Question

You are working with a yeast that can undergo fermentation or respiration. You take equal aliquots of the same yeast culture and grow them providing equal amounts of sugar. However, you culture aliquot A into an airtight bottle (no oxygen) and aliquot B into an open shallow dish (with oxygen, go through glycolysis krebs cycle etc). Will one culture run out of sugar faster?

options:

Culture B will run out of sugar faster

Both cultures will run out of sugar at a similar time

Culture A will run out of sugar faster

Answer 1

Culture A will run out of sugar faster

Yeast can produce energy using two distinct metabolic pathways:

• Absence of Oxygen (fermentation: Culture A)
  • The product of glycolysis which is pyruvate is converted into ethanol and carbon dioxide, and 2 moles of ATP
  • C6H12O6 (aq)------> 2 CH3CH2OH (aq) Ethanol+ 2 CO2 (g) + energy (2 ATP + heat)
• Presence of Oxygen (respiration: Culture B)
  • Pyruvate is converted to acetyl CoA that can be used in the citric acid cycle (Krebs cycle) which produces 36-38 ATP
  • C6H12O6 (aq) + 6 O2(g)--------> 6 H2O (l) + 6 CO2 (g) + energy (36-38 ATP + Heat)

Culture A where anaerobic conditions exist, the rate of glucose metabolism is faster but the measure of ATP generated is smaller. In this culture, the chain cannot be concluded to produce NADH+H+ and it stops at the pyruvate stage so, Culture A will run out of sugar faster.

Culture B which is exposed to aerobic conditions the ATP and Citrate production increases but sugar metabolism is slow. But yeasts continue to process sugar at an even higher rate than in an aerobic environment in order to remunerate for the energy loss

The Pasteur effect explains that in the presence of oxygen the yeast multiples but metabolism will be faster in the absence of oxygen.