Unit 3 - Organisms exchange substances with their environment
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Summary Notes on Gas Exchange - AQA A Level Biology
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Unit 3 - Organisms exchange substances with their environment
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Summary notes on Gas Exchange - AQA A Level Biology
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Unit 3 - Organisms exchange substances with their environment
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For all parts of the specification you MUST learning the phrasing of the Learning Outcomes
3.3.2 Gas exchange
Summary notes
Learning Outcome
a Adaptations of gas exchange surfaces, shown by:
- Gas exchange across the body surface of a single celled organism.
b - In the tracheal system of an insect (tracheae, tracheoles and spiracles)
c - Across the gills of fish (gill lamellae and filaments including the counter-current principle)
d - By the leaves of dicotyledonous plants (mesophyll and stomata).
e Structural and functional compromises between the opposing needs for efficient gas exchange
and the limitations of water loss shown by terrestrial insects and xerophytic plants
f The gross structure of the human gas exchange system limited to the alveoli, bronchioles, bronchi,
trachea and lungs.
g The essential features of the alveolar epithelium as a surface over which gas exchange takes place
h Ventilation and the exchange of gases in the lungs. The mechanism of breathing to include the
role of the diaphragm and the antagonistic interaction between the external and internal
intercostal muscles in bringing about pressure changes in the thoracic cavity.
Students could be given the values of pulmonary ventilation rate (PVR) and one other measure,
requiring them to change the subject of the equation.
PVR = tidal volume x breathing rate
i Be able to interpret information relating to the effects of lung disease on gas exchange and / or
ventilation
j Be able to interpret data relating to the effects of pollution and smoking on the incidence of lung
disease.
k Be able to analyse and interpret data associated with specific risk factors and the incidence of lung
disease.
L Be able to evaluate the way in which experimental data led to statutory restrictions on the
sources of risk factors.
m Be able to recognise correlations and causal relationships
n Be able to use Spearman’s rank correlation.
3.3.2 a - Adaptations of gas exchange surfaces, shown by gas exchange across the body surface
of a single celled organism.
Single-celled organisms have no specialised gas exchange surfaces or transport systems. Instead,
they rely upon diffusion alone to exchange gases. Typically these organisms have flat cells or
foldings in their cell surface membrane. This increases their surface area (and therefore SA:VOL)
and ensures a short diffusion distance. Small multi-cellular organisms without specialised gas
exchange surfaces also typically have flat body shapes for the same reason as above.
Single-celled amoeba Multicellular flatworm
, 3.3.2 b - Adaptations of gas exchange surfaces in the tracheal system of an insect (tracheae,
tracheoles and spiracles)
3.3.2 e - Structural and functional compromises between the opposing needs for efficient gas
exchange and the limitations of water loss shown by terrestrial insects
As multicellular organisms with a small SA:VOL (compared to single celled organisms) , adult
insects cannot rely upon diffusion across their surface to exchange gases – the rate of diffusion
would be too slow. Instead, insects have a gas exchange system called the tracheal system. This is
not too dissimilar in structure to the human gas exchange system. Both systems work by a basic
principle that air enters an opening into the animal’s body and travels down a series of smaller
and smaller tubes deeper into the animal before gas exchange occrs.
Human
component of gas Insect
“equivalent”
exchange system
Mouth Spiracle
Trachea Tracheae
Bronchioles Tracheoles
Cartilage rings to Chitin rings to
support trachea support tracheae
The major differences are that a human only has one opening with the atmosphere (one mouth)
whereas insects typically have many openings (spiracles) along their abdomen. In addition,
Humans exchange gases into the blood from alveoli. The blood then carries oxygen and carbon
dioxide to/from muscles around the body. On the other hand, insect have no transport system so
exchanges gases directly from their tracheal system into muscles (not into blood)
Spiracle
Muscle (opening to
atmosphere)
Tracheae
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