Year 8 Science · Unit 1 · Lesson 6

Why Do Multicellular Organisms Need Transport Systems?

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Learning Goals

Warm Up

Read the graph

The bar chart below shows the surface area : volume (SA:V) ratio for cubes of increasing size. Study the graph, then answer the three questions.

SA:V ratio for cubes of increasing side length 0 1 2 4 6 SA:V ratio 6.0 3.0 1.5 0.75 1 mm 2 mm 4 mm 8 mm Cube side length

Data: calculated from geometric principles (SA = 6s², V = s³)

(a) Describe the trend shown in the graph. What happens to the SA:V ratio as the cube gets larger?

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(b) By how many times does the SA:V ratio decrease when the cube side doubles from 1 mm to 2 mm? Show your working.

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(c) A blue whale has an extremely low SA:V ratio. Use this to explain why a whale needs a transport system, while an amoeba does not.

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Your Turn

Because… chain

Fill in the missing effects. Each cause leads to the next step in the chain.

An organism grows larger
→
SA:V ratio decreases
→
Diffusion alone is inadequate to supply all cells
→
A transport system evolves
→

Overall outcome for the organism:

Show What You Know

1. Imagine you double the side length of a cube from 2 mm to 4 mm. The SA increases by a factor of 4, but the volume increases by a factor of 8. Explain why this matters for cells deep inside the larger organism.

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2. A heart the size of a small car has been measured in blue whales. Use the concept of SA:V ratio and the needs of living cells to explain why such an enormous heart is necessary.

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Wrap Up

In one sentence, what was the main idea of this lesson?

HSCScience · Year 8 Science · Unit 1 · Lesson 6 · Apply Worksheet