Ch 3: Cell Structure and Function: Study Guide for Microbiology

Ch 3: Cell Structure and Function

Major Processes of Living Cells

Living cells carry out essential processes that sustain life. These include:

• Growth: Increase in size and complexity.

• Reproduction: Production of new cells or organisms.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Chemical reactions that provide energy and build cellular components.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on structural and functional differences.

• Prokaryotic Cells: Lack a nucleus; DNA is found in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus surrounded by a nuclear membrane. Examples: Fungi, Protozoa, Algae, and Animals.

Comparison Table:

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Membranous Organelles	Absent	Present
Cell Wall	Usually present	Varies
Size	Small (0.5–2 μm)	Larger (2–100 μm)

Glycocalyces: Composition, Function, and Relevance

The glycocalyx is a sticky, gelatinous layer outside the cell wall, composed of polysaccharides, polypeptides, or both.

• Function: Protection from desiccation, adherence to surfaces, and evasion of host immune responses.

• Relevance to Human Health: Contributes to pathogenicity by helping bacteria evade phagocytosis.

Capsules vs. Slime Layers

• Capsule: Organized, firmly attached glycocalyx; increases virulence.

• Slime Layer: Unorganized, loosely attached glycocalyx; aids in adherence.

Bacterial Flagella: Structure and Function

Flagella are long, whip-like appendages used for motility.

• Structure: Composed of filament, hook, and basal body.

• Function: Enables movement toward or away from stimuli (chemotaxis).

Fimbriae, Pili, and Flagella: Comparison

• Fimbriae: Short, numerous, used for attachment.

• Pili: Longer, fewer, used for conjugation (transfer of DNA).

• Flagella: Long, used for motility.

Bacterial Cell Shapes and Arrangements

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum: Spiral

• Arrangements: Singles, pairs (diplo-), chains (strepto-), clusters (staphylo-)

Peptidoglycan: Sugar and Peptide Portions

Peptidoglycan is a complex polymer forming the bacterial cell wall.

• Sugar Portion: Alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Peptide Portion: Short amino acid chains cross-link the sugars.

Gram-Positive vs. Gram-Negative Cell Walls

Gram staining differentiates bacteria based on cell wall structure.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Stain Color	Purple	Pink

Clinical Implications of Gram-Negative Cell Wall

• Outer membrane contains lipopolysaccharide (LPS), which can act as an endotoxin.

• Gram-negative bacteria are often more resistant to antibiotics due to the outer membrane.

Phospholipid Bilayer and Cytoplasmic Membrane

The cytoplasmic membrane is a phospholipid bilayer, crucial for cell integrity and function.

• Phospholipid Structure: Hydrophilic head and hydrophobic tails.

• Significance: Selective permeability, barrier to ions and molecules.

Fluid Mosaic Model of Membrane Structure

The fluid mosaic model describes the dynamic nature of the membrane.

• Proteins float in or on the fluid lipid bilayer.

• Allows flexibility and movement of components.

Functions of Cytoplasmic Membrane: Permeability

• Controls entry and exit of substances.

• Maintains internal environment.

• Site of energy generation in prokaryotes.

Passive vs. Active Transport Across Membranes

• Passive Processes: No energy required. Includes diffusion, facilitated diffusion, and osmosis.

• Active Processes: Require energy (ATP). Includes active transport and group translocation.

Osmosis and Solution Types

Osmosis is the movement of water across a semipermeable membrane.

• Isotonic: Equal solute concentration inside and outside.

• Hypertonic: Higher solute concentration outside; cell shrinks.

• Hypotonic: Lower solute concentration outside; cell swells.

Bacterial Cytoplasm and Contents

• Contains DNA, ribosomes, enzymes, and inclusions.

• No membrane-bound organelles.

Endospores: Formation and Function

Endospores are dormant, resistant structures formed by some bacteria.

• Formed in response to harsh conditions.

• Resistant to heat, chemicals, and radiation.

Ribosomes and Cytoskeleton

• Ribosomes: Sites of protein synthesis. Prokaryotic (70S), Eukaryotic (80S).

• Cytoskeleton: Network of protein filaments for shape and support.

Archaeal vs. Bacterial Glycocalyces

• Both have glycocalyces, but differ in composition and chemistry.

• Archaeal glycocalyces may be less well-defined.

Eukaryotic Glycocalyces: Composition, Function, Importance

• Composed mainly of carbohydrates.

• Functions in cell recognition, protection, and adhesion.

Prokaryotic vs. Eukaryotic Cell Walls and Membranes

• Prokaryotes: Peptidoglycan cell wall, no sterols in membrane.

• Eukaryotes: Cell wall varies (cellulose, chitin), membranes contain sterols.

Exocytosis vs. Endocytosis

• Exocytosis: Export of materials via vesicles.

• Endocytosis: Import of materials via vesicle formation.

Pseudopods in Eukaryotic Cells

• Extensions of cytoplasm used for movement and engulfing particles.

Cytoplasm of Prokaryotes vs. Eukaryotes

• Prokaryotes: No organelles, simple structure.

• Eukaryotes: Contains organelles, complex structure.

Nonmembranous vs. Membranous Organelles

• Nonmembranous: Ribosomes, cytoskeleton, centrioles.

• Membranous: Nucleus, endoplasmic reticulum, Golgi body, lysosome, peroxisome, vesicle, mitochondrion, chloroplast.

Eukaryotic Cilia and Flagella: Comparison

• Cilia: Short, numerous, used for movement.

• Flagella: Longer, fewer, used for movement.

Ribosomes, Cytoskeletons, and Centrioles: Structure and Function

• Ribosomes: Protein synthesis.

• Cytoskeleton: Shape, support, movement.

• Centrioles: Cell division, organization of microtubules.

Prokaryotic vs. Eukaryotic Ribosomes

• Prokaryotic: 70S (50S + 30S subunits).

• Eukaryotic: 80S (60S + 40S subunits).

Three Filaments of Eukaryotic Cytoskeleton

• Microfilaments: Actin, cell movement.

• Intermediate Filaments: Structural support.

• Microtubules: Tubulin, cell shape, transport, division.

Functions of Eukaryotic Organelles

• Nucleus: Contains genetic material.

• Endoplasmic Reticulum: Protein and lipid synthesis.

• Golgi Body: Modification, sorting, and packaging of proteins.

• Lysosome: Digestion of macromolecules.

• Peroxisome: Breakdown of toxic substances.

• Vesicle: Transport within cells.

• Mitochondrion: ATP production.

• Chloroplast: Photosynthesis.

Endosymbiotic Theory of Organelle Origin

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Explains similarities between these organelles and bacteria.

Evidence for Endosymbiotic Theory

• Mitochondria and chloroplasts have their own DNA, similar to bacterial DNA.

• They have double membranes.

• Ribosomes resemble prokaryotic ribosomes.

• Reproduce independently within the cell.

Additional info: Where original content was brief, academic context was added to ensure completeness and clarity for exam preparation.