Prokaryotes are exclusively single-celled organisms that include the domains Bacteria and Archaea. The term, derived from Greek roots, means “before nucleus,” pointing to their defining characteristic. They are characterized by a relatively simple internal structure, lacking the complex internal compartments found in all other life forms.
The Defining Structural Difference
The fundamental feature separating prokaryotes from eukaryotes (plants, animals, fungi, and protists) is the absence of internal membrane-bound compartments. A prokaryotic cell contains cytoplasm, but this internal space is not partitioned by lipid membranes. This means structures like the endoplasmic reticulum, Golgi apparatus, and mitochondria are not present in prokaryotes.
The genetic material also demonstrates a clear structural distinction. Instead of being housed within a true nucleus surrounded by a nuclear envelope, the DNA of a prokaryote is concentrated in an irregularly shaped region of the cytoplasm known as the nucleoid. All cellular processes, including DNA replication and protein synthesis, occur in the same general area of the cell.
Internal and External Anatomy
Prokaryotes possess several distinct components. The primary genetic information is typically held on a single, long, circular chromosome located in the nucleoid region of the cell. Many prokaryotes also carry smaller, independent, circular DNA molecules called plasmids, which often carry genes that confer specific advantages, such as resistance to antibiotics.
The cytoplasm is the site of metabolism and contains a large number of ribosomes, the molecular machines responsible for synthesizing proteins. Surrounding the cytoplasm is the plasma membrane, a lipid bilayer that controls the passage of substances into and out of the cell.
Outside the plasma membrane, most prokaryotes have a cell wall that provides structural support and protection from the surrounding environment. This cell wall is often encased by an outermost protective layer, such as a capsule or slime layer, which helps the cell adhere to surfaces and resist drying out.
Many prokaryotes also feature external appendages like flagella, long whip-like structures that propel the cell. Shorter, hair-like protein filaments called pili can also be found on the cell surface, which are used for attachment to surfaces or for transferring genetic material between cells in a process known as conjugation.
The Two Domains of Prokaryotes
Prokaryotes are formally separated into two distinct biological domains: Bacteria and Archaea. While both groups share defining structural characteristics, they exhibit significant differences in their biochemistry and evolutionary history. These differences are most apparent in the composition of their cell walls and cell membranes.
The cell walls of Bacteria are primarily composed of a polymer called peptidoglycan. In contrast, Archaea lack peptidoglycan entirely, instead utilizing other materials like pseudopeptidoglycan, proteins, or polysaccharides for structural support. Furthermore, the lipids that make up the cell membranes of Archaea feature ether linkages that enhance stability, differing from those in Bacteria. This specialized membrane chemistry allows many Archaea to thrive as extremophiles, inhabiting environments with high salinity, acidity, or temperature that are inhospitable to most other life forms.
Prokaryotes in the Global Ecosystem
Prokaryotes are the primary drivers of essential biogeochemical cycles, particularly the recycling of nutrients like carbon and nitrogen. For instance, specific types of bacteria are the only organisms capable of nitrogen fixation, converting atmospheric nitrogen gas into forms that plants can absorb to build proteins and nucleic acids.
Prokaryotes also function as the world’s decomposers, breaking down dead organic matter and returning essential elements to the soil and atmosphere. Within the human body, vast communities of bacteria and archaea form the microbiome, which assists in functions like food digestion, nutrient absorption, and the development of the immune system.
While some prokaryotes are known as pathogens that cause disease, others, such as photosynthetic cyanobacteria, play a significant role as producers, generating oxygen and organic compounds that form the base of many aquatic food webs.