Q.36 Which one of the following represents non-growth associated product formation kinetics in a bioprocess system?
X and P denote viable cell and product concentrations, respectively.
In a typical bioprocess, cells grow (X) and produce metabolites or products (P). Depending on the organism and product type, P can be synthesized:
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Along with growth (growth-associated),
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Only after growth stops (non-growth associated), or
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A combination of both.
The question asks which curve specifically represents non-growth associated product formation, where product appears only after biomass stops increasing.
✅ Correct Answer: Option (B)
Why Option (B) is Correct
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X rises during exponential growth and plateaus at a stationary phase.
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P remains low during growth, then increases significantly only after X stops increasing.
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This matches non-growth associated kinetics, seen in:
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Secondary metabolite production
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Penicillin fermentation
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Stress-dependent product formation
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Thus, product formation is separated from biomass growth.
❌ Why the Other Options Are Incorrect
Option (A) – Growth-Associated Product Formation
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Both X and P rise simultaneously.
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Product is synthesized proportionally to cell growth.
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Typical for primary metabolites (e.g., lactic acid, ethanol).
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Not correct because P does not lag behind X.
Option (C) – Mixed Growth + Non-Growth Associated
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X increases and plateaus.
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P increases steadily throughout, even after growth stops.
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Represents Luedeking-Piret mixed kinetics, where:
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Product partly depends on growth (associated)
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And partly continues after growth (non-growth)
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Not purely non-growth associated, so incorrect.
Option (D) – Product Declines After Formation
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P increases first, then decreases due to:
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Product degradation,
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Consumption,
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Toxicity,
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Cell metabolism.
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X grows independently.
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Does not represent sustained non-growth associated formation.
🎯 Conclusion
Non-growth associated metabolite production is marked by no significant product until cells stop multiplying.
Among the given graphs, Option (B) uniquely matches this profile.
📌 Key Takeaway
Non-growth associated products appear mainly during the stationary phase — NOT during exponential cell growth.
