Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and other specialized organelles. These microscopic life forms exhibit immense diversity, found in nearly every environment on Earth, from common soil to extreme habitats. Their widespread presence and varied roles make them fundamental components of global ecosystems. These organisms are divided into two distinct biological domains.
Understanding Life’s Major Divisions
Carl Woese’s three-domain system significantly revised life’s classification, categorizing all cellular life into Bacteria, Archaea, and Eukarya. Woese proposed this system after discovering fundamental differences in ribosomal RNA (rRNA) sequences, indicating deep evolutionary divergences.
Bacteria and Archaea comprise the two prokaryotic domains. The third domain, Eukarya, includes all organisms whose cells possess a membrane-bound nucleus, such as plants, animals, fungi, and protists.
This classification, based on molecular evidence and cellular biochemistry, replaced earlier systems that grouped all prokaryotes. The distinct genetic and biochemical characteristics of Bacteria and Archaea reveal their separate evolutionary paths, despite their shared prokaryotic cell structure.
Characteristics of Bacteria
Bacteria are single-celled organisms found in nearly every habitat on Earth. Their cell walls typically contain peptidoglycan, a unique polymer providing structural support. Bacterial cell membranes primarily consist of fatty acids linked to glycerol by ester bonds, forming a lipid bilayer.
Bacteria exhibit diverse metabolic capabilities, thriving in varied conditions. Many are heterotrophs, photosynthesizers, or chemosynthesizers.
Examples include Escherichia coli, a common gut bacterium, and Lactobacillus. Nitrogen-fixing bacteria convert atmospheric nitrogen into usable forms, and some species are pathogens.
Characteristics of Archaea
The Domain Archaea consists of single-celled organisms that share a prokaryotic cell structure but possess unique features distinguishing them from bacteria. Their cell walls lack peptidoglycan, composed instead of materials like pseudopeptidoglycan, glycoproteins, or S-layers. Archaeal membranes feature ether-linked lipids with branched isoprenoid chains, contrasting with bacterial ester-linked fatty acids.
Archaea are renowned for inhabiting extreme environments, earning them the label “extremophiles,” though many also reside in moderate conditions. They are found in hot springs, highly saline lakes, and anaerobic swamps.
Their diverse metabolic strategies include methanogenesis, a process unique to archaea. Examples include methanogens, halophiles, and thermophiles.
Distinguishing Bacteria from Archaea
Bacteria and Archaea are distinct domains due to fundamental differences in their cellular composition and genetic machinery. A primary distinction lies in their cell wall composition; bacterial cell walls typically contain peptidoglycan, a polymer absent in archaeal cell walls. Instead, archaea have cell walls made of diverse materials like pseudopeptidoglycan or proteins.
Differences extend to their cell membranes as well. Bacterial membranes feature ester-linked fatty acids, while archaeal membranes are characterized by ether-linked phytanyl chains, which can form unique monolayer structures. This ether linkage in archaea provides increased stability, contributing to their survival in harsh environments.
The genetic machinery also differs significantly; Archaea exhibit more similarities with eukaryotes in their ribosomal RNA sequences and gene expression processes, including the complexity of their RNA polymerase and the presence of introns in some genes. These molecular distinctions highlight their separate evolutionary lineages.