Microbiology: Cell Structure & Function 3 Flashcards

Characteristics of Life and Their Distribution in Microbes

• Growth: Increase in size

  • Bacteria, Archaea, Eukaryotes: Occurs in all three groups.

  • Viruses: Growth does not occur.

• Reproduction: Increase in number

  • Bacteria, Archaea, Eukaryotes: Occurs in all three groups.

  • Viruses: The host cell replicates the virus; the virus does not reproduce on its own.

• Responsiveness: Ability to react to environmental stimuli

  • Bacteria, Archaea, Eukaryotes: Occurs in all three groups.

  • Viruses: Reaction to host cells is seen in some viruses.

• Metabolism: Controlled chemical reactions of organisms

  • Bacteria, Archaea, Eukaryotes: Occurs in all three groups.

  • Viruses: Viruses use the host cell’s metabolism for their needs.

• Cellular Structure: Membrane-bound structure capable of all above functions

  • Bacteria, Archaea, Eukaryotes: Present in all three groups.

  • Viruses: Viruses lack a cytoplasmic membrane or cellular structure.

Comparison of Prokaryotic and Eukaryotic Cells

• Prokaryotes (Bacteria and Archaea)

  • Nucleus: Categorized by a lack of a nucleus.

  • DNA/Protein Synthesis: Processes occur simultaneously because there is no nuclear envelope separating them.

  • Chromosomes: Typically possess a single circular chromosome that is not enclosed in a membrane.

  • Size: Smaller cells, generally averaging $\sim 1.0\,\mu m$ in diameter.

  • Organelles: Lack membrane-bound internal structures (organelles).

  • Cell Walls:

    • Bacteria: Composed of peptidoglycan.

    • Archaea: Composed of pseudomurein.

  • Division: Reproduce through binary fission.

• Eukaryotes (Algae, Protozoa, Fungi, Animals, and Plants)

  • Nucleus: Possess a distinct nucleus.

  • Chromosomes: Paired chromosomes contained within a nuclear membrane.

  • Size: Larger cells, ranging from $10\text{--}100\,\mu m$ in diameter.

  • Organelles: Contain various membrane-bound internal structures.

  • Cell Walls: When present, they are composed of polysaccharides.

  • Division: Reproduce through mitosis.

External Structures of Prokaryotic Cells

• Glycocalyces

  • Definition: A gelatinous, sticky substance surrounding the outside of the cell.

  • Composition: Polysaccharides, polypeptides, or both.

  • Function: Protects cells from drying out (desiccation), contributes to virulence, and helps in the formation of biofilms.

  • Types of Glycocalyces:

    • Capsule: Composed of organized repeating units; firmly attached to the cell surface; prevents recognition and phagocytosis by host immune systems.

    • Slime Layer: Loosely attached to the cell surface; water-soluble; allows the cell to attach to surfaces.

• Flagella

  • Definition: Long structures extending beyond the cell surface for movement.

  • Function: Rotation propels the bacterium through the environment. Movement is reversible (counterclockwise or clockwise).

  • Movement Types:

    • Runs: Smooth movement in one direction.

    • Tumbles: Abrupt, random changes in direction.

  • Stimuli Response: Taxis (movement in response to stimuli).

  • Basic Arrangements:

    • Monotrichous: Single polar flagellum.

    • Amphitrichous: Flagella at both poles of the cell.

    • Lophotrichous: A tuft (multiple) of polar flagella.

    • Peritrichous: Flagella covering the entire cell surface.

  • Endoflagella: Found in spirochetes; flagella reside in the periplasmic space (axial filament). The rotation of the axial filament causes the spirochete to corkscrew and move forward.

• Fimbriae and Pili (Nonmotile Extensions)

  • Fimbriae: Sticky, proteinaceous, bristle-like projections used to adhere to other bacteria, hosts, and environmental substances. Shorter than flagella; critical for biofilm development.

  • Pili (Conjugation Pili or Sex Pili): Long hollow tubules ($1\text{--}2$ per cell) used to transfer DNA from one bacterial cell to another through the process of conjugation.

Bacterial Cell Walls

• General Functions

  • Provides structure and shape.

  • Protects the cell from osmotic forces.

  • Assists in attaching to other cells or resisting antimicrobial drugs.

  • Contributes to pathogenicity.

  • Note: Animal cells do not have cell walls, making bacterial walls a primary target for antibiotics.

• Peptidoglycan Structure

  • Chains of alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

  • Sugar chains are attached to other chains by tetrapeptide cross-bridges (four amino acids linked by peptide bonds).

• Gram-Positive Cell Walls

  • Relatively thick layer of peptidoglycan.

  • Contains teichoic acids that link the peptidoglycan layers.

  • Polysaccharides and teichoic acids provide antigenic specificity.

  • Gram stain reaction: Appears purple dye to retaining crystal violet.

  • Survives desiccation well.

• Gram-Negative Cell Walls

  • Thin layer of peptidoglycan.

