Characteristics of Cells and Life

Overview of Cellular Organization

All living organisms, whether single-celled or multicellular, are composed of cells that share fundamental characteristics. Understanding these features is essential for studying microbiology and the diversity of prokaryotic life.

• Basic Shapes: Cells may be spherical, cubical, or cylindrical in form.

• Internal Content: The cytoplasm is surrounded by a cell membrane, providing a controlled environment for cellular processes.

• Genetic Material: Cells contain DNA (chromosomes), ribosomes, and possess metabolic capabilities.

• Cell Types: There are two basic cell types: eukaryotic and prokaryotic.

Comparison of Eukaryotic and Prokaryotic Cells

Cells are classified based on the presence or absence of membrane-bound organelles and a nucleus.

• Eukaryotic Cells: Found in animals, plants, fungi, and protists.

  • Contain membrane-bound organelles that compartmentalize the cytoplasm and perform specialized functions.

  • Have a double-membrane bound nucleus containing DNA chromosomes.

• Prokaryotic Cells: Include bacteria and archaea.

  • Lack a nucleus and other membrane-bound organelles.

Characteristics of Life

Cells exhibit several defining characteristics that are essential for life:

• Reproduction and Heredity: Genome composed of DNA packed in chromosomes; offspring produced sexually or asexually.

• Growth and Development: Cells grow and develop over time.

• Metabolism: Chemical and physical life processes occur within cells.

• Movement and Responsiveness: Cells respond to internal and external stimuli; many organisms are capable of self-propulsion.

• Support, Protection, and Storage: Structures such as cell walls, vacuoles, granules, and inclusions provide support and store materials.

• Transport: Movement of nutrients and waste across cellular membranes.

Prokaryotic Cell Structure

General Features

Prokaryotic cells have a simpler structure compared to eukaryotes, but possess specialized components that enable survival and adaptation.

• Cell (Cytoplasmic) Membrane: A lipid bilayer that controls the movement of substances in and out of the cell.

• Bacterial Chromosome and Nucleoid: Region containing the cell's genetic material.

• Ribosomes: Sites of protein synthesis.

• Cytoplasm: Gel-like matrix filling the cell interior.

• Additional Structures (in some bacteria):

  • Endospore: Dormant, highly resistant cell type.

  • Intracellular Membranes: Specialized for certain metabolic functions.

  • Flagellum: Used for motility.

  • Pilus/Fimbriae: Used for attachment and genetic exchange.

  • Glycocalyx: Surface coating for protection and adhesion.

External Structures of Prokaryotic Cells

Appendages

Prokaryotic cells possess external structures that aid in movement, attachment, and environmental interaction.

• Motility Structures:

  • Flagella: Long, whip-like structures composed of the protein flagellin. They rotate to propel the cell through its environment.

  • Axial Filaments (Periplasmic Flagella): Found in spirochetes, these are internal flagella that enable corkscrew movement.

• Attachment or Channel Structures:

  • Fimbriae: Short, hair-like bristles that function in adhesion to surfaces and other cells.

  • Pili: Longer, tubular structures made of pilin protein, primarily found in Gram-negative bacteria. Pili are involved in conjugation, a process of partial DNA transfer between cells.

• Glycocalyx: A surface coating composed of polysaccharides and/or proteins. It exists in two forms:

  • Slime Layer: Loosely organized and attached.

  • Capsule: Highly organized and tightly attached.

Functions of the Glycocalyx

• Protects cells from dehydration and nutrient loss.

• Inhibits killing by white blood cells (phagocytosis), contributing to pathogenicity.

• Facilitates attachment and biofilm formation.

Cell Envelope Structure

Overview

The cell envelope is an external covering outside the cytoplasm, composed of the cell wall and cell membrane. It maintains cell integrity and is a key feature in bacterial classification.

• Gram-Positive Bacteria: Thick cell wall composed primarily of peptidoglycan and a cell membrane.

• Gram-Negative Bacteria: Outer cell membrane, thin peptidoglycan layer, and cell membrane.

Peptidoglycan Structure and Function

• Determines cell shape and prevents lysis due to osmotic pressure changes.

• Composed of a repeating framework of long glycan chains cross-linked by short peptide fragments.

Gram-Positive Cell Wall

• 20-80 nm thick peptidoglycan layer.

• Contains teichoic acid and lipoteichoic acid for maintenance and enlargement during cell division, and for stimulating immune responses.

• May have a periplasmic space between the cell membrane and cell wall.

Gram-Negative Cell Wall

• Contains both outer and inner membranes, with a thin peptidoglycan layer in between.

• Outer membrane contains lipopolysaccharides (LPS), which can act as endotoxins when released during infections.

• Porin proteins in the upper layer regulate molecule entry and exit.

Comparison Table: Gram-Positive vs. Gram-Negative Cell Walls

Gram Stain Technique

• Differential stain that distinguishes cells with a Gram-positive cell wall from those with a Gram-negative cell wall.

