OPENSTAX MICROBIOLOGY – EXAM QUESTIONS AND CORRECT
ANSWERS (VERIFIED ANSWERS) PLUS RATIONALE | 2027 Q&A
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1. A microbiology student is asked to distinguish cellular organisms from viruses.
Which characteristic is shared by bacteria and eukaryotic cells but not viruses?
A. Ability to contain genetic material
B. Ability to undergo mutation
C. Presence of a cellular structure bounded by a membrane
D. Ability to evolve over generations
Rationale: Cellular organisms have a membrane-bound cellular structure and
metabolic machinery, whereas viruses are acellular entities that depend on host
cells for replication. Both bacteria and eukaryotic cells contain genetic material,
mutate, and evolve, so those characteristics do not distinguish them from
viruses.
2. Which observation provides the strongest evidence that a newly discovered
microorganism is prokaryotic?
A. It contains DNA and ribosomes
, B. Its genetic material is not enclosed within a membrane-bound nucleus
C. It reproduces by cell division
D. It possesses a plasma membrane
Rationale: Prokaryotic cells lack a membrane-bound nucleus, so their DNA occupies
a nucleoid region. DNA, ribosomes, plasma membranes, and cell division occur
in both prokaryotic and eukaryotic organisms.
3. A researcher wants to observe the overall shape and arrangement of bacterial
cells in a living preparation without applying a stain. Which microscopy method is
particularly useful?
A. Bright-field microscopy using appropriate illumination and contrast
techniques
B. Transmission electron microscopy
C. Scanning electron microscopy
D. X-ray crystallography
,Rationale: Bright-field microscopy can be used to examine living cells, although
unstained bacteria often have poor contrast. Electron microscopy generally
requires specialized preparation and is not used for routine observation of living
bacterial cells.
4. A bacterial species appears as spherical cells arranged in long chains. Which
description is most appropriate?
A. Staphylococci
B. Diplococci
C. Streptobacilli
D. Streptococci
Rationale: Cocci are spherical bacterial cells, while strepto- describes a chain
arrangement. Staphylococci form clusters, diplococci occur in pairs, and bacilli
are rod-shaped.
5. During a Gram stain, a bacterium remains purple after the decolorization step.
What does this result most strongly indicate?
A. The organism lacks a cell wall.
B. The organism has an outer membrane rich in lipopolysaccharide.
, C. The organism has a thick peptidoglycan layer that retains the crystal
violet–iodine complex.
D. The organism contains unusually large amounts of cytoplasmic DNA.
Rationale: Gram-positive bacteria have a thick peptidoglycan cell wall that retains
the crystal violet–iodine complex during decolorization. Gram-negative bacteria
have a thinner peptidoglycan layer and generally lose the primary stain during
decolorization.
6. Which cellular structure is primarily responsible for selectively regulating
movement of substances into and out of a bacterial cell?
A. Capsule
B. Plasma membrane
C. Ribosome
D. Nucleoid
Rationale: The plasma membrane forms a selective barrier controlling transport
between the cytoplasm and environment. The capsule provides external
ANSWERS (VERIFIED ANSWERS) PLUS RATIONALE | 2027 Q&A
| INSTANT DOWNLOAD PDF
1. A microbiology student is asked to distinguish cellular organisms from viruses.
Which characteristic is shared by bacteria and eukaryotic cells but not viruses?
A. Ability to contain genetic material
B. Ability to undergo mutation
C. Presence of a cellular structure bounded by a membrane
D. Ability to evolve over generations
Rationale: Cellular organisms have a membrane-bound cellular structure and
metabolic machinery, whereas viruses are acellular entities that depend on host
cells for replication. Both bacteria and eukaryotic cells contain genetic material,
mutate, and evolve, so those characteristics do not distinguish them from
viruses.
2. Which observation provides the strongest evidence that a newly discovered
microorganism is prokaryotic?
A. It contains DNA and ribosomes
, B. Its genetic material is not enclosed within a membrane-bound nucleus
C. It reproduces by cell division
D. It possesses a plasma membrane
Rationale: Prokaryotic cells lack a membrane-bound nucleus, so their DNA occupies
a nucleoid region. DNA, ribosomes, plasma membranes, and cell division occur
in both prokaryotic and eukaryotic organisms.
3. A researcher wants to observe the overall shape and arrangement of bacterial
cells in a living preparation without applying a stain. Which microscopy method is
particularly useful?
A. Bright-field microscopy using appropriate illumination and contrast
techniques
B. Transmission electron microscopy
C. Scanning electron microscopy
D. X-ray crystallography
,Rationale: Bright-field microscopy can be used to examine living cells, although
unstained bacteria often have poor contrast. Electron microscopy generally
requires specialized preparation and is not used for routine observation of living
bacterial cells.
4. A bacterial species appears as spherical cells arranged in long chains. Which
description is most appropriate?
A. Staphylococci
B. Diplococci
C. Streptobacilli
D. Streptococci
Rationale: Cocci are spherical bacterial cells, while strepto- describes a chain
arrangement. Staphylococci form clusters, diplococci occur in pairs, and bacilli
are rod-shaped.
5. During a Gram stain, a bacterium remains purple after the decolorization step.
What does this result most strongly indicate?
A. The organism lacks a cell wall.
B. The organism has an outer membrane rich in lipopolysaccharide.
, C. The organism has a thick peptidoglycan layer that retains the crystal
violet–iodine complex.
D. The organism contains unusually large amounts of cytoplasmic DNA.
Rationale: Gram-positive bacteria have a thick peptidoglycan cell wall that retains
the crystal violet–iodine complex during decolorization. Gram-negative bacteria
have a thinner peptidoglycan layer and generally lose the primary stain during
decolorization.
6. Which cellular structure is primarily responsible for selectively regulating
movement of substances into and out of a bacterial cell?
A. Capsule
B. Plasma membrane
C. Ribosome
D. Nucleoid
Rationale: The plasma membrane forms a selective barrier controlling transport
between the cytoplasm and environment. The capsule provides external
