Prokaryotic and Eukaryotic Cell Structures and Functions

Prokaryotic and Eukaryotic Cells

Cell Size

Cell size is a fundamental characteristic distinguishing prokaryotic and eukaryotic cells. Prokaryotic cells are generally smaller (0.5–5 µm) compared to eukaryotic cells (10–100 µm). This size difference impacts cellular complexity, metabolic rates, and the ability to compartmentalize functions.

Cell Shape and Arrangement

Prokaryotic cells exhibit diverse shapes and arrangements, which are important for identification and classification.

• Coccus: Spherical shape

• Rod (Bacillus): Cylindrical shape

• Spirillum: Spiral shape

• Spirochete: Flexible, spiral-shaped

• Budding and appendaged bacteria: Cells with stalks or hyphae

• Filamentous bacteria: Long, thread-like cells

Arrangement depends on the plane of division and can include diplococci, streptococci, tetrads, sarcinae, and staphylococci.

Prokaryotic Cell Structure

Molecular Composition

Bacterial cells are composed of water, proteins, nucleic acids, lipids, polysaccharides, and small molecules. The cell envelope (membrane and wall) is critical for protection and interaction with the environment.

The Cell Membrane

The cell membrane separates the cytoplasm from the external environment and regulates transport. It is composed of a phospholipid bilayer, proteins, and sometimes steroids (e.g., hopanoids in bacteria).

• Phospholipid bilayer: Provides structural integrity

• Proteins: Support protruding structures, signaling, export of toxins, transport, and energy transfer

Key formula: The cell membrane maintains concentration gradients essential for energy transfer, such as ATP synthesis: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

The Cell Wall

The cell wall prevents osmotic lysis and protects the cell membrane. It is a single, interlinked molecule that encloses the entire cell.

Peptidoglycan Structure

• Composed of glycan chains cross-linked by peptides

• Provides rigidity and shape

Bacterial Cell Wall Types

• Gram-positive: Thick, multilayered peptidoglycan wall with teichoic acids

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS)

• Acid-fast: Complex wall with peptidoglycan and mycolic acid

• Mollicutes: Lack a true cell wall

Gram-Positive Cell Wall

Contains teichoic acids that bind ions and provide flexibility.

Gram-Negative Cell Wall

Features an outer membrane with LPS, a thin peptidoglycan layer, and murein lipoprotein.

Acid-Fast Cell Wall

Contains mycolic acids attached to peptidoglycan, characteristic of Mycobacterium tuberculosis.

Flagella

Flagella are filamentous appendages made of flagellin protein. They allow bacteria to move toward or away from stimuli (taxis) and can be distributed over the cell surface (peritrichous) or at the poles.

Glycocalyx

The glycocalyx is a viscous, gelatinous layer surrounding the cell wall, made of polysaccharide and/or polypeptide. It promotes attachment and protects from environmental harm.

• Capsule: Organized, firmly attached, prevents phagocytosis

• Slime layer: Unorganized, loose, aids in biofilm formation

Pili

Pili are proteinaceous appendages. Fimbriae (attachment pili) adhere cells to surfaces, while conjugation (sex) pili facilitate DNA transfer between cells.

Nucleoid and Plasmids

Bacterial DNA is organized in a nucleoid, which stores genetic information. Plasmids are small, circular DNA molecules outside the nucleoid, often carrying genes for antibiotic resistance or virulence.

Ribosomes

Prokaryotic ribosomes consist of two subunits (30S and 50S), forming a 70S ribosome. They are composed of 60% rRNA and 40% protein and are responsible for protein synthesis.

Inclusions in the Cytoplasm

• Gas vesicles: Provide buoyancy in aquatic bacteria

• Storage granules: Store nutrients (sulfur, phosphate, PHA)

• Magnetosomes: Store magnetite for magnetotaxis

Eukaryotic Cell Structure

Cell Types

Eukaryotic cells include fungi, protozoa, algae, plants, and animals. They are larger and more complex than prokaryotic cells.

The Cell Membrane

Similar to prokaryotes, the eukaryotic cell membrane is a phospholipid bilayer with embedded proteins and steroids. It separates the cytoplasm from the environment and regulates transport.

The Nucleus

The nucleus is a membrane-bound organelle that stores DNA and controls genetic information.

Ribosomes

Eukaryotic ribosomes consist of two subunits (40S and 60S), forming an 80S ribosome. They are found scattered in the cytoplasm, attached to the rough ER, and within mitochondria and chloroplasts.

The Endomembrane System

• Plasma membrane

• Nuclear envelope

• Rough ER: Tubular network with ribosomes, site of protein synthesis and transport

• Smooth ER: Tubular network without ribosomes, involved in nutrient processing and lipid storage

• Lysosomes: Vesicles with digestive enzymes for food digestion, protection, and debris removal

• Golgi apparatus: Modifies and ships proteins, consists of cisternae

Mitochondria

Mitochondria evolved through endosymbiosis, contain bacterial genomes and ribosomes, and are responsible for ATP synthesis.

Cytoskeleton

The cytoskeleton modulates cell shape, provides mechanical strength, enables movement, facilitates intracellular transport, and participates in cell division.

Summary Table: Prokaryotic vs. Eukaryotic Cell Features

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	0.5–5 µm	10–100 µm
Nucleus	Absent	Present
Cell Wall	Peptidoglycan (bacteria), variable (archaea)	Cellulose (plants), chitin (fungi), absent (animals)
Ribosomes	70S	80S
Membrane-bound organelles	Absent	Present
DNA	Circular, nucleoid	Linear, nucleus
Flagella	Made of flagellin	Made of microtubules

Additional info: The notes expand on brief points to provide academic context, definitions, and examples for each cell structure and function, ensuring completeness and clarity for microbiology students.