Microbial Cell Structure and Function: Study Notes for BIOL 305

Microbial Cell Structure and Function

Introduction

This study guide covers the fundamental aspects of microbial cell structure and function, focusing on Bacteria and Archaea. Topics include cell morphology, cell size, cytoplasmic membrane structure and function, nutrient transport, and cell wall composition. These concepts are essential for understanding microbial physiology, adaptation, and classification.

Cells of Bacteria and Archaea

Cell Morphology

Cell morphology refers to the shape and arrangement of microbial cells. While morphology does not directly predict physiology or ecology, it can be influenced by selective forces such as nutrient uptake and motility.

• Major Morphologies of Prokaryotic Cells:

  • Coccus (pl. cocci): Spherical or ovoid cells.

  • Rod/Bacillus: Cylindrical cells.

  • Spirillum: Curved or spiral-shaped cells.

  • Arrangements: Some bacteria remain grouped after division, forming chains (Streptococcus), cubes (Sarcina), or clusters (Staphylococcus).

  • Unusual Shapes: Spirochetes (tightly coiled), appendaged bacteria, filamentous bacteria.

• Selective Forces Affecting Morphology:

  • Optimization for nutrient uptake (small cells, high surface-to-volume ratio).

  • Swimming motility in viscous environments (helical or spiral-shaped cells).

  • Gliding motility (filamentous bacteria).

Cell Size and the Significance of Being Small

Cell size varies widely among prokaryotes and eukaryotes, with important implications for growth and adaptation.

• Size Range:

  • Prokaryotes: 0.2 μm to >700 μm in diameter.

  • Most rod-shaped bacteria: 0.5–4.0 μm wide, <15 μm long.

  • Very large prokaryotes: Epulopiscium fishelsoni, Thiomargarita namibiensis.

  • Eukaryotes: 2 to >600 μm in diameter.

• Surface-to-Volume Ratio (S/V):

  • Higher S/V ratio in small cells allows more efficient nutrient and waste exchange.

  • Small cells tend to grow faster and adapt more rapidly due to higher mutation rates.

  • Eukaryotic cells generally adapt slower than prokaryotes.

• Lower Limits of Cell Size:

  • Cells <0.15 μm in diameter are unlikely due to the need to house essential biomolecules.

  • Open ocean contains "ultramicrobacteria" (0.2–0.4 μm), with streamlined genomes dependent on other organisms for some functions.

Table: Cell Size and Volume of Some Bacteria

Organism	Cell Morphology	Cell Size (μm)	Cell Volume (μm³)
Thiomargarita namibiensis	Coccus in chains	750	200,000,000
Epulopiscium fishelsoni	Rod with tapered ends	80 × 600	150,000,000
Escherichia coli	Rod	1 × 3	2.4
Mycoplasma genitalium	Pleomorphic	0.2	0.0265
Additional info: Table includes other bacteria with varying morphologies and sizes, illustrating the diversity in cell volume and adaptation.

The Cytoplasmic Membrane and Transport

Membrane Structure

The cytoplasmic membrane is a thin, vital barrier that separates the cell's interior from its environment and is essential for maintaining cellular integrity and function.

• Bacterial Cytoplasmic Membrane:

  • Phospholipid bilayer with embedded proteins.

  • Hydrophobic (fatty acids) and hydrophilic (glycerol, phosphate, functional groups) regions.

  • Fatty acids point inward, hydrophilic portions face the cytoplasm or external environment.

  • 8–10 nm wide, stabilized by hydrogen bonds and hydrophobic interactions.

  • Mg2+ and Ca2+ ions stabilize the membrane via ionic bonds.

  • Somewhat fluid, allowing dynamic protein movement.

• Membrane Proteins:

  • Integral membrane proteins: Firmly embedded in the membrane.

  • Peripheral membrane proteins: Anchored to one portion of the membrane.

  • Outer surface proteins bind substrates or process molecules for transport; inner surface proteins are involved in energy-yielding reactions.

• Archaeal Cytoplasmic Membranes:

  • Structurally similar to Bacteria/Eukarya, but chemically distinct.

  • Lipids have hydrophobic isoprenoid tails (not fatty acids), bound to glycerol by ether bonds.

  • Bacteria/Eukarya: Ester linkages; Archaea: Ether linkages.

  • Major lipids: Phosphoglycerol diethers (C20 phytanyl group), diglycerol tetraethers (C40 biphytanyl group).

  • Can form monolayers, bilayers, or mixtures.

Table: General Structure of Lipids

Domain	Linkage Type	Tail Type
Bacteria/Eukarya	Ester	Fatty acids
Archaea	Ether	Isoprenoids

Membrane Function

The cytoplasmic membrane performs several essential functions for the cell.

