Fermentation: Lactic Acid and Alcoholic Fermentation
When oxygen runs short, cells cannot rely on the Krebs’ cycle. Instead, they dispose of pyruvic acid through fermentation — a set of anaerobic reactions that regenerate ##\ce{NAD+}## so that glycolysis can keep running.
This quiz tests your grasp of the two great fermentative routes:
- Alcoholic fermentation in yeast: pyruvic acid → ##\ce{CO2}## + ethanol
- Lactic acid fermentation in some bacteria and in exercising animal muscle
You will also be asked about the enzymes involved, the reoxidation of ##\ce{NADH + H+}## to ##\ce{NAD+}##, and the modest energy yield — less than seven per cent of the energy in glucose.
Read each question carefully; only one option is correct.
Q1. According to the lesson, what are the three major ways in which different cells handle the pyruvic acid produced by glycolysis?
The lesson states that there are three major ways in which different cells handle pyruvic acid: lactic acid fermentation, alcoholic fermentation and aerobic respiration.
Q2. Under what conditions does fermentation take place in many prokaryotes and unicellular eukaryotes?
Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
Q3. In yeast, pyruvic acid is converted to which two products during fermentation?
In fermentation by yeast, the incomplete oxidation of glucose under anaerobic conditions converts pyruvic acid to ##\ce{CO2}## and ethanol.
Q4. Which two enzymes catalyse the conversion of pyruvic acid to ##\ce{CO2}## and ethanol in yeast?
The lesson names pyruvic acid decarboxylase and alcohol dehydrogenase as the enzymes catalysing these reactions.
Q5. Which enzyme catalyses the reduction of pyruvic acid to lactic acid in animal muscle cells during exercise?
In animal cells such as muscles during exercise, pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
Q6. In both lactic acid and alcoholic fermentation, which substance acts as the reducing agent?
The reducing agent is ##\ce{NADH + H+}##, which is reoxidised to ##\ce{NAD+}## in both processes.
Q7. What happens to ##\ce{NADH + H+}## during fermentation?
The lesson states that the reducing agent ##\ce{NADH + H+}## is reoxidised to ##\ce{NAD+}## in both lactic acid and alcohol fermentation.
Q8. Approximately what percentage of the energy in glucose is released during lactic acid and alcohol fermentation?
In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released.
Q9. Which statement best describes the trapping of energy released in fermentation?
The lesson notes that less than seven per cent of the energy in glucose is released and not all of it is trapped as high-energy bonds of ATP.
Q10. Why does the lesson describe fermentation as hazardous?
The lesson states that the processes are hazardous because either acid or alcohol is produced.
Q11. At what concentration of alcohol do yeasts poison themselves to death?
Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.
Q12. Which process requires a supply of ##\ce{O2}## for the complete oxidation of glucose to ##\ce{CO2}## and ##\ce{H2O}##?
For the complete oxidation of glucose to ##\ce{CO2}## and ##\ce{H2O}##, organisms adopt the Krebs' cycle, also called aerobic respiration, which requires ##\ce{O2}## supply.
Q13. In which situation do animal muscle cells reduce pyruvic acid to lactic acid?
In animal cells such as muscles during exercise, when oxygen is inadequate for cellular respiration, pyruvic acid is reduced to lactic acid.
Q14. Which organisms produce lactic acid from pyruvic acid, according to the lesson?
The lesson states that other organisms, like some bacteria, produce lactic acid from pyruvic acid.
Q15. Which of the following correctly pairs a fermentation type with its product?
In alcoholic fermentation, pyruvic acid is converted to ##\ce{CO2}## and ethanol, whereas lactic acid fermentation yields lactic acid.