Prokaryotic Cell Basics

Overview of Prokaryotic Domains

Prokaryotic cells are fundamental to life on Earth, having originated approximately 3.8 billion years ago. They are classified into two distinct domains: Bacteria and Archaea.

• Bacteria and Archaea are both unicellular and lack a membrane-bound nucleus and organelles.

• One key difference: Bacteria have cell walls containing peptidoglycan, while Archaea have pseudopeptidoglycan.

• One similarity: Both domains are prokaryotic and share basic cellular structure.

Prokaryotic Cell Structure and Size

Prokaryotic cells are typically small, which allows for efficient nutrient uptake and waste removal.

• Most prokaryotes range from 0.2 to 2.0 μm in diameter.

• Small size is advantageous for rapid growth and adaptation.

Shapes and Arrangements of Prokaryotes

The shape and arrangement of prokaryotic cells are important for identification and pathogenicity.

• Monomorphic bacteria have a single, consistent shape.

• Pleomorphic bacteria can vary in shape, enhancing survival and transmission.

• Common shapes:

  • Bacilli (rod-shaped)

  • Cocci (spherical)

  • Vibrio (comma-shaped)

  • Stella (star-shaped)

  • Coccobacilli (ovoid)

  • Spirochetes (spiral-shaped)

• Arrangements:

  • Diplococci (paired cocci)

  • Streptococci (chains of cocci)

  • Staphylococci (grapelike clusters)

  • Diplobacilli (paired bacilli)

  • Streptobacilli (chains of bacilli)

  • Palisade (clusters of bacilli)

Binary Fission

Prokaryotic cells reproduce primarily by binary fission, a simple and efficient process.

• DNA is copied.

• Cell grows and chromosomes move to opposite ends.

• A septum forms at the midpoint, dividing the cell.

• Two daughter cells are produced.

Extracellular Structures

Plasma Membrane Structure and Function

The plasma membrane is a thin, flexible phospholipid bilayer that acts as a selective barrier.

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

• Site for metabolic reactions, including ATP synthesis.

• Selective permeability: allows gases, water, and small noncharged substances to diffuse; larger or charged substances require transport proteins.

Membrane Fluidity

Membrane fluidity is essential for cell function and is influenced by temperature and fatty acid composition.

• Warmer temperatures increase fluidity; colder temperatures decrease it.

• Unsaturated fatty acids improve fluidity in cold conditions; saturated fatty acids make membranes more rigid.

Archaea Plasma Membranes

• Bacterial membranes contain linear fatty acids; archaeal membranes contain long-branched fatty acids.

• Some archaea form lipid monolayers for stability in extreme environments.

Cell Wall Structure and Function

Cell walls provide rigidity and protection.

• Bacteria: cell wall contains peptidoglycan.

• Archaea: cell wall contains pseudopeptidoglycan.

Gram Staining and Acid-Fast Properties

Gram staining is a key clinical tool for classifying bacteria based on cell wall structure.

• Gram-positive: thick peptidoglycan layer, stains purple, lacks outer membrane.

• Gram-negative: thin peptidoglycan layer, stains red/pink, has outer membrane with lipopolysaccharide.

• Acid-fast staining detects mycolic acid in cell walls (e.g., Nocardia, Mycobacterium).

• Acid-fast bacteria grow slowly and require long, multidrug therapies.

Clinical Implications of Gram Status

• Gram-negative bacteria are harder to kill due to their outer membrane.

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

• Teichoic acids in Gram-positive cell walls stabilize structure and aid in cell division.

Table: Comparing Gram-Negative and Gram-Positive Bacteria

Transport Mechanisms

Passive and Active Transport

• Passive transport: no energy required; includes diffusion and osmosis.

• Active transport: requires energy; includes primary (ATP-driven), secondary (ion gradient-driven), and phosphotransferase systems.

Diffusion

• Simple diffusion: movement of small, noncharged molecules.

• Facilitated diffusion: uses membrane proteins (channels/carriers).

Osmosis

• Water moves from low solute to high solute concentration.

• Isotonic: no net water movement.

• Hypertonic: cell loses water, plasmolysis occurs.

• Hypotonic: cell gains water, osmotic lysis may occur.

Active Transport Mechanisms

• Primary: uses ATP.

• Secondary: uses ion gradients (symport and antiport).

• Phosphotransferase: transfers phosphate to transported substance (e.g., glucose).

External Structures for Adhesion, Movement, and Protection

Flagella

• Flagella are protein filaments for motility, powered by a rotary mechanism.

• Gram-positive bacteria have two rings; Gram-negative have four rings for anchoring.

• Movement types: chemotaxis (chemical), phototaxis (light), aerotaxis (oxygen).

• Arrangements: monotrichous (single), lophotrichous (tuft), amphitrichous (both poles), peritrichous (all over).

• Periplasmic flagella (axial filaments) allow spirochetes to move in corkscrew motion.

Fimbriae and Pili

• Fimbriae: short, bristle-like, for adhesion and biofilm formation.

• Pili: longer, fewer, for adhesion, movement, and gene transfer (conjugation).

Glycocalyx

• Sticky carbohydrate layer for adhesion, protection, and resistance to desiccation.

• Slime layer: unorganized; capsule: organized and tightly associated.

Intracellular Structures

Nucleoid

• Region containing single, circular chromosome.

Ribosomes

• 70S ribosomes (50S + 30S subunits) synthesize proteins.

Cytoskeleton

• Protein filaments provide structure and support.

Inclusion Bodies

• Storage granules for substances (e.g., carboxysomes, magnetosomes).

Endospores

• Metabolically inactive, highly resistant structures for survival in harsh conditions.

• Produced by Bacillus, Clostridium, and Clostridioides genera.

• Endospores can survive for extended periods, posing challenges in healthcare.

Sporulation

• DNA is copied and packaged with ribosomes and enzymes.

• Spore core is surrounded by resistant layers and released.

Visual Summary: Introduction to Prokaryotic Cells

• Prokaryotic cells are diverse, efficient, and clinically significant.

• Understanding their structure and function is essential for microbiology and healthcare.

Additional info: Some content was inferred and expanded for completeness and clarity, including typical prokaryotic cell sizes, sporulation steps, and clinical implications.