Cell Structure and Function: Prokaryotes vs. Eukaryotes – Study Guide

Cell Structure and Function

Overview: Prokaryotes vs. Eukaryotes

This section explores the fundamental differences and similarities between prokaryotic and eukaryotic cells, focusing on their structures, organelles, and specialized features. Understanding these distinctions is essential for microbiology students, as it forms the basis for studying microbial diversity and physiology.

• Prokaryotes are unicellular organisms lacking a membrane-bound nucleus and organelles. Examples include Bacteria and Archaea.

• Eukaryotes possess a true nucleus and membrane-bound organelles. Examples include Fungi, Protozoa, Algae, plants, and animals.

• Key Differences:

  • Prokaryotic DNA is circular and located in the nucleoid; eukaryotic DNA is linear and enclosed in the nucleus.

  • Prokaryotes lack complex organelles; eukaryotes have mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

Shapes of Bacteria

Bacteria exhibit three primary shapes, which are important for identification and classification.

• Coccus: Spherical-shaped bacteria (e.g., Staphylococcus).

• Bacillus: Rod-shaped bacteria (e.g., Escherichia coli).

• Spirillum: Spiral-shaped bacteria (e.g., Spirillum volutans).

Glycocalyx: Structure and Function

The glycocalyx is a protective outer layer found in many bacteria.

• Structure: Composed of polysaccharides and/or polypeptides.

• Types: Capsule (organized, firmly attached) and slime layer (unorganized, loosely attached).

• Functions: Protection against desiccation, phagocytosis, and aids in adherence to surfaces.

Flagella, Axial Filaments, Fimbriae, and Pili

These structures facilitate movement and attachment in bacteria.

• Flagella: Long, whip-like appendages for motility.

• Axial Filaments: Found in spirochetes; enable corkscrew movement.

• Fimbriae: Short, hair-like structures for attachment.

• Pili: Longer than fimbriae; involved in conjugation (DNA transfer).

Cell Walls: Types and Comparisons

Bacterial cell walls vary in composition, affecting staining and antibiotic susceptibility.

Type	Main Features	Example
Gram-positive	Thick peptidoglycan, teichoic acids	Bacillus subtilis
Gram-negative	Thin peptidoglycan, outer membrane with lipopolysaccharide	Escherichia coli
Acid-fast	Waxy mycolic acids, resistant to staining	Mycobacterium tuberculosis
Archaea	No peptidoglycan; may have pseudopeptidoglycan	Halobacterium
Mycoplasmas	No cell wall; sterols in membrane	Mycoplasma pneumoniae

Protoplast, Spheroplast, and L-form

These are cell wall-deficient forms of bacteria, often induced by antibiotics or environmental stress.

• Protoplast: Gram-positive bacterium with cell wall removed.

• Spheroplast: Gram-negative bacterium with partial cell wall loss.

• L-form: Bacteria that have lost their cell wall and can reproduce.

Plasma Membrane: Structure and Function

The plasma membrane controls the movement of substances in and out of the cell.

• Structure: Phospholipid bilayer with embedded proteins.

• Functions: Selective permeability, transport, energy generation, and cell signaling.

Membrane Transport Mechanisms

Cells use various mechanisms to move substances across membranes.

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

• Facilitated Diffusion: Uses transport proteins for passive movement.

• Osmosis: Diffusion of water across a membrane.

• Active Transport: Requires energy (ATP) to move substances against a gradient.

• Group Translocation: Substance is chemically modified during transport.

Equation for Diffusion Rate:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Nucleoid and Ribosomes

Prokaryotic cells contain a nucleoid and ribosomes for genetic information and protein synthesis.

• Nucleoid: Region containing circular DNA.

• Ribosomes: 70S in prokaryotes, 80S in eukaryotes; site of protein synthesis.

Inclusions

Inclusions are reserve deposits found in prokaryotic cells.

• Types: Metachromatic granules, polysaccharide granules, lipid inclusions, sulfur granules, gas vacuoles.

• Function: Storage of nutrients and metabolic products.

Endospores, Sporulation, and Germination

Endospores are highly resistant structures formed by some bacteria for survival.

• Sporulation: Process of endospore formation under stress.

• Germination: Return to vegetative state when conditions improve.

Flagella: Prokaryotic vs. Eukaryotic

Flagella differ structurally and functionally between prokaryotes and eukaryotes.

• Prokaryotic Flagella: Composed of flagellin, rotates like a propeller.

• Eukaryotic Flagella: Composed of microtubules (9+2 arrangement), moves in a whip-like fashion.

Cell Walls: Prokaryotic vs. Eukaryotic

Comparison of cell wall composition and function.

• Prokaryotic: Peptidoglycan (bacteria), pseudopeptidoglycan (archaea).

• Eukaryotic: Cellulose (plants, algae), chitin (fungi), absent in animals.

Plasma Membranes: Prokaryotic vs. Eukaryotic

• Prokaryotic: No sterols (except mycoplasmas), simple structure.

• Eukaryotic: Contains sterols, more complex with carbohydrates.

Cytoplasm: Prokaryotic vs. Eukaryotic

• Prokaryotic: No cytoskeleton, simple internal structure.

• Eukaryotic: Contains cytoskeleton, complex organization.

Organelles

Organelles are specialized structures within eukaryotic cells.

• Nucleus: Contains genetic material.

• Endoplasmic Reticulum (ER): Protein and lipid synthesis.

• Golgi Complex: Modifies, sorts, and packages proteins.

• Lysosomes: Digestive enzymes for breakdown of macromolecules.

• Peroxisomes: Oxidation of fatty acids and detoxification.

• Mitochondria: ATP production via cellular respiration.

• Chloroplasts: Photosynthesis in plants and algae.

• Centrosomes: Organize microtubules during cell division.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Evidence:

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

  • Double membranes suggest engulfment by ancestral eukaryotes.

  • Ribosomes resemble those of prokaryotes (70S).

  • Replicate independently of the cell cycle.

Example: The origin of mitochondria from an ancestral aerobic bacterium.

Additional info: Academic context and definitions have been expanded for clarity and completeness.