7.3 Fermentation
Fermentation is an alternative pathway of cellular respiration that takes place when oxygen is limited or completely absent. It begins with glycolysis in the cytoplasm and proceeds without oxygen, resulting in the incomplete breakdown of glucose.
Learning Objectives
- State the definition of fermentation.
- Explain the processes of alcohol fermentation and lactic acid fermentation with word equations.
- Describe examples of alcohol fermentation in yeast and paddy plants.
- Describe examples of lactic acid fermentation in Lactobacillus bacteria and human muscle cells during vigorous training.
- Explain the concepts of oxygen debt and oxygen debt repayment in human muscle cells.
- Describe the laboratory experiment used to study yeast fermentation.
- Compare and contrast fermentation with aerobic respiration in terms of similarities and differences.
Comparison of Fermentation Pathways and Oxygen Debt Recovery
After glycolysis in the cytoplasm, pyruvate follows one of two anaerobic pathways. Alcohol fermentation produces ethanol and carbon dioxide, while lactic acid fermentation produces lactic acid.
Alcohol Fermentation
Glucose is broken down incompletely into ethanol, carbon dioxide and 210 kJ of energy. It occurs in yeast and oxygen-starved paddy plants.
Lactic Acid Fermentation
Glucose is broken down into lactic acid and 150 kJ of energy. It occurs in Lactobacillus and human muscle cells during vigorous exercise.
Short Explanation
Definition of Fermentation
Fermentation is the incomplete breakdown of glucose in conditions of limited oxygen or without oxygen. It shares the initial glycolysis stage with aerobic respiration, where glucose is converted to pyruvate in the cytoplasm. After glycolysis, pyruvate undergoes either alcohol fermentation or lactic acid fermentation.
Alcohol Fermentation
The incomplete breakdown of glucose to ethanol, carbon dioxide, and energy.
Word equation: \(\text{Glucose} \rightarrow \text{Ethanol} + \text{carbon dioxide} + \text{energy } (210\text{ kJ})\)
Lactic Acid Fermentation
The breakdown of glucose into lactic acid and energy.
Word equation: \(\text{Glucose} \rightarrow \text{Lactic acid} + \text{energy}\)
Alcohol Fermentation in Yeast
Yeast (Saccharomyces) undergoes alcohol fermentation anaerobically. Ethanol produced is utilised in the brewing of beer and winemaking. Carbon dioxide released causes bread dough to rise.
Alcohol Fermentation in Paddy Plants
Paddy plants growing in waterlogged, oxygen-deficient soil carry out alcohol fermentation to obtain energy. Paddy plant cells possess a higher tolerance to ethanol and produce high quantities of the enzyme alcohol dehydrogenase, which breaks down toxic ethanol into non-toxic carbon dioxide.
Lactic Acid Fermentation in Lactobacillus
The bacterium Lactobacillus carries out milk fermentation to produce yoghurt. It acts on lactose and converts it into lactic acid. Lactic acid coagulates casein to produce the thick texture and sour taste of yoghurt.
Lactic Acid Fermentation in Human Muscle Cells
Occurs during vigorous exercise when the rate of oxygen used by muscles exceeds the rate of oxygen supplied by the circulatory system. The muscle cells enter an oxygen-deficiency state termed oxygen debt. Each glucose molecule yields only 2 ATP molecules or 150 kJ of energy. Accumulated lactic acid causes muscle fatigue and cramps.
Repaying the Oxygen Debt
When vigorous exercise stops, rapid and deep breathing provides excess oxygen intake. This excess oxygen is used to oxidise accumulated lactic acid in the liver and muscles into carbon dioxide, water, and energy. Once all lactic acid has been oxidised and expelled, the oxygen debt is fully repaid.
Experiment: Studying Yeast Fermentation (Activity 7.3)
Problem Statement: What are the products of yeast fermentation?
Hypothesis: Yeast fermentation produces energy, carbon dioxide, and ethanol.
Variables:
- Manipulated variable: Presence of yeast.
- Responding variables: Changes in temperature, change in lime water, and smell of ethanol.
- Fixed variables: Volume of boiled glucose solution and anaerobic condition.
Experimental Setup:
- Boiling tube A: Contains 15 ml of 5% boiled and cooled glucose solution + 5 ml of 5% yeast suspension + a layer of paraffin oil. Connected via a delivery tube to a test tube containing lime water.
- Boiling tube B (Control): Contains 15 ml of 5% boiled and cooled glucose solution + a layer of paraffin oil (without yeast suspension). Connected via a delivery tube to lime water.
Key Precautions & Controls:
- Boiling the glucose solution: Expels any dissolved oxygen to ensure completely anaerobic conditions.
- Cooling the glucose solution before adding yeast: Prevents high temperatures from denaturing yeast enzymes and killing the yeast.
- Layer of paraffin oil: Prevents atmospheric oxygen from dissolving into the glucose solution, maintaining the anaerobic environment.
- Delivery tube submerged in lime water: Tests for the release of carbon dioxide gas.
Expected Observations in Tube A: Temperature rises, lime water turns cloudy, and an alcohol smell is present.
