Prokaryotic and Eukaryotic Cell Structure and Function: Microbiology Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Prokaryotic vs. Eukaryotic Cells

Distinguishing Features

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology. These differences impact cellular organization, function, and complexity.

Prokaryotic cells: Small, simple cells lacking a nucleus or membrane-bound organelles. Found in Bacteria and Archaea.
Eukaryotic cells: Larger, more complex cells with a nucleus and various membrane-bound organelles. Found in plants, animals, fungi, and protists.

Bacterial Cell Components: Structure and Function

External Cell Wall Structures

Bacteria possess various external structures that contribute to protection, adhesion, and survival in diverse environments.

Glycocalyx: Complex layer of polysaccharides surrounding the cell. Function: Protects from dehydration, aids in adhesion, biofilm formation, and camouflage.
Capsule: Tightly bound, ordered layer outside the cell wall. Function: Adhesion, camouflage, anti-phagocytic properties.
Slime Layer: Loose, watery, and sticky layer. Function: Protects from dehydration, adhesion, biofilm formation, camouflage.
Flagella: Long, tail-like structure composed of filament, hook, and basal body. Function: Motility; allows bacteria to move.
Fimbriae: Short, bristle-like protein projections, usually shorter than flagella. Function: Adhesion to surfaces and other cells.
Pili: Hollow tubes, longer than fimbriae but shorter than flagella. Function: Transfer DNA between cells (conjugation); adhesion.

Cell Wall Components

The bacterial cell wall provides structural support and determines cell shape. Its composition varies between Gram-positive and Gram-negative bacteria.

Peptidoglycan: Rigid mesh of polysaccharide and protein. Function: Provides structural support.
Gram-positive cell wall: Thick peptidoglycan layer, teichoic acids, lipoproteins, periplasmic space, inner plasma membrane.
Gram-negative cell wall: Outer cell membrane, thin peptidoglycan layer, LPS in outer membrane, porins, lipoproteins, periplasmic space, inner plasma membrane.
Teichoic and Lipoteichoic Acids: Teichoic acids do not touch the cell membrane; lipoteichoic acids do. Function: Both involved in cell wall structure; lipoteichoic anchor peptidoglycan to plasma membrane. Only in Gram-positive cells.
Porins: Channel proteins in Gram-negative cells. Function: Facilitate diffusion of sugars and other molecules.
LPS (Lipopolysaccharide) and Lipid A: Found in the outer membrane of Gram-negative bacteria. Function: Endotoxin activity; structural integrity.

Internal Cell Components

Bacterial cells contain several internal structures essential for survival and function.

Plasmids: Small, circular DNA pieces not part of the chromosome. Function: May confer survival advantages (e.g., antibiotic resistance).
Periplasmic Space: Space between cell wall and plasma membrane in Gram-negative bacteria. Function: Contains enzymes and transport proteins.
Cell Membrane: Phospholipid bilayer. Function: Active area for metabolism, transport, and energy generation.
Ribosomes: Protein synthesis machinery. Function: Translate mRNA into proteins.
Inclusions: Collections of substances (e.g., nutrients, gas vesicles). Function: Storage for food, wastes, or gases.

Endospores: Structure, Function, and Sporulation

Endospore Structure and Function

Endospores are highly resistant, dormant structures formed by certain bacteria to survive extreme conditions.

Structure: Core (DNA, RNA, proteins, dipicolinic acid, Ca2+), cortex (2 membranes with peptidoglycan), spore coat (layers of keratin-like protein).
Function: Resist extreme heat, toxic chemicals, and nutrient depletion.
Characteristics: Environmentally resistant, low or zero metabolism, resistant to drying, chemicals, heat, and cold. Can remain dormant for long periods and germinate when conditions improve.

Sporulation Process

Sporulation is the process by which a vegetative cell becomes an endospore, typically occurring in five steps:

Cell stops growing and the chromosomal DNA is replicated.
Small part of DNA is engulfed by a larger part.
Forespore is formed as the cortex and coat are produced.
Endospore is released and the cell lyses.

Genera capable of producing endospores: Bacillus and Clostridium.

Transport Processes in Cells

Passive vs. Active Transport

Cells move substances across membranes using passive or active transport mechanisms.

Passive transport: Moves substances along their concentration gradient (high to low) without energy input. Example: Osmosis (water movement).
Active transport: Moves substances against their concentration gradient (low to high), requiring ATP. Example: Phagocytosis.

Equation for active transport energy requirement:

Effects of Solutions on Cells

Hypotonic, Hypertonic, and Isotonic Solutions

The effect of different solutions on cells depends on the presence or absence of a cell wall.

Cells with cell wall (bacteria, fungi, plants):
- Hypotonic: Cell swells; membrane is pushed against the wall, but the wall prevents bursting.
- Hypertonic: Cell loses water; membrane pulls away from the wall, causing cytoplasm and vacuole shrinkage.
- Isotonic: No net movement of water; cell is neither shrunken nor swollen.
Animal cells (no cell wall):
- Hypotonic: Water rushes in, causing the cell to swell and burst.
- Hypertonic: Cell loses water, shrinks, and membrane becomes distorted.

Eukaryotic Cell Components: Structure and Function

Major Organelles

Eukaryotic cells contain specialized organelles that perform distinct functions.

Nucleus: Stores DNA.
Mitochondria: Site of cellular respiration and energy production.
Endoplasmic Reticulum (Rough & Smooth): Protein and lipid synthesis.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Digestion and waste removal.
Peroxisomes: Breakdown of fatty acids and detoxification.
Plasma Membrane: Controls entry and exit of substances.
Cytoskeleton: Maintains cell shape and enables movement.
Ribosomes: Protein synthesis.
Chloroplasts (in plants & algae): Photosynthesis.
Cell Wall (in plants, fungi, algae): Structural support.

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

This table summarizes the key differences between Gram-positive and Gram-negative bacterial cell walls.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
LPS (Lipopolysaccharide)	Absent	Present
Porins	Absent	Present
Periplasmic Space	Small or absent	Large

Summary Table: Bacterial Cell Structures and Functions

Structure	Function
Glycocalyx	Protection, adhesion, biofilm formation
Capsule	Adhesion, anti-phagocytic
Slime Layer	Protection, adhesion
Flagella	Motility
Fimbriae	Adhesion
Pili	DNA transfer, adhesion
Peptidoglycan	Structural support
Plasmids	Genetic advantage
Ribosomes	Protein synthesis
Inclusions	Storage

Additional info: Academic context and definitions have been expanded for clarity and completeness. All major cell structures and processes relevant to introductory microbiology are included for exam preparation.