Organisms exchange substances with their environment

1. Why can't diffusion alone supply large organisms with enough oxygen?

1. They have a large surface area to volume ratio
2. They have a small surface area to volume ratio, so diffusion distance is too great relative to demand
3. They don't need oxygen
4. They have no cells

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B: They have a small surface area to volume ratio, so diffusion distance is too great relative to demand. As volume grows faster than surface area, exchange by diffusion becomes insufficient.

2. How do insects deliver oxygen to their tissues?

1. Through blood only
2. Directly via tracheae and tracheoles from spiracles
3. Through the exoskeleton by osmosis
4. Via gills

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B: Directly via tracheae and tracheoles from spiracles. Air moves through a network of tubes straight to respiring cells.

3. What is the advantage of countercurrent flow in fish gills?

1. It saves energy only
2. It maintains a concentration gradient along the whole gill, maximising oxygen absorbed
3. It prevents water loss
4. It increases blood pressure

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B: It maintains a concentration gradient along the whole gill, maximising oxygen absorbed. Blood is always meeting water with a higher oxygen concentration.

4. Which feature increases the internal surface area for gas exchange in a leaf?

1. Waxy cuticle
2. Air spaces in the spongy mesophyll
3. Guard cells
4. Xylem vessels

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B: Air spaces in the spongy mesophyll. These interconnected spaces let gases reach cells easily.

5. Why might insects close their spiracles?

1. To increase oxygen uptake
2. To reduce water loss
3. To speed up respiration
4. To let in more carbon dioxide

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B: To reduce water loss. This is a trade-off between gas exchange and water conservation.

6. Why do smaller organisms not need specialised gas exchange surfaces?

1. They have a large surface area to volume ratio, so diffusion alone is sufficient
2. They don't respire
3. They are always aquatic
4. They have no cells

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A: They have a large surface area to volume ratio, so diffusion alone is sufficient. Diffusion distance is short relative to their needs.

7. What is the function of tracheoles in an insect?

1. Carry blood
2. Deliver oxygen directly to respiring tissues
3. Filter air
4. Produce ATP

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B: Deliver oxygen directly to respiring tissues. They form the finest branches of the tracheal system.

8. A cube has sides of 2 cm. What is its surface area to volume ratio?

1. 6 : 1
2. 12 : 1
3. 3 : 1
4. 1.5 : 1

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C: 3 : 1. SA = 6 × 2² = 24 cm²; V = 2³ = 8 cm³; 24 ÷ 8 = 3, so 3 : 1. The ratio 6 : 1 belongs to a 1 cm cube.

9. A cubic cell doubles the length of each of its sides. What happens to its surface area to volume ratio?

1. It halves
2. It doubles
3. It stays the same
4. It falls to a quarter

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A: It halves. Surface area increases 4× but volume increases 8×, so the ratio halves (e.g. 6 : 1 for a 1 cm cube becomes 3 : 1 for a 2 cm cube).

10. Why do flying insects often pump their abdomens during flight?

1. To open and close the stomata
2. To ventilate tracheae and keep a steep O₂ gradient
3. To pump haemolymph carrying O₂ to muscles
4. To force oxygen through the cuticle

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B: To ventilate tracheae and keep a steep O₂ gradient. Abdominal pumping moves air in and out of the tracheae, speeding O₂ supply to flight muscles. Insect blood does not carry O₂, and insects have spiracles, not stomata.