Functional Anatomy of Prokaryotic and Eukaryotic Cells: An Overview

Functional Anatomy of Prokaryotic and Eukaryotic Cells

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: Prokaryotes lack membrane-bound organelles; eukaryotes contain various organelles (e.g., mitochondria, endoplasmic reticulum).

• Cell Wall Composition: Prokaryotic cell walls contain peptidoglycan; eukaryotic cell walls, if 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 that perform specialized functions.

In general, prokaryotic cells are structurally simpler than eukaryotic cells.

Characteristic	Prokaryotes	Eukaryotes
DNA	One circular molecule, not in a nucleus	Many linear chromosomes, inside nucleus
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan	Simpler (if present)
Cell Division	Binary fission	Mitosis

Size, Shape, and Arrangement of Bacterial Cells

Bacterial identification often begins with cell shape and arrangement. These features are key 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-

  • Strepto-

  • Coccobacillus: Short, almost spherical rods

Structures External to the Cell Wall

Many bacteria possess structures outside their cell wall that contribute to survival, pathogenicity, and identification.

• Glycocalyx: A sticky, gelatinous layer composed of polysaccharide, polypeptide, or both. It can be a capsule (firmly attached) or a slime layer (loosely attached). Capsules enhance pathogenicity by protecting bacteria from phagocytosis.

• 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; rotate to move the cell toward or away from stimuli (chemotaxis). The H antigen on flagella is used for immune identification.

• Axial Filaments: Rope-like structures for movement, found in spirochetes (e.g., Treponema pallidum – syphilis; Borreliella burgdorferi – Lyme disease).

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

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

The Cell Wall

The cell wall provides structural support, prevents osmotic lysis, and is a major target for antibiotics. Its composition varies between Gram-positive and Gram-negative bacteria.

• Peptidoglycan: A mesh-like polymer of sugars and amino acids. The main sugars are N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by amino acid cross-bridges.

• Functions: Maintains cell shape, protects against osmotic pressure, and contributes to pathogenicity.

• Antibiotic Target: Many antibiotics (e.g., penicillin) inhibit peptidoglycan synthesis.

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; acid-fast staining is used for identification. Mycobacterium causes leprosy and tuberculosis.

• No Cell Wall: Mycoplasma species lack cell walls (e.g., Mycoplasma pneumoniae – "walking pneumonia").

Structures Internal to the Cell Wall

Internal structures are essential for cell function, survival, and reproduction.

• Plasma Membrane: Also called the cytoplasmic or inner membrane. Acts as a selective barrier and contains proteins for transport and signaling. Targeted by antibiotics such as polymyxins (notable for a narrow therapeutic index).

• Cytoplasm: The gel-like substance inside the cell membrane, containing enzymes, nutrients, and the cell's genetic material.

• Nucleoid: Region containing the bacterial chromosome (DNA). May also contain 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 (e.g., streptomycin, gentamicin, erythromycin).

• Endospores: Highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Clostridium botulinum, Bacillus species). Endospores can survive extreme conditions and are not destroyed by boiling.

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

Ribosomes are a major target for antibiotics due to structural differences between prokaryotic and eukaryotic ribosomes. For example, erythromycin inhibits bacterial protein synthesis without affecting eukaryotic cells.

The Evolution of Eukaryotes

The origin of eukaryotic cells is explained by the endosymbiotic theory, which proposes that organelles such as mitochondria and chloroplasts originated from symbiotic prokaryotes engulfed by ancestral eukaryotic cells.

Additional info: The endosymbiotic theory is supported by evidence such as the presence of circular DNA and prokaryote-like ribosomes in mitochondria and chloroplasts.