GCSE · Biology · Edexcel · Spec 1BI0

Need to transport substances in organisms

Picture one cell at the centre of a huge animal, with everything it needs a very long way off. How long would you wait?

Biology · Transport in organisms

Can diffusion do the job alone?

Every living cell needs substances from outside the organism. Pick a size, then follow each route to its end to see what happens.

3 possible routes.

On A living cell needs oxygen and nutrients that start outside the organism. 2 branches to choose from.

Watch out: Bigger does not mean 'more surface, so easier'. As an organism gets wider, its surface area to volume ratio decreases.

Why size is a problem

Reason it through

Why can't a large organism with no transport system rely on diffusion alone?

Link 1 of 4

First link · your turn

Make an organism wider. What happens to its surface area to volume ratio?

Locked — reveal the link above first

Biology · The blood

Who carries what?

The blood is a two-way delivery service. For each substance, decide whether it is carried by the red blood cells or by the plasma.

Still to sort

Red blood cells (0)

Carry oxygen from the lungs to the living cells.

Plasma (0)

Carries nutrients and waste products around the body.

5 of 5 still to sort.

Biology · Diffusion and size

Which idea sounds right to you?

A student is thinking about why a big organism needs a transport system when a tiny one doesn't. Four ideas come up in the discussion.

Which one is closest to what you think right now?

A bigger organism has more surface, so diffusion keeps up with it easily.Diffusion moves particles from an area of low concentration to an area of high concentration.In a huge organism the centre just gets its supplies a bit later, so size is never a real problem.As an organism gets wider, its surface area to volume ratio decreases, so diffusion into the centre takes longer.

How sure are you?

I'm sureI think soI'm guessing

A bigger organism has more surface, so diffusion keeps up with it easily.

This is a common misconception

It's an easy one to believe, because the surface area really does increase as the organism gets wider. The catch is that the volume increases too, and the surface area to volume ratio decreases. Volume sets how far substances have to diffuse to reach the centre, so the distance and the time both go up.

Diffusion moves particles from an area of low concentration to an area of high concentration.

This is a common misconception

That's the wrong way round. Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Swap the direction back and the rest of the lesson makes sense.

In a huge organism the centre just gets its supplies a bit later, so size is never a real problem.

This is a common misconception

A bit later is the trouble. As the diffusion distance increases, the time taken for substances to diffuse into the centre increases, and at some point it is too long for the organism to survive. That is why an organism with nothing to reduce the diffusion distance cannot grow very large.

As an organism gets wider, its surface area to volume ratio decreases, so diffusion into the centre takes longer.

This matches the evidence

Yes. Surface area and volume both increase, but the ratio decreases, and the time for substances to diffuse into the centre increases. This is the reason a large organism needs a transport system.

Wider means a lower surface area to volume ratio, a longer diffusion distance and a longer diffusion time. Never 'more surface, so easier'.

Biology · Write it up

Explain it in your own words

Explain why a large organism needs a transport system, and say what its circulatory system contains and carries. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Volume increases without significantly increasing surface area (the surface area to volume ratio decreases). (1 mark)
So the diffusion distance to the centre increases. (1 mark)
So the time taken for diffusion becomes too long for the organism to survive. (1 mark)
The circulatory system contains blood, blood vessels and the heart, and transports nutrients and waste products around the body. (1 mark)
Nutrients and waste products are exchanged between the blood and the cells at the capillaries. (1 mark)

Model answer

As an organism gets larger, its volume increases without a matching increase in surface area, so the surface area to volume ratio goes down. That makes the distance substances must diffuse to reach the centre longer, and the time taken eventually becomes too long for the organism to survive. A circulatory system solves this. It is made of blood, blood vessels and the heart, and it transports nutrients and waste products around the body. At the capillaries, nutrients leave the blood for the cells and waste products come back into the bloodstream.

A full answer runs in order: ratio, distance, time too long, then the transport system and the exchange at the capillaries.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Diffusion is brilliant over short distances. Past a certain size, it needs help.

