Prokaryotic cells are single-celled organisms that represent the most ancient and abundant form of life on Earth. The name, derived from Greek roots, means “before the nucleus,” which points directly to their defining structural feature. Prokaryotes are the fundamental unit of life for all Bacteria and Archaea. Their simple cellular architecture allows them to colonize every environment and influence the planet’s chemistry and the health of other organisms.
Defining Characteristics of a Prokaryotic Cell
The primary feature distinguishing a prokaryotic cell from a eukaryotic cell is the absence of a true, membrane-bound nucleus. Instead of being enclosed in a separate compartment, the genetic material resides in a dense, irregularly shaped region within the cytoplasm called the nucleoid. This genetic material typically consists of a single, circular chromosome, which is a closed loop of double-stranded deoxyribonucleic acid (DNA). Many prokaryotes also carry smaller, extrachromosomal DNA molecules known as plasmids, which can confer advantageous traits like antibiotic resistance.
The interior of the prokaryotic cell lacks any membrane-bound organelles, such as mitochondria, endoplasmic reticulum, or Golgi apparatus. All metabolic processes occur within the cytoplasm or on the inner surface of the plasma membrane. Prokaryotic cells are smaller than eukaryotic cells, ranging from 0.1 to 5.0 micrometers (µm) in diameter. This small size facilitates the rapid diffusion of ions and molecules, allowing for quick waste removal and nutrient uptake that supports fast reproductive rates.
Nearly all prokaryotes possess a cell wall located outside the plasma membrane, which provides structural support, maintains the cell’s shape, and prevents dehydration. The chemical composition of this wall varies between the two prokaryotic domains, but it serves as a rigid external layer. Some prokaryotes also feature external appendages like flagella for locomotion or pili for surface attachment and the exchange of genetic material.
The Primary Examples: Bacteria and Archaea
Prokaryotic life is classified into two distinct domains: Bacteria and Archaea, separated based on molecular differences despite their similar cellular structure. This classification is part of the three-domain system, which also includes Eukarya. The discovery of Archaea highlighted that prokaryotes represent two separate groups that diverged early in life’s history.
Bacteria represent the most widely known and diverse group of prokaryotes, found in almost every habitat on Earth. They exhibit a variety of characteristic shapes, including spherical cells known as cocci, rod-shaped cells called bacilli, and spiral-shaped cells referred to as spirilli. A defining molecular trait of Bacteria is the presence of the polymer peptidoglycan in their cell walls, a complex structure of linked carbohydrates and proteins that is unique to this domain.
Archaea are genetically and biochemically distinct from Bacteria, notably lacking peptidoglycan in their cell walls, which may instead be composed of pseudopeptidoglycan or S-layer proteins. Many Archaea are described as extremophiles because they thrive in environments considered too harsh for most other life forms. This domain is often considered more closely related to Eukarya than to Bacteria based on similarities in ribosomal RNA (rRNA) sequences and certain molecular mechanisms.
Prokaryotes in Everyday Life
Prokaryotes are integrated into daily human life and the global environment through essential functional roles. Within the human body, vast numbers of Bacteria and Archaea form the human microbiome, particularly in the gut. They assist in breaking down complex carbohydrates and synthesize short-chain fatty acids used by the host for energy, as well as producing essential vitamins like Vitamin K and certain B vitamins.
In the environment, prokaryotes are the primary drivers of global nutrient cycles, which are necessary for maintaining ecological balance. A specific and highly specialized function is nitrogen fixation, a process carried out by certain free-living and symbiotic bacteria, such as those in the genus Rhizobium. These organisms convert atmospheric nitrogen gas (N₂) into ammonia (NH₃), a form that plants can readily absorb and use to build proteins and nucleic acids. This biological process is responsible for fixing over 90 percent of the nitrogen used by living organisms on Earth.
Prokaryotes are also used in industrial and food production processes, most notably through fermentation. Lactic acid bacteria convert sugars into lactic acid, which preserves food and imparts a characteristic sour flavor. This fermentation process is responsible for the production of common foods:
- Yogurt
- Cheese
- Sauerkraut
- Kimchi
The controlled use of these prokaryotic organisms allows for a diverse global food supply.