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

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 15, Coordination and response in plants, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): every learning outcome in section 15.1 (15.1.1 to 15.1.4). The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Why a shoot needs light is revised in Plant nutrition, and how roots take up water is in Transport in flowering plants.

Key definitions

Term	Meaning
Gravitropism	A response in which parts of a plant grow towards or away from gravity
Phototropism	A response in which parts of a plant grow towards or away from light
Tropism	A growth response towards or away from a one-directional stimulus
Positive (tropism)	Growth towards the stimulus
Negative (tropism)	Growth away from the stimulus
Auxin	A plant hormone, made in the shoot tip, that stimulates cell elongation
Clinostat	A device that slowly rotates seedlings so that gravity (or one-sided light) acts on all sides equally; used as a control
Angle of curvature	The angle between the original direction of growth and the new direction, measured with a protractor

The first two definitions are the syllabus wording. Learn them exactly, including “towards or away from”.

15.1.1 and 15.1.2 Which way does each part grow?

Part	Gravity	One-sided light
Shoot	Away: negatively gravitropic	Towards: positively phototropic
Root	Towards: positively gravitropic	Weak or no response; any curve is away from the light

Why the responses help the plant:

• Shoot grows up and towards light → leaves absorb more light for photosynthesis.
• Root grows down → anchors the plant and reaches water and mineral ions in the soil.
• A seed works whichever way up it lands: the root always turns down and the shoot always turns up.

15.1.3 Auxin: the four syllabus points

The syllabus limits the role of auxin in shoot growth to exactly these:

	Syllabus point	Memory hook
(a)	Auxin is made in the shoot tip	Made at the top
(b)	Auxin spreads through the plant from the shoot tip	Moves back from the tip
(c)	Auxin is unequally distributed in response to light and gravity	More on one side
(d)	Auxin stimulates cell elongation	Makes cells longer

The rule that links (c) and (d): the side with more auxin elongates more, so the shoot bends away from that side.

The auxin explanation is for shoots only. For roots, describe the response; you are not asked to explain it with auxin.

Method in steps: shoot bending towards light

1. Auxin made in shoot tip
2. Light from one side
3. More auxin on SHADED side
4. Auxin spreads back from tip to region below
5. Shaded-side cells elongate MORE than lit-side cells
6. Shaded side grows longer → shoot bends TOWARDS light

Method in steps: horizontal shoot curving upwards

1. Auxin made in shoot tip, spreads back
2. Gravity → more auxin on LOWER side
3. Lower-side cells elongate MORE than upper-side cells
4. Lower side grows longer → shoot curves UPWARDS (away from gravity)

What the tip experiments show

Treatment (one-sided light)	Result	Reason
Tip removed	Little growth, no bending	No tip, so no auxin made
Tip covered with a light-proof cap	Grows straight up	Auxin made but not redistributed, so all sides elongate equally
Region below tip covered, tip exposed	Bends towards light	Auxin redistributed at the tip, then spreads down to the covered region
Even light from all sides	Grows straight up	Auxin spread evenly

15.1.4 Investigations

Phototropism (shoots)

• Test: seedlings in a light-proof box with a slit on one side, facing a lamp.
• Control: identical seedlings with light from all sides (or a box with no slit).
• Independent variable: direction of light. Dependent variable: direction or angle of growth.
• Measure: angle of curvature of each shoot with a protractor; calculate a mean.

Gravitropism (roots and shoots)

• Test: soaked bean seeds pinned to damp cotton wool in a Petri dish, roots pointing in different directions; dish kept still, in the dark.
• Control: a dish on a clinostat, also in the dark.
• Result: still dish, roots curve down and shoots up; clinostat, both keep growing in their starting direction.

Phototropism (roots)

• Roots in water or on damp paper in a clear container, lit from one side, compared with an evenly lit container.

Controlled variables to name

• Same type and age of seedling.
• Same temperature.
• Same water supply (keep cotton wool damp).
• Same length of time.
• In a gravitropism test, no light (dark); in a phototropism test, only the direction of light differs.

Small worked reminders

• Mean of angles 25°, 31°, 22°, 30°: (25 + 31 + 22 + 30) ÷ 4 = 108 ÷ 4 = 27°.
• Percentage difference uses the starting or reference value as the divisor: (new − original) ÷ original × 100.
• Growth rate = change in length ÷ time, with units, for example mm per day.

Reading a diagram of a bent shoot

When an exam diagram shows a shoot curving, work backwards in three steps:

1. Find the outer side of the curve. That side grew longer.
2. Longer growth means more cell elongation, so that side had more auxin.
3. Ask why: if light came from the opposite side, it was the shaded side; if the shoot was lying flat, it was the lower side.

The inner side of the curve always faces the light (phototropism) or points upwards (gravitropism in a shoot).

Must-know distinctions

Do not confuse	With
Phototropism (light)	Gravitropism (gravity)
Positive (towards)	Negative (away from)
Cell elongation (cells get longer)	Cell division (more cells)
Auxin unequally distributed	Auxin “destroyed by light”
Shoot bends away from the side with more auxin	Shoot bends towards the side with more auxin (wrong)
Stimulus (the change detected, e.g. light from one side)	Response (the growth that results)
Auxin made in shoot tip	Root tip (not the syllabus point)

Quick self-test

1. Define phototropism.
2. Define gravitropism.
3. State the direction of the gravitropic response of a shoot, and name it.
4. Where is auxin made?
5. What effect does auxin have on cells in the region below the shoot tip?
6. In a shoot lit from one side, which side has more auxin?
7. A shoot is laid horizontally in the dark. Which side elongates more, and which way does the shoot curve?
8. A shoot has its tip covered with a light-proof cap and is lit from one side. Predict and explain the result.
9. Why should a gravitropism investigation be kept in the dark?
10. What is the purpose of a clinostat in a gravitropism investigation?
11. Four shoots curved by 25°, 31°, 22° and 30°. Calculate the mean.
12. A shoot grew from 15 mm to 35 mm in 4 days. Calculate its mean growth rate.

Answers

1. A response in which parts of a plant grow towards or away from light.
2. A response in which parts of a plant grow towards or away from gravity.
3. Upwards, away from gravity: negative gravitropism.
4. In the shoot tip.
5. It stimulates cell elongation.
6. The shaded side.
7. The lower side elongates more, so the shoot curves upwards.
8. It grows straight up. Auxin is still made, but light cannot cause it to be unequally distributed, so all sides elongate equally.
9. So that light cannot cause a response; gravity must be the only one-sided stimulus.
10. It is the control: it rotates the seedlings so gravity acts on all sides equally, showing that the curve in the still dish is caused by gravity.
11. 108 ÷ 4 = 27°.
12. (35 − 15) ÷ 4 = 20 ÷ 4 = 5 mm per day.

Where marks are usually lost

• Definitions that leave out “grow”: a tropism is a growth response, not a movement of the whole plant.
• Definitions that say only “towards light”; the syllabus wording is “towards or away from”.
• Writing “auxin is made in the tip” without saying shoot tip.
• Writing that auxin causes cell division, or “faster growth” without naming cell elongation.
• Saying light destroys auxin, rather than that auxin is unequally distributed with more on the shaded side.
• Getting the direction wrong: the side with more auxin grows longer, so the shoot bends away from that side.
• Swapping positive and negative for shoots and roots in gravitropism.
• Explaining root responses with auxin when the question asks about shoots, or mixing the two.
• Missing the control (clinostat or even light) in a planning question, or not keeping a gravitropism experiment in the dark.
• Giving a mean angle or growth rate without units (° or mm per day).

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.