In simple terms
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Control and coordination in plants
Cambridge 9700 Paper 4 — Control and coordination in plants (15.2). A-Level Notes diagram-backed lesson with premium structure and live visuals.
- 1
Explain the roles of auxins, gibberellins, abscisic acid (ABA), and ethene in plant growth and development.
- 2
Describe and explain the role of auxins in phototropism of shoots and roots.
- 3
Describe and explain the role of auxins in gravitropism of shoots and roots.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 15.2.1
Describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved
- 15.2.2
Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
- 15.2.3
Describe the role of gibberellin in the germination of barley (see 16.3.4)
Explore the concept
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Key Players: Plant Hormones
Plants produce several classes of hormones, each with distinct, yet often overlapping, functions. For your 9700 exam, you need to master the roles of four principal types.
1. Gibberellins
Gibberellins are crucial for breaking seed dormancy and promoting stem growth.
Seed Germination: When a seed absorbs water, the embryo produces gibberellins. These diffuse to the aleurone layer, stimulating the synthesis of hydrolytic enzymes like amylase. Amylase breaks down stored starch in the endosperm into maltose and glucose, which are used for respiration to provide ATP for the embryo's growth.
Stem Elongation: Active gibberellin (GA1) stimulates stem elongation by causing the breakdown of DELLA protein repressors. DELLA proteins normally inhibit growth genes. By removing them, gibberellins allow for cell division and elongation, leading to an increase in internode length.
2. Auxins
Auxins are perhaps the most famous plant hormones, primarily responsible for controlling cell elongation and directional growth (tropisms). They are synthesised in the meristems (growing tips) of shoots and roots.
Cell Elongation: Auxins promote cell elongation by increasing the plasticity of cell walls. They stimulate proton pumps, which lower the pH in the cell wall, activating enzymes (expansins) that break bonds between cellulose microfibrils, allowing the wall to stretch.
Apical Dominance: The auxin produced by the apical bud inhibits the growth of lateral (axillary) buds further down the stem. This ensures the plant grows taller, competing for light, before branching out.
Rooting: Auxins are used commercially in rooting powders to stimulate the growth of roots from cuttings.
Auxins in Action: Tropisms Explained
Now let's focus on auxins, which are central to how plants navigate their environment. Tropisms are directional growth responses towards or away from a stimulus. Auxins achieve this by causing differential growth – meaning one side of a plant organ grows faster than the other, causing a bend.
Phototropism: Chasing the Light
Phototropism is the growth response towards or away from a light source. Shoots are positively phototropic (grow towards light), while roots are generally negatively phototropic or insensitive.
Phototropism in Shoots: Light is detected by photoreceptors (e.g., phototropin) in the shoot tip. Auxin migrates from the illuminated side to the shaded side of the shoot. The higher auxin concentration on the shaded side promotes greater cell elongation, causing the shoot to bend towards the light.
Phototropism in Roots: Roots are far more sensitive to auxin's inhibitory effects. Light exposure can cause some auxin redistribution, leading to increased auxin on the shaded side. This increased auxin concentration inhibits cell elongation on the shaded side, causing the root to bend away from light (negative phototropism), though this response is generally weaker than in shoots.
Gravitropism (Geotropism): Battling Gravity
Gravitropism (also known as geotropism) is the growth response to gravity. Shoots are negatively gravitropic (grow upwards, against gravity), and roots are positively gravitropic (grow downwards, with gravity).
Gravitropism in Shoots: Gravity sensors (statoliths/amyloplasts) in cells detect the direction of gravity. Auxin moves to the lower side of a horizontally placed shoot. The higher auxin concentration on the lower side promotes greater cell elongation. This causes the lower side to grow faster, making the shoot bend upwards.
Gravitropism in Roots: Gravity sensors in the root cap detect gravity. Auxin moves to the lower side of a horizontally placed root. Crucially, the higher auxin concentration on the lower side inhibits cell elongation in roots (as roots are highly sensitive to auxin). The upper side therefore grows faster, causing the root to bend downwards.
3. Abscisic Acid (ABA)
Often called the 'stress hormone', ABA plays a critical role in helping plants cope with adverse environmental conditions.
Stomatal Closure: During water stress (drought), ABA levels in the leaves rise. ABA binds to receptors on the cell surface membranes of guard cells, causing an efflux of K+ ions. This increases the water potential inside the guard cells, causing water to move out by osmosis. The cells become flaccid, and the stomata close, reducing water loss via transpiration.
Seed Dormancy: ABA is responsible for inducing and maintaining seed dormancy. It prevents seeds from germinating during unsuitable conditions (e.g., winter). For germination to occur, ABA levels must fall, and the ratio of gibberellin to ABA must increase.
4. Ethene
Ethene is a unique plant hormone as it is a gas. It is widely known for its role in fruit ripening and senescence.
Fruit Ripening: In climacteric fruits (e.g., bananas, tomatoes, apples), a burst of ethene production triggers ripening. This involves increased respiration, conversion of starch to sugars, softening of cell walls (by enzymes like polygalacturonase), and breakdown of chlorophyll. The process is autocatalytic (positive feedback), meaning ethene stimulates more ethene production.
Leaf Abscission: Ethene, often in conjunction with changing auxin levels, promotes senescence (aging) and the formation of an abscission layer at the base of the leaf petiole. This weakens the connection to the stem, eventually causing the leaf to fall.
A common mistake is to assume auxin always promotes growth. Remember: shoots have an optimal auxin concentration for growth which is much higher than the optimal concentration for roots. In fact, auxin concentrations that promote shoot growth are inhibitory to root growth. This differential sensitivity is key to understanding gravitropism in particular!
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
An experiment was set up to investigate the effect of unilateral light on the growth of an oat coleoptile. The initial length of the coleoptile was 15.0 mm. After 48 hours, the length of the shaded side was measured to be 19.5 mm, and the length of the illuminated side was 16.5 mm. Calculate the percentage elongation for each side and explain the difference.
- 1
Calculate the elongation of the shaded side:
A student set up an experiment where oat coleoptiles were placed horizontally in the dark. After 24 hours, the coleoptiles curved upwards. Explain the role of auxins in this observed response.
- 1
Identify the tropism: The coleoptile bending upwards in response to gravity, in the dark, is an example of negative gravitropism. This eliminates light as a factor, focusing purely on gravity.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is a phytohormone?
A chemical substance produced by a plant that acts as a signal, controlling and regulating growth, development, and responses to the environment. Also known as a plant hormone.
Key takeaways
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- ✓
Seed Germination: When a seed absorbs water, the embryo produces gibberellins. These diffuse to the aleurone layer, stimulating the synthesis of hydrolytic enzymes like amylase. Amylase breaks down stored starch in the endosperm into maltose and glucose, which are used for respiration to provide ATP for the embryo's growth.
- ✓
Stem Elongation: Active gibberellin (GA1) stimulates stem elongation by causing the breakdown of DELLA protein repressors. DELLA proteins normally inhibit growth genes. By removing them, gibberellins allow for cell division and elongation, leading to an increase in internode length.
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