Biology Assignments – Fish ANSWERS
In order to live, fish must extract oxygen from the water and transfer it to their bloodstream.
Water is a thousand times denser (heavier per unit volume) than air, and at 20 C it has 50 times
more viscosity (resistance to flow) than air and contains only 3% as much oxygen as does an
3equal volume of air.
1. Explain why it is more difficult and requires more energy to extract oxygen from water
than from air.
Ventilation of the GE system requires a lot of energy. In addition water contains only 3% as
much oxygen as does an equal volume of air.
2. Describe some of the advantages of gas exchange in an aquatic environment.
Water is a dense medium; therefore it supports the gill structure and prevents it from collapsing.
Organisms in an aquatic environment do not need to worry about the gas exchange surface
drying out.
3. Name the organ system used by fish for gas exchange
Gills consisting of a bony arch, gill filaments and lamellae
4. Describe how fish keep their gas-exchange surface moist.
They live in an aquatic environment
5. Explain how fish provide the support for the gas-exchange surface
Water is a dense medium and so it provides support for the GE surface. Fish do not need
specific support.
6. Explain how the structure of the gas-exchange system of fish allows for a large surface
area.
A single gill of a bony fish consists of a curved gill arch bearing a V-shaped double row of gill
filaments. Each filament has many minute folds in its surface, lamellae, giving it a fuzzy
appearance and increasing the surface area along a given length of filament.
7. Explain how fish ventilate the gas-exchange surface
The pumping action of the mouth passes water over the gills. The pharynx and the opercular
chambers open and water is drawn in through the mouth. The mouth closes and the pharynx
and the opercular chambers contract. The membrane at the edge of the operculum is pushed
open, forcing water over the gills.
8. How do fish prevent the gas-exchange surface from becoming blocked?
Gill rakers strain food and other particles out of the water before it passes over the gill
9. How does the counter-current circulation of bony fish increase the rate of diffusion across
the gas-exchange surface?
By using counter-current circulation in the gills, the blood in the filament-folds travels forward in
the opposite direction to the water flow so that a constant imbalance is maintained between the
lower amount of oxygen in the blood and the higher amount in the water ensuring passage of
oxygen to the blood.
10. Explain how fish transport oxygen from the gas-exchange surface to other body cells and
carbon dioxide form body cells to the GE surface.
At the GE surface oxygen diffuses through the thin membrane into blood vessels surrounding it.
RBC’s contain haemoglobin which carries oxygen. Oxygen binds with the haemoglobin to form
oxyhaemoglobin. As the blood flows through the body cells it changes back to haemoglobin.
Carbon dioxide diffuses into the plasma component of blood which is 90%water. It combines
with water to form carbonic acid and then hydrogen and bicarbonate. When the blood reaches
the GE surface the reaction reverses to release CO2