, AQA A Level Biology: 3.3
Organisms exchange
substances with their
environment
Describe the relationship between the size of an organism and its SA:V ratio
Smaller organisms have high SA:V ratios
Larger organisms have low SA:V ratios
What adaptations do some organisms have for exchange (in terms of SA:V ratio)?
The flattened shape so that no cell is ever far from the surface. The specialised exchange
surfaces with large areas to increase the SA:V ratio ---> lungs in animals, gills in fish etc.
What is the relationship between SA:V ratio and metabolic rate?
Smaller organisms ---> higher SA:V ratio ---> higher metabolic rate.
In endothermic and exothermic animals, metabolic rate is inversely proportional to size.
Describe the adaptations of gas exchange across the body surface of a single-celled
organism?
1. High SA:V ratio so short diffusion pathway allowing oxygen to be absorbed across the
body surface.
2. Cell-membrane is the only barrier - cell wall is no additional barrier.
Describe the adaptions of gas exchange across the tracheal system of an insect?
Insects have an internal network of tracheae tubes for gas exchange, supported by
strengthened rings to prevent collapse.
The tracheae extend and divide into smaller tracheoles, which extend throughout all
body tissues of the insect - oxygen brought directly to the respiring tissues ---> short
diffusion pathway from tracheoles to any body cell.
,How do insects exchange gases?
1. Oxygen used up as cells aerobically respire - [O2] at end of tracheoles is low ---> O2
diffuses down the concentration gradient from atmosphere into tracheoles
2. Contraction of muscles in insects squeezes the tracheae enabling mass movements of
air in and out.
3. Tracheoles are filled with water at their ends; soluble lactate produced by anaerobic
respiration during periods of major activity, lowering wp of muscle cells ---> water drawn
in by osmosis, reducing volume of tracheoles, drawing in air
=> final diffusion pathway is in a gas phase > liquid so faster diffusion, but some water
lost by evaporation.
How do insects limit water loss?
Thin, permeable surface with a large area conflict with the need to conserve water --->
need to compromise and balance opposing needs of exchanging respiratory gases
limiting water loss.
1. Small SA:V ratio - minimises area over which water is lost.
2. Waterproof coverings - over body surfaces - rigid outer-skeleton of chitin, covered with
a waterproof cuticle.
3. Spiracles - can be closed to reduce water loss - conflicts with the need for oxygen so
occurs briefly when insect is at rest.
Explain the limitations of the gas exchange method for insects?
Relying mostly on diffusion for gas exchange means the diffusion pathway needs to be
short ---> limits size of insects.
Fossilised insects were much larger. Suggest how the composition of the atmosphere
then compares to now.
Then, [O2] in atmosphere was higher.
Short diffusion pathway not as essential as now, as larger [O2] gradient so adequate O2
available for insects to be larger.
Describe the adaptations of gas exchange surfaces across the gills of fish?
- Relatively large organisms ---> small SA:V ratio.
, - Waterproof outer covering => specialised internal gas exchange surface (gills) as body
surface inadequate for gas exchange.
- Gills made up of gill filaments, with gill lamellae perpendicular to filaments => increase
SA of gas exchange surface.
How do fish exchange gases?
- Countercurrent exchange principle:
1. Water flowing over gill lamellae flows in opposite direction to blood flow in gill
lamellae.
2. Blood already well loaded with O2 meets water with max [O2] (and vice versa) --->
diffusion as still a gradient.
3. => Maintains a favourable O2 concentration gradient along the entire gas-exchange
surface - gill lamellae.
Suggest why one-way flow of gases is advantageous for the fish.
Less energy is required, because flow does not have to be reversed, which is important
and water is dense and difficult to move.
Suggest difference in gills for more metabolically active fish.
More gill lamellae and more gill filaments => larger SA of gills.
Describe the adaptations of gas exchange surfaces by the leaves of dicotyledonous
plants
1. Volumes of and types of gases being exchanged depend on balance between
respiration and photosynthesis - at times gases from each process can be used in the
other.
2. a) Photosynthesis is taking place:
- Some CO2 from respiration, but most from external air.
- Some O2 used in respiration is from photosynthesis, but most diffuses out of the plant.
