Classification and Characteristics of Prokaryotes and Eukaryotes

Classification of Microorganisms

The Three-Domain System

The three-domain system is a fundamental classification scheme in microbiology, dividing all life into three domains: Bacteria, Archaea, and Eukarya. This system is based on differences in ribosomal RNA (rRNA) sequences and other molecular characteristics.

• Bacteria: Includes most prokaryotic organisms, characterized by peptidoglycan cell walls.

• Archaea: Prokaryotes with unique membrane lipids and often found in extreme environments; lack peptidoglycan in cell walls.

• Eukarya: Includes all eukaryotic organisms such as plants, animals, fungi, and protists; cells contain a nucleus and membrane-bound organelles.

Key Concepts:

• All organisms evolved from cells that formed over 3 billion years ago.

• Horizontal gene transfer played a significant role in early cell evolution.

• The domain Eukarya includes kingdoms Fungi, Plantae, Animalia, and Protista.

• Bacteria and Archaea are prokaryotes, while Eukarya are eukaryotes.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Characteristics of Archaea, Bacteria, and Eukarya

Microorganisms are classified based on their cellular structure and biochemical properties. The table below summarizes the main differences among Archaea, Bacteria, and Eukarya.

Feature	Archaea	Bacteria	Eukarya
Cell Type	Prokaryotic	Prokaryotic	Eukaryotic
Cell Wall	Varies in composition; contains no peptidoglycan	Contains peptidoglycan	Varies in composition; contains carbohydrates
Membrane Lipids	Composed of branched carbon chains attached to glycerol by ether linkage	Composed of straight carbon chains attached to glycerol by ester linkage	Composed of straight carbon chains attached to glycerol by ester linkage
First Amino Acid in Protein Synthesis	Methionine	Formylmethionine	Methionine
Antibiotic Sensitivity	No	Yes	No
rRNA Loop	Lacking	Present	Lacking
Common Arm of tRNA	Lacking	Present	Present

Comparison of Prokaryotic Cells and Eukaryotic Organelles

Prokaryotic Cells vs. Eukaryotic Cells and Organelles

Prokaryotic cells differ from eukaryotic cells in several fundamental ways. Some eukaryotic organelles, such as mitochondria and chloroplasts, share similarities with prokaryotic cells, supporting the endosymbiotic theory of organelle origin.

Feature	Prokaryotic Cell	Eukaryotic Cell	Eukaryotic Organelles (Mitochondria & Chloroplasts)
DNA	One circular; some linear	Linear	Circular
Histones	Absent	Present	Absent
First Amino Acid in Protein Synthesis	Formylmethionine (bacteria)	Methionine	Formylmethionine
Ribosomes	70S	80S	70S
Growth	Binary fission	Mitosis	Binary fission

Classification of Selected Prokaryotes

Gram-Negative Bacteria

Gram-negative bacteria are characterized by a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. They are classified based on their genetic and phenotypic properties.

• Examples include Proteobacteria, Cyanobacteria, and Chlamydiae.

• Gram-negative bacteria are often resistant to antibiotics due to their outer membrane.

Gram-Positive Bacteria

Gram-positive bacteria have a thick peptidoglycan layer and lack an outer membrane. They are classified into groups such as Firmicutes and Actinobacteria.

• Examples include Bacillus, Staphylococcus, and Streptococcus.

• Gram-positive bacteria are generally more susceptible to antibiotics targeting cell wall synthesis.

Domain: Archaea

Archaea are prokaryotes distinct from bacteria, often found in extreme environments such as hot springs, salt lakes, and anaerobic conditions.

• Major groups include methanogens, halophiles, and thermophiles.

• Archaea lack peptidoglycan in their cell walls and have unique membrane lipids.

Additional info: The classification of prokaryotes is based on genetic, biochemical, and structural characteristics. Modern taxonomy uses molecular techniques such as 16S rRNA sequencing to determine evolutionary relationships.