• Permeability Barrier: Prevents passive leakage of solutes; transport proteins move polar/charged molecules against concentration gradients.

• Protein Anchor: Holds transport proteins in place, ensuring high sensitivity and specificity.

• Energy Conservation: Site of generation and dissipation of the proton motive force, crucial for ATP synthesis. $\text{Proton Motive Force (PMF)} = \Delta pH + \Delta \Psi$

Nutrient Transport

Microbial cells use specialized transport systems to import nutrients and export waste products.

• Carrier-Mediated Transport Systems:

  • Show saturation effect and high specificity.

  • Low-affinity transporters operate at high external concentrations; high-affinity transporters function at low concentrations.

  • Some transporters are specific for a single molecule; others transport related classes (e.g., sugars, amino acids).

• Active Transport and Transporters:

  • Cells accumulate solutes against the concentration gradient, requiring energy (proton motive force or ATP).

  • Three major classes:

    • Simple Transport: Only a transmembrane transport protein (e.g., sodium-proton symporter).

    • Group Translocation: Series of proteins; substance is chemically modified during transport (e.g., phosphotransferase system in E. coli).

    • ABC Transport System: Three components—binding protein, transmembrane transporter, ATP-hydrolyzing protein; highly substrate-specific.

• Transport Events:

  • Uniport: One direction transport.

  • Symport: Co-transport of two molecules in the same direction.

  • Antiport: Simultaneous transport of two molecules in opposite directions.

• Examples:

  • Lac permease of Escherichia coli: Symporter for lactose, energy-driven.

  • Phosphotransferase system in E. coli: Group translocation for glucose, fructose, mannose; energy from phosphoenolpyruvate.

  • ABC (ATP-binding cassette) systems: >200 systems in prokaryotes; uptake of organic/inorganic nutrients and trace metals; Gram-negatives use periplasmic binding proteins, Gram-positives use substrate-binding proteins.

Cell Walls of Bacteria and Archaea

Peptidoglycan

Peptidoglycan is a rigid polysaccharide layer that provides strength to bacterial cell walls and is a key determinant in Gram staining.

• Gram-Positive Cell Wall:

  • One layer: thick peptidoglycan.

  • May contain teichoic acids (acidic substances) and lipoteichoic acids.

• Gram-Negative Cell Wall:

  • Two layers: thin peptidoglycan and outer membrane (LPS).

• Peptidoglycan Composition:

  • N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

  • Amino acids (e.g., lysine, diaminopimelic acid).

  • Cross-linking differs between Gram-positive and Gram-negative bacteria.

LPS: The Outer Membrane

The outer membrane, or lipopolysaccharide (LPS) layer, is characteristic of Gram-negative bacteria and provides additional protection.

• Structure:

  • Contains core polysaccharide and O-polysaccharide.

  • LPS replaces most phospholipids in the outer half of the membrane.

  • Endotoxin: Toxic component of LPS.

• Periplasm:

  • Space between cytoplasmic and outer membranes (~15 nm wide).

  • Gel-like consistency; contains enzymes, binding proteins, chemoreceptors, and porins (channels for hydrophilic substances).

• Gram Stain Reaction:

  • Structural differences in cell walls are responsible for Gram stain outcomes.

Archaeal Cell Walls

Archaeal cell walls differ significantly from those of Bacteria, lacking peptidoglycan and often the outer membrane.

• Composition:

  • Polysaccharides, proteins, glycoproteins, or mixtures.

  • Pseudomurein: Polysaccharide similar to peptidoglycan, composed of N-acetylglucosamine and N-acetyltalosaminuronic acid; found in some methanogenic Archaea; resistant to lysozyme and penicillin.

  • Some Archaea lack pseudomurein.

• Examples:

  • Methanosarcina species: Thick polysaccharide walls with glucose, glucuronic acid, galactosamine uronic acid, acetate.

  • Halococcus: Large amounts of sulfate, binds Na+ in the habitat.

Summary Table: Key Differences in Cell Envelope Structure

Feature	Gram-Positive Bacteria	Gram-Negative Bacteria	Archaea
Peptidoglycan	Thick layer	Thin layer	Absent (pseudomurein or other)
Outer Membrane	Absent	Present (LPS)	Absent
Teichoic Acids	Present	Absent	Absent
Lipid Linkage	Ester	Ester	Ether

Conclusion

Understanding microbial cell structure and function is foundational for microbiology. The diversity in cell morphology, size, membrane composition, and cell wall structure reflects the evolutionary adaptations of Bacteria and Archaea to their environments.