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

Control and Coordination in Plants – Venus fly‑trap (Dionaea muscipula)

Learning objective (Cambridge AS & A Level Biology 9700)

Describe the rapid response of the Venus fly‑trap to stimulation of the trigger hairs on the lobes of its modified leaves and explain how the closure of the trap is achieved. Relate this example to the syllabus topic “Control & coordination in plants – neural‑type signalling and rapid movements”.

Why the Venus fly‑trap is a model system

• It generates a true neural‑type electrical signal (action potential) in a plant.
• The movement is extremely fast (≤ 0.3 s) and is driven by a combination of ion fluxes, rapid turgor change and elastic snap‑buckling – a classic example of a rapid, non‑growth movement.
• It illustrates the key syllabus ideas:

  • stimulus detection (mechanosensitive hairs)
  • signal integration (the two‑stimulus rule)
  • propagation of an electrical signal
  • calcium as a second messenger
  • conversion of an electrical signal into a mechanical output

Link to the wider “Control & Coordination” syllabus

The Venus fly‑trap example represents the plant side of the syllabus. For the full A‑Level unit you should also be able to:

• compare plant action potentials with animal nerve‑muscle signalling (e.g. Na⁺/K⁺ channels, refractory period, synaptic transmission)
• explain hormonal coordination (e.g. auxin transport, gibberellin control of seed germination, ABA‑mediated stomatal closure)
• describe homeostatic feedback loops such as blood‑glucose regulation, calcium homeostasis in animal muscle, and the regulation of leaf water potential.

Understanding the similarities (electrical excitability, use of Ca²⁺ as a messenger) and differences (absence of voltage‑gated Na⁺ channels, reliance on turgor rather than contraction) helps you answer comparative questions in Paper 1 and Paper 3.

Comparison with other rapid plant movements

Key morphological features of the Venus fly‑trap

• Leaves are modified into a bilobed “trap” with a flexible midrib acting as a hinge.
• Each inner lobe surface carries 3–4 mechanosensitive trigger hairs.
• Motor cells are located in the outer epidermis of the lobe margins; the inner epidermis remains relatively turgid during closure.
• Marginal interlocking cilia form a watertight seal when the trap is closed.

Sequence of events leading to trap closure

Detailed mechanistic explanation

1. Stimulus detection – Bending of a trigger hair stretches the plasma membrane of a specialised sensory cell. Stretch‑activated mechanosensitive channels (MSL‑type) open, allowing Na⁺ and Ca²⁺ to flow inward, producing a rapid depolarisation.
2. Action‑potential generation – A single hair touch gives a sub‑threshold depolarisation. If a second touch (same or opposite hair) occurs within ≈ 20 s, the two depolarisations summate. When the membrane potential reaches ≈ ‑30 mV the AP is triggered. The resting potential is about ‑120 mV (calculated from the Nernst equation for K⁺, Ca²⁺ and Na⁺).
3. Electrical signal propagation – The AP travels through plasmodesmata and the symplastic pathway, reaching motor cells on both lobes and the midrib within a few hundred milliseconds.
4. Calcium signalling – Depolarisation opens voltage‑gated Ca²⁺ channels (e.g. TPC1) in motor cells. Cytosolic Ca²⁺ rises from ~0.1 µM to >10 µM, acting as a second messenger that activates downstream ion channels.
5. Ion efflux and turgor loss – Ca²⁺ activates outward‑rectifying K⁺ channels (SKOR‑type) and Cl⁻ channels. The rapid loss of K⁺ and Cl⁻ lowers the osmotic potential, causing water to leave the outer epidermal cells. The inner epidermal cells stay relatively turgid, creating a curvature differential.
6. Snap‑buckling (elastic instability) – The differential turgor creates a bending moment on the midrib. When the critical stress is exceeded, the curved midrib undergoes a bistable snap‑buckling transition, moving the lobes together faster than a gradual turgor change could achieve.
7. Sealing and subsequent digestion – Interlocking marginal cilia lock the lobes, forming a watertight chamber. If prey is present, the trap remains closed for several days while digestive enzymes break down the captured material (digestion is outside the AO1 requirement).
8. Recovery (energy considerations) – After digestion the H⁺‑ATPase in the plasma membrane pumps protons out, re‑establishing the electrochemical gradients that drive Na⁺/K⁺/Cl⁻ uptake. ATP is therefore required to restore ion balance and turgor before the trap can reopen.

