Control and coordination in plants

1. Overview

Plants cannot move away from a stimulus, but they can still respond to it. Without nerves or muscles, they use changes in turgor, electrical signals and chemical growth regulators to bring about responses. This section looks at three contrasting examples:

• the rapid trap closure of the Venus fly trap (a fast, turgor-driven movement triggered by an electrical signal),
• the role of auxin in stretching cells during elongation growth by acidifying the cell wall, and
• the role of gibberellin in mobilising stored starch during the germination of barley.

A useful contrast to keep in mind throughout: the trap response is fast and reversible because it uses existing turgor and elastic tension, whereas auxin-driven elongation is slow and irreversible because it involves actual growth.

Key Definitions

• Turgor pressure: the outward pressure of the cell contents against the cell wall when a cell is full of water, caused by water entering the vacuole by osmosis.
• Action potential (in plants): a rapid, transient change in the electrical potential across a plant cell membrane that can spread to neighbouring cells and trigger a response.
• Auxin: a plant growth regulator (chiefly indole-3-acetic acid, IAA) that stimulates the elongation of cells in shoots.
• Acid growth hypothesis: the model in which auxin causes hydrogen ions to be pumped into the cell wall, lowering its pH and loosening it so the cell can elongate.
• Expansin: a wall protein, activated by low pH, that loosens the bonds between cellulose microfibrils so the wall can stretch.
• Gibberellin: a plant growth regulator that, in germinating seeds such as barley, promotes transcription of the gene(s) for amylase, so the enzyme is made to break down stored starch.
• Aleurone layer: the outer layer of living cells surrounding the endosperm of a cereal grain that secretes hydrolytic enzymes during germination.
• Germination: the start of growth of a seed, using stored food reserves, once conditions such as water, oxygen and a suitable temperature are met.

Content

The rapid response of the Venus fly trap

The Venus fly trap (Dionaea muscipula) catches insects using a pair of modified leaves that form a hinged trap. On the inner surface of each lobe are small sensory (trigger) hairs. When an insect touches a hair, it bends the hair and stimulates the cells at its base, generating an action potential — a rapid electrical signal that spreads across the lobes.

Why two touches are needed. A single touch is usually not enough to close the trap. This prevents the plant from wasting energy on a raindrop or piece of debris. Think of it as a short "memory":

1. An insect touches a trigger hair, producing a first action potential — but this is below the threshold needed to close the trap.
2. This first signal is briefly "remembered" as a small electrical change that fades within about 20–30 seconds.
3. If a second touch arrives within that window (a second hair, or the same hair touched again), it adds to the first.
4. The combined signals now reach threshold, and the trap closes.

How the trap actually snaps shut. Once threshold is reached, closure is extremely fast (a fraction of a second) and depends on a sudden change in turgor and lobe shape — not on growth:

• The action potential makes the membranes of the lobe cells more permeable, so ions move and water follows by osmosis. This rapidly changes the turgor of cells on the inner and outer surfaces of each lobe.
• The lobes are held under elastic tension in a curved-outwards (convex) shape, like a bent strip of plastic ready to flip.
• The change in turgor releases that tension, so each lobe snaps from convex to concave (curved-inwards), closing the trap like a spring.

Because the response relies on existing turgor and elastic tension rather than new growth, it is very rapid and reversible — a feature unusual among plant responses.

The role of auxin in elongation growth

Auxin (mainly indole-3-acetic acid, IAA) is a plant growth regulator that stimulates cells in the shoot to elongate. Cell elongation, not cell division, is what makes a young shoot get longer. For a cell to elongate, its cellulose cell wall must be loosened so the wall can stretch as the vacuole takes in water and turgor pushes outward.

Auxin loosens the wall through the acid growth hypothesis:

• Auxin stimulates proton pumps (H⁺-ATPases) in the cell surface membrane.
• These pumps actively transport hydrogen ions (H⁺) out of the cytoplasm and into the cell wall, using ATP.
• This lowers the pH of the cell wall (makes it more acidic).
• The low pH activates wall-loosening proteins called expansins, which break the cross-links and weaken the bonds between cellulose microfibrils.
• The loosened wall can now stretch as water enters the vacuole by osmosis and turgor pressure pushes the wall outwards, so the cell elongates.

The result is irreversible growth in length. The same mechanism underlies tropisms: when auxin becomes unevenly distributed (for example, more on the shaded side of a shoot), the side with more auxin elongates faster, so the shoot bends.

Worked example

Exam-style question: Sections were cut from young oat coleoptiles (a growing shoot tissue). The sections were placed in buffer solutions and their increase in length was measured after 3 hours. The table shows the results.

