Year 8 Science · Unit 1 · Lesson 6
Apply Worksheet
Learning Goals
Odd one out
In each row, three items belong together and one does not. Circle the odd one out and give the reason.
| Row | Items | Reason it is the odd one out |
|---|---|---|
| 1 | oxygen · urea · glucose · water | |
| 2 | heart · blood vessels · blood · roots | |
| 3 | a single-celled diatom · a eucalyptus tree · a koala · a human |
Surface compared with volume
Cubes as model organisms
Oxygen and nutrients enter an organism through its outside surface, but every part of its volume needs them. Scientists compare the two using the surface area to volume ratio (SA:V). It tells you how many square millimetres of surface there are for every cubic millimetre of body.
| Side length (mm) | Surface area (mm²) = 6 × side × side | Volume (mm³) = side × side × side | SA:V (surface area ÷ volume) |
|---|---|---|---|
| 1 | 6 | 1 | 6 |
| 2 | 24 | 8 | 3 |
| 4 | |||
| 8 | 384 | 512 | 0.75 |
Source: calculated from the formulas in the column headings.
1. Using the completed rows, state what happens to the SA:V as the cube gets larger. Quote two values.
2. Complete the 4 mm row of Table 1. Show your working for each value.
SA:V (no unit)
Cause and effect
Complete the chain. Each box is caused by the box before it.
Classify and give a reason: does it need a transport system?
Write yes or no, then a reason about how far its deepest cells are from the surroundings.
| Organism | Yes or no, and the reason |
|---|---|
| A jacaranda tree, 10 m tall | |
| Sea lettuce, a green seaweed whose flat sheet is only two cells thick | |
| A tadpole |
Scenario: a flatworm and an earthworm
Flatworms are tiny, flat animals found under rocks in creeks. They have no heart and no vessels. A flatworm’s body is long, wide and extremely thin, so each of its cells sits a tiny distance from the creek water. An earthworm’s body is thick and round, with a network of vessels and several heart-like tubes that pump blood around it.
3. Explain how the flatworm’s body shape lets it survive with no heart or blood vessels.
4. Both worms are made of many cells. Suggest why the earthworm needs a transport system but the flatworm does not. Link your answer to Table 1.
5. A student writes: “Only animals need transport systems, because plants do not move.” Write a stronger scientific explanation. Use a Claim, Evidence and Reasoning structure.
Odd one out
Row 1: urea (a waste removed from cells; the others are delivered). Row 2: roots (plant transport; the others are animal transport). Row 3: the diatom (one cell, direct exchange; the others are multicellular).
Surface compared with volume
1. As the cube gets larger, the SA:V gets smaller, for example 6 for the 1 mm cube but only 0.75 for the 8 mm cube. [2 marks]
2. Surface area = 6 × 4 × 4 = 96 mm². Volume = 4 × 4 × 4 = 64 mm³. SA:V = 96 ÷ 64 = 1.5. [3 marks: 1 each]
Cause and effect: box 2: volume grows faster than surface area, so SA:V falls and many cells are far from the surface. Box 3: diffusion from the surface is too slow to supply those cells or remove their wastes.
Classify and give a reason
Jacaranda: yes, leaf cells are metres from the roots that take in water. Sea lettuce: no, every cell touches the seawater, so substances diffuse directly. Tadpole: yes, many internal cells are far from its skin.
Go Deeper
3. The body is so thin that every cell is close to the surface, so oxygen diffuses in and wastes diffuse out fast enough without a heart or blood. [2 marks]
4. The earthworm’s thick, round body is like a larger cube in Table 1: lower SA:V, with many cells deep inside. Diffusion from the surface is too slow to reach them, so blood vessels and pumping tubes must carry substances to and from them. The thin flatworm has a high SA:V and no deep cells. [3 marks: deep internal cells; lower SA:V linked to Table 1; transport carries substances to them]
Show What You Know
5. Marking guide: 1 for the claim (the student is incorrect; plants need transport too); 1 for evidence that plants are multicellular and their cells need water, minerals and gases; 1 for evidence that many plant cells are far from the soil or air (for example leaf cells far from the roots); 1 for naming plant transport through roots, stems and leaves; 1 for reasoning that transport means moving substances through the organism, not moving the whole organism, and blood is only one way to do it. [5 marks]
Common error to watch for: reading a smaller SA:V as “less surface area”. A bigger cube has more surface in total; it has less surface for each unit of volume.