Cell Types: Eukaryotic versus Prokaryotic

Organisms can be mostly classified into 2 groups, prokaryotes and eukaryotes, which have fundamental differences at the cellular level. Prokaryotes are unicellular organisms that include 2 of the 3 domains of life: bacteria and archaea. Eukaryotes can be single-celled or multicellular organisms and include plants, animals, fungi, and protozoa. Prokaryotic cells consist of a single cytoplasm-filled compartment enclosed by a cell membrane and cell wall, while eukaryotic cells contain a well-organized nucleus contained by a membrane, along with other membrane-bound organelles.

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Eukaryotic Organisms

Eukaryotic organisms include:

  • Protozoans
  • Algae
  • Fungi
  • Plants
  • Animals

Cellular organization

  • Unicellular
  • Multicellular

Cellular characteristics

  • Size:
    • Large
    • 0.5–100 μm in diameter
  • Organization:
    • Compartmentalized in membrane-bound organelles
    • Shape determined by cytoskeleton
    • Endomembrane system (endoplasmic reticulum, Golgi, lysosomes) containing large ribosomes for protein synthesis
  • DNA:
    • Linear
    • Structured in multiple rod-shaped chromosomes
    • Organized and packaged with histones
    • Chromosomes can be haploid (unpaired) or diploid (paired).
    • Stored in membrane-bound nucleus
    • Transcription occurs inside the nucleus; translation occurs in the cytosol.
  • Cell membrane: lipid bilayer 
  • Cell wall:
    • Only present in some cells
    • Composed of cellulose (plants and algae) or chitin (mollusks, insects, crustaceans, fungi)
  • Motility:
    • Flagella
    • Flexible projections made of microtubules
  • Cell division:
    • Meiosis 
    • Mitosis
  • Transport of matter across cell membrane:
    • Simple and facilitated diffusion
    • Phagocytosis
Eukaryotic cell

An eukaryotic cell and its components

Image: “Animal cell structure” by Mariana Ruiz. License: Public Domain

Prokaryotic Organisms

Prokaryotic organisms include bacteria and archea.

The cellular organization is unicellular.

Cellular characteristics

  • Size:
    • Small
    • 0.5–100 μm in diameter
  • Organization:
    • No true compartmentalization
    • Metabolic reactions occur freely in cytosol.
    • Ribosomes found free in cytosol
    • Lack cytoskeleton
    • Can have “inclusion bodies”: dense polymerized aggregates of proteins or nutrients
  • DNA:
    • Circular 
    • Organized and packaged with nucleoid-associated proteins (NAPs)                                         
    • Chromosomes are exclusively haploid (unpaired).
    • Single chromosome
    • May have additional extrachromosomal segments of DNA called plasmids
    • Found in the nucleoid (nonmembrane-bound area)
    • Transcription and translation both occur in the cytosol.
  • Cell membrane: lipid bilayer 
  • Cell wall:
    • Found in almost all cells
    • Often composed of peptidoglycan
  • Motility: rigid spiral flagella
  • Cell division: binary fission
  • Transport of matter across cell membrane: simple and facilitated diffusion
Structure of bacteria

Structure of a prokaryotic cell

Image: “Average prokaryote cell” by Mariana Ruiz Villarreal. License: Public Domain
Eukaryotes vs prokaryotes

Eukaryote and prokaryote cell comparison

Image: “The cells of eukaryotes (left) and prokaryotes (right)” by Science Primer. License: Public Domain

Clinical Relevance

  • Bacteria: prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Management of bacterial disease generally involves antibiotics; however, antibiotic choice may vary depending on the bacterial structure and metabolism.
  • Fungi: these organisms belong to the eukaryote domain and, like plants, have cell walls and vacuoles, exhibit cytoplasmic streaming, and are immobile. Almost all fungi, however, have cell walls composed of chitin, not cellulose. Fungi do not carry out photosynthesis but obtain their substrates for metabolism as saprophytes (i.e., they obtain their food from dead matter). An infection caused by a fungus is called a mycosis.

References

  1. Lodish H, Berk A, Zipursky SL, et al. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman. Section 1.3, The Architecture of Cells. https://www.ncbi.nlm.nih.gov/books/NBK21665/
  2. Vellai T, Vida G. (1999). The origin of eukaryotes: the difference between prokaryotic and eukaryotic cells. Proc Biol Sci. 266(1428):1571-7. doi: 10.1098/rspb.1999.0817

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