Lesson Eleven: Anaerobic Respiration

• Outcome
• Fermentation
• Alcoholic Fermentation
• Assessment Strategy (15 minutes)
• Group discussion is possible for 5-7 minutes

Outcome

• Define anaerobic respiration
• Describe the process of lactic acid and alcoholic fermentations
• List the products of lactic acid and alcoholic fermentations.

Materials

• dry yeast
• spoon
• small water bottle
• warm sugar solution
• balloon

Anaerobic respiration does not consume oxygen. It occurs in the absence of oxygen. It takes place in the cytoplasm of cells. If no oxygen is available to the cell, the pyruvate resulted from glycolsis undergoes fermentation.  

Suggested teaching Methodology: - Lecture, Group work and experiment. 

Fermentation

Fermentation is the process by which cells release energy without oxygen. Recall that in cellular respiration the cell first breaks glucose into smaller molecules. This releases a small amount of energy only about 2 ATP molecules.  Without oxygen, aerobic respiration cannot take place. In eukaryotic cells, instead of entering the mitochondria, these smaller molecules stay in the cytoplasm, where fermentation occurs.

Essential questions/posing problem 

• What are the two types of fermentation?
• Can you give examples of the two types of fermentation?
• In what respect the two types of fermentation differ from each other?

There are two main types of fermentation: alcoholic fermentation and lactic acid fermentation. Both types of fermentation break sugars down to small molecules. In the absence of oxygen, different reactions occur that produce either alcohol and carbon dioxide or lactic acid. In both cases, a small amount of energy is released.

Alcoholic Fermentation

Yeasts and a few other microorganisms like bacteria carry out alcoholic fermentation. 
The students are supposed to write the equation of alcoholic fermentation. The equation of alcoholic fermentation is as follows;

C6 H12O6     →     CO2 + C2H5OH (Alcohol) + 2 ATP

In alcoholic fermentation, small amount of energy (only 2ATP ) is released. Carbon dioxide and alcohol are produced as by products.  

2 ATP   +

Fig.11.1 Alcoholic fermentation of yeast

Fig.  11.2: Anaerobic respiration in plants and animals

How do your muscles continue to get energy during high levels of activity when there is not enough oxygen? Muscles can get energy from glucose through fermentation

Lactic Acid Fermentation: is a type of anaerobic fermentation that takes place in muscles of humans during rapid exercises. In the case of lactic acid fermentation, one molecule of glucose yields only 2 ATP molecules. Lactic acid is produced as by product.  

Equation of Lactic acid fermentation:

C6 H12O6   è C3H6O3 (Lactic Acid)  + 2 ATP

Lactic acid fermentation process is carried out by some bacteria like the Lactobacillus acidophilus in yogurt.

2ATP   +

Fig. 11.3: Lactic Acid Fermentation

How can you tell if fermentation releases products?

Procedure

• Add half teaspoon of yeast to the empty water bottle.
• Fill the bottle about three-quarters and full with the sugar solution.
• Place the balloon tightly around the mouth of the bottle.
• Gently swirl the bottle to mix the yeast and sugar solution.
• After 20 minutes, observe the balloon and record your observations.

Questions

• What changes did you observe?
• What do you think is the source of energy that caused these changes?
• What accounts for the change in the amount of gas inside the balloon?  

Tips: Microbes especially yeasts have been used from time immemorial for the production of beverages like wine, beer, whisky, brandy or rum. For this purpose the same yeast Saccharomyces cerevisiae used for bread-making and commonly called brewer’s yeast, is used for fermenting malted cereals and fruit juices, to produce ethanol. Do you recollect the metabolic reactions, which result in the production of ethanol by yeast? Depending on the type of the raw material used for fermentation and the type of processing (with or without distillation) different types of alcoholic drinks are obtained. Wine and beer are produced without distillation whereas whisky, brandy and rum are produced by distillation of the fermented broth.

Table 11.4: Comparison of Aerobic and Anaerobic Respiration

Assessment Strategy (15 minutes)

For a real-life application of anaerobic respiration, lead a class discussion on muscle soreness as related to school sports and exercise. Have student’s research lactic acid build-up in muscles, the causes, and any possible treatments.

Group discussion is possible for 5-7 minutes

The students should be paired in groups of 4-5 to discuss the significances of fermentation in daily life.