Question

Yeast can convert glucose into

• A. Starch
• B. Alcohol
• C. Lactic acid
• D. Yogurt

Answer 1

The correct option is B

Alcohol

Explanation for the correct option

(B) Alcohol

• Yeasts are facultative anaerobes or obligate aerobes
• In presence of oxygen, it carries out cellular respiration of carbohydrate source to give ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$
• Yeast undergoes fermentation to give alcohol and ${\mathrm{CO}}_2$
• The glucose is used as a substrate which gives pyruvate which further undergoes reductive decarboxylation in absence of oxygen with the help of specific enzymes to give ethanol and ${\mathrm{CO}}_2$.
• The reaction is as follows:-
• ${\mathrm{C}}_6{\mathrm{H}}_{12}\mathrm{O}\to 2{\mathrm{CO}}_2+2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$

Explanation for incorrect options

(A) Starch

• Starch is the polymer of glucose formed by multiple glucose monomers.
• It is mainly produced by plants and algae as a reserve source of carbohydrates.
• Yeast ferments glucose to give ethanol and ${\mathrm{CO}}_2$ and stores glucose as glycogen.
• Thus, yeast does not convert glucose to starch.

(C) Lactic acid:

• Lactic acid is produced by homo and hetero fermenting bacteria
• The lactic acid-producing bacteria have the enzyme lactic acid dehydrogenase which is responsible for reducing pyruvate (obtained via glucose metabolism) to lactic acid.
• Yeasts lack this enzyme thus, yeast does not convert glucose to lactic acid.

(D) Yogurt:

• Yogurt is the product of milk fermentation reaction done with the help of lactose fermenting bacterial cultures.
• In this lactose is the milk is fermented to give yogurt which is not feasible in yeast cultures.
• Yeasts do not ferment glucose to give yogurt but play important role in its spoilage.

Hence, the correct option is (B). Yeast can convert glucose into alcohol.