🌱 1. Auxin is made
Auxin is made in the shoot tip. Some of it moves down the stem, and it makes the cells just below the tip grow longer.
Co-ordination and Response » Auxin and the Phototropic Response
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.85
- What auxin is and where it is made
- How auxin makes a shoot bend towards light
- How the classic shoot-tip experiments work
- How to explain the results in an exam
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Unlock This CourseThe big idea: auxin is the messenger
You already know that a shoot grows towards light. This is called positive phototropism. Now we find out how it happens. A plant has no nerves or brain, so it uses a chemical messenger instead. The messenger is a plant hormone called auxin.
Key terms:
- Auxin: a plant hormone made in the tip of a shoot. It makes cells grow longer.
- Elongation: cells getting longer. This is how auxin makes a stem grow.
- Shoot tip: the very top of a stem, where auxin is made.
Step by step: how a stem bends towards light
Imagine a seedling on a windowsill. Light comes from one side only. This is what happens:
🌞 2. Light is on one side
The tip detects the light coming from one side. Auxin then moves across the stem to the shaded side, away from the light.
📏 3. Shaded side grows more
The shaded side now has more auxin, so its cells elongate more than the cells on the lit side.
🌱 4. The stem bends
The shaded side grows longer than the lit side, so the stem bends towards the light.
Learn this
Auxin is made in the shoot tip. It moves to the shaded side. It makes the cells on the shaded side elongate more. The stem bends towards the light.
Notice that the stem bends because cells get longer, not because the plant makes more cells. The shaded side is the longer side, so it ends up on the outside of the curve and the tip is turned towards the light.
The classic shoot-tip experiments
Scientists worked out the role of the tip by changing young shoots (such as oat or grass seedlings) and shining light from one side. Each experiment asks one question: what does the tip do?
✂️ Tip removed
The tip is cut off. The shoot does not bend towards the light and grows very little. With no tip, there is no source of auxin.
📦 Tip covered with foil
The tip is covered with foil, so light cannot reach it. The shoot grows straight up and does not bend. The tip must be able to detect the light.
🚧 Barrier under the tip
A thin barrier that auxin cannot pass through is pushed into the stem just below the tip. The result depends on which side it is on (see below).
The barrier experiment in more detail. This shows that something moves down the stem from the tip.
- Barrier on the shaded side: auxin cannot move down the shaded side. The shaded side does not get extra auxin, so it does not grow longer and the stem does not bend.
- Barrier on the lit side: auxin can still move down the shaded side. The stem bends towards the light as normal.
Another test covers only the part of the stem below the tip with foil, leaving the tip in the light. The shoot still bends. This shows that the tip is the part that detects the light, not the rest of the stem.
Worked example
A student covers the tip of one seedling with foil and shines a lamp on one side. A second seedling has its tip uncovered. Both are left for two days.
Uncovered tip: the shoot bends towards the lamp, because auxin moves to the shaded side and the cells there elongate more.
Covered tip: the shoot grows straight up. The tip cannot detect the light, so the auxin is not moved to one side. Both sides grow the same amount.
The only thing changed is the foil, so we can say the tip is needed to detect the light.
Common mistakes
- Saying auxin moves towards the light. It moves to the shaded side.
- Saying the shaded side makes more cells. The cells there elongate more.
- Saying the stem bends away from the light. A shoot bends towards it.
- Forgetting where auxin is made. It is made in the tip, so a seedling with no tip does not bend.
- Not using the results as evidence. In an experiment answer, say what happened and what it shows.
Exam-style question
A student shone light from the left onto three young shoots. Shoot A was left alone. Shoot B had its tip cut off. Shoot C had its tip covered with foil.
(a) Describe what happens to shoot A. [1 mark]
(b) Explain why shoot A responds in this way. [3 marks]
(c) Predict what will happen to shoot C and give a reason. [2 marks]
Model answer
(a) It bends towards the light (1).
(b) Auxin is made in the tip (1). It moves to the shaded side (1). The cells on the shaded side elongate more than the cells on the lit side, so the shoot bends towards the light (1).
(c) It grows straight up and does not bend (1) because the foil stops the tip detecting the light, so auxin is not moved to one side (1).
Exam tip
In an "explain" answer about phototropism, write the chain in order: made in the tip, moves to the shaded side, cells elongate more, stem bends towards the light.