Introduction to Prokaryotic Cells

Overview

Prokaryotic cells are fundamental to the study of microbiology, encompassing the domains Bacteria and Archaea. These cells are structurally simpler than eukaryotic cells, lacking membrane-bound organelles and a nucleus. Understanding their structure, function, and diversity is essential for identifying pathogens and understanding microbial physiology.

Prokaryotic Cell Basics

Domains of Prokaryotes

• Bacteria and Archaea are the two domains of prokaryotic life.

• Both are unicellular and lack a membrane-bound nucleus and organelles.

• Despite similarities, Archaea and Bacteria differ in membrane composition, cell wall structure, and genetic machinery.

Example: Mycoplasma (Bacteria) and Halobacterium (Archaea) are both prokaryotes but differ significantly in their cell envelope structures.

Prokaryotic Cell Structure and Size

• Prokaryotic cells typically range from 0.2 to 750 μm in diameter; most are between 0.5 and 2.0 μm.

• Mycoplasma species are among the smallest, while Thiomargarita namibiensis is among the largest.

• Prokaryotes lack defined organelles but may have protective layers such as plasma membrane, cell wall, and sometimes a capsule.

Shapes and Arrangements

• Shapes:

  • Bacilli – rod-shaped

  • Cocci – spherical

  • Vibrio – comma-shaped

  • Stella – star-shaped

  • Coccobacilli – oval

  • Spirillum – spiral-shaped, rigid

  • Spirochete – spiral-shaped, flexible

• 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

Example: Streptococcus pyogenes forms chains of spherical cells (streptococci).

Surface Area-to-Volume Ratio

• Most nutrients enter prokaryotic cells by diffusion, favoring small cell size due to a high surface area-to-volume ratio.

• As cell size increases, the ratio decreases, limiting efficient nutrient uptake and waste removal.

Equation: For a cube,

Binary Fission

Prokaryotic cells primarily reproduce by binary fission, an asexual process resulting in two genetically identical daughter cells.

• Steps of binary fission:

  1. DNA is copied.

  2. Cell grows.

  3. Copied chromosome is drawn to opposite ends of the cell.

  4. Septum (partition) forms at the midpoint.

  5. Septum completes, dividing the cell into two daughter cells.

Extracellular Structures

Plasma Membrane Structure and Function

• The plasma membrane is a thin, flexible, phospholipid bilayer with selective permeability.

• Proteins in the membrane function as transporters, anchors, receptors, and enzymes.

• Prokaryotic plasma membranes are also sites for metabolic reactions (e.g., ATP synthesis).

• Plasma membranes exhibit selective permeability—gases, water, and small noncharged substances diffuse freely; ions and larger polar substances require transport proteins.

Membrane Fluidity

• The lipid bilayer is a fluid-mosaic—lipids and proteins move laterally within the layer.

• Fluidity is essential for cell function and is affected by:

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

  • Fatty acid content: Unsaturated fatty acids increase fluidity (especially in cold); saturated fatty acids decrease fluidity (more rigid in cold).

Archaeal Plasma Membranes

• Bacterial phospholipids: linear fatty acids joined by ester bonds.

• Archaeal phospholipids: long-branched fatty acids joined by ether bonds (more stable in extreme environments).

• Certain Archaea form monolayers instead of bilayers for added stability.

Cell Wall Structure and Function

• Most prokaryotes have a rigid cell wall outside the plasma membrane.

• Bacterial cell walls contain peptidoglycan (protein and sugar complex).

• Archaeal cell walls contain pseudomurein or other unique polymers.

Gram Staining and Cell Wall Types

• Gram staining differentiates bacteria based on cell wall structure:

• Clinical implications: Gram-negative bacteria are generally more resistant to antibiotics and disinfectants due to their outer membrane.

Acid-Fast Staining

• Detects a waxy lipid called mycolic acid in cell walls (e.g., Mycobacterium).

• Acid-fast bacteria resist conventional Gram staining and require special stains.

• Drugs do not easily cross the waxy cell wall, making infections harder to treat.

Mycoplasma and L-Forms

• Mycoplasma species lack a cell wall, have sterol-enriched membranes, and are pleomorphic.

• L-forms are bacteria that lose their cell wall, often becoming resistant to antibiotics targeting cell wall synthesis.

Transport Across Cell Wall and Plasma Membrane

Passive Transport

• Simple diffusion: Movement of small, nonpolar molecules down their concentration gradient without energy input.

• Facilitated diffusion: Movement of substances down their concentration gradient via membrane proteins (channels or carriers).

Osmosis

• Diffusion of water across a selectively permeable membrane from low solute to high solute concentration.

• Isotonic: No net water movement.

• Hypertonic: Water leaves the cell; cell shrinks (plasmolysis).

• Hypotonic: Water enters the cell; cell may burst (osmotic lysis) if the wall is damaged or missing.

Active Transport

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

• Types:

  • Primary active transport: Direct use of ATP (e.g., proton pumps).

  • Secondary active transport: Uses energy from an ion gradient (e.g., symporters and antiporters).

  • Phosphotransferase system: Transfers a phosphate group to the substance being transported (commonly used for glucose uptake).

External Structures for Adhesion, Movement, and Protection

Flagella

• Filamentous structures built from flagellin protein; provide motility.

• Arrangements:

  • Monotrichous: Single flagellum

  • Lophotrichous: Tuft at one pole

  • Amphitrichous: Flagella at both poles

  • Peritrichous: Flagella all over the cell surface

• Run and tumble system: Allows cells to move toward or away from stimuli (chemotaxis, phototaxis, aerotaxis).

• Periplasmic flagella (axial filaments): Located in the periplasmic space; allow spirochetes to move in a corkscrew motion.

Fimbriae and Pili

• Fimbriae: Short, bristle-like structures for adhesion to surfaces or host tissues; important for colonization and biofilm formation.

• Pili: Longer, less numerous; used for attachment and DNA transfer (conjugation).

Glycocalyx

• Sticky carbohydrate-enriched layer outside the cell wall.

• Slime layer: Unorganized and loosely attached.

• Capsule: Well-organized and tightly associated; protects against desiccation and immune defenses.

Intracellular Structures

Nucleoid Region

• Area within the cytoplasm where the prokaryotic chromosome (usually a single, circular DNA molecule) is located.

• Lacks a surrounding membrane (not a true nucleus).

Ribosomes

• Sites of protein synthesis; composed of rRNA and proteins.

• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits).

Cytoskeleton

• Composed of long protein filaments; provides structure, support, and aids in cell division and shape maintenance.

Inclusion Bodies

• Distinct collections of substances (e.g., nutrients, enzymes) stored within the cell.

• Examples:

  • Carboxysomes: Contain carbon-fixing enzymes.

  • Magnetosomes: Contain magnetic iron for orientation.

Sporulation and Endospores

• Sporulation: Process of forming an endospore—a dormant, highly resistant structure.

• Medically relevant genera: Bacillus and Clostridium.

• Endospores survive extreme conditions (heat, desiccation, chemicals).

• Steps:

  1. Copying DNA

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

  3. Surrounding the spore core with several heat- and chemical-resistant layers

  4. Release of the mature endospore

• High dipicolinic acid content stabilizes DNA and adds heat resistance.

Clinical Relevance

• Understanding prokaryotic structure aids in pathogen identification and informs treatment strategies (e.g., antibiotic selection based on Gram status).

• Clinical cases often require knowledge of cell wall structure, staining properties, and external features for accurate diagnosis.