Metabolic coupling in yeast — When a fermenting Saccharomyces culture becomes metabolically uncoupled, ethanol production becomes...

Introduction:
Yeast can channel carbon to ethanol either as part of growth-associated catabolism or as a non-growth-associated overflow, depending on metabolic coupling between energy generation and biosynthesis. This question probes what happens when a fermenting culture becomes metabolically uncoupled.

Given Data / Assumptions:

• Fermentative metabolism is active (anaerobic or Crabtree effect under high glucose).
• Uncoupling means ATP generation no longer drives proportional biomass synthesis.
• Carbon flux to ethanol can persist independently of growth.

Concept / Approach:
In coupled metabolism, catabolic ATP and reducing power support macromolecular synthesis; product formation is often growth associated. Under uncoupling, ATP dissipation and redox balancing can proceed without parallel biomass accumulation. Ethanol, serving as an electron sink and ATP-yielding product via substrate-level phosphorylation, may continue to form even when net growth stalls, thus appearing non-growth-associated.

Step-by-Step Solution:
Recognize the definition: growth-associated product formation scales with dX/dt; non-growth-associated scales with X (maintenance).Uncoupling reduces biomass yield Yx/s but does not necessarily stop glycolytic flux.Therefore ethanol formation can persist at roughly constant or maintenance-linked rates even with little biomass increase.

Verification / Alternative check:
Fitting Luedeking–Piret parameters often shows a drop in the growth-associated term (alpha) and a rise in the non-growth term (beta) after uncoupling stress (e.g., inhibitors, ionophores, heat shock), consistent with continued ethanol production at low growth.

Why Other Options Are Wrong:

• (a) Ethanol remains a primary catabolic product, not a secondary metabolite.
• (b) Opposite trend; uncoupling weakens growth association.
• (d) Ethanol is not a growth nutrient for yeast under these conditions.
• (e) Ethanol formation is often used to restore redox balance rather than being suppressed.

Common Pitfalls:
Equating “uncoupled” with “no metabolism”; assuming ethanol is only formed during rapid growth.

Final Answer:
non-growth-associated (continues even without net biomass increase).