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Gram-negative Bacteria: Structure, Entry Routes and Endotoxin Quiz
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Which feature of the cell envelope most clearly distinguishes Gram-negative bacteria from Gram-positive bacteria?
- A One phospholipid bilayer without an outer membrane or porin system You picked ✕
- B One membrane beneath a thick peptidoglycan wall You picked ✕
- C A thick polysaccharide capsule replacing both membranes You picked ✕
- D A thick wall containing teichoic and lipoteichoic acids You picked ✕
- E Two membranes with thin peptidoglycan between them You picked ✓
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Incorrect
The answer is E: Two membranes with thin peptidoglycan between them
Explanation
Why E is correct
See The Two-Membrane Envelope. Option E is correct. The envelope is the set of layers that surrounds a bacterial cell. Gram-negative bacteria have an inner (cytoplasmic) membrane, then a periplasmic space (a gel-like, watery compartment) containing a thin peptidoglycan layer, then an outer membrane. Peptidoglycan is the mesh of sugar chains, cross-linked by short peptides, that gives the cell its shape and strength. The outer membrane is asymmetric: its outer layer is rich in lipopolysaccharide (LPS, a large sugar-and-fat molecule) and its inner layer is more phospholipid-like. Porins (protein channels) let small water-soluble molecules through but restrict larger or fat-soluble ones, so the outer membrane is a selective barrier that can keep some antibiotics out. Gram-positive bacteria follow the opposite plan: one membrane covered by a thick peptidoglycan wall. A memory aid: Gram-negative is a two-membrane sandwich with a thin wall as the filling, while Gram-positive is one membrane wearing a thick coat.
- Layers: inner membrane, then a periplasmic space with thin peptidoglycan, then an outer membrane.
- Outer membrane: asymmetric, rich in LPS in its outer layer, and a selective barrier because of its porins.
- Contrast: Gram-positive bacteria have one membrane covered by a thick peptidoglycan wall.
Why the other options are incorrect
- A. One phospholipid bilayer without an outer membrane or porin system: A single bilayer without an outer membrane or its porins lacks the second membrane that defines Gram-negative bacteria. The option also says nothing about a peptidoglycan wall, which Gram-negative bacteria do have. Your answer
- B. One membrane beneath a thick peptidoglycan wall: One membrane beneath a thick peptidoglycan wall is the usual Gram-positive envelope. Gram-negative cells have an additional outer membrane and a much thinner peptidoglycan layer. Your answer
- C. A thick polysaccharide capsule replacing both membranes: A capsule is a separate, optional coat outside the cell wall, usually made of sugars. It cannot replace the membranes that enclose the bacterium. Your answer
- D. A thick wall containing teichoic and lipoteichoic acids: Teichoic and lipoteichoic acids are polymers, or chains of repeating units, that are major components of the thick Gram-positive wall. They do not describe the Gram-negative outer membrane. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which part of lipopolysaccharide (LPS) is chiefly responsible for its endotoxin activity?
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2
A Gram-negative bacterium changes the sugar repeats of its LPS O antigen but leaves lipid A unchanged. Which statement best describes the different roles of these two components?
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