3.3.1 Surface area to volume ratio
13 March 2021 12:16
The relationship between the size of an organism or structure and its surface area to volume ratio.
Changes to body shape and the development of systems in larger organisms as adaptations that
facilitate exchange as this ratio reduces.
Larger organisms have mechanisms to ventilate system:
- Air sacs can be squeezed by muscles to squeeze air in and out.
- Flight muscles alter volume of thorax in insects(e.g. increase so more air is drawn in by
spiracles)
- Some insects have specialised breathing mechanisms e.g. grasshoppers whereby they expand
abdomen, more air enters abdomen and spiracles close at the back of the body and when they
contract abdomen, more air enters back of body as spiracles open at the back of body.
Beneficial as spiracle isn't always open so retains more water.
- Muscle cells respire anaerobically producing lactate. This decreases water potential in cells, so
tracheal fluid moves out of tracheoles into cells through osmosis, decreases pressure in
tracheoles so more air is drawn in from spiracle.
Students should be able to appreciate the relationship between surface area to volume ratio and
metabolic rate
The larger the organism, the smaller the surface to volume ratio.
Single celled organisms have large surface area to volume ratio so can get nutrients through direct
diffusion (absorb oxygen across cell surface)
Big mammals have specialised organ systems to get nutrients as they have low surface area to
volume ratio so has a lower rate of respiration so uses less oxygen.
The greater the mass of an organism, the higher the metabolic rate.
Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more
energy to maintain their body temperature
Organisms exchange substances with environment Page 1
, 3.3.2 Gas exchange
13 March 2021 12:17
Adaptations of gas exchange surfaces, shown by gas exchange:
• across the body surface of a single-celled organism in the tracheal system of an insect (tracheae,
tracheoles and spiracles)
- Many tracheoles so increases gas exchange surface area
- High respiration rate (so oxygen used up quickly) maintains steep concentration gradient.
- Abdominal pumping/tracheal compression increases and decreases volume in thorax by decreasing
and increasing volume in thorax, ventilation maintains steep concentration gradient.
- Diffusion distance kept small as tracheoles have one cell thick membrane and are close to respiring
cells so diffusion pathway is small.
1. Tracheoles have thin walls so short
diffusion distance to cells;
2. Highly branched/large number of
tracheoles so short diffusion distance to
cells;
3. Highly branched/large number of
tracheoles so large surface area (for gas
exchange);
4. Tracheae provide tubes full of air so fast
diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that
moves out (into tissues) during exercise
so faster diffusion through the air to the
gas exchange surface
When insect moves:
Tracheal fluid is at the end of tracheoles. When resting, tracheal fluid seeps into tracheoles. When
insect moves, respires anaerobically so lactate decreases water potential so fluid drawn in by
muscles, as it contains oxygen so it can be used for respiration, lowers pressure in tracheoles, which
draws in more air from spiracle.
Increases surface area for oxygen to diffuse through tracheal walls directly.
• across the gills of fish (gill lamellae and filaments including the counter-current principle)
Organisms exchange substances with environment Page 2
, - Many gill lamella on many gill filaments, increasing surface area
- Capillary network in every lamella, thin diffusion distance
- Maintains concentration gradient by counter current flow
Counterflow system
Water flow is in opposite direction to blood flow in capillaries. This means that high concentration of
oxygen in water always meets low concentration of oxygen in blood, so equilibrium not reached and
concentration gradient maintained throughout length of filament.
• by the leaves of dicotyledonous plants (mesophyll and stomata).
Structural and functional compromises between the opposing needs for efficient gas exchange and
the limitation of water loss shown by terrestrial insects and xerophytic plants.
Insects are surrounded by waxy cuticle for:
- Protection
- Water retention (also open and close spiracle for further water retention)
However, waxy cuticle (made of chitin) doesn’t allow for effective gas exchange therefore insects
have evolved to develop the tracheal system.
Stomata close at night when photosynthesis does not occur, to reduce water loss by evaporation.
Xerophytic plants are plants that are adapted to survive with limited water supply:
- Curled leaves to trap moisture so reduced water potential gradient so water doesn't evaporate
- Hairs to trap moisture
- Sunken stomata to trap moisture
- Thicker waxy cuticle to reduce evaporation of water
Organisms exchange substances with environment Page 3
13 March 2021 12:16
The relationship between the size of an organism or structure and its surface area to volume ratio.
