MCAT Biology : Bacteria and Types of Prokaryotes

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #21 : Bacteria And Types Of Prokaryotes

An unknown cell type has been isolated. It is suspected that this cell is prokaryotic. The presence of which of these cell structures would confirm that the cell is prokaryotic?

Possible Answers:

Flagellum

Spores

Ribosomes

Peptidoglycan cell wall

Cytoplasm

Correct answer:

Peptidoglycan cell wall

Explanation:

Cytoplasm is found in all types of cells. Flagella is found in all cells that are mobile. Spores can be found in bacteria but also fungi. Ribosomes are found in prokaryotic cells albeit smaller in size compared to those in eukaryotic cells. 

Example Question #61 : Microbiology

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

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A scientist is using a Gram stain to investigate Neisseria. If Neisseria is bright pink upon examination after staining, which of the following is likely true?

Possible Answers:

Neisseria is Gram negative, and has abundant peptidoglycan on its surface

Neisseria is Gram positive, and has abundant peptidoglycan on its surface

Neisseria only has a cell wall, and no underlying cell membrane

Neisseria is Gram positive, and has reduced peptidoglycan on its surface

Neisseria is Gram negative, and has reduced peptidoglycan on its surface

Correct answer:

Neisseria is Gram negative, and has reduced peptidoglycan on its surface

Explanation:

Bright pink cells under Gram stain are characteristic of Gram negative status. This is associated with an outer lipid membrane and reduced peptidoglycan. The reduction in peptidoglycan is what leads to a failure to retain the Gram stain, which would have colored the cells purple.

Example Question #1022 : Mcat Biological Sciences

Gram staining prepares bacteria to be visualized under light microscope.

What allows a bacteria to stain positively with gram stain?

Possible Answers:

The bacteria's thick peptidoglycan cell walls

The bacterial sample was pretreated with 3% ethanol

The bacteria is anaerobic

The bacteria's periplasmic space

Correct answer:

The bacteria's thick peptidoglycan cell walls

Explanation:

Gram-positive bacteria are able to retain the crystal violet stain found in gram stain because of their thick peptidoglycan cell walls. Consequently, these bacteria stain dark blue or purple by gram staining. Alternatively, gram-negative bacteria appear red or pink because they are unable to retain the dye due to thinner cell walls and less peptidoglycan.

Example Question #61 : Microbiology

Which of the following cell types does not have a cell wall?

Possible Answers:

Plant cell

Gram negative bacteria

Trichocyte (forms fur and nails)

Fungal cell

Gram positive bacteria

Correct answer:

Trichocyte (forms fur and nails)

Explanation:

No animal cells will have cell walls. Trichocytes are specialized to be hard and resilient, but do not have cell walls.

Fungi, plants, and bacteria do use cell walls. The distinction between gram positive and gram negative bacteria refers to the composition of the cell wall only; both positive and negative bacteria have a cell wall present.

Example Question #1001 : Biology

Which of the following is found in bacterial cells, but not in mature red blood cells?

Possible Answers:

Cell membrane

Mitochondria

Nucleus

DNA

Correct answer:

DNA

Explanation:

Bacterial cells are prokaryotic and lack all membrane-bound organelles. We can thus eliminate mitochondria and nucleus as answers. These structures are not found in bacterial cells or in mature erythrocytes.

All cells, regardless of origin, are surrounded by a cell membrane. This structure is present in both prokaryotes and erythrocytes.

DNA is used to code for the genome of the organism and is found in both prokaryotes and eukaryotes. Bacteria contain a cytsolic genome known as the nucleoid. Red blood cells, however, lack DNA. This is partly due to their lack of a nucleus, and is considered evolutionarily advantageous because no viruses can target red blood cells.

Example Question #61 : Microbiology

Slides containing bacteria are stained with crystal violet and iodine, destained with alcohol, and counterstained with safranin. Under the mircoscope, the bacteria appear a bluish-purple color. Which class of bacteria are on the slide?

