AQA GCSE Biology

Revision Notes

5.5.2 Required Practical: Plant Growth

Effect of Light or Gravity

  • Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings
  • You will:
    • Set up two petri dishes with 3 mustard seeds and allow them to germinate
    • Place one dish on a clinostat and the other to a support on its side
    • Record the direction of growth of both the shoots and the roots for each seed
  • This experiment is not to find out about the factors that affect germination; its important not to get confused

MethodInvestigating the gravitropic response (set-up), IGCSE & GCSE Biology revision notes

Investigating the gravitropic response (set-up)

  • Add some damp cotton wool to two petri dishes
  • Place 3 bean seedlings in the cotton wool in each petri dish
    • A – radicle facing downwards
    • B – horizontally
    • C – radicle (root grows from here) facing upwards
  • Cover each dish with a lid
  • Attach one petri dish to a support so that it’s on its side
  • Attach the second petri dish to a clinostat (as shown in the diagrams above).
  • Place both in a light-proof box (so that the seedlings are in complete darkness), leave for two days and then observe growth of the seedlings

Analysis of resultsInvestigating the gravitropic response (results), IGCSE & GCSE Biology revision notes

Investigating the gravitropic response (results)

  • In the first petri dish all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response)
  • In the second petri dish, all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat – they have shown no gravitropic response at all
  • The experiment needs to be done in a lightproof box in order to cancel out the effect of light on the growth of the seedlings

Exam Tip

Make sure you know what a clinostat is and what it does (it cancels out the effect of light or gravity).

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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