fish + insects

Question 1

Why do bony fish need a specialised gas exchange surface (gills)? (2)

Answer 1

They are multicellular, with a small surface area to volume and impermeable skin

So they need gills for gas exchange

Question 2

How do gills provide a large surface area for gas exchange? (2)

Answer 2

Have thin branches called gill filaments/primary lamellae and gill plates/secondary lamellae

Which further increase their SA

Question 3

What is the counter-current system in fish gills? (4)

Answer 3

Blood flows through gill plates in one direction

Water flows in the opposite direction

Maintaining a large concentration gradient between the water and blood

The concentration of oxygen in the water is always higher than in the blood, maximizing oxygen diffusion into the blood

Question 4

How do fish ventilate their gills? (5)

Answer 4

1. Fish opens its mouth
2. Lowering the floor of the buccal cavity
3. Increasing the volume and decreasing pressure, which sucks water in
4. When the fish closes its mouth, the floor of the buccal cavity is raised
5. Decreasing volume and increasing pressure, which forces water over the gill filaments

Question 5

What is the operculum, and what is its function? (1)

Answer 5

Bony flap that covers and protects the gills in fish

Question 6

Why do insects need a specialised gas exchange surface? (2)

Answer 6

They have a hard exoskeleton unsuitable for gas exchange

So they require a tracheal system to transport oxygen directly to respiring cells

Question 7

How does the tracheal system in insects work? (3)

Answer 7

1. Air enters through spiracles (pores on the insect’s surface)
2. Oxygen travels down its concentration gradient through tracheae and tracheoles, which have thin, permeable walls and lead to individual cells
3. Oxygen dissolves in the fluid inside the tracheoles and diffuses into cells, while carbon dioxide moves in the opposite direction

Question 8

How do insects ventilate their tracheal system? (3)

Answer 8

1. Rhythmic abdominal movements change the volume of their bodies
2. Moving air in and out of the spiracles
3. Larger insects use wing movements to pump their thoraxes

Question 9

How do you dissect the gaseous exchange system of a bony fish? (3)

Answer 9

1. Place the fish on a dissection tray
2. Push back the operculum and use scissors to remove the gills
3. Cut each gill arch through the bone at the top and bottom to see the gill filaments

Question 10

How do you dissect the gaseous exchange system of an insect? (4)

Answer 10

1. Fix the insect (e.g., grasshopper or cockroach) to a dissection board using dissecting pins
2. Cut and remove a piece of the exoskeleton from the abdomen
3. Fill the abdomen with saline solution using a syringe to reveal tracheae, which appear as silver-grey tubes filled with air
4. Examine the tracheae under a microscope using a wet mount slide

Question 11

What structures can be seen in the insect trachea under a microscope? (2)

Answer 11

Rings of chitin in the walls of the tracheae

Which function to support the tracheal tubes