Functional Anatomy of Prokaryotic & Eukaryotic Cells
9 sub-topics · Pages 55–112
1. Introduction
Prokaryotic and eukaryotic cells share fundamental biochemistry but differ profoundly in structural organisation. Prokaryotes lack membrane-bound organelles; their entire cellular machinery — DNA replication, transcription, translation, energy generation — occurs in the single cytoplasmic compartment. This simplicity enables rapid growth but limits functional specialisation compared to eukaryotes.
✏️ Fill in the Blank
1. The rigid polymer found in bacterial cell walls that provides structural support and shape is called _______.
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Peptidoglycan2. The region in prokaryotes where DNA is concentrated, but not enclosed by a membrane, is called the _______.
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Nucleoid🔘 Multiple Choice
1. The lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria is medically significant because:
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Correct: B) Its lipid A component acts as an endotoxin that triggers inflammatory responses and septic shock in humans💬 Open-Ended Questions
1. Explain the significance of the Gram stain in clinical microbiology. Why does the Gram reaction differ between Gram-positive and Gram-negative bacteria, and how does this affect antibiotic selection?
Hint / Guidance
Gram-positive: thick peptidoglycan (20–80 nm) retains crystal violet-iodine complex after decolourisation → purple. Gram-negative: thin peptidoglycan + outer membrane; decolouriser removes lipids from outer membrane allowing dye to wash out → red (safranin). Clinical use: Gram stain result (30 min) guides empirical antibiotic choice before culture results (24–48 h); penicillin/vancomycin target Gram-positive; broader-spectrum agents needed for Gram-negative due to outer membrane barrier.2. What is the function of bacterial endospores and what environmental conditions trigger sporulation? Describe the key steps in endospore formation.
Hint / Guidance
Function: dormant survival structure; resistant to heat, UV, desiccation, chemicals; can survive millions of years. Trigger: nutrient starvation activates Spo0A transcription factor. Steps: (1) axial filament formation; (2) asymmetric cell division creating forespore; (3) engulfment of forespore by mother cell; (4) cortex synthesis (modified peptidoglycan); (5) synthesis of spore coat (keratin-like proteins); (6) accumulation of dipicolinic acid + calcium; (7) maturation and lysis of mother cell. Germination triggered by specific nutrients.2. Morphology
Bacterial cell morphology — cocci (spheres), bacilli (rods), spirilla (helices), and vibrios (curved rods) — is determined by the cytoskeletal proteins that guide cell wall synthesis. Morphology is not merely academic: rod-shaped cells have a higher surface-to-volume ratio than cocci, favouring rapid nutrient acquisition in dilute environments.
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1. Prokaryotic cells divide asexually by a process called _______, in which the cell elongates and splits into two identical daughter cells.
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Binary fission🔘 Multiple Choice
1. In the Gram staining procedure, what is the correct order of steps?
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Correct: B) Crystal violet → iodine → decolouriser (ethanol/acetone) → safranin counterstain2. Which eukaryotic organelle is responsible for cellular respiration and ATP production?
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Correct: C) Mitochondrion💬 Open-Ended Questions
1. What is an endospore? Describe the conditions that trigger sporulation, the process of spore formation, and the mechanisms that make endospores extraordinarily resistant to heat, radiation, and chemicals.
Hint / Guidance
Endospore: dormant, non-reproductive structure formed by Bacillus, Clostridium etc. under nutrient starvation. Sporulation: asymmetric division, engulfment of forespore, cortex (modified peptidoglycan) synthesis, coat protein deposition, maturation. Resistance mechanisms: dipicolinic acid chelates water (dehydration); saturated core; acid-soluble proteins bind DNA and protect it; cortex prevents enzyme access; heat resistance linked to reduced water activity in core.2. Describe the fluid mosaic model of the cell membrane. How do membrane proteins contribute to cell function?