Changes to body shape and the development of systems in larger organisms as adaptations that
facilitate exchange as this ratio reduces.
Larger organisms have mechanisms to ventilate system:
- Air sacs can be squeezed by muscles to squeeze air in and out.
- Flight muscles alter volume of thorax in insects(e.g. increase so more air is drawn in by
spiracles)
- Some insects have specialised breathing mechanisms e.g. grasshoppers whereby they expand
abdomen, more air enters abdomen and spiracles close at the back of the body and when they
contract abdomen, more air enters back of body as spiracles open at the back of body.
Beneficial as spiracle isn't always open so retains more water.
- Muscle cells respire anaerobically producing lactate. This decreases water potential in cells, so
tracheal fluid moves out of tracheoles into cells through osmosis, decreases pressure in
tracheoles so more air is drawn in from spiracle.
Students should be able to appreciate the relationship between surface area to volume ratio and
metabolic rate
The larger the organism, the smaller the surface to volume ratio.
Single celled organisms have large surface area to volume ratio so can get nutrients through direct
diffusion (absorb oxygen across cell surface)
Big mammals have specialised organ systems to get nutrients as they have low surface area to
volume ratio so has a lower rate of respiration so uses less oxygen.
The greater the mass of an organism, the higher the metabolic rate.
Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more
energy to maintain their body temperature
Organisms exchange substances with environment Page 1
, 3.3.2 Gas exchange
13 March 2021 12:17
Adaptations of gas exchange surfaces, shown by gas exchange:
• across the body surface of a single-celled organism in the tracheal system of an insect (tracheae,
tracheoles and spiracles)
- Many tracheoles so increases gas exchange surface area
- High respiration rate (so oxygen used up quickly) maintains steep concentration gradient.
- Abdominal pumping/tracheal compression increases and decreases volume in thorax by decreasing
and increasing volume in thorax, ventilation maintains steep concentration gradient.
- Diffusion distance kept small as tracheoles have one cell thick membrane and are close to respiring
cells so diffusion pathway is small.
1. Tracheoles have thin walls so short
diffusion distance to cells;
2. Highly branched/large number of
tracheoles so short diffusion distance to
cells;
3. Highly branched/large number of
tracheoles so large surface area (for gas
exchange);
4. Tracheae provide tubes full of air so fast
diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that
moves out (into tissues) during exercise
so faster diffusion through the air to the
gas exchange surface
When insect moves:
Tracheal fluid is at the end of tracheoles. When resting, tracheal fluid seeps into tracheoles. When
insect moves, respires anaerobically so lactate decreases water potential so fluid drawn in by
muscles, as it contains oxygen so it can be used for respiration, lowers pressure in tracheoles, which
draws in more air from spiracle.
Increases surface area for oxygen to diffuse through tracheal walls directly.
• across the gills of fish (gill lamellae and filaments including the counter-current principle)
Organisms exchange substances with environment Page 2
, - Many gill lamella on many gill filaments, increasing surface area
- Capillary network in every lamella, thin diffusion distance
- Maintains concentration gradient by counter current flow
Counterflow system
Water flow is in opposite direction to blood flow in capillaries. This means that high concentration of
oxygen in water always meets low concentration of oxygen in blood, so equilibrium not reached and
concentration gradient maintained throughout length of filament.
• by the leaves of dicotyledonous plants (mesophyll and stomata).
Structural and functional compromises between the opposing needs for efficient gas exchange and
the limitation of water loss shown by terrestrial insects and xerophytic plants.
Insects are surrounded by waxy cuticle for:
- Protection
- Water retention (also open and close spiracle for further water retention)
However, waxy cuticle (made of chitin) doesn’t allow for effective gas exchange therefore insects
have evolved to develop the tracheal system.
Stomata close at night when photosynthesis does not occur, to reduce water loss by evaporation.
Xerophytic plants are plants that are adapted to survive with limited water supply:
- Curled leaves to trap moisture so reduced water potential gradient so water doesn't evaporate
- Hairs to trap moisture
- Sunken stomata to trap moisture
- Thicker waxy cuticle to reduce evaporation of water
Organisms exchange substances with environment Page 3
