Functional Anatomy of Prokaryotic and Eukaryotic Cells: An Overview

Functional Anatomy of Prokaryotic and Eukaryotic Cells: An Overview

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells are the two fundamental cell types in biology, distinguished by their structural and functional characteristics. Understanding these differences is essential for identifying microorganisms and their biological roles.

• DNA Organization: Prokaryotes have a single, circular DNA molecule not enclosed in a nucleus, while eukaryotes possess multiple, linear chromosomes within a double membrane-bound nucleus.

• Histones: DNA packaging proteins called histones are absent in prokaryotes but present in eukaryotes.

• Organelles: Membrane-bound organelles are found only in eukaryotes.

• Cell Wall Composition: Prokaryotic cell walls contain peptidoglycan; eukaryotic cell walls, when present, are simpler (animal cells lack cell walls).

• Cell Division: Prokaryotes divide by binary fission; eukaryotes use mitosis.

Organelles are defined as membrane-bound subcellular compartments. In general, prokaryotic cells are structurally simpler than eukaryotic cells.

Characteristic	Prokaryotes	Eukaryotes
DNA	One circular molecule, not in nucleus	Many linear chromosomes, in nucleus
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan	Simpler or absent
Cell Division	Binary fission	Mitosis

Size, Shape, and Arrangement of Bacterial Cells

Bacterial identification often begins with cell shape and arrangement. These features are critical for classification and diagnosis.

• Shapes:

  • Coccus (plural: cocci): Spherical

  • Bacillus (plural: bacilli): Rod-shaped

  • Spiral/Helical:

    • Vibrio: Curved rod, comma-shaped

    • Spirilla: Helical and rigid

    • Spirochetes: Helical and flexible, corkscrew-shaped

• Arrangements of Cocci:

  • Diplo-: Pairs

  • Tetrad: Groups of four

  • Sarcinae: Groups of eight (two tetrads stacked)

  • Strepto-: Chains

  • Staphylo-: Clusters

• Arrangements of Bacilli:

  • Single

  • Diplo- (pairs)

  • Strepto- (chains)

  • Coccobacillus (short, almost spherical)

Structures External to the Cell Wall

Many bacteria possess external structures that contribute to their survival, pathogenicity, and identification.

• Glycocalyx: A sticky, gelatinous layer outside the cell wall, composed of polysaccharide, polypeptide, or both. It can be a capsule (firmly attached) or a slime layer (loosely attached).

• Role in Pathogenicity: Capsules enhance the ability to cause disease by protecting bacteria from phagocytosis. Absence of a capsule can render a strain non-pathogenic.

• Examples of Capsule-Forming Pathogens:

  • Streptococcus pneumoniae – pneumonia

  • Bacillus anthracis – anthrax

  • Klebsiella – pneumonia, UTIs

  • Streptococcus mutans – dental caries, endocarditis

  • Vibrio cholerae – cholera

• Flagella: Tail-like structures for locomotion, rotating to move the cell toward or away from stimuli. The H antigen on flagella is used for immune identification.

• Axial Filaments: Rope-like structures for movement, found in spirochetes such as Treponema pallidum (syphilis) and Borreliella burgdorferi (Lyme disease).

• Fimbriae: Hair-like appendages for adherence to surfaces. Example: Neisseria gonorrhoeae (gonorrhea), E. coli O157:H7 (diarrhea).

• Pili: Used for motility or DNA transfer (conjugation).

The Cell Wall

The cell wall is a rigid structure that maintains cell shape, prevents lysis, and contributes to pathogenicity. It is a major target for antibiotics.

• Function: Prevents osmotic lysis and provides structural support.

• Antibiotic Target: Many antibiotics, such as penicillin, target cell wall synthesis.

• Peptidoglycan: The main component of bacterial cell walls, consisting of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by amino acid cross-bridges.

Characteristic	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acid	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharides (LPS)	Absent	Present
Lipid A (Endotoxin)	Absent	Present
O Polysaccharide	Absent	Present
Lysozyme Susceptibility	Yes	No
Penicillin Susceptibility	High	Low
Gram Stain Color	Purple/Blue	Red

• Acid-Fast Bacteria: Mycobacterium and Nocardia have unique cell walls and are detected by acid-fast staining. Mycobacterium causes leprosy and tuberculosis.

• Bacteria Without Cell Walls: Mycoplasma species lack cell walls (e.g., Mycoplasma pneumoniae causes "walking pneumonia").

Structures Internal to the Cell Wall

Internal structures are essential for cellular function, genetic information, and survival under harsh conditions.

• Plasma Membrane: Also called the cytoplasmic or inner membrane, it acts as a selective barrier and contains proteins for transport and attachment. In Gram-negative bacteria, the outer membrane is also present.

• Antibiotic Target: Polymyxins disrupt the outer membrane of Gram-negative bacteria but have a narrow therapeutic index (risk of toxicity).

• Cytoplasm: The internal fluid containing enzymes, nutrients, and the cell's genetic material.

• Nucleoid: Region containing the bacterial chromosome (DNA) and often plasmids (small, circular DNA molecules that can carry antibiotic resistance genes).

• Ribosomes: Sites of protein synthesis. Bacterial ribosomes differ from eukaryotic ribosomes, making them targets for antibiotics such as streptomycin, gentamicin, and erythromycin.

• Endospores: Highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Clostridium botulinum, Bacillus species) to survive extreme conditions. Endospores are resistant to boiling and many disinfectants.

The Cell Wall and Glycocalyx in Eukaryotes

Eukaryotic cell walls, when present (e.g., in fungi and plants), do not contain peptidoglycan. Yeast cell walls are an example of this difference.

Ribosomes as Antibiotic Targets

Bacterial ribosomes are structurally distinct from eukaryotic ribosomes, allowing selective targeting by antibiotics such as erythromycin.

The Evolution of Eukaryotes

The origin of eukaryotic cells is explained by the endosymbiotic theory, which proposes that certain organelles (e.g., mitochondria, chloroplasts) originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Endosymbiotic Theory: Mitochondria and chloroplasts have their own DNA and ribosomes, supporting their prokaryotic ancestry.