Fermentation
Fermentation is anaerobic respiration that breaks glucose down without oxygen, releasing a small amount of energy, and it occurs as alcoholic fermentation in yeast or lactic acid fermentation in muscle cells and some bacteria.
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Two types of fermentation
| Feature | Alcoholic fermentation | Lactic acid fermentation |
|---|---|---|
| Occurs in | Yeast and some microorganisms | Human muscle cells during vigorous exercise; some bacteria |
| Word equation | Glucose → ethanol + carbon dioxide + energy | Glucose → lactic acid + energy |
| Chemical equation | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy | C₆H₁₂O₆ → 2C₃H₆O₃ + energy |
| Common use / effect | Baking (carbon dioxide raises dough) and brewing (ethanol produced) | Causes muscle fatigue and cramp from lactic acid build-up |
Why fermentation releases little energy
Because no oxygen is used, fermentation only partially breaks down glucose, it does not go through the oxygen-requiring stages that release most of the energy in aerobic respiration. The end products (ethanol or lactic acid) still contain a large amount of unused chemical energy, so only a small amount of ATP is released compared with aerobic respiration.
As a rough comparison, one glucose molecule respired anaerobically releases only a small fraction of the number of ATP molecules released by the same glucose molecule through complete aerobic respiration.
Applications of fermentation
Alcoholic fermentation by yeast is used in baking, where the carbon dioxide produced makes bread dough rise, and in brewing, where the ethanol produced gives alcoholic drinks their alcohol content. Lactic acid fermentation occurs naturally in human muscle cells when oxygen supply cannot keep up with the demand of vigorous exercise, and certain bacteria carry out lactic acid fermentation to produce yogurt from milk.
How it is examined
Exam questions typically ask you to write the word or chemical equation for alcoholic fermentation or lactic acid fermentation, to explain why muscle cells produce lactic acid during intense exercise, or to explain why fermentation releases far less energy than aerobic respiration. You may also be asked to link fermentation to an everyday application, such as bread-making or yogurt production.
Worked exam-style question
Question. A baker mixes yeast with sugar and warm water and leaves the dough to rise for an hour before baking it. During this time, the dough increases noticeably in volume.
(a) Name the type of respiration carried out by the yeast in the dough. (b) Write the word equation for this process.
(c) Explain, in terms of a gas produced, why the dough increases in volume. (d) Suggest why the dough is left in a warm place rather than a cold place, and explain the biological reason.
Model answer. (a) Alcoholic fermentation (anaerobic respiration in yeast). (b) Glucose → ethanol + carbon dioxide + energy.
(c) The carbon dioxide gas produced forms bubbles that get trapped in the dough, causing it to expand and rise. (d) A warm place is used because it is closer to the yeast's optimum temperature, at which its enzymes work fastest; this increases the rate of fermentation, so more carbon dioxide is produced in the same time and the dough rises faster.
Practice question
Try this. A runner sprints as fast as possible for 100 metres and notices her leg muscles feel tight and sore soon after. Explain, in terms of the type of respiration involved, why this happens, and state one way the discomfort is eventually relieved.
Exam tip
Key terms
These respiration terms from Chapter 7 are often confused, keep them separate:
- Cellular respiration, the release of energy from glucose in every living cell.
- Anaerobic respiration, the partial breakdown of glucose without oxygen, of which fermentation is a form.
- Fermentation, anaerobic respiration in yeast (alcoholic) or muscle cells and some bacteria (lactic acid).
- ATP, the molecule that stores the small amount of energy released.
- Aerobic respiration, the complete breakdown of glucose using oxygen, which releases far more energy.
Source:SRC-DSKP-EN
Frequently asked questions
Why do muscle cells produce lactic acid during intense exercise?
What is the word equation for alcoholic fermentation, and where is it applied?
Why does dough need to be left to rise before baking?
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