What you need to know
- 8.11auxinA plant growth substance made in shoot tips that promotes cell elongation. (IAAIndoleacetic acid: the main auxin. Uneven distribution of IAA causes tropisms.) and gibberellinA plant growth substance that causes stem elongation and switches on amylase genes in germinating seeds. bring about plant responses, including effects on transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus..
Phytochrome and flowering are the next two lessons. Root behaviour is not the same as shoot behaviour. Do not give a gibberellin concentration. The amylase assay is Core Practical 18.
By the end, you should be able to…
- Explain shoot phototropism using a redistribution of auxin.
- Describe how gibberellin leads to amylase in a germinating grain.
- State that both hormones can change transcription.
Build your understanding
Auxin and the bending shoot
auxinA plant growth substance made in shoot tips that promotes cell elongation., also called IAAIndoleacetic acid: the main auxin. Uneven distribution of IAA causes tropisms., is made in the shoot tip. It moves down the shoot and causes cells behind the tip to elongate. In even light the distribution is even and the shoot grows straight up. If the light comes from one side, auxin becomes more concentrated on the shaded side of the shoot. Those cells elongate more, so the shoot bends toward the light. That is positive phototropism.
The visible effect is wall loosening and elongation. The specification also wants the molecular effect: auxin changes transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus., so the cell makes proteins that allow that growth. You do not need every geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. name. Do not apply the shoot rule to a root. A high auxin concentration inhibits root elongation. If the question does not mention roots, stay with the shoot.
Gibberellin and the grain
gibberellinA plant growth substance that causes stem elongation and switches on amylase genes in germinating seeds. promote stem elongation. A plant that cannot make them stays very short. They also start the use of food stores in a germinating cereal grain. The embryo produces gibberellin. The hormone reaches the aleurone layer and causes transcription of the amylase gene. Amylase is secreted into the endosperm and hydrolyses starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential. to maltoseA disaccharide of two α-glucose molecules joined by a 1,4 glycosidic bond.. The sugar is then available for respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. and growth.
Without the embryo, or without added gibberellin, that transcription does not start and the starch remains. That is the idea behind Core Practical 18. Gibberellin is a signal to switch a gene on, not an enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. that cuts starch itself.
Hormones, not nerves
Plants do not use the action potentialA rapid reversal of the potential difference across a neurone membrane, caused by Na⁺ ions flowing in, then K⁺ ions flowing out. of 8A for these responses. The growth response is slow because it depends on cell elongation and on making new proteins. A shoot does not bend in the time a reflex takes.
Weedkillers that are synthetic auxins are an application, not a required method. A high, uneven dose can disrupt growth. Do not treat that as a garden instruction.
Follow the process
Shoot in one-sided light: the tip makes auxin (IAA) → auxin becomes more concentrated on the shaded side → cells on that side elongate more, because auxin changes transcription of proteins that loosen the wall → the shoot bends toward the light. Even light means even auxin and straight growth. Do not put the extra auxin on the lit side, and do not use this rule for a root. Germinating grain: the embryo releases gibberellin → the aleurone transcribes the amylase gene → amylase is secreted into the endosperm → starch is hydrolysed to maltose. Gibberellin is the signal. It is not the enzyme that cuts the starch.
NOT: the lit side of a shoot does not have the extra auxin, and gibberellin is not amylase.
- Auxin is made in the shoot tip
- Shade side elongates in a shoot
- Gibberellin comes from the embryo
- Amylase is the enzyme that is then made
Two traps this diagram blocks
- Do not put the extra auxin on the lit side of a shoot.
- Do not say gibberellin itself digests starch.
From the diagram alone
Question
Explain how one-sided light causes a shoot to bend, and state one difference if the organ were a root. [4]
Show a model answer
More auxin accumulates on the shaded side of the shoot, so those cells elongate more and the tip bends toward the light. A high auxin concentration inhibits elongation in a root, so the shoot rule must not be copied across.
| Hormone | Visible response | Transcription link |
|---|---|---|
| Auxin (IAA) | Shoot cells elongate; phototropism | Growth proteins are made |
| Gibberellin | Stem elongation; starch breakdown in grain | Amylase gene switched on in the aleurone |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: a shoot bends toward one-sided light. Y: more auxinA plant growth substance made in shoot tips that promotes cell elongation. reaches the shaded side, so those cells elongate more. Z: the faster side pushes the tip over toward the light. Incomplete: “The lit side grows faster because it has more auxin.” Improved: “Auxin accumulates on the shaded side of a shoot. Extra elongation there is what bends the tip toward the light.”
