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Compare fermentation with aerobic respiration

Yeast can live with or without oxygen. Without it, yeast ferments glucose to ethanol and ${CO}_{2}$ . How many glucose must it ferment to make the $32$ ATP that aerobic respiration makes from one? How much ${CO}_{2}$ does each path release per ATP?

Start from the aerobic total

With oxygen, one glucose gives $4$ ATP directly and sends $10$ NADH and $2$ FADH₂ to the chain, about $32$ ATP in all, as the first worked example found.

4 + 10 (2.5) + 2 (1.5) = 32

Count fermentation’s ATP

Without oxygen the chain stops. Glycolysis still nets $2$ ATP and $2$ NADH. Turning each pyruvate into ethanol releases one ${CO}_{2}$ and spends one NADH, which gives back the NAD⁺ that glycolysis needs. No step after glycolysis makes ATP.

Divide the yields

Each fermented glucose gives $2$ ATP, so matching $32$ ATP takes $32 / 2$ glucose.

\frac{32}{2} = 16

${CO}_{2}$ per ATP in fermentation

Fermentation releases $2 {CO}_{2}$ for its $2$ ATP, one for each.

\frac{2}{2} = 1

${CO}_{2}$ per ATP with oxygen

Aerobic respiration releases all six carbons as ${CO}_{2}$ for about $32$ ATP.

\frac{6}{32} = 0.1875

Compare

Divide one rate by the other. Fermentation releases about $5.33$ times as much ${CO}_{2}$ for each ATP, and it burns through sugar $16$ times as fast for the same ATP.

\frac{1}{0.1875} \approx 5.33

Result

Yeast must ferment $16$ glucose to make the $32$ ATP that one glucose gives with oxygen. Fermentation releases $1 {CO}_{2}$ per ATP and aerobic respiration about $0.19$ , so fermentation releases about $5.33$ times as much ${CO}_{2}$ for each ATP.

Your turn

1. One like the example

A sprinting muscle needs $64$ ATP. How many glucose does it use if it makes them all by lactate fermentation, and how many with oxygen at $32$ ATP per glucose?

Show the answer and explanation

$32$ glucose by fermentation and $2$ with oxygen.

Fermentation gives $2$ ATP per glucose, so $64 / 2 = 32$ . Aerobic respiration gives about $32$ , so $64 / 32 = 2$ . The ratio is $16$ again, whatever the ATP demand.

\begin{array}{l} \frac{64}{2} = 32 \\ \frac{64}{32} = 2 \end{array}

2. A variation

A yeast culture ferments $18 g$ of glucose, $0.1 mol$ at $180 g$ per mole. Each glucose gives $2$ ethanol and $2 {CO}_{2}$ . How many moles of ${CO}_{2}$ does it release, and what mass, at $44.01 g$ per mole?

Show the answer and explanation

$0.2 mol$ of ${CO}_{2}$ , about $8.80 g$ .

$18 / 180 = 0.1 mol$ of glucose, and $2 (0.1) = 0.2 mol$ of ${CO}_{2}$ . The mass is $0.2 \times 44.01 \approx 8.80 g$ . The same culture also makes $0.2 mol$ of ethanol.

\begin{array}{l} \frac{18}{180} = 0.1 \\ 2 (0.1) = 0.2 \\ 0.2 (44.01) \approx 8.80 \end{array}

3. Somewhere new

Cyanide blocks the last protein of the electron transport chain, the one that hands electrons to oxygen. Why does a poisoned cell also stop running the citric acid cycle, and how much ATP can it still make from a glucose?

Show the answer and explanation

The cycle stops because NADH can no longer be oxidized, so NAD⁺ runs out. The cell can still make $2$ ATP per glucose by glycolysis and fermentation.

Oxygen is present but cannot take electrons, so the chain backs up and NADH stays reduced. The cycle needs NAD⁺ and FAD at four of its steps and stops. Only glycolysis, kept going by fermentation, still makes ATP.

Keep exploring

Compare the yields in Math starts from the aerobic total, $4 + 10 (2.5) + 2 (1.5)$ $= 32$ . Write $32 / 2 = 16$ under it, and each row is checked.

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Start from the aerobic total

Count fermentation’s ATP

Divide the yields

CO2CO2​ per ATP in fermentation

CO2CO2​ per ATP with oxygen

Compare

Result

Your turn

1. One like the example

3232 glucose by fermentation and 22 with oxygen.

2. A variation

0.2 mol0.2 mol of CO2CO2​, about 8.80 g8.80 g.

3. Somewhere new

The cycle stops because NADH can no longer be oxidized, so NAD⁺ runs out. The cell can still make 22 ATP per glucose by glycolysis and fermentation.

Keep exploring

Study it, then try it

${CO}_{2}$ per ATP in fermentation

${CO}_{2}$ per ATP with oxygen

$32$ glucose by fermentation and $2$ with oxygen.

$0.2 mol$ of ${CO}_{2}$ , about $8.80 g$ .

The cycle stops because NADH can no longer be oxidized, so NAD⁺ runs out. The cell can still make $2$ ATP per glucose by glycolysis and fermentation.