Plant Responses, Tropisms and Auxin

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.83 — Plants respond to stimuli
• 2.84 — The geotropic and phototropic responses of roots and stems
• 2.85 — The role of auxin in the phototropic response of stems

Specification section 2(j) Co-ordination and response — also taught in ADH and the Water Balance of the Blood, Homeostasis and the Control of Body Temperature, The Nervous System, and the Two Ways a Body Sends a Message, Synapses and the Reflex Arc.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Plant a bean upside down, in total darkness, under a flowerpot. Come back in a week and the root will be heading downwards and the shoot upwards, exactly as if you had planted it the right way up. Nothing about that is obvious. The seed has no eyes to find the light, no sense of balance, no brain to work out which way is up, and no muscles to do anything about it if it had. And yet it is not guessing.

Start with the difference that causes most of the confusion on this topic. An animal responds by MOVING: a muscle contracts, something is pulled, and afterwards the animal is the same animal in a different position. A plant cannot do that — its cells are boxed inside rigid cellulose walls and cemented to their neighbours. So a plant responds by GROWING, which means its responses are slow, take hours or days, happen only at the growing tips, and cannot be undone. A stem that has bent towards a window has not moved towards it; it has been permanently built in that direction.

A growth response where the DIRECTION of growth is determined by the direction of the stimulus is called a TROPISM. Positive means growth towards the stimulus and negative means growth away from it — the words say nothing about whether the response is helpful, only which way it points. Two stimuli matter for this specification: light, giving phototropism, and gravity, giving geotropism.

Which way does what grow, and what does the plant get out of it

1. SHOOTS are positively phototropic — they grow towards light. The leaves are carried into the brightest position available, so more light is absorbed and the rate of photosynthesis rises.
2. SHOOTS are negatively geotropic — they grow away from gravity, which is to say upwards. This gets the leaves clear of the soil and above competing plants even before a single leaf has opened, which matters enormously to a seedling still underground in the dark.
3. ROOTS are positively geotropic — they grow in the direction of gravity, downwards, into the soil. That anchors the plant and takes the root hairs towards water and mineral ions, which are deeper down.
4. ROOTS are negatively phototropic — they grow away from light, which keeps them in the soil and reinforces the same outcome.

Notice that gravity is the more reliable of the two signals, and that this is why the buried bean works. Light may be absent, patchy or coming from the wrong side; gravity is present everywhere, always points the same way, and passes through soil. A seedling germinating in the dark has only gravity to go on, and gravity is enough to get the shoot pointing up and the root pointing down. Light takes over as the main signal for the shoot the moment it breaks the surface.

Think of it like a queue that only grows at the front

Imagine a line of bricklayers building a wall who can only ever add bricks at one end, and who can never take a brick out. If the crew on the left lays thirty bricks while the crew on the right lays twenty, the wall does not need to be pushed to curve — it simply curves as it is built, and stays curved. Now you have the exact mechanism of a bending stem. Nothing bends it. One side is built longer than the other, and the difference in the building is the bend.

How auxin bends a shoot towards the light — the mechanism 2.85 asks for

1. AUXIN is a plant hormone, produced in the TIP of the shoot, which then spreads back down the shoot to the region just below the tip where cells are growing.
2. In a shoot, auxin makes cells ELONGATE — a cell exposed to more auxin takes in more water and stretches longer along the shoot.
3. When light shines on the shoot from one side only, auxin moves laterally away from the lit side, so it becomes more concentrated on the SHADED side.
4. The cells on the shaded side are therefore exposed to more auxin than the cells on the lit side.
5. The shaded side elongates more than the lit side. Since one side of the shoot is now longer than the other, the shoot curves — and it curves towards the light, because the short side is the lit side.
6. The leaves end up facing the light, so more light is absorbed and photosynthesis is faster. The response has improved the plant's position without anything having moved.

Two things about that chain are worth pinning down, because they are where answers go wrong. First, auxin does not make more cells — it makes existing cells LONGER. Second, the shoot bends towards the light even though the auxin gathers on the side away from the light, and that only sounds contradictory until you have said the middle step out loud: the side with more auxin grows longer, and a longer side ends up on the OUTSIDE of a curve. More auxin means more growth means the outside of the bend.