Shaded side has more auxin
In the shoot, not in a root. More elongation on the shade side bends the tip toward the light.
Gibberellin is not amylase
The hormone switches the geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. on. Amylase is the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. that is then made.
Avoid this misconception
The claim: The shoot bends toward the light because the lit side has more auxin and grows faster.
The correction: In a shoot, auxin accumulates on the shaded side. Those cells elongate more, so the shoot bends toward the light.
Put plant hormone action in order
Both plant hormones work by changing transcription. Order phototropism by auxin, then germination by gibberellin.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Auxin or Gibberellin.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Light reaches a shoot from one side. Put the steps of phototropism in order.
1 of 5 in the right place
Roots respond to auxin differently from shoots. Gibberellin also causes stem elongation; that effect is not ordered here. Core Practical 18 measures the amylase.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Where is auxin produced in a shoot, and where does it act?
Show suggested answer
It is produced in the tip and causes elongation of cells behind the tip.
Practice question
Retrieval and explanation · suggested answer
Explain positive phototropism of a shoot in three steps.
Show suggested answer
Light from one side causes more auxin on the shaded side. Cells there elongate more. The shoot bends toward the light.
Practice question
Retrieval and explanation · suggested answer
How does gibberellin lead to starch breakdown in a germinating grain?
Show suggested answer
The embryo releases gibberellin. It causes the aleurone to transcribe the amylase gene. Amylase hydrolyses starch in the endosperm.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A grass seedling has its tip covered with an opaque cap, and another has the tip removed. Both stand in one-sided light. Predict the growth of each. Assume auxin from the tip is required for elongation and for the redistribution.
Show a hint
No tip means no auxin source. A covered tip still has a source but no light difference detected at the tip.
Explore the reasoning
The covered tip can still produce auxin, so the shoot elongates, but the tip does not detect the one-sided light, so auxin is not redistributed and the shoot grows straight. The tipless shoot has no auxin source, so it does not elongate and does not bend. The assumptions are the ones in the question.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the two interconvertible forms of phytochrome.
Plant growth regulators
Practice question
Retrieval and explanation · suggested answer
Name the two interconvertible forms of phytochrome.
Lesson: 8B 6: Phytochrome and Flowering
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A hormone can be the signal a transcription factor waits for
8B · Chemical Control Systems in PlantsMedlinePlus Genetics explains that signals can activate transcription factors, proteins that bind DNA and raise or lower transcription. It does not mention auxin or gibberellin. Those are the…Specification 8.11Read & use the scienceClose article
Why this matters
MedlinePlus Genetics explains that signals can activate transcription factors, proteins that bind DNA and raise or lower transcription. It does not mention auxin or gibberellin. Those are the plant signals in 8.11. The page is the general idea. The aleurone’s amylase gene is the plant example. 8.11: IAA and gibberellins cause responses that include changes in transcription. A scan or growth response is indirect evidence: it cannot alone identify every signalling step or prove the proposed cause.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why is “gibberellin cuts starch itself” inconsistent with a transcription-factor account?
Explore a possible response
Gibberellin is the signal. Transcription factors then change production of amylase. The enzyme, not the hormone, hydrolyses the starch.
Explore the source
- MedlinePlus Genetics: can genes be turned on and off?US National Library of Medicine
Plants read the colour of their neighbours
8B · Chemical Control Systems in PlantsPlants beneath leaves receive less red light relative to far-red light because neighbouring foliage changes the light spectrum. A research paper identified early gene-expression targets in Arabidopsis after…Specification 8.11Read & use the scienceClose article
Why this matters
Plants beneath leaves receive less red light relative to far-red light because neighbouring foliage changes the light spectrum. A research paper identified early gene-expression targets in Arabidopsis after phytochrome signalling during shade avoidance. This extends 8.11 from a textbook light switch to an environmental signal that can alter growth. Phytochrome does not push a stem mechanically; a signalling network changes transcription and hormone responses before cells elongate. The exact gene names in the paper are enrichment. To test whether the response is due to light quality rather than light quantity, an experiment must control both separately.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why would comparing plants under equal light intensity but different red:far-red ratios help test phytochrome signalling?
Explore a possible response
It holds total light intensity similar while changing the spectral cue phytochrome detects. A growth difference would then support an effect of light quality, though other controls are still needed.
Explore the source
- Primary target genes of phytochrome signallingPeer-reviewed research via NIH PubMed Central