Hint / Guidance
Fluid mosaic: phospholipid bilayer with hydrophilic heads outward, hydrophobic tails inward; proteins 'floating' in the bilayer (lateral diffusion); Singer & Nicolson 1972. Integral proteins: span membrane (transport channels, pumps, receptors, enzymes). Peripheral proteins: attached to surface (cytoskeletal anchors, signalling). Functions: selective permeability, active/passive transport, signal transduction, cell-cell recognition, anchoring cytoskeleton.3. Bacterial Cell Shape
The peptidoglycan cell wall is a covalently cross-linked polymer of NAM-NAG sugar chains bridged by short peptides. It acts as a structural corset, preventing osmotic lysis. β-lactam antibiotics (penicillin, ampicillin) block the transpeptidases that form these cross-links — making the cell wall one of the most exploited antibiotic targets.
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1. The bacterial structure that enables movement by rotating like a propeller is called a _______.
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Flagellum🔘 Multiple Choice
1. Prokaryotic ribosomes are 70S, made up of 50S and 30S subunits. This difference from eukaryotic 80S ribosomes is clinically exploited because:
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Correct: B) Many antibiotics (e.g., streptomycin, erythromycin) selectively target 70S ribosomes without affecting eukaryotic 80S ribosomes2. The endosymbiotic theory proposes that mitochondria and chloroplasts originated from:
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Correct: B) Free-living prokaryotes engulfed by a host cell💬 Open-Ended Questions
1. Compare the cell envelope structure of Gram-positive and Gram-negative bacteria. How do these differences affect susceptibility to lysozyme and to antibiotics targeting cell wall synthesis?
Hint / Guidance
Gram-positive: thick peptidoglycan directly accessible; lysozyme cleaves β-1,4 glycosidic bond between NAM and NAG → rapid lysis; penicillin/vancomycin highly effective. Gram-negative: thin peptidoglycan shielded by outer membrane; LPS outer leaflet reduces lysozyme/antibiotic penetration; outer membrane porins limit drug entry; β-lactam antibiotics must traverse porin channels; outer membrane loss of porins → resistance mechanism.2. How do bacteria achieve motility and how is it regulated by chemical gradients (chemotaxis)?
Hint / Guidance
Motility: rotating flagellum driven by H⁺/Na⁺ gradient across basal body motor. CCW rotation → straight run; CW rotation → tumble/reorientation. Chemotaxis: methyl-accepting chemotaxis proteins (MCPs) sense attractants/repellents; signal transduction via CheA (kinase) / CheY (phosphorylated → causes tumbling) / CheB+CheR (adaptation by methylation). Net result: biased random walk toward attractants (e.g., glucose) and away from repellents (e.g., phenol).4. Flagella Arrangement
Flagella are rotating molecular motors driven by the proton-motive force across the plasma membrane. The arrangement of flagella — monotrichous, lophotrichous, amphitrichous, or peritrichous — determines swimming pattern and speed. Chemotaxis allows bacteria to bias their random-walk movement toward attractants and away from repellents, a fundamental survival behaviour.
✏️ Fill in the Blank
1. The region of a prokaryotic cell where the chromosome is located, though it is not bounded by a membrane, is called the _______.
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Nucleoid🔘 Multiple Choice
1. Which bacterial morphology is described as 'curved rod' or comma-shaped?
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Correct: C) Vibrio2. Which structure allows bacteria to adhere to surfaces and to each other during biofilm formation?
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Correct: B) Fimbriae/pili💬 Open-Ended Questions
1. A patient's wound is infected with a bacterium that tests positive for capsule production. How does the capsule contribute to virulence, and what does this mean for the patient's immune response and treatment?
Hint / Guidance
Capsule (polysaccharide or polypeptide) prevents phagocytosis by neutrophils and macrophages (anti-opsonisation); masks surface antigens reducing complement activation; resists desiccation in host. Immune response impaired — antibodies needed to opsonise capsule for effective phagocytosis (type-specific anti-capsular antibodies). Treatment: antibiotics must penetrate capsule to reach cell; capsule-targeted vaccines (e.g., pneumococcal vaccine) prevent initial infection.2. Explain why eukaryotic cells are generally larger than prokaryotic cells and how eukaryotes compensate for diffusion limitations.
