Questions: Prokaryotic Cell Structural Features and Organization

Question 1

A photosynthetic bacterium must concentrate its photosynthetic machinery into a high-density region to function efficiently. Without membrane-bound organelles, how does it accomplish this?

• A. It cannot — photosynthesis is only possible in cells with membrane-bound chloroplasts
• B. It invaginates its inner membrane into extensive internal sheets that house the photosynthetic proteins
• C. It assembles the photosynthetic machinery in the nucleoid region alongside DNA
• D. It expels photosynthetic enzymes into the periplasmic space between membranes

Answer: (B)

Photosynthetic bacteria such as cyanobacteria fold their inner membrane into elaborate internal structures (analogous to thylakoids) that concentrate the photosynthetic machinery. This is membrane invagination rather than membrane-bounded compartmentalization. The key insight is that prokaryotes achieve metabolic specialization not by building separate organelles but by remodeling the plasma membrane itself. Some bacteria also use protein-shell microcompartments (like carboxysomes) for other metabolic functions.

Question 2

What does the Gram stain test reveal about bacterial cell structure?

• A. Whether the bacterium has a nucleus
• B. The size and shape (morphology) of the bacterial cell
• C. Whether the bacterium has a thick peptidoglycan wall (Gram-positive) or a thin wall sandwiched between two membranes (Gram-negative)
• D. The presence or absence of flagella on the cell surface

Answer: (C)

The Gram stain distinguishes two fundamental wall architectures. Gram-positive bacteria retain the crystal violet dye because their thick peptidoglycan layer (20–80 nm) traps it. Gram-negative bacteria lose the dye because their thin peptidoglycan is sandwiched between an inner and outer membrane — the outer membrane is disrupted by the decolorizer and the dye washes out, leaving them pink from the safranin counterstain. This architectural difference has profound implications for antibiotic susceptibility and immune recognition.

Question 3

Prokaryotic cells have no internal organization because they lack membrane-bound organelles.

• T. True
• F. False

Answer: False

This is a fundamental misconception. 'Lacking membrane-bound organelles' and 'lacking organization' are not the same thing. Prokaryotes achieve sophisticated internal organization through multiple strategies: the nucleoid (a defined chromosomal region organized by supercoiling and DNA-binding proteins), protein-shelled microcompartments (like carboxysomes), membrane invaginations for metabolic specialization, inclusion bodies for nutrient storage, and the cytoskeletal proteins that determine cell shape. These cells are highly organized — just not via lipid-membrane compartmentalization.

Question 4

The 70S ribosomes of prokaryotic cells are smaller than the 80S ribosomes found in eukaryotic cytoplasm.

• T. True
• F. False

Answer: True

Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits) while eukaryotic cytoplasmic ribosomes are 80S (60S and 40S subunits). The S stands for Svedberg units, a measure of sedimentation rate that reflects size and shape. This difference is clinically significant: many antibiotics (such as streptomycin, erythromycin, and tetracycline) target the 70S ribosome specifically, disrupting bacterial protein synthesis without affecting the host's 80S ribosomes.

Question 5

How do prokaryotic cells create functionally distinct internal spaces without using lipid-membrane-enclosed organelles?

Model answer: Prokaryotes use at least three strategies to achieve compartmentalization without lipid membranes. First, membrane invagination: photosynthetic and chemosynthetic bacteria fold the inner membrane into internal sheets that concentrate specific enzyme systems. Second, protein-shell microcompartments: polyhedral structures enclosed by a protein (not lipid) shell that sequester metabolic pathways — carboxysomes house carbon fixation enzymes. Third, nucleoid organization: DNA-binding proteins and supercoiling compact the chromosome into a defined nucleoid region, effectively separating genetic material from the ribosome-rich cytoplasm even without a nuclear membrane.

This question targets the core insight: compartmentalization is a functional concept, not a structural one. What matters is concentrating specific molecules or reactions together and separating them from competing reactions. Lipid membranes are one solution; protein shells and membrane geometry are others. Prokaryotes evolved these solutions independently from, and in some cases before, the eukaryotic membrane-organelle system.