Yeast respiration and temperature sim

What the sim shows

Yeast in glucose solution respires anaerobically under a layer of oil: glucose → ethanol + carbon dioxide (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂). Set the water bath, let the tube settle for 5 minutes, then count the bubbles (or read the syringe) for 1 minute. Repeat three times and find the mean (counts to 1 d.p.; syringe means to 1 or 2 d.p.). A fresh tube is used for each temperature. Model: rate = 10 × 2^((T − 20)/10) × A bubbles per minute, where T is the tube temperature in °C and A = 1/(1 + e^((Tmax − 43)/3.5)) is the share of enzyme still active, Tmax being the hottest the tube has been. Without glucose the yeast respires only its small store, at one tenth of this rate; boiled yeast gives none. Below the optimum (about 39 °C) the rate roughly doubles for every 10 °C rise (Q10 ≈ 2); above about 40 °C more and more enzyme is denatured, and this damage is permanent, so a heated tube stays slow when cooled. Why real results differ: counts scatter randomly around the rate (Poisson, seeded); the tube lags behind the bath (time constant 1.1 min), and the first bubbles are partly air pushed out as it warms; every bubble is assumed to be the same size (0.08 cm³), which real bubbles are not; the syringe is read to 0.1 cm³ and the thermometer to 1 °C. The counts are illustrative for 10 % yeast in 5 % glucose. Time is speeded up: 5 minutes of settling takes 5 s and each 1-minute count takes 10 s. Limewater clouding shows that carbon dioxide is present, not how much.

• Set-point slider on the bath controller: 10–70 °C in 5 °C steps
• Yeast in 5 % glucose under oil: no oxygen, so it respires anaerobically
• Bubbles of carbon dioxide, each about 0.08 cm³

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