Life on Earth is organized into cells, the basic structural and functional units of all living organisms. Scientists categorize cellular life into major domains based on internal architecture and evolutionary history. Understanding the cell type that makes up Bacteria and Archaea is key to appreciating the vast diversity of life on our planet.
The Universal Cell Type of Bacteria and Archaea
Bacteria and Archaea are single-celled organisms that share the prokaryotic cell type. The term “prokaryote” translates from Greek to mean “before nucleus,” describing their most distinguishing feature. This cellular design lacks a membrane-bound nucleus to enclose the genetic material. Instead, their DNA is concentrated in the cytoplasm within a region called the nucleoid.
Prokaryotic cells also lack other internal compartments wrapped in membranes, which are a hallmark of more complex life forms. The entire cellular machinery for metabolism, respiration, and protein production occurs within the single volume of the cell. Despite being classified into two separate domains of life, both Bacteria and Archaea share this simple, efficient prokaryotic structure.
Structural Components of Prokaryotic Cells
The internal environment of a prokaryotic cell is the cytoplasm, a jelly-like substance containing dissolved nutrients, enzymes, and salts where all chemical reactions take place. The genetic material resides in the nucleoid region, typically as a single, circular chromosome of double-stranded DNA. Many prokaryotes also carry smaller, independent rings of DNA called plasmids, which often confer traits like antibiotic resistance.
Ribosomes, non-membrane-bound structures, are scattered throughout the cytoplasm and are responsible for synthesizing the cell’s proteins. The cell envelope defines the exterior, including the plasma membrane that regulates the passage of substances. Outside this membrane lies a rigid cell wall, which provides structural support and protection from osmotic stress. Some prokaryotes also possess an outermost sticky capsule or slime layer, which aids in attachment and protects against dehydration or the host’s immune system.
Key Differences Between Bacteria and Archaea
Despite their shared prokaryotic structure, Bacteria and Archaea are chemically and evolutionarily distinct domains. The most significant difference lies in the composition of their cell walls and cell membranes. All bacteria possess a cell wall made of peptidoglycan, a unique polymer that gives the cell its shape and strength.
Archaea do not have peptidoglycan in their cell walls; their walls are composed of various substances, including pseudopeptidoglycan, proteins, or polysaccharides. Their cell membranes also differ fundamentally in lipid structure. Bacterial membrane lipids use ester linkages and straight-chain fatty acids.
In contrast, archaeal lipids utilize ether linkages and branched-chain hydrocarbons, sometimes forming a lipid monolayer instead of a bilayer. This ether-linked membrane provides stability, allowing many Archaea to thrive as extremophiles in environments like hot springs, highly saline water, or deep-sea vents.
The Contrast with Eukaryotic Cells
The prokaryotic cell structure of Bacteria and Archaea contrasts sharply with the eukaryotic cell type found in animals, plants, fungi, and protists. Eukaryotic cells possess a true nucleus, where the DNA is housed within a double membrane, a feature lacking in prokaryotes. Eukaryotes also contain numerous complex, membrane-bound organelles, such as mitochondria for energy production, the endoplasmic reticulum, and the Golgi apparatus for processing and packaging molecules.
These membrane-bound compartments allow eukaryotes to organize and compartmentalize cellular functions, supporting a much higher level of internal complexity. Eukaryotic cells are also significantly larger than prokaryotic cells, typically ranging from 10 to 100 micrometers in diameter, compared to the 0.1 to 5.0 micrometers of a prokaryote. This difference in size and internal organization reflects two fundamentally different strategies for supporting life.