Decay rate: temperature and lipase (RP10) sim

What the sim shows

Decay rate: the effect of temperature on lipase digesting milk. A test tube of 5 cm³ of full-fat milk with 7 cm³ of sodium carbonate solution and 5 drops of phenolphthalein stands in a thermostatic water bath beside a tube of 1 cm³ of lipase solution. The colour standard beside the white tile is 5 cm³ of milk with 8 cm³ of water and no indicator: the colour at the end-point. The sodium carbonate makes the milk alkaline, so the phenolphthalein is pink. Set the bath with its dial, warm both tubes for 5 minutes, then drag the lipase tube onto the milk tube to pour it in and start the stopwatch. Press Wait 30 s and watch the colour. Lipase breaks down fats into fatty acids and glycerol; the fatty acids lower the pH, the pink fades, and at the end-point the phenolphthalein is colourless. Read the stopwatch, type the time in seconds and the rate (rate = 1000 / time, a relative rate: the 1000 only makes the numbers easier to read). Do 3 repeats at each temperature with fresh milk and fresh lipase each time, leave out any anomaly (a repeat whose rate is more than 25 % from the middle one), and type the mean. The graph plots your mean rates against temperature. Up to the optimum, warming makes the lipase and fat molecules move faster and collide more often, so the rate rises; above it, more and more lipase is denatured (its active site changes shape for good), so the rate falls. This models what decomposers do: bacteria and fungi break down dead material with enzymes, so decay needs warmth, water and oxygen, and is faster when warm, up to an optimum. Simplified on purpose: the times and rates are illustrative (≈): real times depend on the milk, the lipase and how you judge the colour, and real values differ; the repeats vary by a seeded amount (up to 6 %), so the same repeat gives the same time if you do it again, and one repeat is a planted anomaly; the end-point is a model threshold: the tube counts as colourless when less than a tenth of the indicator is still pink, and it is drawn white from that moment; a real end-point is a judgement, which is why you compare with a colour standard on a white tile; the stopwatch is stopped at the exact moment of the end-point, even inside a 30 s wait, as if you were watching all the time; the bath holds every temperature exactly and reaches a new setting at once; a real bath takes time to settle, and needs ice to go below room temperature; the fatty acids lower the pH by turning the sodium carbonate into sodium hydrogencarbonate, the pH is worked out from that alone, and the starting pH is set at 10; the lipase starts to denature just above 40 °C, at a rate that rises smoothly and then steeply with temperature, and the damage is permanent and builds up with every 5 minutes above 40 °C; real enzymes start to denature at different temperatures; the volumes are measured exactly, and the tubes are mixed the moment the lipase goes in; the bench apparatus is drawn to scale, and the close-up is not to scale.

• Lit: the bath's set-point dial
• Close-up: the milk tube magnified beside the colour standard on the white tile, the stopwatch, and the colour at each 30 s step
• Dashed ring: where the carried lipase tube will pour

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