The Fermenting Vat and the Missing ATP
A winery technician inoculates grape must with yeast at 25 °C. The must initially contains about 22% sugar and is well aerated during the first hours, then sealed under an airlock. Over three days the yeast population grows rapidly while oxygen is present, but once the airlock closes, growth slows sharply and ethanol accumulates to about 12%. A rival lab claims it can get the same yeast to produce far more ATP per sugar molecule by 'switching the cells to aerobic respiration' inside the sealed vat. The technician measures the CO2 output and notes that it drops after sealing, even though sugar consumption continues. She also observes that the yeast cells are smaller and divide more slowly in the sealed phase.
1. Explain, using ATP yield per glucose, why the sealed vat cannot sustain the same yeast growth rate as the aerated phase.
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Aerobic respiration of one glucose yields roughly 30–32 ATP (up to ~36 in older textbook counts), while alcoholic fermentation yields only 2 ATP per glucose. In the sealed vat oxygen is exhausted, so the cells are forced into fermentation. The energy yield per sugar molecule collapses by more than 10-fold. To meet the same ATP demand, yeast must consume far more glucose and produce ethanol, but the total ATP available per unit sugar is low. This directly explains slower division and smaller cells: growth is ATP-limited. The claim of 'switching to aerobic respiration' inside a sealed vat is impossible because oxygen is the terminal electron acceptor and is not available. No oxygen means no oxidative phosphorylation, regardless of sugar supply.
2. Why does CO2 output drop after sealing even though sugar consumption continues?
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Before sealing, yeast respires aerobically; CO2 is produced in the pyruvate dehydrogenase reaction and the Krebs cycle. After sealing, the only CO2 source is the decarboxylation of pyruvate to acetaldehyde in alcoholic fermentation. Per glucose, aerobic respiration releases about 6 CO2, while fermentation releases 2 CO2. Because ATP yield per glucose is much lower in fermentation, the cells cannot sustain the same metabolic flux, so total CO2 per unit time falls even though sugar is still being consumed. The CO2 drop is therefore consistent with a shift from respiration to fermentation and with reduced growth, not with the cells stopping sugar uptake.