Microbiology Chapter 5 Quiz

Microbial Cell Structure and Function

• Prokaryotic cells have a cell wall, plasma membrane, ribosomes, mesosome, capsule, plasmid, nucleoid, pili, fimbriae, and flagellum.

Cell Morphology

• Morphology describes the shape of an organism.
• Prokaryotic cells can be spherical/ovoid, cylindrical, or curved/spiral.
• After cell division, prokaryotes can exist in various forms, such as cocci, diplococci, streptococci, tetrads, staphylococci, sarcinae, bacilli, diplobacilli, streptobacilli, trichomes, palisade, coccobacilli, vibrio, spirilla, spirochaetes, arcula, filaments, mycelia, and appendaged and pleomorphic bacteria.

Size

• Prokaryotic cells range from 0.2 µm to over 700 µm in diameter.
• Eukaryotic cells range from 2 µm to over 600 µm in diameter.
• The small size of prokaryotes optimizes nutrient uptake, waste elimination, and accelerates growth.

Cytoplasmic Membrane

• The cytoplasmic membrane has a main function of selective permeability, is 8-10 nm wide, and has embedded proteins and fatty acids.
• The membrane is composed of a phospholipid bilayer with hydrophilic regions facing outward and hydrophobic regions facing inward.
• Integral and peripheral membrane proteins are embedded within the membrane and play various roles.

Comparisons of Cytoplasmic Membranes

• Linkages in phospholipids differ between archaea (ether linkages) and bacteria/eukaryotes (ester linkages).
• Archaea use isoprenes instead of fatty acids.
• The cytoplasmic membrane serves as a permeability barrier, holds transport proteins, and is involved in energy conservation through the generation of a proton motive force.

Cell Walls: Peptidoglycan

• Gram-staining distinguishes between gram-positive and gram-negative bacteria based on cell wall structure.
• Gram-negative cell walls have an outer membrane (LPS layer) and a peptidoglycan layer. Gram-positive cell walls primarily consist of peptidoglycan.
• Peptidoglycan is a rigid layer that gives structure and is a target for antibiotics like penicillin.
• Lysozyme (enzymes found in human secretions) can disrupt the peptidoglycan, weakening bacterial cell walls.

Cell Walls (Archaea)

• Archaea cell walls do not contain peptidoglycan.
• The common cell wall type is S-layers—paracrystalline structures.
• Pseudomurein is a polysaccharide similar to peptidoglycan.
• Archaea cell walls are resistant to lysozyme and penicillin.

Cell Surface Structures

• Capsules and slime layers are polysaccharide layers outside the cell wall.

• Capsules are organized and tightly attached, increasing the cell's adherence to surfaces.

• Slime layers are loosely attached and enhance adherence.

• Both capsules and slime layers can help protect bacteria from the host's immune system and contribute to biofilm formation.

• Fimbriae and pili are filamentous protein structures helping organisms attach to surfaces.

• Fimbriae are shorter and more numerous; pili are longer and fewer.

• Conjugative pili facilitate genetic exchange between cells.

• Type IV pili enable twitching motility and attachment to host tissues.

• Hamus are grappling hooks used for attachment and biofilm formation.

Cell Inclusions

• Inclusions function as energy reserves, carbon reservoirs, or play other specific functions.
• Inclusions are often enclosed by thin membranes, reducing osmotic stress.
• Common examples of inclusion bodies are carbon storage polymers (e.g., polyhydroxyalkanoates), inorganic phosphate (polyphosphate granules), sulfur globules, carbonate minerals (allowing for magnetotaxis), and magnetosomes.

Gas Vesicles

• Gas vesicles are conical-shaped, gas-filled structures made of protein.
• These structures are impermeable to water and solutes.
• Gas vesicles help prokaryotes to regulate buoyancy.

Endospores

• Endospores are highly differentiated, resistant structures formed during endosporulation (sporulation).
• They are survival structures that allow cells to endure harsh conditions.
• Endospores are dormant stages of the bacterial life cycle, ideal for dispersal.
• Endospores feature multiple layers (exosporium, spore coat, cortex, core), dipicolinic acid and high calcium concentration, helping them to resist harsh conditions.
• Endospores are formed when growth ceases because of essential nutrient lack.

Eukaryotic Cell Structures

• Eukaryotic cells contain a membrane-enclosed nucleus, numerous organelles such as mitochondria, Golgi complex, lysosomes, endoplasmic reticulum, cytoskeletal elements (microtubules, microfilaments, intermediate filaments, and cilia), smooth/rough ER.
• The nucleus houses the chromosomes (DNA wound around histones)).
• Specialized cell division mechanisms occur, such as mitosis (producing identical diploid cells) and meiosis (producing unique haploid gametes).

Mitochondria, Hydrogenosomes, and Chloroplasts

• Mitochondria and hydrogenosomes are involved in energy metabolism.
• Mitochondria are located in aerobic eukaryotes and have two membranes, including the inner membrane that forms cristae (foldings).
• Mitochondria contain enzymes for respiration and ATP production.
• Hydrogenosomes are found in anaerobic eukaryotes and lack TCA cycle and cristae.
• Chloroplasts are involved in energy metabolism in photosynthetic eukaryotes. They have two membranes and inner membrane-surrounded stroma that contain RubisCO enzymes for CO₂ conversion to organic compounds.
• Chloroplasts have thylakoids, membrane discs containing chlorophyll for light energy capture.

Other Eukaryotic Cell Structures

• Endoplasmic reticulum (ER) is a network of membranes continuous with the nuclear membrane.
• Rough ER has ribosomes to synthesize proteins, and smooth ER synthesizes lipids and carbohydrates.
• Golgi complex modifies and sorts ER products for secretion. Lysosomes contain digestive enzymes, degrading cellular components, while other eukaryotic structures perform various roles in cell structure, motility and storage.
• The cytoskeleton contains elements like microtubules (a- and ẞ-tubulin), microfilaments (actin), and intermediate filaments (keratin) supporting the cell's structure.
• Flagella/cilia are organelles responsible for movement, with microtubules arranged in specific bundles and a special protein (dynein) enabling movement by rotating or beating.

Chemotaxis and Other Taxes

• Taxis is directed movement in response to chemical or physical gradients.
• Chemotaxis is movement in response to chemical gradients.
• Phototaxis is movement in response to light gradients.
• Aerotaxis is movement in response to oxygen gradients.
• Osmotaxis and hydrotaxis are movements in response to osmotic and water concentration gradients, respectively.