Explain the role of microorganisms in the following : (a) Reducing the sugar content in grape juice (b) Production of a fuel

The correct answer is:
(a) Saccharomyces cerevisiae/ Yeast, is used to ferment grape juice/fermentation to convert sugar into ethanol/ alcohol.
(b) Methanogens, anaerobically act on cellulosic material in excreta (dung) of cattle to produce biogas which is used as a fuel.

Step-by-step Explanation:

(a) Reducing the sugar content in grape juice:
Grape juice naturally contains high amounts of sugars, primarily glucose and fructose. To reduce this sugar content and produce beverages like wine, microorganisms are utilized in a process called fermentation.
The specific microorganism commonly employed is the yeast strain Saccharomyces cerevisiae (commonly known as Brewer's or Wine Yeast). Under anaerobic conditions (in the absence of oxygen), this yeast metabolizes the sugars present in the grape juice.
During this biochemical process, the yeast enzymes break down glucose and fructose molecules, converting them into ethanol (alcohol) and carbon dioxide (${\mathrm{CO}}_2$). This effectively reduces the sugar content of the juice while transforming it into an alcoholic beverage.

(b) Production of a fuel:
Microorganisms play a vital role in generating renewable energy sources such as biogas. Animal excreta, particularly cattle dung (gobar), is highly rich in cellulosic organic matter because cattle feed on grass containing cellulose.
A specialized group of anaerobic bacteria known as methanogens (such as Methanobacterium) are used to treat this waste. In an oxygen-free environment (anaerobic digester), these bacteria break down and ferment the cellulosic material present in the dung.
The metabolic activity of methanogens converts the organic compounds into biogas, which is a mixture of gases consisting predominantly of methane (${\mathrm{CH}}_4$) along with carbon dioxide and other trace gases. Methane is highly inflammable, making biogas an excellent, clean-burning eco-friendly fuel for heating, cooking, and electricity generation.