GCSE Biology: Plant Hormones, Tropisms and Plant Disease

Plants cannot move away from danger or towards food, so they control their growth with hormones and defend themselves with barriers and chemicals. This material covers both sides of that story as they appear in separate GCSE Biology: plant hormones and tropisms from the homeostasis and response topic, and plant disease and plant defences from the infection and response topic. Several points are Higher tier only for many students (for example, in the AQA specification the roles and uses of gibberellins and ethene, the uses of auxins, and the signs and identification of plant disease); the questions say so where it matters.

The first part explains phototropism and gravitropism in terms of auxin. You practise the key contrast that examiners test every year: in a shoot lit from one side, auxin collects on the shaded side and stimulates cell elongation, so the shoot bends towards the light; in a horizontal root, auxin collects on the lower side and inhibits elongation, so the root curves downwards. An experiment with seedlings whose tips are removed, covered with foil or left untreated checks that you can apply the idea to an unfamiliar set-up. You then meet the other hormones — gibberellins in seed germination and ethene in cell division and fruit ripening — and the ways people use them: auxins as selective weed killers, rooting powders and in tissue culture; ethene to control ripening during storage and transport; gibberellins to end seed dormancy, promote flowering and increase fruit size.

The second part covers how to recognise a diseased plant (stunted growth, spots, rot, growths, malformed stems or leaves, discolouration, pests), how the cause can be identified (gardening manuals and websites, laboratory testing, kits based on monoclonal antibodies), the effects of nitrate and magnesium deficiency, the damage done by aphids, and the physical, chemical and mechanical defences plants use, from cellulose walls, waxy cuticles and bark to poisons, thorns and mimicry. The named plant pathogens themselves are covered in the separate pathogens material in this category.

Four formats are offered. The quiz has twelve questions with explanations. The flashcards cover the key vocabulary, from auxin and gibberellin to chlorosis and mimicry. The printable written sheet has eight longer questions, including explaining tropisms with auxin, planning the seedling practical, processing rooting-powder results, evaluating the use of weed killers and diagnosing plants from described symptoms; it can be completed by hand and photographed for feedback. The oral exam lets you explain these ideas to an examiner who asks one question at a time and gives brief feedback at the end.

Use the material after studying plant hormones and plant disease, or as revision for the extended-response questions in which plant responses are often tested.

• Explain phototropism in shoots and gravitropism in roots in terms of the unequal distribution of auxin
• Predict the response of seedlings in an unfamiliar experiment on tip removal or covering
• Higher: state the roles of gibberellins and ethene and describe commercial uses of auxins, gibberellins and ethene
• Higher: recognise signs of plant disease and describe how a disease can be identified
• Explain the effects of nitrate and magnesium deficiency and the damage caused by aphids
• Describe physical, chemical and mechanical plant defences

Practice material written by Zestly, based on the DfE GCSE biology subject content and exam-board specification content for separate Biology (for example, AQA GCSE Biology 8461: plant hormones; plant disease; plant defence responses). All data in the questions are illustrative.