Difficulty: Medium
Correct Answer: Cells will redirect ATP to drive active transport and proton pumping that expels lactic acid and H+, shifting energy from growth to maintenance.
Explanation:
Introduction:
Lactococcus lactis is a lactic acid bacterium that primarily generates ATP by substrate-level phosphorylation and reoxidizes NADH by reducing pyruvate to lactic acid. As lactic acid accumulates in the broth, pH drops and undissociated acid diffuses back into the cell, increasing the internal proton load. The cell must spend energy to maintain intracellular pH, causing product formation to decouple from biomass growth (non-growth-associated formation).
Given Data / Assumptions:
Concept / Approach:
When acid stress rises, cells allocate ATP to maintenance: proton extrusion and acid efflux. This shifts ATP away from anabolism and cell division. Thus, lactic acid production continues (for redox balance), but biomass growth slows or plateaus. The key is active transport driven by ATP hydrolysis, not passive diffusion, because the proton gradient and charge differences must be overcome.
Step-by-Step Solution:
Verification / Alternative check:
Maintenance coefficients increase under acid stress. Chemostat or batch data often show constant or rising specific lactic acid formation rates despite reduced specific growth rates at low pH.
Why Other Options Are Wrong:
A: More ATP goes to maintenance, not anabolism. B: NAD+ recycling occurs via lactate dehydrogenase but does not by itself explain non-growth association. C: Diffusion does not require ATP and cannot account for active extrusion against gradients. E: ATP use does not cease; it is redirected to maintenance.
Common Pitfalls:
Confusing redox balancing (NADH to lactate) with energy allocation; non-growth association stems from ATP diversion to homeostasis, not a halt in metabolism.
Final Answer:
Cells will redirect ATP to drive active transport and proton pumping that expels lactic acid and H+, shifting energy from growth to maintenance.
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