Which of the following organisms has sterols in its cytoplasmic membrane?
Answer: C
Correct answer: Mycoplasma
Mycoplasma is the exception among bacteria: having no cell wall, it strengthens its membrane with sterols taken up from the host. That is why it needs a sterol-containing medium to grow, and why it is naturally resistant to penicillin.
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In prokaryotes the genetic material is located in the
Answer: D
Correct answer: all of these
Prokaryotes have no nucleus. The single circular chromosome lies in an irregular region of the cytoplasm called the nucleoid, sometimes also referred to as the nuclear material or the bacterial chromatin body - three names for the same thing, since there is no membrane to define it more precisely.
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Peptidoglycan is found only in the bacterial
Answer: C
Correct answer: cell wall
Peptidoglycan is a structural polymer found only in the bacterial cell wall, where its cross-linked mesh resists the cell’s internal turgor pressure. The membrane is lipid, the glycocalyx is capsule and slime material, and the spore has its own distinctive cortex - which does contain a modified peptidoglycan, but the wall is where the polymer belongs.
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The outer membrane of the Gram-negative cell wall is anchored to the underlying peptidoglycan by means of which of the following?
Answer: A
Correct answer: Braun’s Lipoprotein
Braun’s lipoprotein is the tether: its lipid end is embedded in the inner leaflet of the outer membrane and its protein end is covalently bonded to the peptidoglycan. Without it the outer membrane would not be held in place, and the cell would shed blebs of it.
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Porins are special proteins that act as channels in the outer membrane of gram-negative bacteria.
Answer: A
Correct answer: True
Porins are trimeric proteins forming water-filled channels through the outer membrane, admitting small hydrophilic molecules up to about 600 daltons. They are what make the outer membrane a selective barrier - and their size limit is one reason large antibiotics are so often useless against gram-negative bacteria.
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The location where the bacterial chromosome concentrates is called
Answer: C
Correct answer: nucleoid
The bacterial chromosome is concentrated in an irregular region called the nucleoid. It is not a nucleus: there is no membrane around it, so transcription and translation can proceed together.
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The bacterium that lacks a cell wall is
Answer: B
Correct answer: Mycoplasma
Mycoplasma has no cell wall - its membrane is strengthened with sterols instead - so it is pleomorphic and naturally resistant to penicillin and every other drug that attacks wall synthesis. Treponema, Staphylococcus and Klebsiella all have walls.
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Which of the following bacteria lack a cell wall and are therefore resistant to penicillin?
Answer: B
Correct answer: Mycoplasmas
Penicillin works by blocking the cross-linking of peptidoglycan, so it can only kill an organism that is building a cell wall. Mycoplasmas have no cell wall at all - only a sterol-strengthened membrane - so there is nothing for the drug to act on. Spirochaetes, cyanobacteria and bdellovibrios all have walls.
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Which of the following may contain fimbriae?
Answer: B
Correct answer: Gram-negative bacteria
Fimbriae are short hair-like appendages used for attachment, and they are characteristic of gram-negative bacteria, where they are anchored in the outer membrane. They are not organs of motility - that is the flagellum - but they are important in colonisation and virulence.
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Porins are located in
Answer: A
Correct answer: the outer membrane of gram-negative bacteria
Porins are trimeric barrel proteins spanning the outer membrane, forming water-filled channels through which small hydrophilic molecules diffuse. They are the reason the outer membrane is a selective barrier rather than an impermeable one - and why large antibiotics, which cannot pass the pores, are so often ineffective against gram-negative bacteria.
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