Question

Conversion of sugar into alcohol during fermentation, is due to the direct action of
(a) Temperature
(b) Micro- organisms
(c) Concentration of sugar solution
(d) Zymase

Answer 1

Hint: The conversion of sugar into alcohol during fermentation is done by the unicellular fungi, yeast. Yeast does this by performing incomplete oxidation of pyruvic acid into ethanol through a series of reactions.

Complete Step by Step Answer:
Yeast is able to perform fermentation because of a naturally occurring chemical enzyme in it which is known as zymase. Zymase was first isolated from a yeast cell in the year 1897 by a German chemist named Eduard Buchner who then proved this by fermenting sugar just with the enzyme zymase instead of living yeast cells. Zymase is an enzyme complex that catalyzes the reaction of fermentation in yeast.
Let us understand the process of anaerobic respiration that takes place in yeast to form ethanol.
Firstly, glycolysis takes place in which glucose (6C) is partially oxidized to 2 molecules of pyruvic acid (3C). This pyruvic acid is then acted upon by pyruvic acid decarboxylase and alcohol dehydrogenase to form ethanol and carbon dioxide. The reducing agent ${ NADH+H }^{ + }$ which was formed in glycolysis is reoxidized to ${ NAD }^{ + }$ in this step as well. Very less amount of energy is released in this, less than 7% of the total energy that glucose produces.

Note:
- Even in organisms like yeast where anaerobic respiration takes place, glycolysis occurs. Hence, glycolysis is a universal respiratory pathway that occurs in all living organisms despite their mode of respiration.
- Anaerobic respiration similar to fermentation occurs in some bacteria and our muscle cells where the pyruvic acid formed by glycolysis is converted into lactic acid by lactate dehydrogenase.