D311 Microbiology with Lab - Week 3 Study Guide Quiz 2026 |WGU
1. Which group of microorganisms obtains energy from light and uses organic
compounds as their carbon source?
A. Photoautotrophs
B. Chemoautotrophs
C. Photoheterotrophs
D. Chemoheterotrophs
Answer: C
Rationale: Photoheterotrophs use light for energy but cannot use carbon dioxide as their
sole carbon source, relying instead on organic compounds.
2. An organism that thrives at temperatures between 20°C and 45°C is classified
as a:
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
Answer: B
Rationale: Mesophiles are moderate-temperature-loving microbes, with an optimum
growth temperature around 25-40°C, which includes most human pathogens.
,3. Which enzyme is responsible for neutralizing the toxic superoxide free
radicals in aerobic organisms?
A. Catalase
B. Superoxide dismutase (SOD)
C. Peroxidase
D. DNA polymerase
Answer: B
Rationale: Superoxide dismutase (SOD) converts superoxide radicals into molecular
oxygen and hydrogen peroxide.
4. In which phase of the bacterial growth curve is the population doubling at a
constant maximum rate?
A. Lag phase
B. Log phase
C. Stationary phase
D. Death phase
Answer: B
Rationale: The log (logarithmic) or exponential growth phase is when cellular
reproduction is most active and generation time reaches a constant minimum.
5. What is the primary purpose of using a selective medium?
A. To distinguish between different species of bacteria
B. To provide nutrients for all types of bacteria to grow
C. To suppress the growth of unwanted microbes and encourage the growth of desired ones
D. To determine the oxygen requirements of an organism
Answer: C
Rationale: Selective media contain ingredients that inhibit the growth of certain microbes
while allowing others to grow.
, 6. Which of the following is an example of an indirect method for measuring
microbial growth?
A. Filtration
B. Turbidity
C. Standard plate count
D. Direct microscopic count
Answer: B
Rationale: Turbidity uses a spectrophotometer to measure the cloudiness of a liquid
culture, which is an indirect estimate of cell mass.
7. If a bacterial cell is placed in a hypertonic solution, what will likely occur?
A. The cell will undergo plasmolysis
B. The cell will swell
C. The cell will burst (osmotic lysis)
D. Nothing will happen to the cell
Answer: A
Rationale: In a hypertonic solution, water leaves the cell, causing the cytoplasm to shrink
away from the cell wall, a process called plasmolysis.
8. Which metabolic pathway is common to both fermentation and aerobic
respiration?
A. Krebs Cycle
B. Electron Transport Chain
C. Glycolysis
D. Chemiosmosis
Answer: C
Rationale: Glycolysis is the initial stage of carbohydrate catabolism and does not require
oxygen, making it a precursor to both aerobic respiration and fermentation.
1. Which group of microorganisms obtains energy from light and uses organic
compounds as their carbon source?
A. Photoautotrophs
B. Chemoautotrophs
C. Photoheterotrophs
D. Chemoheterotrophs
Answer: C
Rationale: Photoheterotrophs use light for energy but cannot use carbon dioxide as their
sole carbon source, relying instead on organic compounds.
2. An organism that thrives at temperatures between 20°C and 45°C is classified
as a:
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
Answer: B
Rationale: Mesophiles are moderate-temperature-loving microbes, with an optimum
growth temperature around 25-40°C, which includes most human pathogens.
,3. Which enzyme is responsible for neutralizing the toxic superoxide free
radicals in aerobic organisms?
A. Catalase
B. Superoxide dismutase (SOD)
C. Peroxidase
D. DNA polymerase
Answer: B
Rationale: Superoxide dismutase (SOD) converts superoxide radicals into molecular
oxygen and hydrogen peroxide.
4. In which phase of the bacterial growth curve is the population doubling at a
constant maximum rate?
A. Lag phase
B. Log phase
C. Stationary phase
D. Death phase
Answer: B
Rationale: The log (logarithmic) or exponential growth phase is when cellular
reproduction is most active and generation time reaches a constant minimum.
5. What is the primary purpose of using a selective medium?
A. To distinguish between different species of bacteria
B. To provide nutrients for all types of bacteria to grow
C. To suppress the growth of unwanted microbes and encourage the growth of desired ones
D. To determine the oxygen requirements of an organism
Answer: C
Rationale: Selective media contain ingredients that inhibit the growth of certain microbes
while allowing others to grow.
, 6. Which of the following is an example of an indirect method for measuring
microbial growth?
A. Filtration
B. Turbidity
C. Standard plate count
D. Direct microscopic count
Answer: B
Rationale: Turbidity uses a spectrophotometer to measure the cloudiness of a liquid
culture, which is an indirect estimate of cell mass.
7. If a bacterial cell is placed in a hypertonic solution, what will likely occur?
A. The cell will undergo plasmolysis
B. The cell will swell
C. The cell will burst (osmotic lysis)
D. Nothing will happen to the cell
Answer: A
Rationale: In a hypertonic solution, water leaves the cell, causing the cytoplasm to shrink
away from the cell wall, a process called plasmolysis.
8. Which metabolic pathway is common to both fermentation and aerobic
respiration?
A. Krebs Cycle
B. Electron Transport Chain
C. Glycolysis
D. Chemiosmosis
Answer: C
Rationale: Glycolysis is the initial stage of carbohydrate catabolism and does not require
oxygen, making it a precursor to both aerobic respiration and fermentation.
