Plants have no nerves or muscles, yet a Venus fly trap snaps shut in a fraction of a second, shoots grow towards light and a barley grain turns its starch store into sugar on cue. Electrical signals, proton pumps and plant hormones make it happen.
The Venus fly trap has modified leaves with two lobes, each carrying sensitive trigger hairs. Bending a hair produces a receptor potential; if two hairs are touched within about 20 seconds, action potentials spread across the lobes. These trigger rapid movement of ions and water, changing the turgor of cells in the lobes, so the lobes flip from convex to concave and the trap snaps shut in a fraction of a second. Auxin, made in shoot tips, stimulates elongation growth: it binds to a receptor and stimulates proton pumps to pump H+ into the cell wall; the acidic wall activates expansins, which loosen the links between cellulose microfibrils, so water entering by osmosis stretches the wall and the cell elongates. In a germinating barley grain, the embryo releases gibberellin, which travels to the aleurone layer and switches on the gene for amylase; amylase hydrolyses starch in the endosperm to maltose, providing sugars for the embryo's respiration and growth.
Fig. 1: auxin stimulates proton pumping into the cell wall; the acidified wall loosens and the cell elongates as it takes up water
Section 01
A trap that snaps.
Choose Venus fly trap and press Go once: one hair touched, the trap stays open. Press Go again to trigger the second touch and watch the lobes snap shut. Then switch to the auxin and gibberellin views and press Go for each.
The trap: a modified leaf with two lobes joined by a midrib; each lobe has stiff, sensitive trigger hairs on its inner surface and stiff bristles along its edge.
Stimulus: an insect bending a trigger hair opens ion channels in cells at the base of the hair, producing a receptor potential.
Two touches: a single touch does not close the trap. If a second hair (or the same one again) is touched within about 20 seconds, action potentials travel across the lobes. Needing two touches stops the trap closing on raindrops or debris.
Closure: the action potentials cause ions to move and water to shift rapidly between cells; cell walls in the lobes loosen as protons are pumped into them, cells change turgor, and the curved lobes flip from convex to concave, closing in about 0.1 to 0.3 s.
Afterwards: the bristles interlock to trap the insect; further struggling sets off more action potentials, the lobes seal tightly and glands secrete digestive enzymes, so the plant gains nitrogen from its prey.
Examiner trap
The Venus fly trap has no muscles and no nerves. Its rapid movement comes from action potentials and changes in turgor and cell wall properties, not from contraction.
Section 02
Auxin and the acid wall.
In the auxin view, press Go and watch the yellow H+ ions pile up in the walls of the cells behind the shoot tip before the cells begin to stretch upwards.
Source: auxin (IAA) is made in the shoot tip and moves back to the zone of elongation just behind it.
Receptor: auxin binds to a receptor protein in the target cell.
Proton pumps: this stimulates ATP-driven proton pumps in the cell surface membrane to pump H+ out of the cytoplasm into the cell wall, so the pH of the wall falls.
Loosening: the low pH activates proteins called expansins, which break hydrogen bonds between cellulose microfibrils and the other polysaccharides of the wall, so the wall becomes more extensible.
Elongation: water enters the cell by osmosis; the turgor pressure stretches the loosened wall and the cell gets longer.
Examiner trap
Auxin does not make the cell divide here, and it is the cell wall that becomes acidic, not the cytoplasm. The key steps are protons pumped into the wall, acidic wall, loosening, then water uptake and elongation.
Section 03
Gibberellin wakes a barley grain.
In the gibberellin view, press Go. Follow the blue gibberellin from the embryo to the aleurone layer, the pink amylase that appears there, and the orange maltose made as the white starch grains disappear.
Water uptake: the grain absorbs water (imbibition), which activates the embryo.
Gibberellin released: the embryo produces gibberellin, which diffuses to the aleurone layer, a protein-rich layer around the endosperm.
Gene switched on: gibberellin causes the breakdown of DELLA proteins that normally stop a transcription factor from working, so the gene for amylase is transcribed and the enzyme is made.
Starch broken down: amylase is secreted into the endosperm and hydrolyses starch to maltose (and then glucose).
Embryo grows: the sugars diffuse to the embryo and are used in respiration, releasing energy for growth, and as raw material for new cells.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the Venus fly trap view, what happens when Go is pressed only once?
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In the auxin view, what builds up in the cell walls before the cells elongate?
Quick check+10 XP
In the gibberellin view, where is amylase made?
Quick check+10 XP
What does the lab say amylase produces from starch in the barley grain?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Why does the Venus fly trap close only when two trigger hairs are touched within a short time?
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A plant shoot is treated with a chemical that inhibits proton pumps. What would be the effect of adding auxin?
Exam style+20 XP
Barley grains with the embryo removed are soaked in water with gibberellin added. What would you expect?
Exam style+20 XP
Which sequence describes the action of auxin in elongation growth?
Skill unlocked
Control and coordination in plants, mastered.
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