How we know the tip is the part that senses light — the Darwins' experiment, 1880

1. Charles Darwin and his son Francis grew coleoptiles — the pale protective sheath around the first shoot of a grass seedling — and lit them from one side. Intact, they bent towards the light.
2. Cut the very tip off, and the shoot did not bend at all, although it went on growing.
3. Cover the tip with a little opaque cap, leaving the rest exposed, and it did not bend either.
4. Cover the tip with a TRANSPARENT cap instead, and it bent normally — so it was the darkness, not the cap, that had stopped it.
5. Cover the lower part of the shoot with an opaque collar but leave the tip exposed, and it bent normally again.
6. So the light is detected at the tip — and the bending happens lower down, in a part of the shoot that need not see any light at all. Something, the Darwins concluded, must travel from the tip down to the region that bends. It was another forty-six years before Frits Went caught that something in a block of agar and named it auxin.

That experiment is worth more than the fact it establishes, because it shows you how to reason about any plant-response question you are given. Ask two things: where is the stimulus being detected, and where is the growing happening? If those are in different places, then something has to travel between them — and in a plant, with no nerves available, the only way to send a message is a chemical that spreads through the tissue. Slow, unfocused, and impossible to switch off quickly, which is exactly why plant responses take days.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Tropism

A growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Positive means growth towards the stimulus, negative means growth away from it.

Learn this definition · Auxin

A plant hormone produced in the tip of a shoot which stimulates the elongation of cells in the region behind the tip.

The vocabulary, stated so it cannot be confused

Phototropism

A growth response in which the direction of growth is determined by the direction of light.

Geotropism

A growth response in which the direction of growth is determined by gravity.

Positive tropism

Growth TOWARDS the stimulus. It does not mean 'a good response'.

Negative tropism

Growth AWAY FROM the stimulus.

Coleoptile

The pale protective sheath around the first shoot of a grass seedling — the standard subject of tropism experiments because it is straight, fast-growing and easy to cut.

Elongation

An existing cell becoming longer by taking in water, not the production of new cells. This is what auxin causes in a shoot.

Roots and shoots — the four responses, and the advantage of each

Phototropism in a shoot — the six marking points, in order

1. Auxin is produced in the tip of the shoot.
2. Light shines on one side of the shoot only.
3. Auxin moves to the shaded side, so its concentration there is higher.
4. Auxin causes cell elongation, so the cells on the shaded side elongate more than those on the lit side.
5. One side of the shoot is now longer than the other, so the shoot curves towards the light.
6. The leaves receive more light, so the rate of photosynthesis increases.

The Darwins' coleoptile experiment — the results to be able to quote

1. Intact coleoptile, lit from one side → bends towards the light.
2. Tip removed → does not bend (but continues to grow).
3. Tip covered with an opaque cap → does not bend.
4. Tip covered with a transparent cap → bends normally, which proves the cap itself was not the cause.
5. Lower part covered, tip exposed → bends normally.
6. Conclusion: the light is detected at the TIP, the bending occurs BELOW the tip, so a substance must be transmitted downwards from the tip.

Two things a bending shoot is NOT doing

A. IT IS: growing unequally. The shaded side elongates more than the lit side, so the shoot is built into a curve and stays curved.

B. IT IS NOT: moving, bending, or leaning towards the light. Nothing pushes or pulls it, and the curve cannot be reversed — only grown out of.

Not this: Auxin collects on the shaded side, so surely the shoot should bend away from the light.

This: The side with more auxin elongates MORE, and the longer side is on the outside of a curve. A longer shaded side therefore puts the bend towards the light.

What auxin does to a cell — the word the mark scheme wants

A. ELONGATION: an existing cell takes in water and becomes longer along the shoot. This is auxin's effect, and it is the word to write.

B. NOT cell division: auxin does not give the shaded side more cells. 'Grows faster' and 'more growth' are also too vague to be credited.

