Prokaryotic and Eukaryotic Cells

Overview of Cell Types

Bacterial cells are classified into two major types based on their cellular organization: prokaryotic cells and eukaryotic cells. Understanding the differences between these cell types is fundamental in microbiology.

• Prokaryotic cells: Lack a true nucleus and membrane-bound organelles. Bacteria and Archaea are prokaryotes.

• Eukaryotic cells: Possess a true nucleus and membrane-bound organelles. Examples include fungi, protozoa, plants, and animals.

Size, Shape, and Arrangement of Bacterial Cells

Bacterial Size

Bacteria vary in size, typically ranging from 0.2 to 2.0 micrometers in diameter. Size can influence nutrient uptake and growth rate.

Major Prokaryotic Shapes and Arrangements

Bacteria are classified by their shapes and arrangements, which are important for identification and understanding their physiology.

• Coccus (pl. cocci): Spherical-shaped bacteria.

  • Arrangements: diplococci (pairs), streptococci (chains), tetrads (groups of four), sarcinae (cubical packets), staphylococci (clusters).

• Bacillus (pl. bacilli): Rod-shaped bacteria.

  • Arrangements: diplobacilli (pairs), streptobacilli (chains), coccobacilli (short rods).

• Spiral: Curved or spiral-shaped bacteria.

  • Types: vibrio (comma-shaped), spirillum (rigid spiral), spirochete (flexible spiral).

• Unusual shapes: Some bacteria exhibit unique shapes and can be monomorphic (single shape) or pleomorphic (variable shapes).

The Structure of a Prokaryotic Cell

External to the Cell Wall

Prokaryotic cells possess several structures external to the cell wall that contribute to their survival and pathogenicity.

• Glycocalyx: A gelatinous polymer surrounding the cell wall.

  • Capsule: Organized and firmly attached. Some strains of Streptococcus pneumoniae produce a capsule, which increases pathogenicity by protecting against phagocytosis.

  • Slime layer: Unorganized and loosely attached.

• Flagella (flagellum): Long, whip-like appendages used for motility.

  • Enable movement via chemotaxis (movement toward/away from chemicals) and phototaxis (movement toward/away from light).

• Fimbriae: Short, hair-like structures used for attachment to surfaces.

Cell Wall Structure and Function

The bacterial cell wall is a complex, semi-rigid structure that maintains cell shape, protects against osmotic lysis, and contributes to pathogenicity.

• Composition: Primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Functions:

  • Maintains shape

  • Prevents rupture from osmotic pressure

  • Provides clinical relevance (e.g., target for antibiotics)

Major Groups Based on Gram Staining

Bacteria are classified into two major groups based on the Gram stain, a differential staining technique.

• Gram-positive cells:

  • Thick peptidoglycan layer

  • No outer membrane

  • Penicillin sensitive

  • Stain purple in Gram stain

• Gram-negative cells:

  • Thin peptidoglycan layer

  • Outer membrane present (contains lipopolysaccharide)

  • Penicillin resistant

  • Stain pink/red in Gram stain

Atypical Cell Walls

Some bacteria lack typical cell walls or have unusual wall structures.

• No cell wall:

  • Mycoplasma genus: lacks cell wall, has sterols in plasma membrane

  • Archaea: may lack cell walls or have unusual walls

• Acid-fast cell walls:

  • Present in Mycobacterium tuberculosis

  • High lipid content (mycolic acid)

  • Stain red with acid-fast stain, will not take up dyes for Gram stain

Key Terms and Definitions

• Peptidoglycan: A polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria.

• Gram stain: A differential staining technique that classifies bacteria as Gram-positive or Gram-negative based on cell wall properties.

• Capsule: A well-organized glycocalyx that protects bacteria from phagocytosis and increases virulence.

• Fimbriae: Short, bristle-like structures that help bacteria adhere to surfaces.

• Flagella: Long, whip-like appendages that provide motility.

• Acid-fast stain: A differential stain used to identify bacteria with high lipid content in their cell walls, such as Mycobacterium.

Relevant Equations

• Bacterial cell volume (approximate for a sphere):

• Bacterial surface area (approximate for a sphere):

Examples and Applications

• Pathogenicity: Encapsulated strains of Streptococcus pneumoniae are more likely to cause disease due to resistance to phagocytosis.

• Antibiotic targeting: Penicillin targets peptidoglycan synthesis, making Gram-positive bacteria more susceptible.

• Identification: Gram staining is a routine laboratory method for classifying and identifying bacteria.

Additional info: Academic context and definitions have been expanded for clarity and completeness.