Prokaryotic Cell Structure and Function – Comprehensive Study Notes

Prokaryotic Cell Structure and Function

Cell Morphology and Arrangement

Prokaryotic cells exhibit a variety of shapes and arrangements, which are important for identification but do not necessarily predict physiology, ecology, or pathogenicity.

• Morphology: Common shapes include cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and others.

• Arrangement: Determined by the plane of cell division; examples include diplococci (pairs), streptococci (chains), tetrads (groups of four), sarcinae (cubical packets), and staphylococci (clusters).

• Functional Adaptation: Certain shapes may excel at specific functions, such as motility or nutrient absorption.

• Example: Streptococcus forms chains due to division in one plane, while Staphylococcus forms clusters due to random division.

Colony Morphology

Colony morphology is the first step in describing a new bacterium and includes several observable characteristics.

• Size: Punctiform, small, moderate, large

• Color/Transparency: Translucent, transparent, opaque, shiny, dull

• Texture: Dry, viscous (sticky), mucoid, brittle

• Elevation: Flat, raised, convex, umbonate, crateriform, draughtsman

• Form: Round, irregular, filamentous, rhizoid, punctiform

• Margin: Entire, lobate, scalloped, filiform, undulate, curled, serrate

• Example: A mucoid, convex, round colony with an entire margin may indicate Klebsiella pneumoniae.

Size, Surface-to-Volume Ratio, and Growth/Mutation Rates

Cell size and surface-to-volume (S:V) ratio are critical for nutrient uptake, waste exchange, and evolutionary adaptation.

• Diameter Range: 0.2 μm to 700 μm, most between 0.5–4 μm

• S:V Ratio: Decreases as cell size increases

• Higher S:V Ratio:

  • Better nutrient and waste exchange

  • Faster growth rate with less nutrient use

  • Higher mutation rate, leading to faster evolution

• Formula: $ext{Surface Area} = 4 ext{π}r^2$ $ext{Volume} = rac{4}{3} ext{π}r^3$ $ext{S:V Ratio} = rac{ ext{Surface Area}}{ ext{Volume}}$

Cytoplasmic Contents

The cytoplasm of prokaryotes is highly concentrated with solutes, mainly proteins and ribosomes, and contains various organelles and molecules.

• Water Content: ~70% water, but with high solute concentration (275 g/L)

• Main Components: Proteins, ribosomes, DNA (nucleoid), RNA, regulatory factors

• High Osmotic Pressure: Due to high solute concentration

Organelle/Molecule	Composition	Function
DNA nucleoid	DNA, RNA, proteins	Genetic information storage and gene expression
Ribosomes	Proteins, rRNA	Protein synthesis
Inclusion bodies	Varied	Storage of nutrients and energy
Gas vesicles	Protein	Buoyancy regulation

Inclusion Bodies – Energy Storage & Osmotic Pressure Reduction

Inclusion bodies are specialized structures for storage and osmotic regulation.

• Structure: Thin membrane partitions “sac” from cytoplasm

• Function: Store nutrients for times of deprivation

• Types:

  • Carbon: polyhydroxyalkanes (PHA), glycogen

  • Sulfur, polyphosphate (for rapid ATP production), nitrogen

  • Magnetosomes: iron oxide storage, impart magnetotaxis (movement in response to magnetic fields)

• Example: Magnetotactic bacteria use magnetosomes to orient in aquatic environments.

Gas Vesicles – Buoyancy

Gas vesicles are protein-bound structures that regulate cell buoyancy, especially in aquatic environments.

• Structure: Protein shells exclude everything but gases

• Function: Fill with gas to float, empty to sink

• Occurrence: Found in many aquatic phototrophic bacteria, such as cyanobacteria

• Example: Cyanobacteria use gas vesicles to position themselves for optimal photosynthesis.

Genomes of Prokaryotes

Prokaryotic genetic material is organized differently from eukaryotes, with unique features for adaptation.

• Chromosome: 99% have a single, closed, circular chromosome aggregated in the nucleoid

• Plasmids: Small, circular DNA molecules that confer unique properties (e.g., antibiotic resistance)

• Gene Count: Prokaryotes have 500–10,000 genes (compact), eukaryotes have 20,000–25,000 (more non-coding DNA)

• Example: Plasmid-encoded resistance genes in Escherichia coli

Ribosomes

Ribosomes are the molecular machines for protein synthesis, differing in size and composition between prokaryotes and eukaryotes.

• Abundance: Up to 20,000 per cell (~30% dry weight)

• Prokaryotic Ribosome: 70S (large subunit: 50S, small subunit: 30S)

• Eukaryotic Ribosome: 80S (large subunit: 60S, small subunit: 40S)

• Protein Content: Prokaryotic large subunit has 31 proteins; small has 21

The Cytoplasmic Membrane

In Bacteria

The bacterial cytoplasmic membrane is a phospholipid bilayer with unique properties.

• Composition: Fatty acids esterified to glycerol

• Structure: Amphipathic phospholipid bilayer (hydrophobic and hydrophilic regions)

• Physical Properties: Physically weak, requiring cell wall support

In Archaea

Archaeal membranes differ in lipid composition and structure.

• Composition: Amphipathic phospholipid bilayer or monolayer

• Monolayer: 20C phytanyl (isoprene units) attached to glycerol via ether linkage

• Bilayer: 40C biphytanyl

• Physical Properties: Also physically weak

Membrane Proteins

Membrane proteins are essential for transport, signaling, and energy conversion.