What you need to know

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Have a goA drop of ink lands in a glass of still water. Your mate insists the ink will just sit there 'because particles don't care'. Which way does the ink drift overall?
Away from the drop, into the water that has little ink.
Diffusion is net movement from higher concentration (the drop) to lower concentration (the clear water).
As an organism gets wider, its surface area and volume both increase, but its surface area to volume ratio decreases.
Surface area affects the diffusion rate. Volume affects the distance diffusion has to cover.
Have a goCube P is much wider than cube Q. Which cube has the smaller surface area to volume ratio, and which has the longer diffusion distance?
Cube P, the wider one, has both.
The ratio decreases as width increases, and volume is what affects the distance.
The wider the organism, the longer it takes for substances to diffuse into the centre.
With no way of reducing the diffusion distance, an organism cannot grow very large, because eventually diffusion takes too long for it to survive.
Have a goThe cells at the centre of a very large organism with no transport system are running short. Has diffusion stopped working, or is something else going on?
It's too slow: the time for substances to reach the centre is too long to survive.
The problem is the time taken over a big distance, not diffusion switching off.
The circulatory system transports nutrients and waste products around the body, and contains blood, blood vessels and the heart.
Red blood cells transport oxygen from the lungs to all living cells, where it is needed for cellular respiration.
Plasma transports water, glucose and other nutrients, plus the waste products carbon dioxide and urea.
Capillaries are where nutrients and waste products are exchanged between the blood and the cells: nutrients leave the blood, waste comes back in.

The big picture

Diffusion only works well over short distances. As an organism gets wider, its surface area to volume ratio decreases and substances take longer to diffuse into the centre, until the time is too long to survive. A circulatory system (blood, blood vessels and the heart) fixes this by carrying nutrients and waste around the body: red blood cells carry oxygen, plasma carries water, glucose and other nutrients plus waste such as carbon dioxide and urea, and the exchange with cells happens at the capillaries.

Key points

1Diffusion is the net movement of particles from higher to lower concentration, and it copes well over short distances.

2As width increases, surface area to volume ratio decreases and the time for substances to diffuse into the centre increases.

3An organism with no mechanism for reducing the diffusion distance cannot grow very large.

4The circulatory system (blood, blood vessels and the heart) transports nutrients and waste products around the body.

5Red blood cells carry oxygen; plasma carries water, glucose and other nutrients, and waste such as carbon dioxide and urea.

6Nutrients leave the blood and waste products enter it at the capillaries.

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Worked example

Problem

A body cell deep in the tissues needs glucose, and it has carbon dioxide waste to get rid of. Describe how the circulatory system deals with both.

⚠ Watch out

Thinking a bigger organism has more surface, so diffusion keeps up. The surface area does increase, but so does the volume, and the surface area to volume ratio decreases. The diffusion distance and the time taken both go up.

🧠

Memory hook

Big body, long wait. Blood is the delivery van: red blood cells drive the oxygen, plasma carries the rest, and the capillaries are the front doors.

✓

Check yourself

Without looking back: pick one substance the blood carries that isn't oxygen, say whether the plasma or the red blood cells carry it, and say whether it leaves or enters the blood at the capillaries.

Flashcards

(12)

What is diffusion?

The net movement of particles from an area of higher concentration to an area of lower concentration.

As the width of an organism increases, what happens to its surface area, its volume and its surface area to volume ratio?

Surface area and volume both increase, but the surface area to volume ratio decreases.

What does surface area affect in diffusion?

The diffusion rate.

What does volume affect in diffusion?

The distance over which diffusion has to occur.

As an organism gets wider, what happens to the time for substances to diffuse into the centre?

It increases.

Why can an organism with no mechanism for reducing the diffusion distance not grow very large?

The diffusion distance increases with size, so at some point the time for diffusion into the centre is too long for it to survive.

Why does increasing volume without significantly increasing surface area limit an organism's viability?

The time taken for substances to diffuse over the large distances eventually becomes too long.

What is the role of the circulatory system?

To transport nutrients and waste products around the body.

What three parts does the circulatory system contain?

Blood, blood vessels and the heart.

What do red blood cells transport, and why?

Oxygen, from the lungs to all living cells, where it is needed for cellular respiration.

What does plasma transport?

Water, glucose and other nutrients, and waste products including carbon dioxide and urea.

What decides whether diffusion alone can supply an organism's cells?

How far substances must diffuse. A small organism has a short distance, but a large one with no way of reducing it takes too long to supply its centre.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Can diffusion do the job alone?

Who carries what?

Which idea sounds right to you?

Explain it in your own words

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