Year 8 Science · Unit 1 · Lesson 6
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s explanation
Every cell in a large organism needs water, gases and nutrients, and its wastes must be removed. A single-celled organism gets its oxygen through blood vessels that run through its cell. As an organism gets bigger, its surface area to volume ratio increases, so it has more surface for each part of its body. Many cells in a large organism are deep inside, far from the surroundings. Plants do not need a transport system because they are too small.
1. Write the three corrections.
Double the size
Treat both models as cubes and complete Table 1. A cube has 6 faces, each side × side; its volume is side × side × side. SA:V = total face area ÷ volume.
| Model | Side (mm) | Outside surface (mm²) | Body volume (mm³) | SA:V |
|---|---|---|---|---|
| Small | 3 | |||
| Large | 6 |
Source: model values; you calculate them.
2. Complete Table 1. Show your working for the large cube.
Large cube SA:V (no unit)
3. The side length doubled. Work out how many times the surface area and the volume each increased. Use this to explain what happens to the oxygen supply of cells at the centre of the larger organism if it has no transport system.
Surface area × Volume ×
Evaluate the claim
Someone claims
“Insects don’t need lungs, because tiny air tubes called tracheae carry oxygen from holes in their body wall straight to their tissues. So an insect could grow to any size. It would just need more tubes. The giant insects in old sci-fi movies could live today.”
Fact file
Oxygen moves along the tracheae mainly by diffusion (some large insects help by pumping their bodies). Insects do have a simple tube-shaped heart, but their blood carries very little oxygen.
4. Identify one part of the claim that is supported by science.
5. Explain why “an insect could grow to any size” is not supported. Use the fact file and your answers from Your Turn.
6. A scientist wants to test whether the tracheal system limits insect size. Suggest two pieces of evidence or measurements that would help, and say what result would support the idea.
7. Giant wētā from New Zealand are among the heaviest insects in the world. Like other insects, they rely on a tracheal system, not lungs and blood, to deliver oxygen to their cells. Suggest why even the heaviest insects are still much smaller than animals such as dogs or kangaroos.
8. Compare how insects and mammals get oxygen to the cells deep inside their bodies. Then evaluate the statement: “The bigger an organism is, the more its cells depend on a transport system.”
Find the errors
1. (i) A single cell has no blood vessels; oxygen diffuses across its cell membrane. (ii) SA:V decreases as size increases (less surface per part of the body). (iii) Plants do need transport: they are multicellular and move substances through roots, stems and leaves. [3 marks]
Double the size
2. Small: SA = 6 × 3 × 3 = 54 mm²; V = 27 mm³; SA:V = 2. Large: SA = 6 × 6 × 6 = 216 mm²; V = 216 mm³; SA:V = 1. [3 marks]
3. SA ×4 (216 ÷ 54); volume ×8 (216 ÷ 27). Volume outgrows the surface oxygen enters through (SA:V halves) and the centre is twice as far away, so diffusion reaches central cells too slowly and they run short of oxygen. [4 marks: 2 factors; outgrows surface; central cells short]
Evaluate the claim
4. Insects have no lungs, and tracheae do carry air from openings in the body wall to the tissues. [1 mark]
5. Diffusion along tracheae is only fast over short distances. A bigger insect has longer tubes and its volume outgrows its surface (as in the cubes). More tubes do not fix the distance, and the blood carries little oxygen, so size is limited. [3 marks]
6. Any two, with a supporting result: rear insects in extra oxygen (they grow larger); compare how much body space tracheae take in large and small species (much more in large); compare fossil insect sizes with past oxygen levels (biggest when oxygen was higher). [3 marks: 1 per piece of evidence, 1 result]
Show What You Know
7. Tracheae rely mostly on diffusion, which only works over short distances. In a dog-sized insect, deep cells would be too far from the air openings to be supplied. Mammals avoid this by pumping oxygen-carrying blood. [2 marks]
8. Marking guide: 1 insect method (tracheae carry air to tissues, mainly by diffusion); 1 mammal method (lungs, and a heart pumping oxygen-carrying blood to every cell); 1 similarity or difference (both bring oxygen close to every cell; insect blood carries little oxygen); 1 SA:V or distance explains why size matters; 1 evidence from the sheet (cubes or insect size limit); 1 judgement with a limitation (mostly agree, but shape matters too: a thin, flat body keeps cells near the surface). [6 marks]
Common error to watch for: saying insects have “no heart” or “no blood”. They have both; their blood just does not do most of the oxygen carrying.