A full summary of unit 2.2 adaptations to gas exchange for the syllabus of WJEC (Wales). It contains all the details that A-level students should memorise before entering the exam hall. There are also some answering techniques and answer formats for exam-type questions.
Adaptations for Gas Exchange
Gaseous exchange
= the exchange of gases between an organism & its environment across a respiratory surface by diffusion
Importance:
Transfer materials
Removal of waste
Obtain enough O2 to meet rapid metabolic demands
Properties of respiratory surfaces
Large surface area to volume ratio – fast rate in gas exchange
Thin – shortens diffusion pathway
Permeable to respiratory gases
Moist – dissolve gases
May hv good blood supply
*Surface area to volume ratio
Limits cell size
Estimate efficiency of exchange of materials
Vol inc., SA doesn’t inc. at same rate
Larger the organism, smaller the SA:V ratio, lower the efficiency
Unicellular organisms (e.g. Amoeba)
Prokaryotic
Thin, (partially) permeable cell membrane
Very large SA:V ratio
Slow-moving ∴ low metabolic rate, low O2 demands
Multicellular organisms
Hv aggregated cells ∴ small SA:V ratio -> need additional features
e.g. ventilation mechanism – maintain conc. gradient across respiratory surface
internal transport system – move gases between respiratory surface & respiring cells
respiratory pigment (e.g. Hb) – inc. O2 carrying capacity
, Flatworm & earthworm
Property Flatworm Earthworm
Shape Flat shape Tubular/cylindrical shape
Inc. surface area for gas exchange ∴increase SA:V ratio
Habitat Aquatic Damp soil
Prevent from desiccation
Respiratory surface Moist cells for gas exchange Moist cells for gas exchange
Secretes mucus? X Yes, to keep skin moist
Circulatory system? X Yes
Maintain diffusion gradient across skin
surface
Respiratory pigments? X Yes, haemoglobin
Inc. oxygen-carrying capacity
Insects
Prevent water loss by:
1. Impermeable exoskeleton
Rigid, thin, waxy layer, covered by waterproof chitin & protein
2. Small SA:V ratio
Minimise area over which water is lost
Tracheal system
Tracheae supported by strengthened chitin rings to prevent collapsing
Each trachea open/close to air through several paired holes/spiracles
-> exists in pairs & hv valves to control opening/closing
Tracheoles – inc. SA for gas exchange
Thin permeable walls – shorten diffusion distance
Carry air into air sacs for storage, use in very hot conditions
End of tracheoles are fluid filled & close to muscle fibres
O2 dissolves into fluid, diffuse directly into muscle cells
No need for blood circulatory system/respiratory pigments
During rest:
1. Gas exchange happens almost entirely by diffusion
2. O2 diffuses down the conc. gradient towards actively respiring cells
3. CO2 diffuses away from respiring cells
4. Simple diffusion is enough as O2 demand low
During activity:
1. Ventilation ensures mass flow of respiratory gases into/out of insect
2. Some spiracles open and others close whilst dilating and constricting abdomen
3. Air drawn in through spiracles of thorax
4. Abdomen move rhythmically to pump air out of spiracles
5. Air flows in one direction
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