Plant tropisms and auxin sim
Grow a bean seedling with light from one side, on its side in the dark or turning on a clinostat, wait a day at a time, measure each tip's angle with a protractor, graph your readings and see where the auxin goes in the shoot and the root.
Plant responses: phototropism, gravitropism and auxin · covers the IGCSE, GCSE topic
What the sim shows
Plant tropisms and auxin, after AQA required practical 8. A germinating broad bean seedling grows on damp cotton wool in a Petri dish that stands on its edge on a clinostat. The bench is in a dark room, so the lamp is the only light. Light from one side: auxin, a plant hormone made in the tip, moves to the shaded side; the cells there elongate more, so the shoot bends towards the light (positive phototropism). Seedling on its side, in the dark: auxin collects on the lower side. In the shoot it makes the lower cells elongate more, so the shoot bends up (negative gravitropism). In the root it slows elongation, so the upper side grows more and the root bends down (positive gravitropism). Clinostat on: it turns the seedling slowly (about 2 turns an hour), so each side faces down, and towards the light, in turn. Gravity and the light still act, but the one-sided effect evens out: auxin stays even and there is no bending. This is the control. Upright in the dark: the shoot grows straight up, taller, thinner and paler than in the light (etiolated). Simplified on purpose: angles, growth rates and the time to bend are illustrative (≈), kept small so the seedling stays in the dish (shoot 4.5 mm a day in light and 6.5 mm in the dark, root 5.5 mm a day). Auxin is drawn as dots, a model of how much there is, not real particles, and the close-up shows a few large cells where a real tip has thousands. This is the simple model taught at GCSE: auxin is redistributed (moved to one side) and changes how fast cells elongate. Here the root responds to gravity only; the roots of some plants also bend slightly away from light, which is not modelled. One seedling is shown; a real experiment uses several in each run and takes a mean. Each tip angle is measured from the vertical (straight up for a shoot, straight down for a root); a reading within 3° is accepted.
- Lit line: the protractor's reading (tip direction)
- Orange dots in the close-up: auxin (a model of how much)
- Pale wedge: light from the lamp
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
More biology sims
- Energy flow and pyramids simTurn an efficiency dial for a field chain or an oak tree chain and watch the pyramids of numbers, biomass and energy redraw, see where each level's energy goes, find how many levels the chain can support, and work out transfer efficiency from a data card.
- Exchange surface designer simDrag an alveolus, villus or root hair model onto the stand, then drag its wall thicker or thinner, drag the folding, microvilli or hair to change the surface area, and turn the flow pump knob, watching the rate of exchange from Fick's law and graphing your own readings.
- Food web builder simPin arrows between pond organisms from the food to the feeder, label each organism's level (one can feed at two), then take a species out or put in a perch or a disease and step the seasons, predicting each population's rise or fall and reading why it changed.
- Hydrogencarbonate indicator in light and dark simPlace the lamp, slide a foil sleeve and a gauze sleeve over tubes of pondweed in hydrogencarbonate indicator, wait an hour at a time, and read each tube against the colour chart: purple in bright light, red in dim light, yellow in the dark, and the control stays red.
- Immune response and vaccination simGive a training arm a vaccine, later meet the live pathogen, wait a day or a week at a time, watch phagocytes and lymphocytes at work in the blood, and compare the primary and secondary antibody responses of a vaccinated and an unvaccinated person on the graph.
- Leaf structure and gas exchange simCut across a model leaf with a blade, name each tissue of the cut face as it glows, and follow one CO₂ molecule through an open stoma and the air spaces into a chloroplast, then switch the lamp off to see the stomata close.