Prokaryotic cells, encompassing the domains Bacteria and Archaea, are the most ancient and abundant forms of life on Earth. These single-celled organisms are structurally simple compared to eukaryotic cells (animals, plants, and fungi). The defining characteristics of prokaryotes revolve around their internal organization, external boundaries, and genetic information management.
Lack of Internal Compartmentalization
A defining characteristic of prokaryotic cells is the absence of a true nucleus and other internal structures enclosed by membranes. Unlike eukaryotic cells, which utilize organelles to divide cellular functions, all metabolic processes occur within the cytoplasm. The cytoplasm is a jelly-like substance that fills the cell, holding water, enzymes, nutrients, and molecules.
The genetic material is concentrated in an irregularly shaped region called the nucleoid. This region is distinct but lacks a surrounding membrane. Within this shared space, the cell performs energy production, protein synthesis, and waste processing simultaneously.
Ribosomes are the only internal structures consistently found in all prokaryotic cells, functioning as the sites of protein synthesis. These particles are composed of RNA and protein, but they are not enclosed by a membrane. Specialized compartments, sometimes called micro-compartments, can exist in some bacteria, but these are typically protein-shelled structures used to organize specific enzymes.
Structural Components and External Layers
The integrity of the prokaryotic cell is maintained by layers that provide protection and shape. The outer layer is often a sticky capsule or slime layer, composed of polysaccharides. This layer allows adherence to surfaces, protects against dehydration, and shields pathogenic bacteria from the host’s immune system.
Beneath the capsule lies the cell wall, a rigid structure preventing rupture from osmotic pressure. Bacteria utilize peptidoglycan, a polymer of sugars cross-linked by peptides. Archaea lack peptidoglycan, featuring cell walls made of pseudopeptidoglycan, glycoproteins, or protein.
The plasma membrane is the innermost boundary, a thin lipid bilayer surrounding the cytoplasm. This membrane is selectively permeable, regulating the passage of ions and molecules. Many prokaryotes also possess external appendages, such as flagella for motility. Shorter pili are used for attachment and facilitating the exchange of genetic material between cells.
Organization of Genetic Material
The prokaryotic genome is organized simply compared to the multiple, linear chromosomes found in eukaryotes. The primary genetic material is a single, circular chromosome located within the nucleoid region. This DNA molecule carries the genes for the organism’s growth, metabolism, and reproduction.
Many prokaryotes also carry small, extra-chromosomal, circular DNA molecules known as plasmids. These plasmids replicate independently and often carry genes that confer advantageous traits, such as antibiotic resistance. Exchanging these plasmids is a mechanism for rapid adaptation.
Prokaryotes reproduce primarily through binary fission, a rapid, asexual process. The circular chromosome is duplicated, and the cell divides into two identical daughter cells. This process is efficient and allows for fast population growth under favorable conditions.
Size and Domain Classification
Prokaryotic cells are minute, significantly smaller than their eukaryotic counterparts. The typical diameter ranges from 0.1 to 5.0 micrometers (µm). This small scale allows ions and molecules to diffuse rapidly throughout the cell, supporting high metabolic rates.
Prokaryotes are not a single, unified group but are formally classified into two distinct domains of life: Bacteria and Archaea. Bacteria are widespread, encompassing many familiar species, including those that are pathogenic or beneficial in human biology and ecosystems. The Archaea were initially recognized as extremophiles, thriving in habitats with extreme temperatures, salinity, or pH, though they are now known to be ubiquitous in nature.
While both domains share the basic prokaryotic structure—lacking a nucleus and membrane-bound organelles—they are separated due to fundamental differences in their biochemistry. These distinctions include the chemical composition of their cell walls and the structure of their cell membranes, indicating a deep evolutionary divergence. The three-domain system of classification recognizes Bacteria, Archaea, and Eukarya as the three primary branches of life.