Ch 2 · 15.2 Control and coordination in plants · AS & A Level Biology 9700
Topic 2 of 2 in Control and coordination — AS & A Level Biology 9700 — 48 lessons.
Introduction
Plants have no nervous system, yet they respond to stimuli. Some responses are fast and electrical, like the snapping of a Venus fly trap; most are slower and controlled by plant hormones such as auxin and gibberellin. (Syllabus section 15.2.)
Key ideas — The Venus fly trap: a rapid response
The Venus fly trap (Dionaea muscipula) grows in nitrogen-poor soil and gets extra nitrogen by catching insects. Each trap is a modified leaf with two lobes, edged with stiff bristles. On the inner surface of each lobe are three or more sensitive trigger hairs.
🧑🏫 Teacher's explanation
Think of the Venus flytrap like a high-security pressure-sensitive trap. A single accidental bump (like a raindrop) isn't enough to trigger the alarm, preventing the plant from wasting energy. It requires two distinct 'pings' within a short window to confirm a real prey item is present. Once the threshold is met, the plant converts an electrical signal into a massive, rapid shift in water pressure, causing the lobes to snap shut.
📖 Definitions
🔍 Worked example — Easy
A Venus flytrap's trigger hair is stimulated. If the first stimulation occurs at t = 0 seconds and the second stimulation occurs at t = 15 seconds, will the trap close? Explain why based on the physiological threshold.
🔍 Worked example — Standard
Describe the sequence of events following the second stimulation of a trigger hair that leads to the physical closure of the lobes.
🔍 Worked example — Exam-style
A researcher observes a Venus flytrap in a nitrogen-poor environment. If the plant's ability to generate action potentials is inhibited by a chemical that blocks voltage-gated ion channels, predict the effect on the plant's survival and explain why.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe how the stimulation of the trigger hairs leads to the closure of the trap. [3] (b) Explain why the plant does not close its trap upon a single stimulation of a trigger hair. [2] [5 marks]
Key ideas — How the trap closes
When the lobes are open, they are convex (bulging outwards), held in a strained shape. The action potentials cause rapid changes in the cells of the lobes:
🧑🏫 Teacher's explanation
Think of the Venus flytrap like a spring-loaded mousetrap. Before the trap is triggered, it is like a bent ruler held under tension; it wants to snap back to a different shape. When the trigger hairs are touched, it's like releasing the catch on that ruler.
📖 Definitions
🔍 Worked example — Easy
A student observes a Venus flytrap. Describe the change in the shape of the leaf lobes when the trap is triggered.
🔍 Worked example — Standard
Explain how the rapid movement of water leads to the closure of the Venus flytrap.
🔍 Worked example — Exam-style
The Venus flytrap trap reopens over several days. Explain the biological process responsible for this reopening.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Auxin and elongation growth
Auxin (IAA) is made in the growing tips of shoots and moves to the region just behind the tip, where it stimulates cell elongation. It works by the acid growth mechanism:
🧑🏫 Teacher's explanation
Think of a plant cell wall like a tight, stiff corset holding a balloon in place. If you want the balloon to expand, you first have to loosen the laces of the corset. In plants, auxin is the signal that tells the cell to 'loosen the laces'.
📖 Definitions
🔍 Worked example — Easy
Identify the sequence of events in the acid growth hypothesis when auxin concentration increases at a specific site in a shoot tip.
🔍 Worked example — Standard
A researcher observes that a plant shoot is growing towards a light source. Explain why the side of the shoot in the shade is growing faster than the side in the light.
🔍 Worked example — Exam-style
Calculate the change in pH if the concentration of H^+ ions in the cell wall increases from 1 × 10^-7 mol/dm^3 to 1 × 10^-5 mol/dm^3.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Gibberellin and the germination of barley
A barley grain contains an embryo and a large endosperm of stored starch, surrounded by a thin aleurone layer.
🧑🏫 Teacher's explanation
Think of a barley grain like a locked pantry. The endosperm is the pantry full of food (starch), but the door is locked. The embryo is the person inside who needs food to grow. To get the food, the embryo sends a 'messenger' (gibberellin) to the pantry door (the aleurone layer) to unlock it. Once unlocked, the pantry releases tools (amylase) that turn the bulky, unusable food into small, easy-to-eat snacks (maltose/glucose) that can pass through the door to the embryo.
📖 Definitions
🔍 Worked example — Easy
A student observes a barley seed. Describe the first step required to initiate the germination process.
🔍 Worked example — Standard
Explain the sequence of events that leads to the production of maltose from starch in a germinating barley grain.
🔍 Worked example — Exam-style
A mutant strain of barley is unable to produce gibberellin. Predict and explain the effect of this mutation on the germination process.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Explain how auxin causes cells behind a shoot tip to elongate. [4]
🔍 Worked example — Easy
A Venus flytrap lobe has sensory hairs. If a stimulus (like an insect) triggers the hairs, describe the immediate physical change that leads to the trap closing.
🔍 Worked example — Standard
Explain the role of gibberellin in the germination of barley. [3]
🔍 Worked example — Exam-style
Explain how auxin causes cells behind a shoot tip to elongate. [4]
⚠️ Common errors
Common mistakes
Practice
Answers