Edexcel IGCSE Biology · Spec 2.51-2.53
Why Multicellular Organisms Need Transport Systems
Covers explain why diffusion is sufficient for unicellular organisms, Explain why multicellular organisms need transport systems and Recall the substances transported in plant phloem.
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Why Multicellular Organisms Need Transport Systems
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Explained
Why size decides whether diffusion is enough
A single celled organism such as Amoeba has no heart, no blood and no vessels, and manages perfectly well. A human could not survive for minutes without them. The difference is entirely a matter of size, and the reasoning is worth following because the same argument reappears in gas exchange, in the small intestine and in the kidney.
Surface area to volume ratio
As an organism gets larger, its volume grows faster than its surface area. Imagine a cube of side 1 cm: the surface area is 6 square cm and the volume is 1 cubic cm, a ratio of 6 to 1. Now a cube of side 2 cm: the surface is 24 square cm and the volume 8 cubic cm, a ratio of 3 to 1. The organism has doubled in size and halved its ratio.
Surface area is what supplies the organism; volume is what needs supplying. So a small organism has a large surface area for its needs, and a large one does not.
Distance
The second problem is how far substances have to travel. In a single cell, everything is close to the surface, so diffusion covers the distance quickly. Diffusion is fast over short distances and extremely slow over long ones, so it simply cannot supply the cells at the centre of a large organism.
A large organism also has a higher total demand for oxygen and glucose, because it has far more respiring cells.
What a transport system does
It moves substances in bulk, by mass flow, which is far faster than diffusion. Blood carries oxygen from the lungs to every tissue in under a minute.
It also keeps the concentration gradients steep at the exchange surfaces. Blood arriving at an alveolus is low in oxygen, so oxygen keeps diffusing in, and blood leaving takes that oxygen away so the gradient is maintained. Without the flow, the gradient would disappear and diffusion would stop.
Note that diffusion has not been replaced. It still does the final short step at every exchange surface. The transport system carries substances the long distances so that diffusion only ever has to work over a few micrometres.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks what is transported in phloem and awards one mark for sucrose or sugars and one for amino acids. In the guidance column it says: ignore glucose. It also allows water and plant hormones as alternatives.
Glucose is the answer most students give, and it earns nothing. Plants convert glucose to sucrose for transport because sucrose is less reactive and is not used in respiration on the way, so it arrives where it is needed. The mark scheme is not being pedantic; it is checking whether you know that conversion happens.
Xylem is the other half of the pair and carries water and mineral ions, in one direction only, upwards. Phloem carries sucrose and amino acids in both directions, from source to sink, which is why the process has its own name, translocation.
Answering the standard question
If asked why a large organism needs a transport system, give three linked points rather than one. The surface area to volume ratio is small, so there is not enough surface to supply the volume. The diffusion distance to the innermost cells is large, so diffusion alone would be too slow. And the total demand is high, because there are many respiring cells.
If asked why a single celled organism does not need one, give the converse of each. Examiners credit the converse, but only if you actually write it out.
Spec 2.51-2.53
What you need to know
- Explain why diffusion is sufficient for unicellular organisms
- Explain why multicellular organisms need transport systems
- Recall the substances transported in plant phloem
Active recall
Quick check
Answer each question before opening the answer.
Why is diffusion sufficient for many unicellular organisms?
They have a large surface area-to-volume ratio and short diffusion distances
Give two reasons multicellular organisms need transport systems
Small surface area-to-volume ratio, long diffusion distances or high metabolic demand
Which two organic substances are carried in phloem?
Sucrose and amino acids
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