Multiple Answers: S. cerevisiae ferments which carbohydrates in traditional wheat bread to generate CO2 necessary for 'rising?' Select all that are true.

Question

BlackTom AI · Accepted Answer

S. cerevisiae ferments glucose and maltose in bread dough to produce the CO2 that makes the dough rise.

1. Relevant knowledge points and concepts
- Wheat flour starch is an abundant polysaccharide; endogenous amylases (and amylase activity from malted grains or flour) hydrolyze starch into maltose (a disaccharide of two glucose units) and some free glucose.  
- S. cerevisiae possesses hexose transporters and glycolytic enzymes to ferment glucose, and MAL genes encoding maltose permease and maltase to import and hydrolyze maltose into glucose.  
- Alcoholic fermentation: glucose → 2 ethanol + 2 CO2 (per glucose). Yeast cannot hydrolyze lactose because it lacks beta-galactosidase and the lactose transport system.

2. Step-by-step reasoning
- Step A: Source of fermentable sugars in dough: starch → (amylases) → maltose + some glucose. Thus the principal fermentable carbohydrates available are maltose and glucose.  
- Step B: Uptake and metabolism: glucose is taken up directly and processed by glycolysis to pyruvate, then decarboxylated and reduced to yield ethanol and CO2. Maltose is imported or hydrolyzed to two glucose molecules by yeast maltase, then enters glycolysis.  
- Step C: Stoichiometry (intermediate steps shown):  
  - Glucose fermentation: C6H12O6 → 2 C2H5OH + 2 CO2. (Each glucose gives 2 CO2.)  
  - Maltose first hydrolyzes: C12H22O11 + H2O → 2 C6H12O6, then fermentation of those two glucoses yields 4 CO2 total. Thus maltose is a major CO2 source.

3. Why the correct answers are correct
- Glucose: directly fermentable to CO2 by yeast.  
- Maltose: produced from starch and hydrolyzed/fermented by yeast, yielding CO2.

4. Why the other options are incorrect
- Lactose: a disaccharide of glucose + galactose but S. cerevisiae lacks beta-galactosidase and lactose transporters, so it cannot ferment lactose (unless engineered).  
- Galactose: although S. cerevisiae can metabolize galactose via the Leloir pathway under certain conditions, free galactose is not a normal component of wheat flour dough in appreciable amounts, and glucose repression prevents its use until glucose is depleted—so it is not one of the principal sugars generating CO2 in traditional wheat bread.

Conclusion: The fermentable carbohydrates in traditional wheat bread that generate CO2 are Glucose and Maltose, matching the provided answer.

Multiple Answers: S. cerevisiae ferments which carbohydrates in traditional wheat bread to generate CO2 necessary for 'rising?' Select all that are true.多项选择题

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