Toxic by-products and yields — Accumulated toxins in a bioreactor often lower biomass yield because...

Introduction:
Biomass yield Yx/s reflects how efficiently substrate carbon and energy are converted into new cell material. When toxins accumulate, cells spend more resources on survival rather than growth. This question targets the energy-allocation mechanism behind reduced yields.

Given Data / Assumptions:

• Toxins may include organics, oxygen radicals, acids, or solvents.
• Cells possess maintenance energy requirements for homeostasis (repair, ion gradients).
• Total ATP generation from metabolism is finite at given conditions.

Concept / Approach:
Maintenance models partition ATP usage into growth-associated (anabolism) and non-growth-associated (homeostasis). Toxins raise maintenance demands (e.g., efflux pumps, repair, chaperone activity), diverting ATP from biosynthesis. Consequently, less biomass is produced per unit substrate, reducing Yx/s despite similar or higher substrate consumption.

Step-by-Step Solution:
Recognize that toxic stress increases maintenance energy m_ATP.ATP budget shifts from polymerization and macromolecule synthesis to repair and protection.Thus, ATP diversion from anabolism to homeostasis lowers biomass yield.

Verification / Alternative check:
Chemostat data fitted with Pirt-type relationships (q_s = m + mu/Yx/s) show increased maintenance term under stress, matching reduced yields. Transcriptomics often reveals upregulation of stress-response genes with higher ATP demand.

Why Other Options Are Wrong:

• (a) and (c) misstate the role of NAD+ (a redox carrier) relative to ATP budgeting.
• (b) Suggests the opposite direction of diversion.
• (e) Ribosome allocation is important but not the primary, general explanation for yield drop due to toxins.

Common Pitfalls:
Blaming only substrate limitation; ignoring proton leak, efflux, or repair costs; conflating redox balance with ATP availability.

Final Answer:
ATP is diverted away from anabolism to homeostasis (maintenance).