• Gram-positive cells retain crystal violet and stain purple.

• Gram-negative cells lose crystal violet and stain red from the safranin counterstain.

• Important for bacterial classification, diagnosis, and guiding treatment.

Atypical Cell Walls

• Some bacteria (e.g., Mycobacterium and Nocardia) have Gram-positive cell wall structure with lipid mycolic acid (cord factor), conferring resistance and pathogenicity.

• Basis for the acid-fast stain used in diagnosis.

• Some bacteria (e.g., Mycoplasma) lack a cell wall; their membrane is stabilized by sterols and they are pleomorphic.

Internal Structures of Prokaryotic Cells

Cytoplasm

• Dense, gelatinous solution of sugars, amino acids, and salts.

• Composed of 70-80% water.

• Serves as a solvent for materials used in all cell functions.

Genetic Material

• Chromosome: Single, circular, double-stranded DNA molecule containing all genetic information required by the cell.

• Plasmids: Small, circular, double-stranded DNA molecules not essential for growth but useful in genetic engineering and horizontal gene transfer.

Ribosomes

• Made of 60% ribosomal RNA and 40% protein.

• Consist of two subunits: large (50S) and small (30S) in prokaryotes, forming a 70S ribosome.

• Site of protein synthesis.

Inclusions and Granules

• Intracellular storage bodies for nutrients and other substances.

• Vary in size, number, and content; used when environmental sources are depleted.

Cytoskeleton

• Internal network of protein polymers closely associated with the cell wall, providing structural support.

Endospores

• Produced by some Gram-positive genera (e.g., Clostridium, Bacillus, Sporosarcina).

• Two-phase life cycle:

  • Vegetative cell: Metabolically active and growing.

  • Endospore: Dormant, highly resistant to heat, drying, freezing, radiation, and chemicals.

• Not a means of reproduction; germination returns the cell to vegetative growth.

• Endospores can survive for millions of years and require pressurized steam at 120°C for 20-30 minutes for destruction.

Bacterial Shapes, Arrangements, and Sizes

Basic Shapes

• Coccus: Spherical

• Bacillus: Rod-shaped

• Coccobacillus: Very short and plump

• Vibrio: Gently curved

• Spirillum: Helical, comma, twisted rod

• Spirochete: Spring-like

• Branching filaments: Filamentous forms

Arrangements

• Dependent on pattern of division and how cells remain attached after division.

• Cocci: Singles, diplococci (pairs), tetrads (groups of four), irregular clusters (staphylococci), chains (streptococci), cubical packets (sarcina).

• Bacilli: Singles, diplobacilli (pairs), chains, palisades.

Classification Systems for Prokaryotes

Methods of Classification

• Microscopic morphology

• Macroscopic morphology (colony appearance)

• Bacterial physiology

• Serological analysis

• Genetic and molecular analysis

Bergey's Manual of Determinative Bacteriology

• Comprehensive resource for prokaryote classification based on genetic information (phylogenetic).

• Two domains: Archaea and Bacteria.

• Five major subgroups with 25 different phyla.

Taxonomic Groups of Prokaryotes

• Domain Archaea: Primitive, adapted to extreme habitats and modes of nutrition.

• Domain Bacteria:

  • Phylum Proteobacteria: Gram-negative cell walls.

  • Phylum Firmicutes: Mainly Gram-positive with low G+C content.

  • Phylum Actinobacteria: Gram-positive with high G+C content.

Diagnostic Scheme for Medical Use

• Identification based on phenotypic qualities, cell wall structure, shape, arrangement, and physiological traits.

Species and Subspecies

• Species: Collection of bacterial cells sharing a similar pattern of traits.

• Strain/Variety: Culture derived from a single parent that differs in structure or metabolism (biovars, morphovars).

• Type: Subspecies showing differences in antigenic makeup (serotype/serovar), susceptibility to viruses (phage type), and pathogenicity (pathotype).

Prokaryotes with Unusual Characteristics

Free-Living Nonpathogenic Bacteria

• Photosynthetic Bacteria:

  • Cyanobacteria (blue-green algae): Gram-negative, possess thylakoids with photosynthetic pigments and gas inclusions.

  • Green and Purple Sulfur Bacteria: Contain bacteriochlorophyll, do not produce oxygen during photosynthesis.

• Gliding and Fruiting Bacteria: Gram-negative, move over moist surfaces, form complex multicellular structures.

Domain Archaea

• Live in extreme habitats (extremophiles): heat, salt, acid pH, pressure, and atmosphere.

• Includes methane producers, hyperthermophiles, extreme halophiles, and sulfur reducers.

• Cell Wall: Archaea do not have peptidoglycan in their cell walls.

Comparison Table: Three Cellular Domains

Example: Cyanobacteria are important oxygen producers in aquatic environments, while Clostridium species form endospores that can survive harsh conditions.

Additional info: Some details on cell wall composition and diagnostic schemes were expanded for clarity and completeness.