More Virtual Labs — Run the Experiment on Screen
Four interactive simulators for the processes behind the practicals: osmosis in a red blood cell, the rate of photosynthesis, diffusion, and enzyme kinetics. Change one variable at a time, watch what happens, and read the science underneath — the same thinking the IB asks for in your own investigations.
💧 Osmosis & tonicity 🌿 Photosynthesis rate 🌀 Diffusion ⚗️ Enzyme kinetics
💧 Osmosis & Tonicity — the Red Blood Cell Test
Red blood cells have no cell wall, so they are perfect osmosis detectors. Their cytoplasm is equivalent to a 0.9% sodium chloride solution. Move the slider to change the solution around the cell and watch water cross the membrane by osmosis — always toward the side with the higher solute concentration (the lower water potential).
What to notice: below 0.9% the solution is hypotonic — water enters, the cell swells, its biconcave dimple disappears, and in very dilute solutions it bursts (haemolysis). At 0.9% it is isotonic — water moves in and out equally and the cell keeps its shape. Above 0.9% the solution is hypertonic — water leaves and the cell shrivels (crenation). This is why IV fluids are made at 0.9%: any other concentration would damage the patient's own cells. Model note: cell volume is shown relative to normal, following the water-potential gradient; real cells also regulate volume actively.
🌿 Photosynthesis — Hunting the Limiting Factor
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The rate of photosynthesis is set by whichever raw material runs shortest — the limiting factor. Set the CO₂ level, then sweep the light slider: the curve shows the rate at every light intensity for your chosen CO₂.
What to notice: at low light the rate climbs steeply with light — light is limiting. Then the curve flattens: more light no longer helps because the leaf cannot get CO₂ fast enough — CO₂ is limiting, and only raising CO₂ lifts the plateau. Glasshouse growers use exactly this: extra CO₂ (about 1,000 ppm) raises yields, but only when light is already plentiful. This is the theory behind the leaf-disk practical in the Practical Lab.
🌀 Diffusion — Why Temperature Speeds It Up
Particles never stop moving. Watch 100 dye particles start crowded on the left and spread by random motion alone — no pump, no energy from the cell — until they are evenly mixed. Then raise the temperature and reset: in water, warming helps twice — the molecules move faster AND the water itself gets thinner (less viscous) — so the dye spreads much sooner.
What to notice: the net movement is from high to low concentration even though every particle moves randomly — more particles happen to leave the crowded side than return. Diffusion is fast over micrometres (across a cell) but hopelessly slow over centimetres, which is why large organisms need transport systems — and why cells stay small. Net movement stops at dynamic equilibrium: particles keep moving, but the numbers on each side stay equal. The model uses the real physics of diffusion in water (Stokes–Einstein: rate ∝ absolute temperature ÷ viscosity) — which is why hot water spreads the dye so much faster than the small √T change in molecular speed alone would suggest.
⚗️ Enzyme Kinetics — Michaelis–Menten in Motion
For a typical enzyme, the rate rises with substrate concentration and then plateaus at Vmax, when every active site is busy. Km is the substrate concentration giving half of Vmax — a low Km means the enzyme reaches speed at low substrate levels. Add an inhibitor and see the two classic patterns.
The model: v = Vmax[S] ÷ (Km + [S]), with Vmax = 100 relative units and Km = 2.0 mM (Ki = 1.5 mM). A competitive inhibitor fights for the active site: apparent Km rises, Vmax is unchanged — enough substrate outcompetes it. A noncompetitive inhibitor binds elsewhere and disables enzymes: Vmax falls, Km is unchanged, and extra substrate cannot rescue the rate. Many medicines work exactly this way — statins competitively inhibit a cholesterol-making enzyme.