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15.2 Control and Coordination in Plants

Syllabus
9700–2028–2029
Topic
15.2
Level
A2

A Venus flytrap closes rapidly when mechanosensory signals reach the leaf trap

The Venus flytrap converts touch to a rapid electrical response that closes its leaf trap. Sensory hairs detect movement, and the resulting signal causes hinge cells to swell so the two leaf lobes fold together.

  1. An insect touches the sensory hairs on the leaf.
  2. Calcium ion channels open in hinge cells at the base of a sensory hair, so calcium ions enter and generate a receptor potential.
  3. Repeated stimulation is required: two hairs stimulated together, or one hair stimulated twice within a short interval, can reach the condition for an action potential. The action potential is propagated across the trap cells; without the required repeat stimulation, the trap resets.
  4. Water moves into the hinge cells, making them swell. This change in cell shape causes the two lobes to fold together and close the trap.

Continued movement by prey can maintain the closed state. Further stimulation can cause calcium ions to enter gland cells and stimulate release of digestive enzymes, but this is a downstream response after the electrical closure pathway.

This is plant electrical coordination, not an animal nervous pathway: the card does not require neurones, synapses, or neurotransmitters. The electrical signal is the trigger, while water-driven hinge-cell swelling provides the immediate mechanical change that closes the trap.

Auxin acidifies cell walls to promote elongation

Auxin promotes elongation by activating proton pumping and making the cell wall easier to extend. Wall loosening must be paired with water entry and turgor pressure for the cell to lengthen.

  1. Auxin binds to receptors and stimulates proton pumps in the cell-surface membrane.
  2. The pumps use ATP to move H+ from cytoplasm into the cell wall, lowering wall pH.
  3. Acidic conditions activate expansins, which disrupt links between cellulose microfibrils so the wall can stretch.
  4. K+ enters through channels, lowering cell water potential; water enters by osmosis.
  5. Rising turgor pressure pushes against the loosened wall and the cell elongates.

Auxin does not supply water or lengthen cellulose directly. Water entry alone cannot produce sustained elongation unless the wall has been loosened; this objective does not require a shaded-side phototropism explanation.

Gibberellin removes DELLA repression to mobilise barley starch

After a barley seed absorbs water, the embryo produces gibberellin. This hormone signals aleurone cells to activate genes for hydrolytic enzymes that mobilise food stored in the endosperm.

  1. Gibberellin moves to the aleurone layer and binds to its receptor.
  2. Receptor signalling causes DELLA repressor proteins to be broken down.
  3. DELLA proteins normally inhibit factors that promote transcription; removing them allows transcription of genes including the gene for amylase.
  4. Aleurone cells synthesise and secrete amylase into the starchy endosperm.

Amylase hydrolyses starch to maltose, which can be converted to glucose. Soluble sugars move to the embryo and are used in respiration and as materials for growth, supporting germination and early seedling development.

Gibberellin is the signal, not the hydrolytic enzyme. Its key gene-control role is removal of DELLA repression; amylase then performs starch hydrolysis.