# Bioprocess Engineering Questions and Answers – Cell Growth Kinetics

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Cell Growth Kinetics”.

1. Estimate the value of “rX”,which is the volumetric rate of biomass production.
a) kg m-1 s-1
b) kg ms-1
c) kg m-3s-1
d) kg m-3s-3

Explanation: Rate of cell growth is described by the equation:
rX = μx
where rX is the volumetric rate of biomass production with units of, for example, kg m-3 s-I, x is viable cell concentration with units of, for example, kg m-3, and μ is the specific growth rate. Specific growth rate has dimensions T-1.

2. Which phase has the condition of specific growth rate “μ = 0”?
a) Lag phase
b) Log phase
c) Stationary phase
d) Death phase

Explanation: In death phase, the cells lose viability and lyse. Therefore, μ=0.

3. Which phase has the condition of specific growth rate “μ ≈ μmax”?
a) Lag phase
b) Log phase
c) Growth phase
d) Death phase

Explanation: In growth phase, Growth achieves its maximum rate. Therefore, μ ≈ μmax.

4. Which condition is correct according to the growth of cells in beginning?
a) Cells are in small amount
b) Cells are in medium amount
c) Cells are in large amount
d) Cells are negligible in amount

Explanation: Growth always appears much slower at the beginning of culture because the number of cells present is small.

5. The specific growth rate of cells is independent on the concentration of nutrients in the medium.
a) True
b) False

Explanation: The specific growth rate of cells is dependent on the concentration of nutrients in the medium. Often, a single substrate exerts a dominant influence on rate of growth; this component is known as the growth-rate-limiting substrate or, more simply, the growth-limiting substrate. The growth-limiting substrate is often the carbon or nitrogen source, although in some cases it is oxygen or another oxidant such as nitrate.
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6. Microbial growth is the result of both cell division and change in cell size.
a) True
b) False

Explanation: Growth for multicelluar organisms is typically measured in terms of the increase in size of a single organism, microbial growth is measured by the increase in population, either by measuring the increase in cell number or the increase in overall mass. Cell division process begins with cell elongation, which requires careful enlargement of the cell membrane and the cell wall, in addition to an increase in cell volume.

7. The lag phase constitute of _________
a) No change in number, but an increase in mass
b) Change in number but decrease in mass
c) No change in number and decrease in mass
d) Constant number and mass

Explanation: The lag phase is the time period when there is no immediate increase in cell number of microorganisms as they are introduced into fresh culture medium. The microorganisms during this phase are trying to acclimatize to the new culture medium as it may be different from the previous one in which the microorganism was growing. Typically cells in the lag period are synthesizing RNA, enzymes, and essential metabolites that might be missing from their new environment (such as growth factors or macromolecules), as well as adjusting to environmental changes such as changes in temperature, pH, or oxygen availability. They can also be undertaking any necessary repair of injured cells.

8. In the accelerated phase, cell starts to _________
a) increase and the division rate increases to reach a maximum
b) decrease and the division rate increases to reach a maximum
c) increase and the division rate decreases to reach a maximum
d) increase and the division rate increases to reach a minimum

Explanation: This is a brief transient period during which cells start growing slowly. In fact, acceleration phase connects the lag phase and log phase.

9. What is the generation time if 100 bacterial cells growing logarithmically for 5 hours produced 1.7×106 cells?
a) 0.351 generations/hour
b) 0.353 generations/hour
c) 0.355 generation/hour
d) 0.356 generation/hour

n = $$\frac{(log(1.7×10^6) – log 100)}{log 2}$$
= 14.053 generation in 5 hours.
g = 5 / 14.053 = 0.35579 = 0.356 (rounding to 3 decimal points)

10. Which of the following is used to grow bacterial cultures continuously?
a) Chemostat
b) Thermostat
c) Haemostat
d) Coulter counter

Explanation: A chemostat (from chemical environment is static) is a bioreactor to which fresh medium is continuously added, while culture liquid containing left over nutrients, metabolic end products and microorganisms are continuously removed at the same rate to keep the culture volume constant.

11. Starvation proteins are produced by a culture during which of the following segments of growth curve?
a) Lag phase
b) Log phase
c) Stationary phase
d) Death phase

Explanation: When microbes lack the nutrients necessary for growth, they enter stationary phase. In cases when energy sources are still present in the environment, they must decide whether to continue to use their metabolic program to harvest the available energy. Here we characterized the metabolic response to a variety of types of nutrient starvation in Escherichia coli and Bacillus subtilis.

12. The average size of cells during exponential phase of growth is _________
a) Greater than the lag phase
b) Lesser than the lag phase
c) Equal to lag phase
d) Constant

Explanation: During this phase bacteria divides continuously at constant rate and the number of bacteria increase exponentially. In this phase all bacteria are in their rapid stage of cell division and show balanced growth. Due to rapid cell division, bacteria have smallest size in this phase.

13. Generation time is the time required for a microbial population to double in number.
a) True
b) False

Explanation: A generation time is simply the time it takes for one cell to become two. When growing exponentially by binary fission, the increase in a bacterial population is by geometric progression. If we start with one cell, when it divides, there are 2 cells in the first generation, 4 cells in the second generation, 8 cells in the third generation, and so on. The generation time is the time interval required for the cells (or population) to divide.

14. Mean growth rate constant is the number of generations per unit time, often expressed as generations per hour.
a) True
b) False

Explanation: The growth rate can be expressed in terms of mean growth rate constant (k), the number of generations per unit time. Mean generation time or mean doubling time (g), is the time taken to double its size.

15. How else can we measure biomass?
a) Rheometry
b) Nephelometer
c) Spectrophotometer
d) Mass spectrometry

Explanation: A spectrophotometer optically determines the absorbance or transmission of characteristic wavelengths of radiant energy (light) by a chemical species in solution. Each molecule absorbs light at certain wavelengths in a unique spectral pattern because of the number and arrangement of its characteristic functional groups, such as double bonds between carbon atoms. According to the Beer-Lambert law, the amount of light absorbed at these wavelengths is directly proportional to the concentration of the chemical species.

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