The classification of all life on Earth begins at the cellular level, distinguishing organisms based on their internal architecture. Life is broadly divided into two foundational cell types: prokaryotic and eukaryotic. Prokaryotic cells are simpler and more ancient, while eukaryotic cells are characterized by greater internal complexity and compartmentalization. Understanding which organisms possess the prokaryotic structure requires looking at the highest level of biological organization, a system that groups all living things based on their evolutionary and genetic histories.
Defining Prokaryotic Cell Structure
Prokaryotic cells are defined primarily by the absence of a membrane-bound nucleus. Instead of being enclosed within a membrane, the genetic material, typically a single circular chromosome, resides in a dense region of the cytoplasm known as the nucleoid. The name “prokaryote” itself reflects this structure, meaning “before kernel,” referencing the lack of a true nuclear structure.
These cells also lack other membrane-bound organelles, such as mitochondria, the Golgi apparatus, and the endoplasmic reticulum. All metabolic activities, including energy production and protein synthesis, occur directly within the cytoplasm. Prokaryotes are generally much smaller than eukaryotic cells, typically measuring between 0.1 and 5 micrometers. They possess a plasma membrane, a cell wall for protection and shape, and ribosomes for protein production.
The Three Domains of Biological Classification
The highest taxonomic rank for classifying organisms is the Domain system, which was established by Carl Woese in 1990. This system organizes all cellular life into three primary groups: Archaea, Bacteria, and Eukarya. This framework replaced older classification systems by using a more fundamental biological marker than physical appearance or morphology.
The three domains are distinguished based on differences in the gene sequences for ribosomal RNA (rRNA), a molecule present in all living cells. By comparing these sequences, scientists can determine the evolutionary relationships between different groups of organisms. This molecular analysis revealed that life diverged into three distinct evolutionary lineages early in its history.
Identifying the Prokaryotic Domains
The two domains composed entirely of prokaryotic cells are Bacteria and Archaea. Members of both these domains are single-celled organisms that share the defining prokaryotic features. Organisms within the Bacteria domain are the most widely known prokaryotes, encompassing everything from disease-causing pathogens to beneficial species.
The domain Bacteria is incredibly diverse, including organisms like photosynthetic cyanobacteria, gut-dwelling Escherichia coli, and various species involved in nutrient cycling. In contrast, the third domain, Eukarya, contains all organisms whose cells have a true nucleus and complex internal structure. This domain includes all plants, animals, fungi, and protists.
Why Archaea and Bacteria Are Separate Domains
Despite sharing the prokaryotic cell structure, Bacteria and Archaea are separated into two distinct domains due to differences in their molecular biology and biochemistry. One of the most telling distinctions is found in the composition of their cell membranes.
Bacterial cell membranes are constructed with fatty acids linked to glycerol via ester bonds, a structure similar to that found in Eukarya. Archaeal membranes, however, use branched hydrocarbon chains, specifically phytanyl, which are connected to glycerol through ether bonds. These ether linkages are chemically more stable, which is thought to aid the survival of many Archaea in extreme environments.
A second major difference lies in the composition of the cell wall. While the cell walls of Bacteria contain peptidoglycan, a unique polymer of sugars and amino acids, the cell walls of Archaea completely lack it. Instead, they may be composed of pseudopeptidoglycan, complex polysaccharides, or various proteins. Furthermore, Archaea possess genetic machinery that shares more similarities with Eukarya, such as the presence of histone-like proteins that associate with DNA, a feature not typically found in Bacteria.