Cultivation systems: continuous culture MCQs With Answer

Cultivation systems: continuous culture MCQs With Answer

Introduction: Continuous culture systems are central to bioprocess engineering and vaccine/drug biomanufacturing. This quiz collection focuses on continuous cultivation concepts such as chemostats, turbidostats, perfusion and cell‑retention methods, steady state, washout, dilution rate, Monod kinetics, productivity and control strategies. Designed for M.Pharm students, the questions probe theory, calculations and practical implications in process design, optimization and troubleshooting. Working through these MCQs will strengthen your understanding of how continuous operation affects growth rate, substrate limitation, yield, oxygen transfer and scale‑up, and how to apply continuous culture principles to improve productivity and product quality in pharmaceutical bioprocesses.

Q1. What parameter in a chemostat is directly controlled to set the growth rate of the microbial culture?

• Feed substrate concentration
• Dilution rate (flow rate/working volume)
• Temperature
• pH

Correct Answer: Dilution rate (flow rate/working volume)

Q2. In a chemostat at steady state, which equality holds true for biomass concentration (X), specific growth rate (μ) and dilution rate (D)?

• μ = 0
• μ > D
• μ = D
• D = 0

Correct Answer: μ = D

Q3. What is the primary operational difference between a chemostat and a turbidostat?

• Chemostat controls turbidity, turbidostat controls temperature
• Chemostat maintains constant flow, turbidostat varies flow to keep biomass constant
• Chemostat operates intermittently, turbidostat is batch only
• Chemostat uses cell retention, turbidostat does not

Correct Answer: Chemostat maintains constant flow, turbidostat varies flow to keep biomass constant

Q4. In a chemostat with Monod kinetics, increasing the dilution rate near the maximum specific growth rate results in which phenomenon?

• Increased substrate inhibition
• Washout of biomass
• Decreased dissolved oxygen concentration only
• Instantaneous adaptation to new feed

Correct Answer: Washout of biomass

Q5. Which of the following definitions best describes washout in continuous culture?

• The biomass concentration becomes negative
• The specific production rate becomes zero
• The dilution rate exceeds the organism’s maximum growth rate, causing culture loss
• The substrate concentration becomes limiting but biomass remains constant

Correct Answer: The dilution rate exceeds the organism’s maximum growth rate, causing culture loss

Q6. For a chemostat operating under substrate‑limited steady state, which relationship between residual substrate concentration (S) and dilution rate (D) is expected when using Monod parameters?

• S increases with increasing D until washout
• S decreases as D increases monotonically
• S remains constant regardless of D
• S becomes zero at any D

Correct Answer: S increases with increasing D until washout

Q7. Perfusion culture differs from classical continuous stirred tank chemostat primarily by what feature?

• Continuous addition of fresh medium without biomass removal
• Cell retention with removal of spent medium to achieve high cell density
• Operation at fixed low biomass concentration
• Use only for microbial systems, not mammalian cells

Correct Answer: Cell retention with removal of spent medium to achieve high cell density

Q8. Which cell‑retention device is commonly used in perfusion to retain mammalian cells while allowing medium exchange?

• Membrane bioreactor or tangential flow filtration
• Simple gravity decantation
• Static mixer
• Bubble column without retention

Correct Answer: Membrane bioreactor or tangential flow filtration

Q9. In continuous culture, the space‑time yield (product per reactor volume per time) is maximized by optimizing which of the following?

• pH setpoint only
• Dilution rate relative to specific production rate and biomass yield
• Feed osmolarity exclusively
• Sterility assurance level

Correct Answer: Dilution rate relative to specific production rate and biomass yield

Q10. Which mathematical expression describes steady-state mass balance for substrate in a chemostat?

• D(Sin – S) = μX/Y
• μ = μmax S/(Ks + S)
• qP = rP/X
• τ = V/F

Correct Answer: D(Sin – S) = μX/Y

Q11. If μmax = 0.8 h−1 and Ks = 0.05 g/L, at what approximate substrate concentration S will μ reach half μmax according to Monod kinetics?

• S = 0.025 g/L
• S = 0.05 g/L
• S = 0.1 g/L
• S = 0.8 g/L

Correct Answer: S = 0.05 g/L

Q12. In a continuous bioreactor, how does increasing the dilution rate affect volumetric productivity when specific production rate qP is constant?

• Volumetric productivity decreases linearly with D
• Volumetric productivity increases linearly with D
• Volumetric productivity remains unchanged
• Volumetric productivity becomes independent of biomass

Correct Answer: Volumetric productivity increases linearly with D

Q13. Which control strategy is most appropriate to prevent washout when operating close to μmax in a chemostat?

• Operate at a fixed high dilution rate without monitoring
• Implement feedback control on biomass or turbidity to adjust feed rate
• Remove aeration entirely
• Use only batch operation

Correct Answer: Implement feedback control on biomass or turbidity to adjust feed rate

Q14. In a cascade of two chemostats in series for product formation, what is the main advantage compared to a single chemostat of equal total volume?

• Lower oxygen transfer requirement always
• Better control of sequential metabolic phases and higher overall conversion
• Reduced need for sterility
• Higher washout risk without benefit

Correct Answer: Better control of sequential metabolic phases and higher overall conversion

Q15. Which statement about maintenance energy in continuous cultures is correct?

• Maintenance energy is negligible at very low dilution rates
• Maintenance energy is the energy required for growth only
• Maintenance energy reduces biomass yield and becomes significant at low specific growth rates
• Maintenance energy increases growth rate directly

Correct Answer: Maintenance energy reduces biomass yield and becomes significant at low specific growth rates

Q16. What is the effect of substrate overflow metabolism (e.g., acetate production in E. coli) on continuous culture productivity?

• Always increases biomass yield
• Can decrease product yield and inhibit growth at high dilution rates
• Has no impact on downstream processing
• Eliminates the need for aeration

Correct Answer: Can decrease product yield and inhibit growth at high dilution rates

Q17. For aerobic continuous cultures, which parameter often becomes limiting at high cell densities and thus limits achievable dilution rate?

• pH buffering capacity
• Dissolved oxygen transfer rate (kLa and OTR)
• Feed substrate purity
• Antifoam concentration

Correct Answer: Dissolved oxygen transfer rate (kLa and OTR)

Q18. In a continuous process producing a growth‑associated product, maximizing specific growth rate will generally have what effect on specific product formation rate (qP)?

• qP always decreases
• qP is independent of μ for strictly growth‑associated products and increases with μ
• qP becomes zero
• qP is inversely proportional to substrate concentration only

Correct Answer: qP is independent of μ for strictly growth‑associated products and increases with μ

Q19. Which experimental observation indicates that a chemostat is not at steady state?

• Constant cell concentration and constant outlet substrate
• Oscillations in biomass concentration over time
• Stable dissolved oxygen and pH
• Constant optical density with time

Correct Answer: Oscillations in biomass concentration over time

Q20. When scaling up a continuous bioreactor, which scale‑up criterion is most critical for maintaining similar oxygen supply to the culture?

• Geometric similarity only
• Constant volumetric oxygen transfer coefficient (kLa) or constant oxygen transfer rate per volume
• Keeping impeller diameter fixed irrespective of volume
• Matching inlet feed temperature only

Correct Answer: Constant volumetric oxygen transfer coefficient (kLa) or constant oxygen transfer rate per volume

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