Glucose Concentration and Yeast Fermentation
A student investigates how glucose concentration affects the rate of fermentation in yeast. She adds 5 g of dried yeast to 100 mL of glucose solution at each of five concentrations and counts the bubbles leaving the tube in five minutes. Bubble counts for three repeats at each concentration are: 0%: 2, 5, 3; 2%: 11, 13, 10; 5%: 22, 24, 25; 10%: 25, 27, 26; 20%: 9, 12, 11. The room stayed at 22 °C. She mixed the 20% solution last, using the same water supply, and it stood for 10 minutes before the yeast went in; she noticed the beaker felt cooler than the others had.
1. Describe the pattern in the data and identify which result should not be trusted. Justify your choice using the information in the setup.
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The bubble count rises steeply from about 3 at 0% glucose to about 24 at 5%, levels off at 10% (mean 26), then falls sharply at 20% (mean about 11).
The 20% result should not be trusted. It is not a fair comparison with the others:
- It was made last and stood for 10 minutes, so the yeast was added to a solution that had been sitting, not freshly mixed.
- The beaker felt cooler. Fermentation is enzyme-controlled, and a lower temperature would slow gas production on its own.
So the drop at 20% could be caused by temperature or delay rather than by the high sugar concentration. Until the 20% trial is repeated freshly made at 22 °C, the data support a rise up to 10% but say nothing reliable about what happens above it.
2. The student concludes: 'Yeast ferments fastest at 10% glucose.' Evaluate this conclusion.
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It is only partly supported.
Supported: the rate clearly increases between 0% and 5% and is highest in the tested range at 10%.
Not supported:
- The 20% point is confounded (temperature/delay), so the fall-off is not established.
- No concentrations between 10% and 20% were tested, so the true peak could be at 12%, 15% or anywhere in between. 'Fastest at 10%' should be 'fastest of the five concentrations tested'.
- Only one trial per concentration exists in the sense that the three repeats come from one batch each - nothing shows the result would repeat on another day.
To test the claim properly she would need fresh solutions at, say, 8%, 10%, 12%, 15% and 20%, all made and used identically at 22 °C.
3. What does the 0% glucose result of about 3 bubbles per five minutes tell you about how the other counts should be interpreted?
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It is a baseline, not a zero. Even with no glucose added, the tube produces about 3 bubbles in five minutes - from dissolved gas coming out of solution, residual sugar in the yeast preparation, or the small amount of air introduced when the tube was set up.
This means the glucose-driven response is the count minus the baseline:
- 2%: 11 − 3 ≈ 8 bubbles of extra activity
- 5%: 24 − 3 ≈ 21
- 10%: 26 − 3 ≈ 23
Subtracting the baseline sharpens the comparison, especially at low concentrations, where 3 bubbles is a large fraction of the total. Without it she would overstate activity at 2% and understate how steep the rise to 5% really is.