Possible Answers:

Helicobacter

Gram-positive bacteria

Mycobacteria

Gram-negative bacteria

Correct answer:

Gram-positive bacteria

Explanation:

The technique explained in the question is the "Gram stain". Bacteria that are Gram-positive will stain a bluish-purple color, while Gram-negative bacteria will stain a red or pink color. This difference in color occurs because of the large cell wall present on Gram-positive bacteria, and is a response to increased peptidoglycan expression. Additionally, some bacteria (such as mycobacteria and helicobacter) are not easily stained using this method.

Example Question #61 : Microbiology

What is the shape of a streptobacillus bacterium?

Possible Answers:

Rigid helix

Non-rigid helix

Round

Rod-shaped

Correct answer:

Rod-shaped

Explanation:

Bacilli are rod-shaped bacteria and "strepto" indicates organization in a chain. Streptobacillus bacteria are a class of bacteria that are rod-like in shape and tend to congregate in chains of cells.

Cocci are spherically shaped, and spirilli are rigid helices. Spirochetes are non-rigid helices.

Example Question #21 : Bacteria And Types Of Prokaryotes

A scientist discovers a new bacteria growing on a plant in his house. He brings it into the lab to study its growth. He grows the new bacteria alongside a known obligate anaerobic bacteria, and sees that after an hour, the new bacteria has not grown as much as the anaerobic bacteria. What classification most likely describes the type of the new bacteria?

Possible Answers:

Aerobe

Microaerobic

Obligate aerobe

Obligate anaerobe

Facultative anaerobe

Correct answer:

Facultative anaerobe

Explanation:

Since the obligate anaerobe grows well, we know that both bacteria are grown in an anaerobic environment.

A facultative anaerobe can grow in an aerobic or anaerobic environment. It simply grows better when oxygen is present; therefore, we would see slower growth from a facultative anaerobe than an obligate anaerobe when there is no oxygen present. If the bacteria were aerobic, it would not grow at all in the anaerobic environment.

Example Question #71 : Microbiology

An unidentified class of bacteria are placed into a test tube containing liquid and set on top of a lab bench. After the liquid and bacteria has a chance to settle, the bacteria are found distributed throughout the liquid (as opposed to located primarily at the top or bottom of the liquid). What type of bacteria is in the tube?

Possible Answers:

Facultative anaerobic bacteria

Obligate anaerobic bacteria

Obligate aerobic bacteria

Microaerophile

Correct answer:

Facultative anaerobic bacteria

Explanation:

In order for the bacteria to be distributed throughout the liquid, it must be able to survive in either the presence or absence of oxygen. Facultative anaerobic bacteria can surive in both cases, although, they prefer oxygen. In the absence of oxygen, they can produce energy through fermentation. Both microaerophiles and obligate aerobic bacteria require oxygen to survive and, in this experiment, would be found near the top of the tube. Oxygen is toxic to obligate anaerobic bacteria, which would be found at the bottom.

Example Question #71 : Microbiology

Which of the following produces ATP without utilizing oxygen, even when oxygen is available?

Possible Answers:

Obligate aerobes

Microaerophiles

Aerotolerant organisms

Facultative anaerobes

Obligate anaerobes

Correct answer:

Aerotolerant organisms

Explanation:

Anaerobic environments lack oxygen, while aerobic environments contain oxygen. Generally, aerobic respiration is more energetically efficient than anaerobic respiration mechanisms. Aerotolerant organisms, however, will continue to use anaerobic means of energy production, even in the presence of oxygen. These are a special class of organism that can survive in either an aerobic or anaerobic environment.

Obligate anaerobes require an anaerobic environment and find oxygen to be toxic. These organisms will not be able to produce ATP in the presence of oxygen. Obligate aerobes require the presence of oxygen in order to survive and cannot produce ATP without oxygen utilization. Facultative anaerobes can switch between metabolism mechanisms, using cellular respiration in the presence of oxygen and fermentation or anaerobic respiration in its absence. Microaerophiles are aerobic organisms, but require a particularly low level of oxygen in order to survive.

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