Cell Structure and Function in Microbiology Ch3-1

Cell Structure and Function

Characteristics of Life in Microbes

Microbes exhibit several fundamental processes of life, which distinguish living organisms from non-living entities such as viruses. These characteristics include growth, reproduction, responsiveness, metabolism, and cellular structure.

• Growth: Increase in size; occurs in all cellular microbes but not in viruses.

• Reproduction: Increase in number; all microbes reproduce, but viruses rely on host cells for replication.

• Responsiveness: Ability to react to environmental stimuli; present in all microbes, some viruses react to host cells.

• Metabolism: Controlled chemical reactions; all microbes metabolize, viruses use host metabolism.

• Cellular Structure: Membrane-bound structure capable of all above functions; present in all microbes, absent in viruses.

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth	Occurs in all	Does not occur
Reproduction	Occurs in all	Host cell replicates virus
Responsiveness	Occurs in all	Reaction to host cells in some viruses
Metabolism	Occurs in all	Uses host cell's metabolism
Cellular Structure	Present in all	Lacks cellular structure

Types of Cells: Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on structural features. This distinction is fundamental in microbiology.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; include bacteria and archaea; typically 1.0 µm or smaller.

• Eukaryotes: Have a nucleus and internal membrane-bound organelles; include algae, protozoa, fungi, animals, and plants; typically 10–100 µm.

Example: Epulopiscium fishelsoni is a giant bacterium initially mistaken for a eukaryote due to its size, but later identified as prokaryotic based on cellular features.

External Structures of Bacterial Cells

Glycocalyces

Bacterial cells may possess a glycocalyx, a gelatinous, sticky substance surrounding the cell, composed of polysaccharides, polypeptides, or both. Glycocalyces are classified as capsules or slime layers.

• Capsule: Organized, firmly attached, may prevent recognition by host immune system.

• Slime Layer: Loosely attached, water-soluble, aids in surface attachment.

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. Not all bacteria possess flagella.

• Structure: Composed of filament, hook, and basal body; basal body anchors to cell wall.

• Function: Rotation propels bacteria; movement can be clockwise or counterclockwise, resulting in 'runs' and 'tumbles'.

• Arrangement: Flagella may be monotrichous, lophotrichous, amphitrichous, or peritrichous.

Fimbriae and Pili

Fimbriae and pili are surface appendages distinct from flagella, involved in adhesion and genetic exchange.

• Fimbriae: Sticky, bristlelike projections; shorter than flagella; important in biofilm formation.

• Pili: Special type of fimbriae; longer than fimbriae but shorter than flagella; involved in DNA transfer (conjugation).

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structural support, shape, and protection from osmotic forces. They are composed primarily of peptidoglycan, a polymer of sugars and amino acids.

• Peptidoglycan: Consists of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide crossbridges.

• Shape: Cell walls give bacteria characteristic shapes (cocci, bacilli, etc.).

• Antibiotic Target: Cell wall synthesis is targeted by antibiotics.

Gram-Positive vs. Gram-Negative Cell Walls

Bacterial cell walls are classified as Gram-positive or Gram-negative based on their structure and staining properties.

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, purple after Gram stain, may contain mycolic acid.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), pink after Gram stain; lipid A in LPS can cause toxic effects.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present (with LPS)
Gram stain color	Purple	Pink
Mycolic acid	May be present	Absent

Bacteria Without Cell Walls

Some bacteria, such as Mycoplasma, lack cell walls and are often mistaken for viruses due to their small size. However, they possess other prokaryotic features such as ribosomes.

Bacterial Cytoplasmic Membranes

Structure

The cytoplasmic membrane is a phospholipid bilayer containing integral and peripheral proteins. The fluid mosaic model describes its dynamic nature.

• Phospholipid Bilayer: Provides selective permeability and structural integrity.

• Proteins: Integral and peripheral proteins facilitate transport and communication.

Function

The membrane controls passage of substances, maintains gradients, and in photosynthetic bacteria, harvests light energy.

• Selective Permeability: Only certain substances can cross.

• Transport Mechanisms: Includes passive (diffusion, facilitated diffusion, osmosis) and active processes.

• Electrical Gradient: Maintains membrane potential.

Passive Transport

Passive transport processes do not require energy and include diffusion, facilitated diffusion, and osmosis.

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement via protein channels.

• Osmosis: Diffusion of water across a semipermeable membrane.

Example: Effects of isotonic, hypertonic, and hypotonic solutions on cells.

Summary Table: Cell Structure and Function

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Organelles	Absent	Present
Cell wall	Peptidoglycan (if present)	Cellulose/chitin (if present)
Size	1.0 µm or smaller	10–100 µm
Examples	Bacteria, Archaea	Algae, Protozoa, Fungi, Animals, Plants

Additional info: Where original content was brief, academic context was added to clarify definitions, examples, and comparisons. All images included are directly relevant to the adjacent explanations and reinforce key concepts in cell structure and function.