• Polarity: Matches location in membrane (hydrophobic regions embedded, hydrophilic regions exposed)

• Types: Integral (span membrane), peripheral (attached to surface)

Functions of the Cytoplasmic Membrane

The cytoplasmic membrane serves as a selective barrier, anchor for proteins, and site of energy conservation.

1. Gatekeeper – Selective Permeability:

   • Nonpolar + small molecules: permeable via simple passive diffusion

   • Water: osmosis through aquaporins

   • Facilitated diffusion: uncommon

   • Polar/charged/large molecules: require active transport via transport proteins

2. Anchor Catalytic Proteins: Enzymes and structural proteins are anchored for cellular processes

3. Energy Conservation/Consumption:

   • Proton Motive Force (PMF): energy for active transport, motility, ATP generation

   • Equation: $ext{PMF} = ext{ΔpH} + ext{Δψ}$

ABC Transporters – Active Transport Proteins

ABC (ATP-Binding Cassette) transporters are the most common type of active transport protein in prokaryotes.

• Solute Specificity: Each transporter is specific for a particular solute

• Structure: Four core polypeptides (2 integral for channel, 2 peripheral for ATP hydrolysis)

• Additional Proteins: Soluble scavenger proteins may bind and deliver solutes

• Mechanism: ATP hydrolysis provides energy for solute transport

• Example: Import of amino acids, sugars, or ions

The Prokaryotic Cell Wall

The cell wall provides shape, rigidity, and protection against osmotic lysis. It is present in all bacteria and archaea, with some exceptions.

• Function: Prevents cell bursting due to high internal solute concentration

• Variation: Significant differences between Gram-positive (G+) and Gram-negative (G-) bacteria; archaea have unique walls

The Bacterial Cell Wall

Bacterial cell walls are primarily composed of peptidoglycan, with structural differences between G+ and G- types.

• G+: Thick peptidoglycan layer

• G-: Thin peptidoglycan layer plus an outer membrane

• Antibiotic Target: Peptidoglycan synthesis is targeted by penicillin; lysozyme breaks existing bonds

• Structure:

  • Glycans: Strength in “X” direction via β(1,4) glycosidic bonds

  • Peptides: Strength in “Y” direction via peptide cross-linking

Peptidoglycan

Peptidoglycan is a mesh-like polymer of sugars and amino acids, providing structural integrity.

• Structure: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β(1,4) bonds

• Cross-linking: Peptide chains connect layers, forming a strong, cable-like network

• Example: Staphylococcus aureus cell wall

Gram Positive vs. Gram Negative Cell Walls

Gram Positive Cell Wall

• Peptidoglycan: ~90%, very thick (20–80 nm)

• Teichoic and Lipoteichoic Acids: Bind Ca2+ and Mg2+ for import; LTA acts as a major exotoxin

Gram Negative Outer Surface

• Peptidoglycan: Small percentage (2–10 nm)

• Outer Membrane: Phospholipid bilayer + protein + lipopolysaccharide (LPS)

• LPS: Major endotoxin, repels antibiotics and toxic substances

• Porin Trimers: Allow entry/exit of solutes (specific and non-specific)

• Periplasm: Contains hydrolytic enzymes, binding proteins, chemoreceptors, biosynthesis enzymes

Feature	Gram Positive	Gram Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present (with LPS)
Endotoxin	Absent	Present (LPS)

Archaeal Cell Walls

Archaeal cell walls differ fundamentally from bacterial cell walls.

• Pseudomurein: Similar to peptidoglycan but with β(1,3) bonds; never true peptidoglycan

• Evolutionary Implications: Reflect divergence from a common ancestor

S Layers – Alternative Archaeal Cell Walls

S layers are crystalline arrays of protein or glycoprotein, providing additional protection and selectivity.

• Structure: Interlocking protein/glycoprotein molecules

• Function: Extreme selectivity for transport, shape/rigidity, resistance to osmotic pressure

• Example: S layer in Halobacterium

Cell Surface Layers

Some prokaryotes possess additional surface layers for protection, attachment, and motility.

• Capsule: Tight matrix, excludes small particles, strongly attached to cell wall

• Slime Layer: Loosely attached, does not exclude small particles, aids in motility

• Functions:

  • Attachment to surfaces/host tissues/biofilm formation

  • Motility

  • Virulence factors (e.g., capsule prevents immune recognition)

  • Bind water to maintain hydration

• Example: Capsule in Streptococcus pneumoniae prevents immune system recognition

Fimbriae and Pili

Fimbriae and pili are surface appendages involved in attachment, motility, and genetic exchange.

• Fimbriae: Numerous, allow attachment to surfaces

• Pili: Longer, fewer, involved in adhesion, extension/retraction, and transformation

• Type IV Pili: Specialized for motility and DNA uptake

Flagella and Motility

Flagella are complex structures enabling motility in many prokaryotes.

• Structure: Anchored in cell membrane and wall; analogous structure in archaea (archaellum)

• Types:

  • Polar: Single flagellum at one or both ends

  • Lophotrichous: Bundles at one end

  • Amphitrichous: Bundles at both ends

  • Peritrichous: Scattered over surface

• Movement: Powered by proton motive force; reversible or unidirectional

• Example: Escherichia coli uses peritrichous flagella for slow, unidirectional movement

Taxes (Taxis)

Prokaryotes can move in response to environmental stimuli.

• Chemotaxis: Movement toward or away from chemicals

• Phototaxis: Movement in response to light

• Aerotaxis: Movement in response to oxygen

• Osmotaxis: Movement in response to osmotic pressure

Additional info: These notes provide a comprehensive overview of prokaryotic cell structure and function, suitable for exam preparation in a college-level microbiology course.