Model answer [4 marks]

A shoot is illuminated from one side. Explain how auxin causes it to grow towards the light. [4]

Auxin is produced in the tip of the shoot and moves to the shaded side, so the concentration of auxin is higher on the side away from the light. Auxin stimulates cell elongation, so the cells on the shaded side elongate more than those on the lit side. The shaded side of the shoot therefore becomes longer than the lit side. This causes the shoot to curve towards the light, so the leaves absorb more light for photosynthesis.

Model answer [3 marks]

Explain the advantage to a seedling of its root being positively geotropic. [3]

The root grows in the direction of gravity, which is downwards into the soil. This anchors the plant firmly so it is not easily pulled or blown out. It also takes the root and its root hairs deeper into the soil, towards the water and mineral ions the plant needs, which are more reliably available below the surface.

Model answer [3 marks]

A coleoptile whose tip has been covered with an opaque cap does not bend when lit from one side. Explain what this shows. [3]

The rest of the coleoptile was still lit and still growing, so the failure to bend cannot be explained by a lack of light or a lack of growth. It shows that light is detected at the tip and nowhere else. Since the bending normally occurs below the tip, a substance must be transmitted from the tip down to the growing region.

Mark-losing trap. Plants GROW in response to a stimulus; they do not move towards it. Use the word growth in every tropism answer.

Mark-losing trap. Auxin causes cells to ELONGATE. 'Grow faster' and 'divide more' are not the same claim and are not credited.

Mark-losing trap. Positive and negative mean towards and away from the stimulus — never 'good' and 'bad'.

Mark-losing trap. Shoots: positively phototropic, negatively geotropic. Roots: the reverse of both. Get one wrong and the whole answer unravels.

Mark-losing trap. State WHERE auxin is made — the tip of the shoot — whenever the question says 'explain'. It is a marking point on its own.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — A plant root grows downwards, in the direction of gravity. What is this response called?
2. Grade 7 · Explain [3 marks] — A bean seed is planted upside down and germinates in complete darkness under the soil. The shoot still grows upwards. Explain how the seedling manages this without any light.
3. Grade 8 · Explain [4 marks] — A young shoot on a windowsill is lit strongly from one side only. Explain how auxin causes it to curve towards the window. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — A shoot is lit from one side for 24 hours. Cells in the growing region on the shaded side then have a mean length of 65 µm, and cells on the lit side have a mean length of 50 µm. Calculate the percentage by which the mean cell length on the shaded side exceeds that on the lit side.
5. 9+ · Analyse [5 marks] — Four coleoptiles are lit from the left. A is intact. B has had its tip cut off. C has an opaque cap over its tip. D has an opaque collar around its middle but an exposed tip. A and D bend towards the light; B and C do not. Select every statement that these results, taken together, justify.

The people behind this science

Two ways into the same idea — the one who proved the tip can sense, and the one who ruled that plants cannot sense at all. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one who proved the tip can sense

The experiment on this page is his. In 1880, with his son Francis, Darwin covered the tips of grass seedlings and showed that a plant detects light in one place and bends in another — which forced the conclusion that a message travels between them. He went further than most people realise, writing that the tip of a root acts like the brain of one of the lower animals. He never identified the substance; he proved there had to be one.

• “Why did you cover the tip rather than the part that was actually bending?”
• “What made you use a transparent cap as well as an opaque one?”
• “What did you think was travelling down the shoot?”
• “Why did you compare the tip of a root to a brain?”
• “How can something with no nerves behave as though it has them?”

Aristotle — the one who ruled that plants cannot sense at all

Aristotle divided living things by what their soul could do: plants had the nutritive powers of growth and reproduction, and animals alone had perception and the ability to respond to what they perceived. That ranking survived for two thousand years and is exactly the claim this lesson overturns — a shoot detects the direction of light, and does something about it. His error is worth arguing with rather than dismissing, because the reasoning behind it is careful and the counter-evidence needed two millennia to arrive.

• “Why did you place plants below animals in your account of the soul?”
• “What would you say to a shoot that detects the direction of light and turns towards it?”
• “Does responding to something require being aware of it?”
• “What would count, for you, as proof that a plant senses its surroundings?”

Carry on through the course

• Previous lesson: Homeostasis and the Control of Body Temperature
• Next lesson: The Nervous System, and the Two Ways a Body Sends a Message
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification