Plant responses — Edexcel International A Level Biology
Phytochrome, auxin (IAA) and gibberellins bring about plant responses by changing transcription; core practical 18 tests amylase in grains.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Phytochrome is a light-sensitive plant pigment that exists in two forms. Red light converts the inactive form Pr to the active form Pfr, and far-red light converts Pfr back to Pr. Pfr moves into the nucleus and alters transcription of genes, controlling responses such as germination, stem elongation and flowering.
Auxin (IAA) is a plant growth substance made in shoot tips that moves down the shoot. It promotes cell elongation by increasing acidification of the cell wall and by changing transcription. Unequal distribution of auxin causes bending: shoots grow towards light and roots grow downwards.
Gibberellins are plant growth substances that stimulate stem elongation and seed germination. In a germinating cereal grain, the embryo releases gibberellin, which switches on transcription of the amylase gene in the aleurone layer. Amylase digests starch to sugars that the embryo respires and uses to grow.
In core practical 18, halves of germinating cereal grains are placed on starch agar. Amylase made in the grains digests the starch, so after flooding with iodine solution a clear zone forms around each half. The size of the clear zone shows how much amylase was produced, and the roles of the embryo and gibberellin can be tested.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of coordination, response and gene technology on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Which wavelength of light converts Pr to Pfr?
Show answer
Red light (about 660 nm).
Which form of phytochrome is active?
Show answer
Pfr.
How does auxin make cells elongate?
Show answer
It increases proton pumping into the cell wall; the acidic wall becomes more extensible and the cell takes in water.
Where is amylase made in a germinating cereal grain?
Show answer
In the aleurone layer, under the control of gibberellin from the embryo.
Frequently asked questions
What is phytochrome and what does it do?
Phytochrome is a light-sensitive protein pigment in plants that exists as Pr and Pfr. Red light converts Pr to the active Pfr, and far-red light converts it back. Pfr moves into the nucleus and changes which genes are transcribed, controlling responses such as seed germination, stem elongation and flowering.
How does auxin cause phototropism?
Auxin (IAA) is moved to the shaded side of a shoot by carrier proteins. There it increases proton pumping into cell walls and switches on genes for elongation, so the cells on the shaded side elongate more than those on the lit side. The shoot therefore bends towards the light.
How do gibberellins affect seed germination?
In a cereal grain, the embryo releases gibberellin, which diffuses to the aleurone layer and switches on transcription of the amylase gene. The amylase digests starch in the endosperm to maltose and glucose, which the embryo uses in respiration for energy and growth.
How can you show that a germinating seed makes amylase?
Place halves of germinating grains on starch agar, incubate, then flood with iodine solution. Starch stains blue-black, but amylase digests it, leaving a clear zone around the grain. Measuring the diameter of the zone shows how much amylase was made, and a boiled grain acts as a control.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026