Cambridge O Level Biology 5090: Coordination and response in plants (5090)

This study guide teaches Topic 15, Coordination and response in plants, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in section 15.1 (15.1.1 to 15.1.4). The 5090 syllabus is not tiered, so every candidate studies all of it. The topic includes a practical outcome, and “tropic responses” is one of the experimental contexts the syllabus lists for the practical papers.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. Why light matters to a plant is covered in Plant nutrition, and water uptake by roots is in Transport in flowering plants. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
15.1.1	Describe gravitropism as a response in which parts of a plant grow towards or away from gravity
15.1.2	Describe phototropism as a response in which parts of a plant grow towards or away from light
15.1.3	Explain the role of auxin in controlling shoot growth, limited to: (a) auxin is made in the shoot tip; (b) auxin spreads through the plant from the shoot tip; (c) auxin is unequally distributed in response to light and gravity; (d) auxin stimulates cell elongation
15.1.4	Investigate gravitropism and phototropism in shoots and roots

Key words before you start

Plants cannot move from place to place, but they do respond to their surroundings. They respond mainly by growing in a particular direction.

Term	Meaning
Stimulus	A change in the surroundings that an organism can detect, such as light coming from one side
Response	What the organism does as a result, here a change in the direction of growth
Tropism	A growth response of part of a plant towards or away from a stimulus that comes from one direction
Positive	Growth towards the stimulus
Negative	Growth away from the stimulus

The name of each tropism tells you the stimulus: photo- means light, gravi- means gravity.

15.1.1 Gravitropism

Gravitropism is a response in which parts of a plant grow towards or away from gravity.

• Roots grow towards gravity, downwards. They are positively gravitropic.
• Shoots grow away from gravity, upwards. They are negatively gravitropic.

A seed can land in the soil in any position. Whichever way up it lies, the root curves down and the shoot curves up. This gets the root into the soil, where it anchors the plant and takes up water and mineral ions, and gets the shoot up towards the light.

15.1.2 Phototropism

Phototropism is a response in which parts of a plant grow towards or away from light.

• Shoots grow towards light coming from one side. They are positively phototropic.
• Light is needed for photosynthesis, so a shoot that bends towards the light lets its leaves absorb more light energy.

You will investigate roots as well as shoots (15.1.4). Shoots respond strongly to one-sided light. In seedlings, roots respond far less to light than shoots do; any response you see in roots is usually small and away from the light. Always describe what your own results show, using “towards” or “away from”.

Summary of the responses

Part	Response to gravity	Response to one-sided light
Shoot	Grows away: negative gravitropism	Grows towards: positive phototropism
Root	Grows towards: positive gravitropism	Little or none; any curve is away from the light

15.1.3 How auxin controls shoot growth

Auxin is a plant hormone: a chemical that is made in one part of the plant and affects growth in another part. The syllabus limits what you need to four points.

Point	What it means
(a) Auxin is made in the shoot tip	Cells at the very tip of the shoot produce auxin
(b) Auxin spreads through the plant from the shoot tip	It moves back from the tip into the region just below it, where cells are growing
(c) Auxin is unequally distributed in response to light and gravity	Light from one side, or gravity acting on a shoot that is not upright, makes more auxin end up on one side than the other
(d) Auxin stimulates cell elongation	Cells with more auxin get longer more than cells with less auxin

Put (c) and (d) together and you get bending. If one side of a shoot has more auxin, the cells on that side elongate more. That side becomes longer than the other side, so the shoot curves away from the side with more auxin.

Phototropism in a shoot, step by step

1. Auxin is made in the shoot tip.
2. Light comes from one side only.
3. More auxin ends up on the shaded side of the shoot than on the lit side.
4. Auxin spreads back from the tip to the region below it.
5. Cells on the shaded side elongate more than cells on the lit side.
6. The shaded side grows longer, so the shoot bends towards the light.

If light comes evenly from all sides, auxin is spread evenly. Cells on all sides elongate equally and the shoot grows straight up.

Gravitropism in a shoot, step by step

Lay a potted seedling on its side, in the dark so that light cannot affect it.

1. Auxin is made in the shoot tip and spreads back from it.
2. Gravity causes more auxin to collect on the lower side of the horizontal shoot.
3. Cells on the lower side elongate more than cells on the upper side.
4. The lower side grows longer, so the shoot curves upwards, away from gravity.

The syllabus limits the auxin explanation to shoots. You must describe how roots respond, but you are not asked to explain root responses using auxin.

Worked example 1: what the tip does

Four sets of oat seedlings were given light from one side for two days.

Set	Treatment	Result
A	Untreated	Grew and bent towards the light
B	Tip cut off	Grew very little and did not bend
C	Tip covered with a light-proof cap	Grew straight up
D	Tip uncovered, region below covered with a light-proof sleeve	Grew and bent towards the light

Explain each result using auxin.

