Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
A group of students performed an experiment to study anaerobic respiration in yeast. They prepared a sugar solution, added yeast, and sealed the flask with a delivery tube dipped in lime water. Within a few hours, the lime water turned milky and a distinct smell of alcohol was detected. When they measured the sugar content of the solution after 24 hours, it had decreased significantly. One student noted that the yeast population stopped growing after the alcohol concentration reached a certain level. The teacher explained that the yeast was carrying out a specific type of fermentation, converting glucose to simpler products under oxygen-free conditions. The students were asked to identify the enzymes involved, calculate the ATP yield, and explain why yeast growth eventually stopped.
Question 1: Name the two enzymes involved in alcoholic fermentation and state the immediate product formed by each enzyme.
- Pyruvic acid decarboxylase catalyses the conversion of pyruvic acid to acetaldehyde and CO2; the milky lime water confirms CO2 production.
- Alcohol dehydrogenase then catalyses the reduction of acetaldehyde to ethanol, using NADH+H+ as the reducing agent.
- The overall products of alcoholic fermentation are ethanol and CO2, both of which were detected in the experiment — alcohol by smell and CO2 by the lime water test.
Question 2: Calculate the net ATP synthesised per molecule of glucose during this fermentation and explain the significance of NADH reoxidation in the process.
- Glycolysis consumes 2 ATP and produces 4 ATP by substrate-level phosphorylation, giving a net gain of 2 ATP per glucose; no additional ATP is produced during the fermentation steps themselves.
- NADH+H+ produced during glycolysis (when PGAL is oxidised to BPGA) must be reoxidised to NAD+; this occurs during the reduction of acetaldehyde to ethanol by alcohol dehydrogenase.
- Reoxidation of NADH to NAD+ is essential because NAD+ is required as an electron acceptor at the PGAL oxidation step in glycolysis; without it, glycolysis would halt and no ATP at all could be produced anaerobically.
Question 3: Explain why yeast growth stopped after the alcohol concentration reached approximately 13%. How would a distiller obtain a beverage with 40% alcohol content from this fermented solution?
- Yeast cells are poisoned by the ethanol they produce; when ethanol concentration in the medium reaches approximately 13%, the concentration becomes lethal to the yeast cells themselves, causing them to die.
- Once yeast cells die, fermentation ceases entirely regardless of available sugar; the biological ceiling on natural fermentation is therefore set by yeast's own ethanol toxicity threshold.
- To obtain a beverage with 40% alcohol (e.g., whisky or vodka), a distiller uses distillation — a physical process that exploits the lower boiling point of ethanol (78°C) compared to water (100°C) to concentrate and separate ethanol from the fermented mixture, far exceeding what biological fermentation alone can achieve.