Aerobic and anaerobic respiration: Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.

The idea

Yeast can respire without oxygen in a process called fermentation. Glucose is converted into ethanol and carbon dioxide, releasing a smaller amount of energy than aerobic respiration. This pathway matters economically. In bread-making, yeast ferments sugars in the dough and the carbon dioxide bubbles become trapped, making the dough rise; the ethanol evaporates during baking. In brewing and winemaking, yeast ferments sugars to produce ethanol, the alcohol in drinks, while carbon dioxide may be retained for fizz or released. The method is to name the process, give the word equation glucose → ethanol + carbon dioxide, then link each product to its use.

What earns the marks

• Name the process as fermentation and state that it is anaerobic respiration in yeast.
• Give the word equation glucose → ethanol + carbon dioxide.
• Explain that carbon dioxide produced by yeast makes bread dough rise.
• Explain that ethanol produced by yeast is the alcohol in alcoholic drinks.
• Recognise that fermentation releases less energy than aerobic respiration.
• Use the term economic importance to describe how fermentation is used on a commercial scale in baking and brewing.

Common mistakes and how to put them right

• Confusing fermentation with aerobic respiration.

  Put it right by stating that fermentation occurs without oxygen and produces ethanol, not water.

• Saying yeast produces alcohol in bread and that this remains in the baked bread.

  Put it right by explaining that the ethanol evaporates during baking, while carbon dioxide makes the dough rise.

• Treating fermentation as a human process rather than a yeast process.

  Put it right by naming yeast cells as the organisms that carry out anaerobic respiration.

Exam tips

• Define fermentation first, then give the equation, then apply it to bread and drinks; this order covers all parts of the statement.
• Use the phrase economic importance when explaining large-scale uses, and name the specific product responsible for each use.
• If asked why bread rises, focus on carbon dioxide gas trapped in the dough rather than on ethanol.