Introductory points.
- Internal cell environment different from outside/external.
- Exchange takes place at exchange surfaces.
- To enter/leave an organism, most substances must cross cell plasma membranes.
Comment on mass transport and its necessity.
Mass transport maintains the final diffusion gradients that bring substances to and from the cell membranes of individual cells.
Helps to maintain relatively stable environment of tissue fluid.
Most cells are too far from exchange surfaces to rely on diffusion alone to supply tissue fluid/remove waste from fluid.
Large organisms need a mass transport system, as diffusion of a substance to necessary cells would take too long, even if the outer surface could supply enough of the substance.
How does SA:V ratio affect exchange?
Smaller organisms have high SA:V ratios —> increase efficiency of gas exchange
What adaptations do some organisms have for exchange (in terms of SA:V ratio)?
- Flattened shape so that no cell is ever far from the surface.
- Specialised exchange surfaces with large areas to increase the SA:V ratio —> lungs in animals, gills in fish etc.
List common features of exchange surfaces.
- Thin so short diffusion distance.
- Selectively permeable.
- Movement of environmental medium to maintain concentration gradient.
Why are some exchange surfaces located inside organisms?
Many exchange surfaces are thin - need to be located inside organisms as they’re easily damaged.
=> therefore need to have a method of moving the external environmental medium over the surface —> ventilation of lungs etc.
What is the relationship between SA:V ratio and metabolic rate?
Smaller organisms —> higher SA:V ratio —> higher metabolic rate.
In endothermic and ectothermic animals, metabolic rate is inversely proportional to size.
How do single-celled organisms exchange gases?
- High SA:V ratio therefore oxygen absorbed across the body surface.
- Cell-membrane is the only barrier - cell wall is no additional barrier.
Structure of insects’ gas exchange system? How do insects exchange gases?
- Have an internal network of tracheae tubes for gas exchange, supported by strengthened rings to prevent collapse.
- 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?
- Oxygen used up as cells aerobically respire - [O2] at end of tracheoles is low.
—> O2 diffuses down gradient from atmosphere into tracheoles.
(opposite for CO2)
- Contraction of muscles in insects can squeeze the tracheae enabling mass movements of air in and out.
- 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.
- 1-3 all lead to faster diffusion.
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.
Limitations of insect gas exchange method?
Relying mostly on diffusion for gas exchange means the diffusion pathway needs to be short —> limits size of insects.
Suggest what causes spiracles to open.
Increasing [CO2].
Fossil 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.
Structure of fish gas exchange system?
- Relatively large organisms —> small SA:V ratio.
- Waterproof, gas-tight 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.
What are the advantages of the counter-current exchange principle?
80% of O2 in water absorbed, rather than 50% that would be absorbed if blood flow in same direction as water due to the equilibrium that would be reached.
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.
Comment on respiration-photosynthesis balance’s effect on plant gas exchange.
- 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.
- 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.
- b) Photosynthesis is not taking place:
- In the dark etc., O2 diffuses into leaf as it is constantly being used by cells during aerobic respiration.
Adaptations of the leaf for gas exchange.
- No living cell is far from the external air —> close to sources of O2/CO2.
- Diffusion takes place in gas phase - faster than in water.
- Many small stomata - no cell is far from a stoma and also therefore a short diffusion pathway.
- Many interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come into contact with mesophyll cells.
- Large SA of mesophyll cells for rapid diffusion.
Describe stomata and comment on their importance.
= Minute pores occurring mainly on leaves, especially on their underside.
- Each stoma surrounded by a pair of guard cells - can open and close the stomatal pore, controlling the rate of gas exchange.
- This is important as terrestrial organisms lose their water by evaporation —> must balance conflicting needs of gas exchange and water loss control by closing stomata at times when water loss would be excessive.
NB = some herbicides kill plants by shutting stomata —> respn and photo could continue for a short time by exchanging gases between them, but not indefinitely.
Comment on conflict between water loss and gas exchange.
- Terrestrial organisms must limit their water loss without compromising the efficiency of their gas exchange surfaces.
- Air at exchange surfaces (within the body) is almost 100% saturated with water vapour —> less evaporation of water from the exchange surface.
How do xerophytic plants limit water loss?
Can’t have a small SA:V ratio as large SA required for exchange and photosynthesis.
- Certain plants (xerophytes) have evolved a range of other adaptations to limit their water loss by transpiration.
1. Thick cuticle - waxy cuticle - as thickness increases, less water can escape via this route.
2. Rolling up of leaves - trap a region of still air under lower epidermis - high wp created as air becomes saturated with water - no wp gradient between inside/outside of leaf so no water loss.
3. Hairy leaves - traps a layer of air.
4. Stomata in pits/grooves - traps a layer of air.
