Q.69. Shifting a Saccharomyces cerevisiae culture from fermentative to aerobic respiratory mode will
(A) decrease carbon dioxide production
(B) increase alcohol production
(C) increase glucose consumption
(D) decrease ATP generation per mole of glucose
Shifting Saccharomyces cerevisiae from fermentative to aerobic respiratory mode enhances efficient energy production through the TCA cycle and oxidative phosphorylation. The correct answer is (A) decrease carbon dioxide production.
Correct Answer
The correct option is (A) decrease carbon dioxide production. In fermentative metabolism, glucose yields 2 ethanol and 2 CO2 per molecule via glycolysis and pyruvate decarboxylation. Respiratory mode fully oxidizes glucose to CO2 via TCA cycle, but under glucose-limited aerobic conditions, CO2 production drops due to lower flux and higher biomass yield.
Option Explanations
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(A) Decrease carbon dioxide production: Fermentation produces 2 CO2 per glucose; respiration produces ~6 CO2 but at slower rates with higher efficiency, reducing total CO2 in steady-state cultures.
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(B) Increase alcohol production: Incorrect, as alcohol (ethanol) forms only in fermentation; respiration diverts pyruvate to acetyl-CoA, halting ethanol synthesis.
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(C) Increase glucose consumption: Wrong; fermentation enables rapid glucose uptake (Crabtree effect), while respiration slows it for efficiency in aerobic, low-glucose setups.
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(D) Decrease ATP generation per mole of glucose: False; fermentation yields ~2 ATP, respiration ~16-30 ATP via oxidative phosphorylation.