2. b) Photosynthesis is not taking place:
Organisms exchange
substances with their
environment
Describe the relationship between the size of an organism and its SA:V ratio
Smaller organisms have high SA:V ratios
Larger organisms have low SA:V ratios
What adaptations do some organisms have for exchange (in terms of SA:V ratio)?
The flattened shape so that no cell is ever far from the surface. The specialised exchange
surfaces with large areas to increase the SA:V ratio ---> lungs in animals, gills in fish etc.
What is the relationship between SA:V ratio and metabolic rate?
Smaller organisms ---> higher SA:V ratio ---> higher metabolic rate.
In endothermic and exothermic animals, metabolic rate is inversely proportional to size.
Describe the adaptations of gas exchange across the body surface of a single-celled
organism?
1. High SA:V ratio so short diffusion pathway allowing oxygen to be absorbed across the
body surface.
2. Cell-membrane is the only barrier - cell wall is no additional barrier.
Describe the adaptions of gas exchange across the tracheal system of an insect?
Insects have an internal network of tracheae tubes for gas exchange, supported by
strengthened rings to prevent collapse.
The tracheae extend and divide into smaller tracheoles, which extend throughout all
body tissues of the insect - oxygen brought directly to the respiring tissues ---> short
diffusion pathway from tracheoles to any body cell.
,How do insects exchange gases?
1. Oxygen used up as cells aerobically respire - [O2] at end of tracheoles is low ---> O2
diffuses down the concentration gradient from atmosphere into tracheoles
2. Contraction of muscles in insects squeezes the tracheae enabling mass movements of
air in and out.
3. Tracheoles are filled with water at their ends; soluble lactate produced by anaerobic
respiration during periods of major activity, lowering wp of muscle cells ---> water drawn
in by osmosis, reducing volume of tracheoles, drawing in air
=> final diffusion pathway is in a gas phase > liquid so faster diffusion, but some water
lost by evaporation.
How do insects limit water loss?
Thin, permeable surface with a large area conflict with the need to conserve water --->
need to compromise and balance opposing needs of exchanging respiratory gases
limiting water loss.
1. Small SA:V ratio - minimises area over which water is lost.
2. Waterproof coverings - over body surfaces - rigid outer-skeleton of chitin, covered with
a waterproof cuticle.
3. Spiracles - can be closed to reduce water loss - conflicts with the need for oxygen so
occurs briefly when insect is at rest.
Explain the limitations of the gas exchange method for insects?
Relying mostly on diffusion for gas exchange means the diffusion pathway needs to be
short ---> limits size of insects.
Fossilised insects were much larger. Suggest how the composition of the atmosphere
then compares to now.
Then, [O2] in atmosphere was higher.
Short diffusion pathway not as essential as now, as larger [O2] gradient so adequate O2
available for insects to be larger.
Describe the adaptations of gas exchange surfaces across the gills of fish?
- Relatively large organisms ---> small SA:V ratio.
, - Waterproof outer covering => specialised internal gas exchange surface (gills) as body
surface inadequate for gas exchange.
- Gills made up of gill filaments, with gill lamellae perpendicular to filaments => increase
SA of gas exchange surface.
How do fish exchange gases?
- Countercurrent exchange principle:
1. Water flowing over gill lamellae flows in opposite direction to blood flow in gill
lamellae.
2. Blood already well loaded with O2 meets water with max [O2] (and vice versa) --->
diffusion as still a gradient.
3. => Maintains a favourable O2 concentration gradient along the entire gas-exchange
surface - gill lamellae.
Suggest why one-way flow of gases is advantageous for the fish.
Less energy is required, because flow does not have to be reversed, which is important
and water is dense and difficult to move.
Suggest difference in gills for more metabolically active fish.
More gill lamellae and more gill filaments => larger SA of gills.
Describe the adaptations of gas exchange surfaces by the leaves of dicotyledonous
plants
1. Volumes of and types of gases being exchanged depend on balance between
respiration and photosynthesis - at times gases from each process can be used in the
other.
2. a) Photosynthesis is taking place:
- Some CO2 from respiration, but most from external air.
- Some O2 used in respiration is from photosynthesis, but most diffuses out of the plant.
2. b) Photosynthesis is not taking place:
