In fermentation to produce ethanol, what is the substrate?

• A. Water
• B. Sugars
• C. Ethene
• D. Ethanol

Answer: (B)

Substrate means the starting material that is changed in a reaction. In fermentation to make ethanol, the starting material is sugars. Yeast cells break down sugars like glucose through glycolysis in the absence of oxygen, converting them into ethanol and carbon dioxide. Water mostly acts as the medium in which the reaction occurs, not as something that is transformed. Ethene is involved in other industrial processes and is not part of this fermentation, and ethanol is the product formed, not the starting material.