Chapter 1: Structure of Prokaryotic Cells

Comprehensive question-and-answer flashcards covering prokaryotic cell structures, external appendages, cell envelope architecture, Gram staining differentiation, endospore formation, and bacterial morphology and arrangement.

What is the resolution limit of the unaided human eye, and what is the typical size dimension of microorganisms?

The unaided human eye cannot visualise objects smaller than $1\text{ mm}$. Most microorganisms fall within the micrometre ($\mu\text{m}$) dimensions.

Why are viruses considered microbes despite not being living organisms?

Viruses are microscopic and capable of causing infection and disease. Structurally, they are simpler than cells and consist of a protein covering that encases a small amount of nucleic acid (DNA or RNA).

What is the descending order of the eight taxonomic ranks in biological classification?

Domain, kingdom, phylum (or division), class, order, family, genus, and species.

Which three domains of living organisms were established by Carl Woese and George Fox?

Domain Bacteria, Domain Archaea, and Domain Eukarya.

What are the distinguishing characteristics of organisms in the Domain Archaea?

They are prokaryotic organisms distinct from Bacteria that often inhabit extreme environments (such as deserts, volcanoes, and the Antarctic) or produce unique metabolic by-products such as methane.

What are the four formatting rules of the binomial nomenclature system?

1. The genus name is written first, followed by the species name.
2. The genus name is capitalized, while the species name is lowercase.
3. Both names must be underlined separately, or written in italics.
4. The genus name may be abbreviated to its initial letter, but the species name cannot be abbreviated.

What illness is associated with Bacillus cereus, and how is it classically transmitted?

Bacillus cereus causes 'Fried Rice Syndrome', classically contracted by consuming fried rice that has been left sitting at room temperature for hours (such as at buffets).

How do prokaryotic and eukaryotic cells differ in chromosome structure and organelle compartmentalization?

Prokaryotic cells have a single circular chromosome located in an unbound nucleoid region and lack membrane-bound organelles. Eukaryotic cells have linear chromosomes enclosed by a nuclear membrane and contain membrane-bound organelles (such as ER, mitochondria, and lysosomes).

How do prokaryotic and eukaryotic ribosomes compare in sedimentation coefficient and abundance?

Prokaryotic ribosomes are fewer and smaller ($70\text{S}$), whereas eukaryotic ribosomes are more plentiful and larger ($80\text{S}$).

In which fungal group can plasmids naturally be found?

Plasmids are found in yeasts.

What is the typical diameter of a bacterial cell, and what functional benefit does this size confer?

Bacteria typically range from $0.5\text{--}1.0\,\mu\text{m}$ in diameter. Their small size provides a high surface area to volume ratio, facilitating extensive contact between the cell surface and the surrounding environment.

Identify the four flagellar arrangements shown in the diagram.

Monotrichous (single polar flagellum), Amphitrichous (flagella at opposite poles), Lophotrichous (a tuft/cluster of flagella at one pole), and Peritrichous (flagella distributed over the entire cell surface).

How does the motility mechanism of bacterial flagella differ from eukaryotic flagella?

Bacterial flagella rotate $360^\circ$ like a propeller and are composed of flagellin protein, whereas eukaryotic flagella move by undulating back and forth.

Identify the two indicated structures on these interacting bacterial cells and state their functions.

The sex pilus is a hollow protein tube used to transfer DNA from a donor cell to a recipient during bacterial conjugation. The fimbriae are fine, hair-like bristles that mediate adhesion to host cells, surfaces, and other bacteria.

How do fimbriae contribute to the pathogenicity of Escherichia coli?

Fimbriae enable Escherichia coli to form tight attachments to host epithelial cells in the intestines, initiating tissue colonization; mutant strains lacking fimbriae are unable to establish infections.

What is the difference between positive chemotaxis and negative chemotaxis?

Positive chemotaxis is directional movement toward a favourable chemical stimulus (such as a nutrient source), whereas negative chemotaxis is movement away from a repellent or harmful substance.

Which pathogenic bacteria use flagella to penetrate host mucosal linings, and what diseases do they cause?

Helicobacter pylori uses flagella to bore through the stomach lining (causing stomach ulcers), and Vibrio cholerae uses flagella to penetrate the small intestine (causing cholera).

Identify the two types of glycocalyx layers shown in diagrams (a) and (b), and describe their structural distinctions.