  • Contains an outer membrane (bilayer) outside the peptidoglycan.

  • Lipid A (Endotoxin): Located in the outer membrane; can cause fever, vasodilation, inflammation, shock, and blood clotting when dead cells disintegrate.

  • Gram stain reaction: Appears pink or red after accepting the counterstain (safranin).

  • Periplasm: The space between the cytoplasmic membrane and the outer membrane containing the peptidoglycan and various proteins.

• Atipical Cell Walls

  • Archaea: Walls are composed of pseudomurein (lacks NAM and D-amino acids) or are cell-wall-less.

  • Mycoplasmas: Bacteria that naturally lack cell walls entirely; their plasma membranes contain sterols for stability.

Comparison of Gram-Positive and Gram-Negative Bacteria (Detailed)

• Characteristics Table:

  • Peptidoglycan Layer: Thick (multilayered) in Gram+; Thin (single-layered) in Gram-.

  • Teichoic Acids: Present in many Gram+; Absent in Gram-.

  • Periplasmic Space: Granular layer in Gram+; Periplasm in Gram-.

  • Outer Membrane: Absent in Gram+; Present in Gram-.

  • LPS Content: Virtually none in Gram+; High in Gram-.

  • Lipid/Lipoprotein Content: Low in Gram+ (except acid-fast bacteria); High in Gram-.

  • Flagellar Structure: $2$ rings in basal body (Gram+); $4$ rings in basal body (Gram-).

  • Toxins: Gram+ produces Exotoxins; Gram- produces both Endotoxins and Exotoxins.

  • Resistance to Physical Disruption: High in Gram+; Low in Gram-.

  • Lysozyme Disruption: High in Gram+; Low in Gram-.

  • Susceptibility to Penicillin/Sulfonamide: High in Gram+; Low in Gram-.

  • Susceptibility to Streptomycin/Chloramphenicol/Tetracycline: Low in Gram+; High in Gram-.

  • Resistance to Drying: High in Gram+; Low in Gram-.

Bacterial Morphology (Shapes)

• Bacillus: Rod-shaped.

• Coccus: Spherical.

• Spiral: Includes Vibrio, Spirillum, and Spirochete.

• Archaea Shapes: Can include Cocci, Bacillus, oblong with irregular protrusions, and square shapes with bright spots.

Prokaryotic Cytoplasmic Membranes

• Structure

  • Phospholipid Bilayer: Composed of hydrophilic (phosphate) heads and hydrophobic tails.

  • Peripheral Proteins: Situated on the membrane surface.

  • Integral/Transmembrane Proteins: Penetrate or span the entire membrane.

  • Fluid Mosaic Model: The membrane is as viscous as olive oil; proteins move freely, and phospholipids rotate and move laterally. It is self-sealing.

• Function

  • Selective Permeability: Controls passage of substances; naturally impermeable to most substances.

  • Maintenance of concentration and electrical gradients.

  • Harvests light energy in photosynthetic bacteria and is the site of energy production (ATP synthesis).

Transport Processes Across Bacterial Membranes

• Passive Transport (Requires No Energy)

  • Diffusion: Movement of small or lipid-soluble chemicals (e.g., $O_2$, $CO_2$) down their electrochemical gradient through the phospholipid bilayer.

  • Facilitated Diffusion: Movement down an electrochemical gradient via nonspecific channels or specific carrier proteins (permeases) for substances like glucose, fructose, urea, and vitamins.

  • Osmosis: Net movement of water across a selectively permeable membrane.

    • Isotonic Solution: Solute concentration is the same inside and outside; no net movement of water.

    • Hypertonic Solution: Higher solute concentration outside the cell; water moves out (causing crenation in wall-less cells or plasmolysis in walled cells).

    • Hypotonic Solution: Lower solute concentration outside the cell; water moves in (causing wall-less cells to burst; walled cells become turgid).

• Active Transport (Requires ATP)

  • Active Transport: Uses ATP-dependent carrier proteins to move substances against their electrochemical gradient (e.g., $Na^+$, $K^+$, $Ca^{2+}$, $H^+$, $Cl^-$).

    • Uniport: One substance moved in one direction.

    • Antiport: Two substances moved in opposite directions.

    • Symport (Coupled Transport): Two substances moved in the same direction simultaneously.

  • Group Translocation: The substance is chemically modified during transport so it cannot escape the cell (e.g., glucose, mannose, fructose).

Cytoplasm of Prokaryotes

• Cytosol: Liquid portion containing a single circular DNA molecule in the nucleoid region.

• Inclusions: Reserve deposits of chemicals (e.g., PHB granules).

• Ribosomes: Sites of protein synthesis; size is 70S.

• Cytoskeleton: Maintains cell shape and aids in movement.

• Endospores

  • Dormant form produced by bacteria like Bacillus and Clostridium as a defensive strategy against unfavorable conditions (limited nutrients).

  • Extremely resistant to heat, radiation, and chemicals.

