Crabtree effect phenotype: in the presence of oxygen, how does a Crabtree-positive microorganism (e.g., Saccharomyces cerevisiae) behave at high sugar concentrations?

Introduction / Context:
The Crabtree effect describes aerobic fermentation: certain yeasts preferentially ferment glucose to ethanol at high sugar concentrations despite sufficient oxygen for respiration. This behavior influences reactor design, oxygen transfer, and product spectrum in industrial fermentations like brewing and bioethanol production.

Given Data / Assumptions:

• Organism exhibits Crabtree positivity (e.g., S. cerevisiae).
• Oxygen is available but not necessarily limiting.
• Glucose concentration is high.

Concept / Approach:
At high glucose, glycolytic flux saturates downstream respiratory pathways, and regulatory networks (e.g., glucose repression) down-regulate respiration. Carbon is diverted to ethanol formation with ATP yield per glucose lower than respiration but higher volumetric rates due to glycolysis throughput. As glucose drops, cells shift back to respiratory metabolism (diauxic shift if ethanol is later consumed).

Step-by-Step Solution:
High glucose triggers glucose repression of respiratory genes and activates fermentation pathways.Even with oxygen present, pyruvate predominantly converts to ethanol (and CO2) rather than entering the TCA cycle extensively.Observed outcomes: high ethanol production, heat release, and oxygen uptake below purely respiratory demand.

Verification / Alternative check:
Batch profiles show ethanol accumulation during early high-glucose phase under aerobic conditions, followed by ethanol consumption after glucose depletion (diauxic growth), confirming the effect.

Why Other Options Are Wrong:
Low sugar scenarios do not typically elicit the Crabtree fermentation response.Pure respiration at high sugar conflicts with glucose repression dynamics.

Common Pitfalls:
Confusing Crabtree (aerobic fermentation due to high glucose) with Pasteur effect (oxygen inhibiting fermentation); assuming oxygen presence guarantees respiration dominance.

Final Answer:
Ferments at high sugar even in the presence of dissolved oxygen