(a) Slime layer: a loose, unorganized shield that protects against dehydration and nutrient loss while aiding adhesion. (b) Capsule: a dense, tightly bound layer composed of repeating polysaccharide or protein units that prevents phagocytosis by white blood cells.

What structural polymer is shown here, and what are its chemical building blocks?

Peptidoglycan (murein). It consists of alternating glycan residues of $\beta, 1\rightarrow 4$ linked NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid), cross-linked at adjacent NAM molecules by peptide chains of four amino acids.

Name the four reagents applied during the Gram staining procedure in order of use.

1. Crystal violet (primary stain)
2. Gram's iodine (mordant)
3. Alcohol wash ($95\%$ ethanol; decolorizer)
4. Safranin (counterstain)

Why does the alcohol wash step decolorize Gram-negative bacteria but not Gram-positive bacteria?

Alcohol dehydrates the thick peptidoglycan layer of Gram-positive cells, trapping the crystal violet-iodine complexes inside. In Gram-negative cells, alcohol dissolves the outer lipid membrane and disrupts the thin peptidoglycan layer, allowing the crystal violet-iodine complexes to wash out.

Identify which diagram represents a Gram-positive cell wall and which represents a Gram-negative cell wall.

The left diagram represents a Gram-positive cell wall (thick peptidoglycan layer containing teichoic acid and lipoteichoic acid, lacking an outer membrane). The right diagram represents a Gram-negative cell wall (thin peptidoglycan layer surrounded by an outer membrane with lipopolysaccharides and porins).

What are the quantitative differences in peptidoglycan thickness and percentage of cell wall between Gram-positive and Gram-negative bacteria?

Gram-positive cell walls have a peptidoglycan thickness of $20\text{--}80\text{ nm}$ ($60\%$ of cell wall). Gram-negative cell walls have a peptidoglycan thickness of $8\text{--}11\text{ nm}$ ($10\text{--}20\%$ of cell wall).

What is the biological function of porin proteins in the Gram-negative outer membrane?

Porins form transmembrane channels that allow small, hydrophilic nutrients to enter the cell while blocking larger, harmful molecules such as bile salts.

Identify the two circular DNA elements shown in this bacterium and compare their roles.

The bacterial chromosome (nucleoid) is a single, circular double-stranded DNA molecule that carries essential genes for cell survival and reproduction. The plasmid is a smaller, self-replicating circular DNA molecule that carries non-essential genes conferring advantageous traits such as antibiotic resistance or toxin production.

What physical property is quantified by Svedberg ($\text{S}$) units during ultracentrifugation?

The sedimentation rate of macromolecular structures; heavier, more compact structures sediment more rapidly and are assigned higher $\text{S}$ values.

What two-phase life cycle is illustrated in this diagram?

Sporulation (formation of a dormant, resistant endospore within a mother cell during environmental stress) and spore germination (conversion of an endospore back into a metabolically active vegetative cell under favourable conditions).

Which two bacterial genera are capable of producing endospores, and how quickly can spore germination take place?

Clostridium and Bacillus (both Gram-positive rod-shaped bacteria). Germination can occur as quickly as $90\text{ minutes}$ once triggered by water and an activating agent.

How do spirilla and spirochetes differ in morphology and mechanism of motility?

Spirilla are rigid helices with $1\text{--}20$ helical turns that move via polar flagella. Spirochetes are flexible, spring-like helices with $3\text{--}70$ turns that move via $2\text{--}100$ periplasmic flagella (axial filaments).

Identify the coccal arrangements produced by cell division in (a) one plane, (b) two perpendicular planes, and (c) several planes.

(a) One plane: diplococci (pairs) and streptococci (chains). (b) Two perpendicular planes: tetrads (clusters of 4) and sarcinae (cubical packets of $8\text{--}64$ cells). (c) Several planes: staphylococci (irregular grape-like clusters).

Identify the four morphological forms and arrangements of bacilli shown in the diagram.

From left to right: coccobacillus (short, oval rod), bacilli (single rods), diplobacilli (pairs of rods attached end-to-end), and streptobacilli (chains of rods attached end-to-end).

Why is the variety of cellular arrangements in bacilli more limited than in cocci?

Bacilli only divide across a single transverse plane perpendicular to their long axis, restricting arrangements to single cells, pairs (diplobacilli), or chains (streptobacilli).