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Tropisms in plantsIB MYP Biology: Revision notes

Section 1

What is a tropism?

A tropism is a growth response of part of a plant to a stimulus from one direction. Plants cannot move from place to place, but they can change the direction in which they grow.

  • Growth towards the stimulus is a positive tropism.
  • Growth away from the stimulus is a negative tropism.

This helps plants to get what they need: light for photosynthesis, and water and mineral ions from the soil.

Key termstropismpositive tropismnegative tropism

Section 2

Phototropism, gravitropism and hydrotropism

  • Phototropism is the response to light. Shoots grow towards light (positive phototropism), so the leaves absorb more light for photosynthesis.
  • Gravitropism (also called geotropism) is the response to gravity. Roots grow downwards (positive gravitropism), which anchors the plant. Shoots grow upwards (negative gravitropism), which lifts leaves towards the light.
  • Hydrotropism is the response to water. Roots grow towards water (positive hydrotropism), so the plant can absorb the water it needs.
Key termsphototropismgravitropismgeotropismhydrotropism
Exam tip

Say which part of the plant you mean. Shoots and roots respond differently to gravity.

Section 3

Auxin and differential growth

Auxin is a plant hormone made in the tip of the shoot. It moves through the plant and makes shoot cells elongate (grow longer).

When the auxin is spread out unevenly, the two sides of a shoot grow at different rates. This is differential growth, and it makes the shoot bend.

  • Light from one side: auxin moves to the shaded side. Cells on the shaded side elongate more, so the shoot bends towards the light.
  • Gravity: auxin collects on the lower side. In a shoot this makes the lower side grow faster, so the shoot bends upwards. In a root, the higher concentration of auxin inhibits elongation, so the upper side grows faster and the root bends downwards.
Key termsauxinelongatedifferential growth
Common mistake

Auxin does not make a shoot grow towards the light by moving to the lit side. It moves to the shaded side.

Section 4

Investigating tropisms

To test a tropism, change one factor and keep the others the same.

  • Phototropism: grow seedlings in boxes with light from one side and from all sides. The independent variable is the direction of light; the dependent variable is the angle the shoot bends (or its direction of growth).
  • Control variables: the type and age of seedlings, temperature, water, and the time left.
  • Reliability: use many seedlings, repeat, and calculate a mean.

A suitable hypothesis gives a reason, for example: 'Shoots will bend towards light because auxin moves to the shaded side.'

Key termsindependent variabledependent variablecontrol variablehypothesis

Must Know

  • Tropisms are growth responses to a stimulus from one direction
  • Shoots: positive phototropism, negative gravitropism. Roots: positive gravitropism, positive hydrotropism
  • Auxin makes shoot cells elongate and moves to the shaded side or the lower side
  • Uneven auxin causes differential growth, so the plant bends
  • In investigations, change one variable and control the others

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Tropisms in plants

  1. A sunflower seedling is grown in a pot on a classroom windowsill in Cape Town. After a week its shoot has bent towards the window, while its roots have grown downwards into the soil.
    Explain why it is an advantage to the seedling that its shoot grows towards the light.2 marks
  2. Auxin is a plant hormone made in the tip of a growing shoot. In an experiment, light shines onto a shoot from one side only.
    Explain how the movement of auxin makes the shoot bend towards the light.2 marks
  3. A student investigates whether cress seedlings respond to light. She grows 20 seedlings in each of two identical boxes kept at the same temperature and given the same amount of water. Box 1 has a small hole in one side, so light enters from one side only. Box 2 is lit equally from all sides. After five days she measures how far each shoot has bent from the vertical. The mean angle was 38 degrees in Box 1 and 3 degrees in Box 2.
    State a testable hypothesis for this investigation, with a scientific reason, and identify the independent and dependent variables.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).