Tropic responses — Cambridge IGCSE Biology
Gravitropism and phototropism, how auxin controls shoot growth, and how to investigate tropisms.
Revision notes
2 short notes, in the order of the specification. Each one in short:
Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source, and gravitropism is the same response to gravity. Shoots grow towards light and away from gravity. Auxin, made in the shoot tip, is unequally distributed and stimulates cell elongation, so the side with more auxin grows more.
Investigating tropisms
PracticalTropisms are investigated by growing young seedlings, such as cress or beans, on damp cotton wool in clear dishes. For phototropism, light reaches them from one side through a slit in a box. For gravitropism, a dish is turned on its side in the dark. Shoots bend towards light and upwards; roots bend away from light and downwards.
Exam questions
5 exam-style questions (14 marks), each with its mark scheme.
Answer the questionsFlashcards
6 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of coordination and response on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Define phototropism.
Show answer
A response in which parts of a plant grow towards or away from the direction of the light source.
In which direction does a root grow in response to gravity?
Show answer
Towards gravity (downwards).
Supplement Where is auxin made in a shoot?
Show answer
In the shoot tip.
Supplement What does auxin do to shoot cells?
Show answer
It stimulates cell elongation.
Frequently asked questions
What is phototropism?
Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Shoots grow towards the light, while roots grow away from the light. It can be shown by growing seedlings in a box where light enters from one side only, then recording which way the shoots and roots bend.
How does auxin make a shoot bend towards light?
Supplement Auxin makes a shoot bend towards light because it builds up on the shaded side. Auxin is made in the shoot tip and diffuses through the plant. When light comes from one side, it is unequally distributed, and cells on the shaded side elongate more than cells on the lit side, so the shoot bends.
What is gravitropism?
Gravitropism is a response in which parts of a plant grow towards or away from gravity. Shoots grow away from gravity, so they grow upwards, and roots grow towards gravity, so they grow downwards. It is investigated by turning a dish of germinating seeds on its side and keeping it in the dark.
How do you investigate phototropism in seedlings?
Grow seedlings of the same species and age on damp cotton wool in identical clear dishes. Put one set in a box with a slit so light reaches them from one side only, and keep a control set with light from all sides or in the dark. After several days, record the direction in which shoots and roots grow.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026