Hint / Guidance
Diffusion: rate inversely proportional to distance; at larger diameters, diffusion of nutrients/waste is too slow for metabolic needs. Eukaryotic solutions: (1) endomembrane system increases reaction surface area; (2) cytoskeleton + motor proteins provide active intracellular transport; (3) compartmentalisation (organelles) concentrates reactions; (4) mitochondria bring ATP synthesis close to sites of use; (5) endocytosis brings extracellular material directly inside. Average prokaryote: 1–10 µm; eukaryote: 10–100 µm.5. Gram - versus Gram + Cell Walls
The critical difference between Gram-positive and Gram-negative cell walls goes beyond peptidoglycan thickness. Gram-positive walls contain teichoic acids that regulate cation balance and are important in pathogen-host interactions. Gram-negative walls include an outer membrane with LPS — a potent endotoxin that triggers the inflammatory cascade responsible for septic shock.
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1. Short, hair-like appendages on the bacterial cell surface used for adhesion to surfaces or host cells are called _______.
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Fimbriae (pili)🔘 Multiple Choice
1. A capsule surrounding a bacterial cell primarily functions to:
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Correct: B) Protect the cell from phagocytosis by host immune cells and from desiccation2. Gram-negative bacteria differ from Gram-positive bacteria in having:
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Correct: C) An outer membrane containing lipopolysaccharide💬 Open-Ended Questions
1. Describe the endosymbiotic theory for the origin of mitochondria and chloroplasts. What structural and genomic evidence supports this theory?
Hint / Guidance
Theory (Lynn Margulis): ancestral eukaryote engulfed α-proteobacterium (→ mitochondrion) and cyanobacterium (→ chloroplast) without digesting them. Evidence: both organelles have double membranes (inner = original bacterial membrane); contain their own circular DNA; 70S ribosomes sensitive to bacterial antibiotics; divide by binary fission; 16S-like rRNA in organelle genome closely related to α-proteobacteria (mitochondria) and Cyanobacteria (chloroplasts).6. Gram-Negative Outer Membrane
The Gram-negative outer membrane is a selective permeability barrier. Porins — trimeric β-barrel proteins — allow passage of small hydrophilic molecules (sugars, amino acids) while excluding large hydrophobic molecules, including many antibiotics. Reduced porin expression is a major resistance mechanism in clinical pathogens such as Pseudomonas aeruginosa and Klebsiella pneumoniae.
🔘 Multiple Choice
1. Which structure is present in prokaryotic cells but absent in eukaryotic cells?
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Correct: C) Nucleoid region (no membrane-bound nucleus)2. Which eukaryotic organelle is believed to have originated from an endosymbiotic α-proteobacterium?
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Correct: C) Mitochondrion3. Which of the following is a function of the bacterial capsule?
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Correct: C) Protection from phagocytosis and desiccation💬 Open-Ended Questions
1. Explain how the structure of the bacterial flagellum allows it to rotate and drive motility. How does chemotaxis work at the molecular level?
Hint / Guidance
Flagellum: hollow filament (flagellin protein) attached via hook to basal body embedded in membrane. Basal body contains MS ring (inner membrane), C ring, L and P rings (outer membrane). Rotation powered by proton flow through MotA/MotB stator complexes (proton motive force). Chemotaxis: methyl-accepting chemoreceptors detect attractants/repellents → signal transduction via CheA kinase and CheY phosphorylation → CheY-P binds flagellar switch → tumbling (counterclockwise → clockwise rotation, disperses flagella); absence of CheY-P → smooth swimming. Gradient sensing by receptor adaptation via methylation.7. Plasma Membrane
The plasma membrane is the universal cellular boundary — a fluid phospholipid bilayer studded with transport proteins, receptors, and, in prokaryotes, the electron transport chain components. It is the site of oxidative phosphorylation, lipid synthesis, and secretion systems. Many antimicrobials (polymyxins, daptomycin) target the bacterial plasma membrane, disrupting its integrity and killing the cell rapidly.
🔘 Multiple Choice
1. Gram-negative bacteria are generally more resistant to certain antibiotics because:
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Correct: B) They possess an outer membrane containing lipopolysaccharide (LPS) that acts as a barrier2. Penicillin kills bacteria by:
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Correct: C) Inhibiting peptidoglycan cross-linking💬 Open-Ended Questions
1. List and explain at least four structural differences between prokaryotic and eukaryotic cells. For each difference, describe its functional significance.
