Questions: Bacterial Cell Structure

Question 1

Which of the following best describes what the Gram stain reveals about bacterial cell structure?

• A. Whether the bacterium has flagella for motility
• B. The thickness of the peptidoglycan layer and presence of an outer membrane
• C. Whether the bacterium carries plasmids
• D. The number of ribosomes in the cell

Answer: (B)

The Gram stain differentiates bacteria based on cell wall architecture. Gram-positive bacteria retain the crystal violet dye because of their thick peptidoglycan layer; Gram-negative bacteria do not, because their thin peptidoglycan is sandwiched between two membranes. Flagella, plasmids, and ribosome count are not detected by this stain.

Question 2

Most bacteria have flagella that allow them to move toward nutrients and away from toxins.

• T. True
• F. False

Answer: False

Flagella are present only in motile species. Many bacteria are entirely non-motile and lack flagella altogether. Even among species capable of producing flagella, expression can vary by environmental condition. Motility is a useful trait but not a universal feature of prokaryotes.

Question 3

What is the functional difference between the bacterial cell wall and the cell membrane?

Model answer: The cell wall provides structural rigidity and protects against osmotic lysis; the cell membrane (plasma membrane) controls selective transport of molecules into and out of the cell.

These are two distinct layers with different roles. The peptidoglycan wall is a structural scaffold — it gives the bacterium its shape and resists bursting under osmotic pressure. The lipid bilayer membrane beneath it is the true permeability barrier, regulating what enters (nutrients) and exits (waste, secreted proteins) via channels, pumps, and passive diffusion.