BIOD 171 Module 3: Exam Portage Learning Questions and
Answers 2026 Practice Set
1. Which metric unit is commonly used in microbiology to measure the size of
bacterial cells?
A) Nanometer (nm)
B) Micrometer (
m)
C) Millimeter (mm)
D) Kilometer (km)
Rationale: Most bacteria are between 1–10
m in size, making the micrometer the standard unit of measurement. Viruses, which
are much smaller, are typically measured in nanometers.
2. Resolution is best defined as the:
A) Ability to make an object appear larger
B) Ability to distinguish two points that are close together as separate
C) Difference in light absorbance between a specimen and the background
D) Ability to see the internal organelles of a virus
Rationale: Resolution determines the clarity of an image; if resolution is poor, two
distinct objects will appear as a single blurred image.
3. What is the total magnification of a specimen viewed with a 10x ocular lens and
a 40x objective lens?
A) 50x
B) 100x
C) 400x
D) 4000x
Rationale: Total magnification is calculated by multiplying the ocular lens power by the
objective lens power (10
40 = 400).
CliffsNotes +1
, 4. Which type of microscopy is most appropriate for viewing the swimming
motility of a living, unstained microalgae cell?
A) Bright-field
B) Phase-contrast
C) Transmission Electron (TEM)
D) Fluorescence
Rationale: Phase-contrast microscopy amplifies slight differences in light refraction,
allowing for the visualization of live, transparent samples without the need for heat-fixing
or staining.
5. What is the primary purpose of "heat-fixing" a bacterial smear before staining?
A) To provide a dark background for contrast
B) To make the bacteria move faster
C) To adhere the cells to the slide so they aren't washed away during staining
D) To make the cell wall more permeable to oxygen
Rationale: Heat-fixing kills the bacteria and firmly attaches them to the glass surface,
ensuring the sample remains intact through multiple washing steps.
6. In the Gram stain procedure, which reagent acts as the "mordant"?
A) Crystal Violet
B) Gram's Iodine
C) 95% Ethanol
D) Safranin
Rationale: Iodine acts as a mordant by forming a complex with crystal violet, effectively
"trapping" the dye within the thick peptidoglycan layer of Gram-positive cells.
Milne Publishing +3
7. After a successful Gram stain, Gram-negative bacteria will appear _____ due to
the _____ counterstain.
A) Purple; Crystal Violet
B) Pink/Red; Safranin
C) Green; Malachite Green
D) Blue; Methylene Blue
Answers 2026 Practice Set
1. Which metric unit is commonly used in microbiology to measure the size of
bacterial cells?
A) Nanometer (nm)
B) Micrometer (
m)
C) Millimeter (mm)
D) Kilometer (km)
Rationale: Most bacteria are between 1–10
m in size, making the micrometer the standard unit of measurement. Viruses, which
are much smaller, are typically measured in nanometers.
2. Resolution is best defined as the:
A) Ability to make an object appear larger
B) Ability to distinguish two points that are close together as separate
C) Difference in light absorbance between a specimen and the background
D) Ability to see the internal organelles of a virus
Rationale: Resolution determines the clarity of an image; if resolution is poor, two
distinct objects will appear as a single blurred image.
3. What is the total magnification of a specimen viewed with a 10x ocular lens and
a 40x objective lens?
A) 50x
B) 100x
C) 400x
D) 4000x
Rationale: Total magnification is calculated by multiplying the ocular lens power by the
objective lens power (10
40 = 400).
CliffsNotes +1
, 4. Which type of microscopy is most appropriate for viewing the swimming
motility of a living, unstained microalgae cell?
A) Bright-field
B) Phase-contrast
C) Transmission Electron (TEM)
D) Fluorescence
Rationale: Phase-contrast microscopy amplifies slight differences in light refraction,
allowing for the visualization of live, transparent samples without the need for heat-fixing
or staining.
5. What is the primary purpose of "heat-fixing" a bacterial smear before staining?
A) To provide a dark background for contrast
B) To make the bacteria move faster
C) To adhere the cells to the slide so they aren't washed away during staining
D) To make the cell wall more permeable to oxygen
Rationale: Heat-fixing kills the bacteria and firmly attaches them to the glass surface,
ensuring the sample remains intact through multiple washing steps.
6. In the Gram stain procedure, which reagent acts as the "mordant"?
A) Crystal Violet
B) Gram's Iodine
C) 95% Ethanol
D) Safranin
Rationale: Iodine acts as a mordant by forming a complex with crystal violet, effectively
"trapping" the dye within the thick peptidoglycan layer of Gram-positive cells.
Milne Publishing +3
7. After a successful Gram stain, Gram-negative bacteria will appear _____ due to
the _____ counterstain.
A) Purple; Crystal Violet
B) Pink/Red; Safranin
C) Green; Malachite Green
D) Blue; Methylene Blue