Why two stimuli are required (the “two‑stimulus rule”)

The rule prevents wasteful closure on non‑prey stimuli such as raindrops or wind. The plant integrates depolarisations over a short time window (≈ 20 s). Only when the summed depolarisation exceeds the AP threshold does the full cascade proceed, providing a simple decision‑making mechanism.

Membrane‑potential calculations (optional AO2 extension)

For a typical motor cell (internal K⁺ ≈ 150 mM, external K⁺ ≈ 5 mM):

\[

E{K}= \frac{RT}{zF}\ln\frac{[K^+]{out}}{[K^+]_{in}} \approx -92\ \text{mV}

\]

Similar calculations for Na⁺ and Ca²⁺ give potentials of about ‑30 mV and +120 mV respectively. The weighted sum of these ions yields a resting membrane potential of roughly ‑120 mV. Depolarisation to ‑30 mV therefore represents a change of ≈ 90 mV, sufficient to open voltage‑gated Ca²⁺ channels.

Practical investigation – “Measuring the action potential of a Venus fly‑trap”

Sample data set & practice questions (AO2)

Voltage trace (mV) recorded after the second stimulus

Questions for practice

1. Identify the approximate voltage threshold required for trap closure from the data.
2. Calculate the average latency (time from second stimulus to peak voltage) if the oscilloscope shows a latency of 0.12 s for the 5 s interval and 0.15 s for the 10 s interval.
3. Explain why the trap does not close when the interval is ≥ 20 s, linking your answer to signal integration.
4. Suggest two improvements to reduce experimental error and justify how they would increase data reliability.

Glossary (AO1 quick‑reference)

• Action potential (AP) – A rapid, self‑propagating change in membrane potential caused by coordinated ion fluxes; in plants it mainly involves Na⁺/Ca²⁺ influx followed by K⁺/Cl⁻ efflux.
• Calcium signalling – Transient rise in cytosolic Ca²⁺ that acts as a universal second messenger to activate downstream proteins such as ion channels.
• Mechanosensitive ion channel – A membrane protein that opens in response to mechanical stress (e.g., stretch‑activated MSL channels in trigger hairs).
• Voltage‑gated Ca²⁺ channel (e.g., TPC1) – Opens when the membrane depolarises, allowing Ca²⁺ influx that triggers downstream responses.
• Plasmodesmata – Cytoplasmic channels linking adjacent plant cells, permitting electrical and chemical signal transmission.
• Snap‑buckling – A rapid change in curvature of a thin structure when a critical stress threshold is exceeded; stores elastic energy that is released in a millisecond‑scale movement.
• Turgor pressure – Hydrostatic pressure within a plant cell generated by water entering the vacuole; loss of turgor causes cell shrinkage.
• Elastic instability – A condition where a structure cannot maintain its shape under load and abruptly shifts to a new configuration (basis of snap‑buckling).
• H⁺‑ATPase – An ATP‑driven proton pump that re‑establishes ion gradients after a rapid movement, allowing the trap to reopen.

Key points to remember for exam (Paper 1 & Paper 3)

• The Venus fly‑trap’s rapid movement is not driven by growth; it relies on ion‑driven turgor changes and a mechanical snap‑buckling of the midrib.
• Two separate stimulations of trigger hairs within ~20 s are required to reach the action‑potential threshold (≈ ‑30 mV).
• Plant APs involve Na⁺/Ca²⁺ influx**, depolarisation, and subsequent K⁺/Cl⁻ efflux that leads to water loss.
• Ca²⁺ acts as a universal second messenger linking the electrical signal to the mechanical response.
• Snap‑buckling provides a bistable system – the trap can exist in an open or closed state with a rapid transition between them.
• After closure, ATP‑dependent pumps restore ion gradients so the trap can reopen and be ready for the next prey.