7.3 - Fermentation

What Is Fermentation?

• Fermentation is the incomplete breakdown of glucose when oxygen is limited or absent.
• Its metabolic pathway differs from aerobic respiration after glycolysis.
• After glycolysis, pyruvate undergoes either alcohol fermentation or lactic acid fermentation.

Types of Fermentation

Alcohol Fermentation

• Glucose is broken down incompletely into ethanol, carbon dioxide and energy.

$\text{Glucose}\to \text{Ethanol}+\text{Carbon dioxide}+\text{Energy (210 kJ)}$

• Yeast:
  • Ethanol is used to make beer and wine.
  • Released carbon dioxide raises dough in bread making.
• Paddy plants:
  • Paddy plants growing in waterlogged, oxygen-deficient areas can carry out alcohol fermentation.
  • Ethanol is toxic to most plants, but paddy cells have a higher tolerance for ethanol than other species.
  • Paddy plants produce large quantities of alcohol dehydrogenase, which breaks down ethanol into non-toxic carbon dioxide.

Lactic Acid Fermentation

• Glucose is broken down into lactic acid and energy.

$\text{Glucose}\to \text{Lactic acid}+\text{Energy}$

• Human muscle cells:
  • During vigorous activity, oxygen is used faster than the circulatory system supplies it.
  • Muscles become oxygen-deficient and incur an oxygen debt.
  • Glucose is not broken down completely; each glucose molecule produces only two ATP molecules or $150\text{ kJ}$ of energy.
  • Lactic acid accumulates to a concentration that can cause muscular fatigue and cramps.
  • After vigorous activity stops, excess oxygen oxidises lactic acid into carbon dioxide, water and energy.
  • The oxygen debt is paid when all lactic acid has been removed.
• Lactobacillus:
  • Lactobacillus ferments milk to produce curd.
  • It converts lactose (milk sugar) into lactic acid.
  • Lactic acid coagulates casein (milk protein) into curd or yogurt and causes yogurt’s sour taste.

Experiment: Studying Fermentation in Yeast

• Problem statement: What products are obtained from fermentation in yeast?
• Hypothesis: Yeast fermentation produces energy, carbon dioxide and ethanol.
• Manipulated variable: Presence of yeast.
• Responding variables: Temperature change, change in limewater and ethanol smell.
• Constant variables: Volume of boiled glucose solution and anaerobic conditions.
• Add 15 ml of boiled, cooled 5% glucose solution to boiling tubes A and B; add 5 ml of 5% yeast suspension to A only.
• Add paraffin oil to both tubes to maintain anaerobic conditions. Connect each tube to limewater and leave for one hour.
• Compare the initial and final temperatures, limewater changes and solution smells.

Comparison between Aerobic Respiration and Fermentation

Similarities

• Break down glucose and convert it into chemical energy.
• Produce chemical energy in the form of ATP.
• Begin with glycolysis, in which glucose is converted into pyruvate.
• Occur in yeast, bacteria, animals and plants.
• Begin in the cytoplasm.

Differences

Aspect	Aerobic respiration	Fermentation
Breakdown of glucose	Complete; oxygen is present	Incomplete; oxygen is absent or limited
Site	Cytoplasm and mitochondrion	Cytoplasm
Water	Produced	Not produced
Oxidation products	Carbon dioxide and water	Ethanol and carbon dioxide, or lactic acid
Energy per glucose molecule	$2898\text{ kJ}$	$210\text{ kJ}$ (alcohol) or $150\text{ kJ}$ (lactic acid)