Cell Structure and Function

Overview

This chapter explores the structural and functional diversity of cells, focusing on prokaryotic, archaeal, and eukaryotic cell types. Understanding these differences is fundamental to microbiology, as cell structure determines cellular processes, adaptation, and classification.

Comparison of Prokaryotic and Eukaryotic Cells

• Size: Prokaryotic cells (bacteria and archaea) are generally smaller (0.5–5 μm) than eukaryotic cells (10–100 μm).

• Structural Complexity: Prokaryotes lack membrane-bound organelles; eukaryotes possess complex organelles (nucleus, mitochondria, etc.).

• Genome Organization: Prokaryotes have a single, circular chromosome in the nucleoid; eukaryotes have multiple, linear chromosomes within a nucleus.

• Compartmentalization: Eukaryotic cells are highly compartmentalized, allowing specialized functions; prokaryotes have limited compartmentalization.

Phospholipid Structure and Bilayer Formation

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Bilayer Formation: In aqueous environments, phospholipids spontaneously arrange into bilayers, with tails inward and heads outward.

• Chemical Properties: The bilayer is selectively permeable, forming the basis of cellular membranes.

Bacterial External Structures

• Glycocalyces: Gelatinous, sticky substances surrounding the cell; includes capsules and slime layers.

• Capsules: Organized, firmly attached glycocalyx; protects against desiccation and phagocytosis.

• Slime Layers: Loosely attached, unorganized glycocalyx; aids in adherence to surfaces.

• Pili: Hair-like appendages used for attachment and conjugation (transfer of genetic material).

• Fimbriae: Short, numerous projections for attachment to surfaces and other cells.

Flagellar Arrangements and Motility

• Monotrichous: Single flagellum at one pole.

• Lophotrichous: Tuft of flagella at one pole.

• Amphitrichous: Flagella at both poles.

• Peritrichous: Flagella distributed over the entire cell surface.

• Motility: Flagella rotate to propel cells; arrangement affects movement patterns.

Archaeal External Structures

• Archaeal Flagella: Structurally distinct from bacterial flagella; powered differently.

• Pili and Fimbriae: Present in some archaea, but differ in protein composition.

• Glycocalyces: May be present; composition varies from bacteria.

Gram-Positive vs. Gram-Negative Bacterial Cell Walls

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides (LPS), periplasmic space.

• Peptidoglycan: Polymer of N-acetylglucosamine and N-acetylmuramic acid cross-linked by peptides.

Archaeal Cell Walls

• Composition: Lack peptidoglycan; may contain pseudopeptidoglycan, proteins, or polysaccharides.

• Chemical Differences: Unique sugars and linkages; resistant to lysozyme and antibiotics targeting peptidoglycan.

Membrane Transport Mechanisms

• Simple Diffusion: Passive movement of solutes down concentration gradient.

• Facilitated Diffusion: Passive transport via membrane proteins.

• Active Transport: Energy-dependent movement against gradient; uses ATP or proton motive force.

• Group Translocation: Substance chemically modified during transport (e.g., phosphotransferase system).

Solute and Water Movement Across Membranes

• Solute Transport: Direction and mechanism depend on concentration gradients and membrane permeability.

• Osmosis: Movement of water across membrane toward higher solute concentration.

• Cellular Consequences:

  • Hypotonic: Water enters cell; risk of lysis.

  • Hypertonic: Water leaves cell; risk of plasmolysis.

  • Isotonic: No net water movement; cell remains stable.

Comparative Lipid Composition of Cytoplasmic Membranes

• Bacteria: Phospholipids with ester-linked fatty acids.

• Archaea: Ether-linked lipids; branched isoprenoid chains.

• Eukaryotes: Phospholipids, sterols (e.g., cholesterol) for membrane fluidity.

• Functional Consequences: Membrane stability and adaptation to extreme environments.

Bacterial Cytoplasm Organization

• Nucleoid: Region containing the bacterial chromosome.

• Ribosomes: 70S ribosomes for protein synthesis.

• Inclusion Bodies: Storage granules (e.g., glycogen, polyphosphate).

• Cytoskeletal Elements: Proteins for cell shape and division.

Prokaryotic vs. Eukaryotic Ribosomes

• Prokaryotic Ribosomes: 70S (composed of 50S and 30S subunits).

• Eukaryotic Ribosomes: 80S (composed of 60S and 40S subunits).

• Function: Both synthesize proteins, but differ in sensitivity to antibiotics.

Archaeal Cytoplasm

• Similarities: Nucleoid, ribosomes, inclusion bodies.

• Differences: Ribosome structure and protein composition; unique metabolic pathways.

Endospore Formation

• Process: Sporulation occurs under adverse conditions; involves complex cellular changes.

• Selective Advantage: Endospores are highly resistant to heat, desiccation, chemicals, and radiation.

• Example: Bacillus and Clostridium species form endospores.

Eukaryotic Cell Organelles: Structure and Function

• Nucleus: Contains genetic material; site of transcription.

• Nucleolus: Ribosome synthesis.

• Rough Endoplasmic Reticulum (RER): Protein synthesis and processing.

• Smooth Endoplasmic Reticulum (SER): Lipid synthesis and detoxification.

• Golgi Apparatus: Protein modification, sorting, and packaging.

• Lysosomes: Digestive enzymes for intracellular digestion.

• Peroxisomes: Breakdown of hydrogen peroxide and fatty acids.

• Mitochondria: ATP production via cellular respiration.

• Chloroplasts: Photosynthesis in plants and algae.

• Cytoskeleton: Structural support, cell movement, and division.

Endosymbiotic Theory

• Origin: Mitochondria and chloroplasts evolved from engulfed prokaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes similar to prokaryotes, replication by binary fission.

Comparison of Eukaryotic and Prokaryotic Cell Walls and Membranes

• Eukaryotic Cell Walls: Present in plants, fungi, algae; composed of cellulose, chitin, or other polysaccharides.

• Prokaryotic Cell Walls: Peptidoglycan (bacteria), pseudopeptidoglycan or other materials (archaea).

• Cytoplasmic Membranes: Eukaryotes contain sterols; prokaryotes generally lack sterols.

Adaptation to Environments

• Bacteria: Diverse cell wall and membrane structures allow survival in varied environments.

• Archaea: Unique membrane and cell wall chemistry enables life in extreme conditions.

• Eukaryotes: Compartmentalization and specialized organelles support complex multicellular life.

Key Table: Comparison of Cell Types

Key Equations

• Osmosis: Water movement across membrane is governed by osmotic pressure difference: Where is osmotic pressure difference, is gas constant, is temperature, and is solute concentration difference.

• Diffusion Rate: Where is flux, is diffusion coefficient, is concentration gradient.

Additional info: Academic context and examples were added to clarify cell structure, membrane transport, and adaptation mechanisms.