Hint / Guidance
(1) Nucleus: eukaryotes have membrane-bound nucleus separating transcription from translation; prokaryotes have nucleoid allowing simultaneous transcription-translation. (2) Organelles: eukaryotes have mitochondria, ER, Golgi; prokaryotes lack them — all metabolism at plasma membrane/cytoplasm. (3) Cell wall: prokaryotes have peptidoglycan (antibiotic target); eukaryotes have chitin (fungi) or cellulose (plants). (4) Ribosomes: 70S vs 80S — basis for selective antibiotic targeting. (5) Cell size: prokaryotes 0.2–10 µm, eukaryotes 10–100 µm.2. A researcher observes that two morphologically identical spherical bacteria behave very differently in Gram staining, antibiotic susceptibility tests, and clinical infections. How can you reconcile this observation, and what molecular techniques would you use to definitively identify each?
Hint / Guidance
Morphological similarity does not equal relatedness (convergent evolution). Gram staining difference: one Gram-positive, one Gram-negative — different cell wall architecture. Antibiotic susceptibility: Gram-positive susceptible to penicillin; Gram-negative may be resistant due to outer membrane. Molecular identification: 16S rRNA gene sequencing for species-level ID; MALDI-TOF mass spectrometry for rapid clinical ID; whole-genome sequencing for complete characterisation including virulence genes and resistance determinants.8. Ribosomes
Prokaryotic ribosomes (70S, composed of 50S and 30S subunits) are the molecular machines of protein synthesis. Their structural differences from eukaryotic 80S ribosomes are exploited by a wide range of clinically important antibiotics: tetracyclines and aminoglycosides bind the 30S subunit; macrolides, chloramphenicol, and lincosamides bind the 50S subunit — all selectively inhibiting bacterial (not host cell) translation.
✏️ Fill in the Blank
1. The plasma membrane is primarily composed of a phospholipid _______ with embedded proteins.
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Bilayer🔘 Multiple Choice
1. The high surface-to-volume ratio of bacteria is significant because it:
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Correct: B) Allows rapid exchange of nutrients and waste with the environment💬 Open-Ended Questions
1. A bacterium is placed in a hypotonic solution after its cell wall is enzymatically removed (creating a protoplast). Predict and explain what happens to this cell.
Hint / Guidance
Water enters by osmosis (concentration gradient drives water into cell with higher solute concentration inside); protoplast swells; without rigid peptidoglycan wall to resist turgor pressure, the cell membrane ruptures (osmotic lysis). In intact bacteria, the cell wall prevents lysis by withstanding internal osmotic pressure of ~2–5 atm.2. Compare the structure and function of prokaryotic and eukaryotic flagella. Are they evolutionarily related?
Hint / Guidance
Prokaryotic flagella: hollow protein filament (flagellin) rotated by a proton-motive force-driven basal body motor; 20 nm diameter; chemotaxis. Eukaryotic flagella: '9+2' microtubule axoneme (tubulin) powered by dynein ATPase; 200 nm diameter; sinusoidal beating. Not homologous — different proteins, different mechanism, convergent evolution for motility.9. Eukaryotic Cells
Eukaryotic microorganisms — fungi, protozoa, and algae — are distinguished by compartmentalisation: a nucleus houses the genome, mitochondria perform aerobic respiration, the ER and Golgi process proteins, and lysosomes recycle cellular components. The endosymbiotic origin of mitochondria and chloroplasts from ancient prokaryotes is supported by their own circular DNA, 70S ribosomes, and binary fission.
✏️ Fill in the Blank
1. Bacterial ribosomes have a sedimentation coefficient of _______ S.
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70🔘 Multiple Choice
1. Which statement about bacterial endospores is correct?
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Correct: B) Endospores are highly resistant dormant structures formed under nutrient starvation that can survive extreme conditions💬 Open-Ended Questions
1. Describe three mechanisms bacteria use for motility. For each, explain the structural basis and the ecological advantage it provides.
Hint / Guidance
(1) Flagella: rotating protein filament powered by proton motive force; enables chemotaxis toward nutrients and away from toxins. (2) Type IV pili (twitching): pilus extension, attachment, retraction; colonisation of surfaces, biofilm formation. (3) Gliding motility: movement along surfaces without flagella (mechanism involves focal adhesion complexes); allows navigation through soil particles and along surfaces.2. Explain the significance of the LPS (lipopolysaccharide) layer in Gram-negative bacteria from both a structural and immunological perspective.