BIOREACTOR ENGINEERING: CELL GROWTH & CONDITIONS

During which phase of the microbial growth curve does cell division occur at a constant and maximum rate?

• Lag phase

• Log (exponential) phase

• Stationary phase

• Death phase

Answer explanation

In the log phase, cells divide at their maximum rate, and the population size doubles at regular intervals.

Which phase is characterized by metabolic adjustments without cell division?

• Lag phase

• Log phase

• Stationary phase

• Decline phase

Answer explanation

During the lag phase, cells adapt to new environmental conditions by synthesizing enzymes and preparing for division, but they do not divide.

Which of the following statements about growth-associated product formation is correct?

• It occurs primarily during the lag phase.

• The product is synthesized as cells actively divide.

• It happens when the culture reaches the stationary phase.

• It is linked to secondary metabolite production.

Answer explanation

Growth-associated products are directly tied to microbial growth, typically during the log (exponential) phase. Examples include primary metabolites like ethanol and amino acids.

Non-growth-associated product formation is characterized by:

• Continuous synthesis during the exponential phase

• Production occurring after active growth has stopped

• Direct correlation with biomass increase

• Dependence on nutrient availability during cell division

Answer explanation

Non-growth-associated products are synthesized during the stationary phase, often in response to nutrient limitation or stress, as seen with secondary metabolites like antibiotics.

In mixed growth-associated product formation:

• The product is only produced in the absence of growth.

• Product formation occurs during both growth and stationary phases.

• It happens exclusively in continuous fermentation.

• Product formation is independent of substrate concentration.

Answer explanation

Mixed growth-associated products are produced during both the exponential and stationary phases, with production rates linked to biomass and independent factors (e.g., maintenance energy).

For a product to be classified as growth-associated, its production rate must:

• Decrease with increased biomass

• Be proportional to the cell growth rate

• Peak during the stationary phase

• Be independent of substrate concentration

Answer explanation

Growth-associated products are synthesized alongside biomass formation; their production rate is directly linked to cell division.

What is the main consequence of exceeding the optimal temperature range for microbial growth?

• Increased metabolic activity

• Enhanced oxygen transfer

• Enzyme denaturation and loss of cell viability

• Stabilization of the cell membrane

Answer explanation

High temperatures can denature enzymes and damage cellular structures, ultimately killing the microorganism or halting growth.

What is the impact of a pH lower than the optimal range on microbial growth?

• Enhanced enzyme activity

• Denaturation of proteins and enzymes

• Increased metabolic efficiency

• No effect on microbial structure

Answer explanation

Extreme pH values can disrupt the structure and function of enzymes and proteins, inhibiting growth or causing cell death.

What happens to obligate aerobes under oxygen-limited conditions?

• They switch to anaerobic respiration.

• Their growth rate decreases and may slowly died.

• They produce ATP through fermentation.

• Microbial contamination occurs.

Answer explanation

Obligate aerobes cannot survive without oxygen because they rely on it for energy production through aerobic respiration.

In anaerobic fermentation, dissolved oxygen should be:

• Maintained at high levels.

• Eliminated entirely.
  

• Reduced but not removed.
  

• Increased gradually.

Answer explanation

Anaerobic fermentation requires an oxygen-free environment because oxygen inhibits anaerobic pathways, such as those producing ethanol or lactic acid.