Gas Exchange

Question 1

How does oxygen move through the insect?

Answer 1

1. Oxygen diffuses in through the spiracles
2. Spiracle closes
3. Oxygen moves through the trachea into the tracheoles
4. Oxygen delivered directly to the respiring tissues

Question 2

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange.

Answer 2

1. Tracheoles have thin walls so short diffusion distance to cells
2. Highly branched/large number of tracheoles so short diffusion distance to cells
3. Highly branched/large number of tracheoles so large surface area (for gas exchange)
   4.Tracheae provide tubes full of air so fast diffusion (into insect tissues)
4. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface
   OR
   Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange)
5. Body can be moved (by muscles) to move air so maintains diffusion/concentration gradient for oxygen/carbon dioxide

Question 3

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill.

Answer 3

1. Water and blood flow in opposite directions
2. Maintains diffusion/concentration gradient of oxygen OR Oxygen concentration always higher (in water)
3. (Diffusion) along length of lamellae/filament/gill/capillary

Question 4

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange.

Answer 4

1. Large surface area provided by many lamellae over many gill filaments
2. Increases diffusion/makes diffusion efficient
3. Thin epithelium/distance between water and blood
4. Water and blood flow in opposite directions/countercurrent
5. (Point 4) maintains concentration gradient (along gill)/equilibrium not reached
6. As water always next to blood with lower concentration of oxygen
7. Circulation replaces blood saturated with oxygen
8. Ventilation replaces water (as oxygen removed)

Question 5

Describe the gross structure of the human gas exchange system and how we breathe in and out.

Answer 5

1. Named structures: trachea, bronchi, bronchioles, alveoli
2. Above structures named in correct order
3. Breathing in: diaphragm contracts and external intercostal muscles contract
4. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in)
5. Breathing out: diaphragm relaxes and internal intercostal muscles contract
6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out)

Question 6

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf. (4)

Answer 6

1. (Carbon dioxide enters) via stomata
2. (Stomata opened by) guard cells
3. Diffuses through air spaces
4. Down diffusion gradient

Question 7

Explain why plants grown in soil with very little water grow only slowly.

Answer 7

1. Stomata close

2. Less carbon dioxide (uptake) for less photosynthesis/glucose production