Gas exchange in single-celled organisms, insects and fishAQA A-Level Biology: Revision notes
Section 1
Single-celled organisms
In a single-celled organism such as Amoeba, oxygen and carbon dioxide diffuse across the cell surface membrane.
- The SA:V is large, so there is plenty of surface for the volume of cytoplasm
- The diffusion distance is very short
- Oxygen is used in aerobic respiration, so its concentration inside stays low and diffusion continues
No specialised gas exchange system is needed.
Section 2
The insect tracheal system
Insects have an internal system of air-filled tubes.
- Spiracles: openings on the surface of the thorax and abdomen that can open and close
- Tracheae: larger tubes that carry air into the body, supported by rings of chitin so they do not collapse
- Tracheoles: very fine branching tubes with thin walls that end next to or inside respiring cells
Oxygen diffuses along the tracheae and tracheoles down a concentration gradient and directly into the cells; carbon dioxide diffuses out. Branching gives a large surface area and short diffusion distance, and diffusion in air is fast. Blood does not carry oxygen.
Section 3
Insects: gas exchange against water loss
A gas exchange surface must be thin and moist, but this causes evaporation. Terrestrial insects show a compromise.
- Spiracles open to allow oxygen in and carbon dioxide out, but this lets water vapour escape
- Spiracles are closed as much as possible to reduce water loss, and opened when oxygen demand rises
- A waterproof cuticle and hairs around the spiracles (trapping moist air) reduce the water loss further
- The small number of spiracles limits the surface for evaporation
Do not say the spiracles are permanently open or closed. They open and close, which is the compromise.
Section 4
Gas exchange in fish
Water holds much less dissolved oxygen than air, so fish need very efficient gills.
- Each gill has many gill filaments, each with many gill lamellae (a very large surface area)
- Lamellae have a thin epithelium (short diffusion distance) and many capillaries (maintaining the gradient)
- Water is ventilated over the gills using the mouth and operculum
Water flows over the lamellae in one direction as blood flows through them in the opposite direction, which is the counter-current principle.
Section 5
Counter-current flow explained
With parallel flow, blood and water would flow in the same direction. Oxygen would diffuse until the saturation of both was equal, so only about half of the oxygen could be absorbed.
With counter-current flow, water meets blood of lower oxygen saturation at every point along the lamella. The concentration gradient is maintained along the whole length, equilibrium is never reached and a high percentage (more than 80%) of the oxygen is absorbed.
Worked example: water entering 90% saturated and leaving 20%: (90 − 20) ÷ 90 × 100 = 77.8% of the oxygen absorbed.
In a counter-current answer say gradient is maintained along the whole length, so equilibrium is not reached.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gas exchange in single-celled organisms, insects and fish
- Amoeba is a single-celled organism that lives in freshwater ponds. It takes in oxygen and loses carbon dioxide across its cell surface membrane and has no gas exchange organs.Explain how a concentration gradient for oxygen is maintained across the cell surface membrane of Amoeba.2 marks
- A locust is a terrestrial insect. Air enters through openings along the surface of its thorax and abdomen and passes into branching tubes that end next to the muscle cells. The outer surface of the locust is covered by a waterproof layer.Explain how the locust's gas exchange system is a compromise between gas exchange and limiting water loss.2 marks
- In a bony fish, water passes over the gills and flows across the gill lamellae in the opposite direction to the flow of blood in the capillaries inside each lamella. Water entering the gills is 90% saturated with oxygen and leaves 20% saturated. Blood entering the lamellae is 5% saturated with oxygen and leaves 85% saturated.Explain how counter-current flow allows the fish to absorb a high percentage of the oxygen from the water.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).