Expected Observations in Tube B: No change in temperature, lime water remains clear, no ethanol smell.
Comparison Between Aerobic Respiration and Fermentation
Similarities:
- Both are breakdown processes of glucose that convert chemical energy into ATP.
- Both processes begin in the cytoplasm.
- Both processes begin with glycolysis, where glucose is converted into pyruvate.
- Both occur in living organisms such as yeast, bacteria, animals, and plants.
- Both produce chemical energy in the form of ATP.
Differences:
- Oxygen requirement: Aerobic respiration requires oxygen; fermentation occurs without oxygen or in limited oxygen conditions.
- Site of occurrence: Aerobic respiration occurs in the cytoplasm and mitochondrion; fermentation occurs only in the cytoplasm.
- Water production: Aerobic respiration produces water; fermentation does not produce water.
- Oxidation of glucose: Aerobic respiration completely oxidises glucose into carbon dioxide and water; fermentation incompletely oxidises glucose into ethanol and carbon dioxide, or lactic acid.
- Energy generated per glucose: Aerobic respiration generates 2898 kJ (approximately 38 ATP); fermentation generates 210 kJ (alcohol fermentation) or 150 kJ (lactic acid fermentation) (2 ATP).
Try to Answer First
Answer in your mind, then press “Check Answer”.
What are the products of alcohol fermentation in yeast, and what is the total energy yield per molecule of glucose?
Why does lime water turn cloudy during the yeast fermentation experiment?
Explain why an athlete breathes heavily and rapidly for several minutes after finishing a 100-metre sprint.
Common Mistakes
- Writing water (\(H_2O\)) as a product of fermentation. Fermentation does not produce water.
- Stating that fermentation takes place in mitochondria. Fermentation occurs entirely in the cytoplasm.
- Assuming paddy plants do not produce toxic ethanol; they do produce ethanol, but survive because their cells have a high tolerance and produce alcohol dehydrogenase to break it down into carbon dioxide.
- Stating that carbon dioxide is produced during lactic acid fermentation in human muscles. Lactic acid fermentation produces lactic acid and energy only; no carbon dioxide is released.
- Forgetting to cool the boiled glucose solution before adding yeast in Activity 7.3, which would kill the yeast cells due to excessive heat.
Concept Misunderstandings
Fermentation produces the same amount of ATP as aerobic respiration.
Fermentation yields only 2 ATP molecules (150 kJ to 210 kJ) per glucose molecule because glucose is only partially oxidised, whereas aerobic respiration yields 2898 kJ (around 38 ATP) through complete oxidation.
Oxygen debt occurs after exercise is completed.
Oxygen debt is incurred during vigorous exercise when oxygen consumption exceeds oxygen supply; it is repaid after the exercise stops during the recovery phase.
Paraffin oil produces carbon dioxide in the fermentation experiment.
Paraffin oil acts solely as a physical barrier to block atmospheric oxygen from entering the boiled glucose solution, maintaining an anaerobic medium.
Summary
- Fermentation is the incomplete breakdown of glucose under anaerobic or oxygen-limited conditions, taking place strictly in the cytoplasm.
- Alcohol fermentation produces ethanol, \(\text{CO}_2\), and 210 kJ of energy per glucose molecule. It is utilized by yeast in baking and brewing, and by paddy plants via alcohol dehydrogenase.
- Lactic acid fermentation produces lactic acid and 150 kJ (2 ATP) per glucose molecule. It occurs in Lactobacillus (curdling milk to make yoghurt) and human skeletal muscle cells during strenuous training.
- Oxygen debt represents the cumulative deficit of oxygen during vigorous exertion, which is repaid post-exercise as excess oxygen oxidises toxic lactic acid into \(\text{CO}_2\), \(H_2O\), and energy.
- Compared to aerobic respiration, fermentation does not produce water, does not involve mitochondria, and releases significantly less energy.
Short Activity
Objective: test your understanding of fermentation pathways, oxygen debt and the yeast fermentation experiment.
A. Objective Quiz
Which of the following word equations correctly represents alcohol fermentation?
How do paddy plants survive alcohol fermentation in waterlogged conditions?
B. Fill in the Blanks
During milk fermentation, the bacterium Lactobacillus converts lactose into , which coagulates casein to produce yoghurt.
In Activity 7.3, a layer of is added on top of the glucose solution to maintain an anaerobic condition by preventing oxygen from dissolving into the solution.
C. Matching / Drag and Drop
Drag each item to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Item to Its Description
Write the correct item next to each description.
| Description | Item |
|---|---|
| 1. Enzyme produced by paddy plants to break down ethanol into non-toxic carbon dioxide. | |
| 2. The oxygen-deficiency state incurred when oxygen demand exceeds oxygen supply during vigorous exercise. | |
| 3. Bacterium that ferments milk sugar to produce the sour taste and thick texture of yoghurt. | |
| 4. Metabolic process that produces water and yields 2898 kJ of energy per glucose molecule. |
Answer choices: Alcohol dehydrogenase • Oxygen debt • Lactobacillus • Aerobic respiration
