Introduction to Prokaryotic Cells: Structure, Function, and Diversity

Introduction to Prokaryotic Cells

Overview of Prokaryotes

Prokaryotic cells are among the earliest forms of life on Earth, dating back approximately 3.8 billion years. They are classified into two domains: Bacteria and Archaea. Unlike eukaryotic cells, prokaryotes lack a membrane-bound nucleus and organelles, and are typically unicellular.

• Bacteria and Archaea are the two main types of prokaryotic cells.

• Eukaryotic cells belong to the domain Eukarya.

• Prokaryotes are characterized by their simple structure and lack of compartmentalization.

Prokaryotic Cell Morphology

Size, Shape, and Arrangement

Prokaryotic cells exhibit a wide range of sizes, shapes, and arrangements, which are important for their survival and transmission.

• Monomorphic bacteria have a single, consistent shape.

• Pleomorphic bacteria can change shape, aiding in adaptation and transmission.

• Typical size range: 0.2 – 750 nm (most between 0.5 – 2.0 μm).

• Smallest: Mycoplasma species; Largest: Thiomargarita namibiensis.

Common Shapes

• Bacilli (rod-shaped)

• Cocci (spherical)

• Vibrio (comma-shaped)

• Stella (star-shaped)

• Coccobacilli (ovoid)

• Spirochetes (spiral-shaped, corkscrew motion)

Cell Arrangements

• Diplococci: pairs of cocci

• Streptococci: chains of cocci

• Staphylococci: grape-like clusters

• Diplobacilli: pairs of bacilli

• Streptobacilli: chains of bacilli

• Palisade: clusters of bacilli

Prokaryotic Cell Division

Binary Fission

Prokaryotic cells reproduce asexually through binary fission, a process that ensures genetic continuity.

1. DNA is replicated.

2. Cell grows in size.

3. Chromosomes are segregated to opposite ends.

4. A septum forms at the midpoint.

5. The septum divides the cell into two genetically identical daughter cells.

Cell Envelope: Plasma Membrane and Cell Wall

Plasma Membrane

The plasma membrane is a thin, flexible phospholipid bilayer that acts as a selective barrier, controlling the entry and exit of substances.

• Composed of phospholipids and proteins (proteins can make up half the membrane mass).

• Functions of membrane proteins: transport, anchoring, signal reception, enzymatic activity.

• Site of metabolic reactions, including ATP synthesis.

• Exhibits selective permeability: allows gases, water, and small nonpolar molecules to diffuse freely; ions and large polar molecules require transport proteins.

Membrane Fluidity

• Described as a "fluid-mosaic" model; lipids and proteins move laterally within the layer.

• Fluidity is essential for function and is influenced by:

  • Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

  • Fatty acid composition: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

  • Bacteria: Linear fatty acids; Archaea: Branched fatty acids.

  • Certain Archaea form lipid monolayers for extreme environments.

Cell Wall Structure and Function

• Provides rigidity and protection.

• Bacteria: Cell wall contains peptidoglycan.

• Archaea: Cell wall contains pseudopeptidoglycan.

Gram Staining and Cell Wall Types

Gram staining differentiates bacteria based on cell wall structure, which is clinically significant.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (20–80 nm)	Thin (2–8 nm)
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Teichoic Acids	Present	Absent
Gram Stain Color	Purple	Pink/Red

• Gram-negative bacteria are generally more resistant to chemicals due to their outer membrane.

• Gram-positive bacteria are more sensitive to agents targeting peptidoglycan but retain moisture and resist mechanical stress better.

Acid-Fast Bacteria

• Contain mycolic acid (waxy lipid) in their cell walls.

• Detected by acid-fast staining (cells appear red/pink).

• Examples: Mycobacterium, Nocardia.

• Grow slowly and are resistant to many drugs due to their impermeable cell wall.

Cell Wall Variants: Mycoplasma and L-forms

• Mycoplasma: Lack a cell wall, have sterol-rich membranes, are pleomorphic, and often live inside host cells.

• L-forms: Bacteria that have lost their cell wall; can persist in hostile environments and evade antibiotics targeting cell wall synthesis.

Transport Across the Plasma Membrane

Passive Transport

• Simple Diffusion: Movement of small, noncharged molecules, gases, and lipid-soluble substances down their concentration gradient.

• Facilitated Diffusion: Movement of substances down their gradient with the help of membrane proteins (channels or carriers).

• Osmosis: Diffusion of water from areas of low solute concentration to high solute concentration.

Active Transport

• Requires energy (often ATP) to move substances against their concentration gradient via transport proteins.

External Structures of Prokaryotic Cells

Flagella

• Filamentous structures made of flagellin; provide motility.

• Move via a rotary propeller mechanism.

• Enable movement toward or away from stimuli (chemotaxis, phototaxis, aerotaxis).

Arrangement	Description
Monotrichous	Single flagellum
Lophotrichous	Tuft of flagella at one pole
Amphitrichous	Flagella at both poles
Peritrichous	Flagella all over the cell surface

Fimbriae

• Short, bristle-like protein structures.

• Enable adhesion to surfaces and biofilm formation.

• Common in Gram-negative bacteria.

Pili

• Longer, less numerous than fimbriae.

• Involved in adhesion, movement, and gene transfer (conjugation).

Glycocalyx

• Sticky, carbohydrate-rich layer outside the cell wall.

• Protects against desiccation, antibiotics, and immune responses.

• Slime layer: Loosely organized; Capsule: Well-organized and tightly attached.

Intracellular Structures

Nucleoid Region

• Location of the single, circular DNA chromosome in prokaryotes.

Ribosomes

• Sites of protein synthesis; composed of RNA and protein.

• Prokaryotic ribosomes are 70S, made of 50S (large) and 30S (small) subunits.

Cytoskeleton

• Network of protein filaments providing structural support and shape.

Inclusion Bodies

• Storage sites for nutrients and other substances.

• May be membrane-bound or insoluble granules.

Endospores

• Metabolically inactive, highly resistant structures formed by certain bacteria (e.g., Bacillus, Clostridium).

• Enable survival under extreme conditions (heat, desiccation, chemicals).

• Can persist on surfaces for long periods, posing challenges in healthcare settings.

Sporulation Process

1. DNA replication

2. Packaging of DNA, ribosomes, and enzymes into the spore coat

3. Formation of protective layers

4. Release of the mature endospore

• When conditions improve, endospores germinate into vegetative cells.

• Medically important spore-formers include Bacillus anthracis (anthrax), Clostridium tetani (tetanus), Clostridium botulinum (botulism), Clostridium perfringens (gas gangrene), and Clostridioides difficile (severe diarrhea).

Additional info: The notes above expand on the original outline by providing definitions, examples, and clinical relevance for each structure and process. Where slides were referenced, standard textbook content was inferred to ensure completeness.