  • Formation (7 Steps):

    1. DNA is replicated.

    2. Cytoplasmic membrane invaginates to form a forespore.

    3. Membrane grows and engulfs the forespore within a second membrane; vegetative DNA disintegrates.

    4. A cortex of peptidoglycan is deposited between membranes; dipicolinic acid and calcium accumulate.

    5. Spore coat forms around the endospore.

    6. Endospore matures (increased resistance).

    7. Endospore is released from the original cell.

Comparison of Archaea and Bacteria External/Internal Features

• Glycocalyx: Polypeptide or polysaccharide in both.

  • Flagella:

    • Archaea: $10\text{--}14\,nm$ diameter; grows at the base; rotates in bundles CW and CCW.

    • Bacteria: $\sim 20\,nm$ diameter; grows at the tip; rotates CW (tumbles) or CCW (runs).

• Pili: None discovered in Archaea; present in some Bacteria.

• Hami: Present in some Archaea (for attachment); absent in Bacteria.

• Membrane Lipids: * Archaea: Ether linkages; some have single lipid layers. * Bacteria: Ester linkages in a phospholipid bilayer.

Eukaryotic Cell Structure and Function

• Glycocalyces

  • Not as organized as prokaryotic capsules.

  • Anchors animal cells, strengthens the cell surface, provides protection against dehydration, and functions in cell-to-cell recognition.

• Cell Walls

  • Plants: Cellulose.

  • Fungi: Cellulose, chitin, and/or glucomannan.

  • Algae: Variety of polysaccharides.

• Cytoplasmic Membrane

  • Fluid mosaic containing steroid lipids (e.g., cholesterol) to help maintain fluidity.

  • Endocytosis: Importing substances.

    • Phagocytosis: Import of solid substances (via pseudopods).

    • Pinocytosis: Import of liquid substances.

  • Exocytosis: Export of wastes or secretions.

• Nonmembranous Organelles

  • Flagella: Structurally different from prokaryotes; filaments anchored by a basal body (no hook); composed of a "$9 + 2$" arrangement of microtubules; movement is undulating (rhythmic wave) rather than rotation.

  • Cilia: Shorter and more numerous; coordinated beating propels cells or moves substances past the surface.

  • Ribosomes: 80S ($60S$ and $40S$ subunits). Smaller 70S ribosomes are found in mitochondria and chloroplasts.

  • Cytoskeleton: Network of tubulin microtubules ($25\,nm$), actin microfilaments ($7\,nm$), and intermediate filaments ($10\,nm$).

  • Centrioles and Centrosome: Centrioles ($9+0$ arrangement) play a role in mitosis, cytokinesis, and flagella/cilia formation. The centrosome is the region where centrioles are found.

• Membranous Organelles

  • Nucleus: Often the largest organelle; contains most of the DNA. Houses the nucleoplasm, chromatin, and nucleoli (RNA synthesis). Surrounded by a double-membrane nuclear envelope with nuclear pores.

  • Endoplasmic Reticulum (ER): Flattened hollow tubules continuous with the nuclear envelope.

    • Rough ER (RER): Studded with ribosomes; protein synthesis.

    • Smooth ER (SER): No ribosomes; lipid synthesis and detoxification.

  • Golgi Body: Flattened sacs that receive, process, and package large molecules in secretory vesicles for export.

  • Lysosomes: Contain catabolic/digestive enzymes for breakdown of nutrients or damaged cells.

  • Peroxisomes: Contain enzymes (like catalase) to degrade poisonous wastes.

  • Mitochondria: Two membranes; interior matrix contains 70S ribosomes and a circular molecule of DNA; primary site of ATP production.

  • Chloroplasts: Light-harvesting structures in photosynthetic eukaryotes; contain two phospholipid membranes, 70S ribosomes, and their own DNA.

Endosymbiotic Theory

• Concept: Eukaryotes formed from the union of small aerobic prokaryotes with larger anaerobic prokaryotes.

• Evolution:

  • The smaller prokaryotes became internal parasites.

  • The parasites lost the ability to exist independently, while the larger cell became dependent on them for aerobic ATP production.

  • Aerobic prokaryotes evolved into mitochondria.

  • A similar scenario (involving photosynthetic bacteria) led to the origin of chloroplasts.

• Evidence: Mitochondria and chloroplasts have double membranes, their own circular DNA, and 70S ribosomes.

Summary Comparison of Archaea, Bacteria, and Eukaryotes

• Nucleus: Absent in Archaea/Bacteria; Present in Eukaryotes.

• Membrane-bound Organelles: Absent in Archaea; Present in few Bacteria; Various types present in all Eukaryotes.

• Ribosomes: 70S in Archaea/Bacteria; 80S in Eukaryotic cytosol; 70S in Eukaryotic organelles.

• Chromosomes: Commonly single and circular in Archaea/Bacteria; Linear and more than one per cell in Eukaryotes.

• Cell Wall: Archaea (Polysaccharides/Proteins, no peptidoglycan); Bacteria (Peptidoglycan); Eukaryotes (Polysaccharides when present).