1. Exchange
Exchange between organisms and the environment
Gas exchange in single celled organisms and insects
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)
Structural and functional compromises between the opposing needs
for efficient gas exchange and the limitation of water loss shown by
terrestrial insects
Single celled organisms
They have a large surface area to volume ratio – oxygen is absorbed by
diffusion across their body surface which is covered by a CSM – CO2 also diffuses
out across their body surface.
Insects
Air moves through spiracles (which can be opened and closed by valves) into
the tracheae which are highly branched and supported by rings of chitin to
prevent from collapsing.
They branch into tracheoles
Diffusion gradient – when cells are respiring oxygen is used up and its
concentration towards the ends of the tracheoles falls – creates a diffusion
gradient that causes gaseous oxygen to diffuse from the atmosphere along the
tracheae and tracheoles to the cells. CO2 produced during respiration creates
diffusion gradient in the opposite direction – gaseous CO2 diffuse to
atmosphere – diffusion in air is much more rapid than in water.
Mass transport + abdominal pumping – contraction of muscles in insects can
squeeze the trachea enabling mass movements of air in and out – speeds up
exchange of respiratory gases
Water in ends of tracheoles – during periods of intense activity, muscle cells
around tracheoles respire anaerobically produces lactate which is soluble and
lowers water potential – water moves into the cells from tracheoles by
osmosis – water in ends of tracheoles decrease in volume and draws air in –
final diffusion pathway is gas rather than liquid- increases the rate air moved in
tracheoles but higher water evaporation
The valves in the spiracles can be closed to prevent water loss
Limitations
, Relies mostly on diffusion
For diffusion to be effective diffusion pathway needs to be short which
limits the size that insects can be
Exam question
Describe how the structure of the insect gas exchange system:
• provides cells with sufficient oxygen
• limits water loss.
1. Spiracles, tracheae, tracheoles;
2. Spiracles allow diffusion (of oxygen) OR
(Oxygen) diffusion through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface area (for exchange);
Accept ‘network’ or ‘large number’ for highly branched
If tracheae/tracheoles confused, penalise once only
4. Tracheole (walls) thin so short diffusion distance (to cells)
OR
Highly branched tracheoles so short diffusion distance (to cells)
OR
Tracheoles enter cells so short diffusion distance;
5. Tracheole walls are permeable to oxygen/air;
6. Cuticle/chitin/exoskeleton (impermeable) so reduce water loss;
Allow prevents water loss
2. Spiracles (can) close so no/less water loss
OR
Spiracles have valves so no/less water loss;
8. Hairs around spiracles reduce water loss;
Gas exchange in fish
Gas exchange across the gills of fish (gill lamellae and filaments
including the counter-current principle)
Gills are made up of gill filaments, at right angles are gill lamella which
increase the surface area.
Counter current system - water brought in in one direction and blood in
another - there will always be a concentration gradient between them
, Blood and water run in opposite directions
Blood will always be passing water with a higher concentration of oxygen
This maintains the concentration gradient across the length of the
gill - maximising the amount of oxygen taken into the blood through
diffusion
If the blood and water ran parallel to each other then only 50% of the oxygen
would be absorbed as it would reach an equilibrium
Gas exchange in the leaf of a plant
Gas exchange by the leaves of dicotyledonous plants (mesophyll and
stomata).
Structure Function
Waxy cuticle Prevents water loss from transpiration
Thin to absorb sunlight
Prevention of pathogens entering
Structural support
Guard cell Controls when the stomata opens and closes
Reduce water loss out of the stomata through
transpiration
Spongy mesophyll Air spaces where the gas can be exchanged
between the cells
Chloroplasts for photosynthesis
Palisade cells
packed close together with many chloroplasts
in order to have a high surface area to
increase the rate of photosynthesis
Air spaces Allows for gas exchange, allows a diffusion
pathway
Xylem
Exchange between organisms and the environment
Gas exchange in single celled organisms and insects
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)
Structural and functional compromises between the opposing needs
for efficient gas exchange and the limitation of water loss shown by
terrestrial insects
Single celled organisms
They have a large surface area to volume ratio – oxygen is absorbed by
diffusion across their body surface which is covered by a CSM – CO2 also diffuses
out across their body surface.
Insects
Air moves through spiracles (which can be opened and closed by valves) into
the tracheae which are highly branched and supported by rings of chitin to
prevent from collapsing.
They branch into tracheoles
Diffusion gradient – when cells are respiring oxygen is used up and its
concentration towards the ends of the tracheoles falls – creates a diffusion
gradient that causes gaseous oxygen to diffuse from the atmosphere along the
tracheae and tracheoles to the cells. CO2 produced during respiration creates
diffusion gradient in the opposite direction – gaseous CO2 diffuse to
atmosphere – diffusion in air is much more rapid than in water.
Mass transport + abdominal pumping – contraction of muscles in insects can
squeeze the trachea enabling mass movements of air in and out – speeds up
exchange of respiratory gases
Water in ends of tracheoles – during periods of intense activity, muscle cells
around tracheoles respire anaerobically produces lactate which is soluble and
lowers water potential – water moves into the cells from tracheoles by
osmosis – water in ends of tracheoles decrease in volume and draws air in –
final diffusion pathway is gas rather than liquid- increases the rate air moved in
tracheoles but higher water evaporation
The valves in the spiracles can be closed to prevent water loss
Limitations
, Relies mostly on diffusion
For diffusion to be effective diffusion pathway needs to be short which
limits the size that insects can be
Exam question
Describe how the structure of the insect gas exchange system:
• provides cells with sufficient oxygen
• limits water loss.
1. Spiracles, tracheae, tracheoles;
2. Spiracles allow diffusion (of oxygen) OR
(Oxygen) diffusion through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface area (for exchange);
Accept ‘network’ or ‘large number’ for highly branched
If tracheae/tracheoles confused, penalise once only
4. Tracheole (walls) thin so short diffusion distance (to cells)
OR
Highly branched tracheoles so short diffusion distance (to cells)
OR
Tracheoles enter cells so short diffusion distance;
5. Tracheole walls are permeable to oxygen/air;
6. Cuticle/chitin/exoskeleton (impermeable) so reduce water loss;
Allow prevents water loss
2. Spiracles (can) close so no/less water loss
OR
Spiracles have valves so no/less water loss;
8. Hairs around spiracles reduce water loss;
Gas exchange in fish
Gas exchange across the gills of fish (gill lamellae and filaments
including the counter-current principle)
Gills are made up of gill filaments, at right angles are gill lamella which
increase the surface area.
Counter current system - water brought in in one direction and blood in
another - there will always be a concentration gradient between them
, Blood and water run in opposite directions
Blood will always be passing water with a higher concentration of oxygen
This maintains the concentration gradient across the length of the
gill - maximising the amount of oxygen taken into the blood through
diffusion
If the blood and water ran parallel to each other then only 50% of the oxygen
would be absorbed as it would reach an equilibrium
Gas exchange in the leaf of a plant
Gas exchange by the leaves of dicotyledonous plants (mesophyll and
stomata).
Structure Function
Waxy cuticle Prevents water loss from transpiration
Thin to absorb sunlight
Prevention of pathogens entering
Structural support
Guard cell Controls when the stomata opens and closes
Reduce water loss out of the stomata through
transpiration
Spongy mesophyll Air spaces where the gas can be exchanged
between the cells
Chloroplasts for photosynthesis
Palisade cells
packed close together with many chloroplasts
in order to have a high surface area to
increase the rate of photosynthesis
Air spaces Allows for gas exchange, allows a diffusion
pathway
Xylem