• A: auxin made in the tip is unequally distributed, with more on the shaded side, so cells on that side elongate more and the shoot bends towards the light.
• B: with no tip, no auxin is made (point a), so there is little cell elongation and no bending.
• C: auxin is still made, so the shoot grows. But light cannot reach the tip, so auxin is not redistributed. It spreads back evenly, all sides elongate equally, and the shoot grows straight.
• D: light reaches the tip, so auxin is unequally distributed there. It spreads back to the covered region (point b), where cells on the shaded side elongate more. Covering the region below the tip does not stop bending.

Worked example 2: cell lengths

A student cut a thin section through a shoot that had bent towards light. The mean cell length was 60 µm on the lit side and 84 µm on the shaded side. Calculate how much longer the shaded-side cells are, as a percentage, and explain the difference.

• Difference = 84 − 60 = 24 µm.
• Percentage = 24 ÷ 60 × 100 = 40% longer.
• Explanation: more auxin was on the shaded side, and auxin stimulates cell elongation, so the shaded-side cells elongated more. Note that the cells got longer; the answer is not “more cells” on the shaded side.

15.1.4 Investigating gravitropism and phototropism

Seedlings are used because they grow quickly and the curve is easy to see within a few days. Cress, mustard, maize or bean seedlings all work. Large bean seeds are useful for roots, because the root is easy to see and measure.

Investigating phototropism in shoots

1. Grow two groups of seedlings, from the same batch of seeds, in identical pots of the same compost.
2. Put one group in a light-proof box with a slit in one side, facing a lamp. This is one-sided light.
3. Put the control group where light reaches it evenly from all sides, or in a box with no slit.
4. Keep temperature, water and the time allowed the same for both groups.
5. After two or three days, measure the angle of curvature of each shoot from the vertical, using a protractor, and work out a mean.

The independent variable is the direction of light. The dependent variable is the direction or angle of growth.

Investigating gravitropism in roots and shoots

1. Soak bean seeds, then pin them to damp cotton wool or blotting paper lining a Petri dish or a cork disc. Point the young roots in different directions: down, up and sideways.
2. Keep the dish in the dark, so that light cannot cause a response.
3. Set up a clinostat with a second dish. A clinostat slowly turns the seedlings round, so gravity acts on every side of the root and shoot in turn. This is the control.
4. After two or three days, draw or photograph the seedlings.

Expected result: in the still dish, every root has curved downwards and every shoot upwards, whichever way it started. On the clinostat, roots and shoots keep growing in the direction they started, because no side was affected by gravity for longer than any other.

For phototropism in roots, grow seedlings with their roots in water or on damp paper in a clear container, light one side only, and compare with a container lit evenly.

Worked example 3: processing angle data

Five shoots in one-sided light had these angles of curvature after 48 hours: 32°, 35°, 28°, 41°, 34°.

• Sum = 32 + 35 + 28 + 41 + 34 = 170°.
• Mean = 170 ÷ 5 = 34°. The range is 28° to 41°.
• Using five seedlings, not one, makes the mean more reliable, because seedlings vary.

Worked example 4: root growth rate

A bean root on a still dish grew from 12 mm to 30 mm long in 3 days.

• Growth = 30 − 12 = 18 mm.
• Rate = 18 ÷ 3 = 6 mm per day.

Making the investigation valid

• Use seedlings of the same type and the same age.
• Keep the dark for gravitropism, or the direction of light for phototropism, as the only stimulus that differs between groups.
• Use enough seedlings in each group to calculate a mean.
• Keep the cotton wool damp, so the seedlings do not dry out.
• Measure every angle from the same reference line (the vertical, or the original direction of growth).

Common errors

• Giving the root tip, or “the leaves”, as where auxin is made. The answer the syllabus expects is the shoot tip.
• Writing that light “destroys” auxin. Use the syllabus idea: auxin is unequally distributed, with more on the shaded side.
• Writing that auxin makes cells divide. The syllabus point is that auxin stimulates cell elongation.
• Saying the shoot bends towards the side with more auxin. It bends away from that side, because that side grows longer.
• Calling shoots “positively gravitropic”. Shoots grow away from gravity, so they are negatively gravitropic.
• Leaving out the control. A clinostat, or seedlings in even light, shows the curve is caused by the one-sided stimulus.
• Forgetting to keep gravitropism experiments in the dark, so that light also affects the result.
• Using “phototropism” and “gravitropism” interchangeably. Name the stimulus correctly.

Where to go next

Fix the facts with the revision notes, then test yourself with the practice questions. Plant cell structure is revised in Cell structure and organisation.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 15: Coordination and response in plants (15.1 Coordination and response in plants), published by Cambridge Assessment International Education.