Treatment	Buffer pH	Mean increase in length / mm
Buffer only	7.0	0.4
Buffer only	5.0	1.6
Buffer + proton-pump inhibitor	5.0	1.5
Buffer + proton-pump inhibitor	7.0	0.3

Using the acid growth hypothesis, explain why elongation was greater at pH 5.0 than at pH 7.0, and why the proton-pump inhibitor had little effect on the results. [4]

Model answer:

• At the lower pH (5.0) the cell wall is more acidic, which activates expansins; these loosen the bonds between cellulose microfibrils so the wall can stretch and the cells elongate more.
• At pH 7.0 the wall is not acidic enough to activate expansins, so the wall stays rigid and there is little elongation.
• Normally auxin works by stimulating proton pumps to move H⁺ into the wall; a proton-pump inhibitor would block this and reduce growth.
• Here the acidic buffer already supplies the low pH directly, so the wall is loosened without needing the pumps — this is why the inhibitor made little difference at pH 5.0 (and the still-low result at pH 7.0 confirms it is the pH, not the pumps, that matters in this setup).

How the 4 marks split: this is an explain question, so each mark needs a causal link, not just a fact. Marks 1-2 are the chain low pH → activates expansins → wall loosens → cell stretches; the inhibitor point is the "twist" mark — you only score it by recognising that the acidic buffer bypasses the proton pumps, so blocking the pumps cannot stop loosening that the buffer is already causing.

The role of gibberellin in germination of barley

A barley grain (a cereal seed) stores food as starch in its endosperm. When the seed germinates, this starch must be broken down into soluble maltose and glucose that the growing embryo can absorb and use for respiration and growth. Gibberellin controls this mobilisation of stored food.

The sequence of events is:

• When the seed takes up water, the embryo produces and releases gibberellin.
• The gibberellin diffuses to the aleurone layer, a layer of living cells surrounding the endosperm.
• Gibberellin acts as a chemical signal that promotes transcription of the gene(s) for the enzyme amylase in the aleurone cells, so mRNA for amylase is synthesised.
• The aleurone cells synthesise and secrete amylase into the endosperm.
• Amylase hydrolyses the stored starch into maltose (and other soluble sugars).
• These soluble sugars diffuse to the embryo, providing substrate for respiration and the raw materials for growth.

This is a clear example of a plant hormone controlling gene expression: gibberellin does not break down starch itself — it triggers the production of the enzyme that does.

Worked example

Exam-style question: In an experiment, barley grains were cut in half. Half-grains containing the embryo and half-grains with the embryo removed were each placed on a starch-containing agar plate. After two days the plates were flooded with iodine solution. A clear zone (no blue-black colour) appeared in the agar only around the half-grains that contained an embryo. Explain these results. [4]

Model answer:

• The clear zone shows that starch has been broken down (hydrolysed) by amylase in that area, so iodine gives no blue-black colour there.
• Half-grains containing the embryo released gibberellin, which diffused to the aleurone layer.
• Gibberellin promoted transcription of the gene(s) for amylase in the aleurone cells, so they synthesised and secreted amylase that digested the starch in the agar.
• Half-grains with the embryo removed produced no gibberellin, so no amylase was made and the starch was not broken down (the agar stayed blue-black).

Key Equations

This topic is qualitative; there are no equations to learn. Focus instead on describing the mechanisms precisely and in the correct sequence.

Common Mistakes to Avoid

• Saying the trap closes because the plant "decides" to. Closure is a physical response: an action potential triggered by the trigger hairs causes ion movement and water loss by osmosis, changing turgor so the lobes snap shut. Use mechanism, not intention.
• Forgetting that two stimulations are usually needed. A single touch normally does not close the trap; explain that requiring two touches (or one hair touched twice) within a short time stops the plant wasting energy on non-prey.
• Saying auxin "makes cells divide". Auxin causes cells to elongate, not divide. The growth in length comes from cells stretching after the wall is loosened.
• Stating that auxin "softens the wall" without the mechanism. Be specific: auxin stimulates proton pumps that move H⁺ into the wall, lowering the pH, which activates expansins that loosen the cellulose microfibrils so the wall can stretch.
• Using vague directional language. Say that hydrogen ions are pumped out of the cytoplasm into the cell wall, and that ions move across or through the membrane, rather than "into" the membrane. It is the membrane that changes permeability, not "the cell" in a vague sense.
• Claiming gibberellin breaks down the starch itself. Gibberellin is a signal: it promotes transcription of the genes for amylase in the aleurone layer, and it is the amylase that hydrolyses the starch.
• Confusing which part of the seed produces what. The embryo produces the gibberellin; the aleurone layer produces the amylase; the endosperm stores the starch.

Exam Tips

• For the Venus fly trap, build your answer as a sequence: stimulus (trigger hair touched) → action potential → ion and water movement → turgor change → lobes snap shut. Marks are usually given for the linked steps in order.
• When explaining auxin, name the actual mechanism (proton pumping, low pH, expansins) rather than just saying "auxin causes elongation" — this is where the higher marks lie.
• Use precise vocabulary: hydrolyses (not "melts" or "dissolves") for amylase acting on starch, and secrete for the aleurone releasing enzymes.
• In experiment-based questions on barley, link the result back to the presence or absence of the embryo as the source of gibberellin — this is the controlled comparison the question is testing.
• Remember the trap response is fast and reversible (turgor-driven), whereas auxin-driven elongation is slow and irreversible (growth) — make sure your answer matches the type of response being asked about.