What is the general role of plant hormones?

They coordinate responses to environmental changes and regulate growth and development.

How does ethene promote leaf abscission in deciduous plants?

As auxin from the ageing leaf falls, ethene stimulates cells in the abscission zone at the base of the petiole to make cellulase, which digests their walls so the leaf separates.

How does abscisic acid cause stomatal closure?

It causes potassium ions to leave guard cells, reducing their turgor pressure and causing the stomata to close.

OCR A-Level Biology H420

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Plant and Animal Responses

Plant hormones and their commercial uses

4 things to know
Components 01 and 03

Overview

Auxins and apical dominance, gibberellins in germination and stem elongation, abscisic acid in abscission and stomatal closure, and ethylene, rooting powders and weed killers.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

What do ethene, gibberellins and auxin do in leaf abscission, seed germination and apical dominance?

Process

What are auxins and what processes are they involved in?

Process

Explain the mechanism by which gibberellins promote seed germination.

Key term

Define apical dominance.

Questions from this subtopic

Each one with the model answer underneath.

1
What is the general role of plant hormones?
Model answer
They coordinate responses to environmental changes and regulate growth and development.
2
How does ethene promote leaf abscission in deciduous plants?
Model answer
As auxin from the ageing leaf falls, ethene stimulates cells in the abscission zone at the base of the petiole to make cellulase, which digests their walls so the leaf separates.
3
Explain the mechanism by which gibberellins promote seed germination.
Model answer
Gibberellins stimulate the production of amylase enzymes that break down starch reserves into sugars, providing energy for the growing embryo.
4
How does abscisic acid cause stomatal closure?
Model answer
It causes potassium ions to leave guard cells, reducing their turgor pressure and causing the stomata to close.
5
What are auxins and what processes are they involved in?
Model answer
Auxins are plant hormones that promote cell elongation and are involved in phototropism, geotropism, and apical dominance.
6
Define apical dominance.
Model answer
The phenomenon where the main central stem grows more strongly than lateral branches, inhibiting their growth.

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