Prokaryotes: Structure, Function, and Diversity

Prokaryotes: Overview and Classification

Introduction to Prokaryotes

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and organelles. They are classified into two domains: Bacteria and Archaea. Prokaryotes are fundamental to the study of microbiology and play essential roles in ecosystems, health, and biotechnology.

Cell Theory and Prokaryotic Cells

The cell theory, based on the work of Schleiden, Schwann, and Virchow, states:

• All organisms are composed of cells.

• Cells are the basic units of organization of all organisms.

• Cells arise only from pre-existing cells.

Shared Features of Prokaryotic and Eukaryotic Cells

Both prokaryotic and eukaryotic cells possess:

• Plasma membrane

• Ribosomes

• DNA organized in chromosomes

Prokaryotic Cell Morphologies and Arrangements

Common Cell Shapes

Prokaryotic cells exhibit several distinct morphologies, which are important for identification and classification.

• Coccus: Spherical

• Bacillus: Rod-shaped

• Vibrio: Curved rod

• Spirillum: Spiral-shaped

• Spirochete: Long, flexible spiral

Cell Arrangements

Prokaryotic cells can be found in various arrangements depending on their division patterns.

• Coccus: Single spherical cell

• Diplococcus: Pair of cocci

• Tetrad: Group of four cocci

• Streptococcus: Chain of cocci

• Staphylococcus: Cluster of cocci

Microscopic Examples

Bacterial Cell Structure

Essential Components

All bacterial cells contain:

• Cell membrane

• Cytoplasm

• Ribosomes

• Chromosome (DNA) in the nucleoid

Additional Structures

Most bacterial cells also possess:

• Cell wall

• Plasmids

• Glycocalyx (slime layer or capsule)

Specialized Structures

Some bacteria have:

• Second cell membrane

• Flagella, pili, and/or fimbriae

• Inclusion bodies

Bacterial Cytoplasmic Structures

Nucleoid

• Region containing the prokaryotic chromosome (double-stranded, supercoiled DNA)

• Encodes for approximately 4,000 genes

Ribosomes

• Sites of protein synthesis

• Composed of rRNA and proteins

• Consist of a large (50S) and small (30S) subunit, forming a 70S ribosome

Protein Synthesis

Ribosomes catalyze the formation of peptide bonds between amino acids, resulting in polypeptide chains.

Plasmids

• Small, circular DNA molecules

• Encode non-essential functions (e.g., antibiotic resistance)

Inclusion Bodies

Serve as storage sites for nutrients and other substances.

• Granules: Store phosphate, glycogen, or sulfur

• Vesicles: Membrane-bound, may store gases or iron oxide

Cell Envelope: Structure and Function

Membrane Structure

The cell envelope consists of one or two plasma membranes and a cell wall.

• Membranes are composed of phospholipids and proteins

• Control transport in and out of the cell

Fluid Mosaic Model

The membrane is a dynamic, non-rigid structure with amphipathic phospholipids forming a bilayer.

• Integral proteins: Span the membrane, involved in transport and signaling

• Peripheral proteins: Attached to the surface, with various functions

Membrane Transport

The plasma membrane is selectively permeable due to the amphipathic nature of phospholipids.

• Controls movement of molecules

• Maintains cellular homeostasis

Transport Mechanisms

Diffusion

Simple diffusion allows small, non-charged molecules (e.g., O2, CO2) to move across the membrane down their concentration gradient.

• Passive process; no energy required

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane toward higher solute concentration.

• Can cause osmotic pressure, affecting cell volume and function

Facilitated Diffusion

Facilitated diffusion moves ions and molecules through membrane channels or carriers.

• Passive; no energy required

• Equilibrium is reached when solute concentration is equal on both sides

Active Transport

Active transport moves solutes against their concentration gradient (from low to high concentration).

• Requires energy (usually ATP)

• Involves specific transport proteins

Bacterial Appendages and Surface Structures

Glycocalyx

The glycocalyx is a coating of repeating polysaccharides or glycoproteins outside the cell membrane.

• Functions: attachment, protection, prevention of water loss, nutrient trapping

• Types: Slime layer (loose), Capsule (firm), S-layer (proteinaceous)

Cell Wall

The cell wall provides shape and structural support, preventing osmotic lysis.

• Contains peptidoglycan, a polymer of NAG and NAM

• Site of action for several antibiotics

Gram Staining and Cell Wall Types

Gram Positive vs. Gram Negative

Gram staining differentiates bacteria based on cell wall structure.

• Gram Positive: Thick peptidoglycan layer, teichoic acids, stains purple

• Gram Negative: Thin peptidoglycan layer, outer membrane, stains pink

Atypical Cell Walls

• Mycolic acid: Found in Mycobacterium and Nocardia; acid-fast staining required

• Contributes to resistance and pathogenicity

Bacterial Endospores

Endospore Formation and Germination

Some bacteria form endospores under nutrient-limited conditions.

• Vegetative state: Metabolically active

• Endospore: Dormant, highly resistant to heat, chemicals, radiation, and desiccation

• Sporulation: Formation of endospore

• Germination: Return to vegetative state when conditions improve

Bacterial Appendages

Flagella

Flagella are responsible for motility in aqueous environments.

• Arrangements: Monotrichous (single), Amphitrichous (both ends), Lophotrichous (tuft at one end), Peritrichous (all over)

• Movement: "Tumble and run" mechanism; chemotaxis and phototaxis

Pili and Fimbriae

• Composed of pilin protein

• Functions: Attachment, communication, conjugation (genetic exchange)

Archaea: Unique Features

Extremophiles

Archaea are often found in extreme environments.

• Thermophiles: High temperatures

• Halophiles: High salt or acid concentrations

• Methanogens: Produce methane, anaerobic

Archaeal Cell Walls

• May be composed of polysaccharides or pure protein

• Lack true peptidoglycan

• Some lack a cell wall entirely

Summary Table: Gram Positive vs. Gram Negative Cell Walls

Feature	Gram Positive	Gram Negative
Peptidoglycan Thickness	Thick (20–80 nm)	Thin (1–3 nm)
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Stain Color	Purple	Pink

Additional info: This guide expands on the original notes with definitions, examples, and academic context to support foundational understanding of prokaryotic cell biology.