Control and coordination in plants

Biology A2 · Control and coordination. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology A2 topics

The idea

Plants have no nervous system, but they still respond to their surroundings. Most responses are slow and are controlled by plant hormones (plant growth regulators), which act on cells with the right receptors.

The syllabus uses three examples. The Venus fly trap gives a rapid response: touching its sensory hairs produces action potentials that make the trap snap shut. Auxin makes cells in a growing shoot get longer by making their walls acidic. Gibberellin starts the breakdown of the starch store when a barley seed germinates.

In each case, learn the chain of events from the stimulus to the response.

Rules to remember

Venus fly trap: bending a sensory hair opens ion channels in cells at its base → receptor potential → action potentials spread across the lobes
The trap closes only if hairs are stimulated twice within a short time (two hairs, or one hair twice); this avoids wasting energy on rain or debris
Closure: rapid movement of ions and water changes the turgor of cells in the lobes, which flip from convex to concave
Auxin: stimulates proton pumps to move H⁺ into the cell wall → the low pH activates expansins → links between cellulose microfibrils loosen → water enters by osmosis and turgor stretches the wall
Barley germination: seed absorbs water → embryo makes gibberellin → gibberellin diffuses to the aleurone layer → aleurone cells make amylase
Amylase hydrolyses starch in the endosperm to maltose, which is turned into glucose for respiration and growth of the embryo

Common mistake

Do not say that auxin makes cells divide or that it stretches the wall itself. Auxin makes the wall acidic so that it loosens; the force that makes the cell longer is turgor pressure from water entering by osmosis.

Worked example

Barley seeds with their embryos removed are soaked in gibberellin together with a chemical that stops transcription. Will the starch in the endosperm be broken down?

Gibberellin acts on the aleurone layer by switching on transcription of the gene for amylase.
With transcription blocked, no mRNA for amylase is made, so no amylase is synthesised.
Without amylase, starch in the endosperm is not hydrolysed to maltose.

Answer: No. The starch stays, because gibberellin works by switching on the synthesis of amylase.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Full-course notes

Keep going

← Control and coordination in mammals Passage of information from parents to offspring →Take a Biology A2 test