# Bioprocess Engineering Questions and Answers – Oxygen Uptake in Cell Cultures

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Oxygen Uptake in Cell Cultures”.

1. What do you mean by “NA” in mass transfer?
a) Rate of oxygen transfer per unit volume of fluid
b) Rate of oxygen transfer per unit volume of gas
d) Rate of oxygen transfer per unit mass of solid

Explanation: NA is the rate of oxygen transfer per unit volume of fluid (gmol m-3s-I), NA is the rate of Oxygen transfer of component A.

2. The solubility of oxygen in aqueous solutions is about?
a) 20 ppm
b) 15 ppm
c) 10 ppm
d) 5 ppm

Explanation: The solubility of oxygen in aqueous solutions at ambient temperature and pressure is only about 10 ppm. This amount of oxygen is quickly consumed in aerobic cultures and must be constantly replaced by sparging.

3. What is the unit of “Qo”, the oxygen uptake rate per volume of broth?
a) gl s-1
b) g-1l s
c) g-1l-1 s
d) g l-1s-1

Explanation: From the following equation:
Qo = qox
where x is cell concentration. Typical units for qo are g g-1 s-l, and for Qo, g 1-1 s-1.

4. Which Substrate is consumed rapidly in fermentation process?
a) Fructose
b) Glucose
c) Sucrose
d) Lactose

Explanation: Choice of substrate for the fermentation can also significantly affect oxygen demand. Because glucose is generally consumed more rapidly than other sugars or carbon-containing substrates, rates of oxygen demand are higher when glucose is used. For example, maximum oxygen-consumption rates of 5.5, 6.1 and 12 mmol l-1 h-1 have been observed for Penicillium mould growing on lactose, sucrose and glucose, respectively.

5. Single cells are smaller than gas bubbles?
a) True
b) False

Explanation: Single cells are much smaller than gas bubbles, the liquid film surrounding each cell is much thinner than that around the bubbles and its effect on mass transfer can generally be neglected. On the other hand, if the cells form large clumps, liquid-film resistance can be significant.
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6. At steady state, there is accumulation of oxygen at any location in the fermenter?
a) True
b) False

Explanation: At steady state there can be no accumulation of oxygen at any location in the fermenter; therefore, the rate of oxygen transfer from the bubbles must be equal to the rate of oxygen consumption by the cells.

7. Maximum rate of oxygen transfer occurs when?
a) CAL = 0
b) CAL < 0
c) CAL >= 1
d) CAL > 1

Explanation: First, let us estimate the maximum cell concentration that can be supported by the fermenter’s oxygen-transfer system. For a given set of operating conditions, the maximum rate of oxygen transfer occurs when the concentration-difference driving force (C*AL – CAL) is highest, i.e. when the concentration of dissolved oxygen CAL is zero.

8. A strain of Azotobacter vinelandii is cultured in a 15 m3 stirred fermenter for alginate production. Under current operating conditions kLa is 0.17 s-1. Oxygen solubility in the broth is approximately 8 x 10-3 kg m-3. The specific rate of oxygen uptake is 12.5 mmol g-1 h-1. What is the maximum possible cell concentration?

a) 10 gl-1
b) 15 gl-1
c) 20 gl-1
d) 12 gl-1

Explanation: From the equation given in the question:

= 1.2×104 g m-3 = 12 gl-1.

9. Refer to Q8 and, calculate the bacteria suffer growth inhibition after copper sulphate is accidently added to the fermentation broth. This causes a reduction in oxygen uptake rate to 3 mmol g-1 h-1. What maximum cell concentration can now be supported by the fermenter?
a) 50 gl-1
b) 10 gl-1
b) 25 gl-1
d) 55 gl-1

Explanation: Assume that addition of copper sulphate does not affect C*AL or kLa. If qo is reduced by a factor of 12.5/3 = 4.167, xmax is increased to:
xmax = 4.167 (12 gl-1) = 50 gl-1
To achieve the calculated cell densities all other conditions must be favourable, e.g. sufficient substrate and time must be provided.

10. Which type of microbes grow well in the condition of enriched oxygen?
a) Obligate anaerobes
b) Obligate aerobes
c) Facultative anaerobes
d) Facultative aerobes

Explanation: Most cells are aerobic and therefore need to be supplied with oxygen to grow (obligate aerobes). Some find oxygen toxic and grow badly in its presence (obligate anaerobes), while others grow better when oxygen is present, but continue to do so if its absence (facultative anaerobes).

11. Beer may be produced by ________________
a) Fermentation of rice
b) Germination of rice
c) Germination of barley
d) Fermentation of grapes

Explanation: Malt extract is frequently used in the brewing of beer. Its production begins by germinating barley grain in a process known as malting, immersing barley in water to encourage the grain to sprout, then drying the barley to halt the progress when the sprouting begins.

12. In anaerobic condition sugar in dough is converted into?
a) Alcohol
b) Water
c) Glucose
d) Sucrose

Explanation: Fermentation is nothing but an anaerobic respiration process in which the substrate is any organic material other than glucose like alcohol. Saccharomyces cerevisiae (yeast) produces an enzyme named zymogen which ferment the alcohol. It is an anaerobic process and substrate is not glucose. The energy production is much less in fermentation process.

13. The lowest yield of ATP is in ____________
a) Aerobic respiration
b) Aerobic fermentation
c) Anaerobic respiration
d) Fermentation

Explanation: Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle, and the electron transport chain does not run. Because the electron transport chain isn’t functional, the NADH made in glycolysis cannot drop its electrons off there to turn back into NAD+.

14. The population of microbes is maintained in which phase for long time using continuous culture system?
a) Lag
b) Log
c) Exponential
d) Stationary

Explanation: The log phase is a period characterized by cell doubling. The number of new bacteria appearing per unit time is proportional to the present population. If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles with each consecutive time period.

15. Cell density is controlled by increasing or decreasing flow of culture medium in which state?
a) Turbidostat
b) Chemostat
c) Continuous culture
d) Synchronus culture

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.

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