
Gas exchange in fish
1. Mouth opens
2. Operculum closes
3. Buccal cavity floor lowers
4. Contracted muscles of operculum cause it to bulge outwards
5. Volume of opercular cavity
increases
6. Pressure decreases
7. Mouth closes
8. Floor of buccal cavity raises
9. Volume of opercular cavity
decreases
10. Pressure increases
11. Operculum opens due to high
pressure
12. Water flows out of cavity
Gas exchange in insects
Spiracles – hair on surface of insect that reduce water loss;
prevent dust and parasites entering. Open and close in response
to CO2 levels and determine rate of respiration through muscles
contracting and relaxing
Trachea – kept open by rings of chitin; end at respiring tissue; have fluid in them
when insect is less active to reduce dehydration
Air sacs – reservoir of air in larger insects; increase respiratory efficiency by
providing a large surface area for gas exchange; expand and contract using
movements of muscles
Mammalian gas exchange
Breathing
1. Diaphragm contracts and flattens
2. External intercostal muscles contract and internal
intercostal muscles relax
3. Ribs move upwards and outwards
4. Volume of thoracic cavity increases
5. Pressure decreases
6. Air moves down the pressure gradient passively from
the external atmosphere into the lungs
, Chemoreceptors
There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to
changes in carbon dioxide levels in the blood.
If the level is too high (the pH would drop, enzyme action would be affected with serious results),
impulses are sent from these cells to the inspiratory part of the centre so that breathing rate
increases.
This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes
in.
CORE PRACTICAL 7: Dissect an insect to show the structure of the gas exchange system, taking into
account the safe and ethical use of organisms.
ii Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces
in the leaf and lenticels.
Stomata – control water loss by transpiration; open in light and close in dark with large surface area;
site of gas exchange
Lenticel – pore on trunks of trees allowing direct gas exchange with the environment across
otherwise impermeable bark
Adaption Purpose
Large surface area To absorb more light
Thin Short distance for carbon dioxide to diffuse
into leaf cells
Chlorophyll Absorbs sunlight to transfer energy into
chemicals
Network of veins To support the leaf and transport water,
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