The term “prokaryotic” describes the single-celled organisms that represent the earliest forms of life on Earth. These microscopic organisms, which include all bacteria and archaea, are foundational to the planet’s ecosystems and were the exclusive life forms for billions of years. Their cellular design is fundamentally simpler than the cells that make up animals, plants, and fungi. Understanding the prokaryotic cell means looking inside a tiny, efficient machine that has mastered survival across nearly every environment.
The Defining Characteristics of Prokaryotic Cells
The most notable feature of a prokaryotic cell is the absence of a true, membrane-bound nucleus. Unlike more complex cells, the genetic material within a prokaryote is not enclosed by an internal membrane structure. Instead, the cell’s single, typically circular, chromosome is concentrated in a specific, irregularly shaped region of the cytoplasm called the nucleoid.
Prokaryotes also generally lack other internal membrane-bound compartments, or organelles, such as mitochondria, the Golgi apparatus, or the endoplasmic reticulum. The cell’s interior, the cytoplasm, is a dense, gel-like substance where all metabolic activities occur. Functions like energy production and protein synthesis take place directly within the cytoplasm or on the inner surface of the cell membrane. This streamlined structure contributes to the organism’s small size and rapid rate of reproduction.
Internal Organization and Structures
The cytoplasm holds the cell’s machinery, including thousands of ribosomes, which are molecular complexes responsible for synthesizing proteins from genetic instructions. These ribosomes are slightly smaller than those found in more complex cells, specifically measuring 70S instead of 80S. The main genetic material is typically a single, double-stranded loop of DNA located in the nucleoid region.
Many prokaryotes also carry smaller, separate rings of extrachromosomal DNA called plasmids, which often contain genes that provide an advantage, such as antibiotic resistance. Surrounding the cytoplasm is the plasma membrane, a lipid bilayer that acts as a selective barrier, controlling the movement of substances into and out of the cell.
Outside the plasma membrane, most prokaryotes have a rigid cell wall, which provides structural support, maintains the cell’s shape, and protects it from bursting due to osmotic pressure. Other external structures include:
- The capsule, a sticky layer of carbohydrate polymers that helps the cell adhere to surfaces and evade immune responses.
- Flagella, whip-like appendages used for movement.
- Pili, hair-like structures used to attach to surfaces or exchange genetic material with other cells.
The Two Major Domains of Prokaryotic Life
Prokaryotes are divided into two distinct biological domains: Bacteria and Archaea. Although both groups share the fundamental prokaryotic structure, they have significant differences in their biochemistry and evolutionary history. Bacteria are the more commonly known group, encompassing a vast number of species found in nearly every habitat, from soil and water to the human body, such as Escherichia coli.
Archaea are often associated with extreme environments, earning them the nickname “extremophiles.” They can be found thriving in hot springs, highly saline lakes, and deep-sea hydrothermal vents. A primary distinction between the two domains lies in the composition of their cell walls and membranes. Bacterial cell walls contain a polymer called peptidoglycan, while archaeal cell walls lack this substance, instead using structures made of pseudopeptidoglycan, glycoproteins, or proteins. Furthermore, the lipids in the archaeal cell membrane are connected differently than those in bacteria, often forming a stronger, more heat-resistant structure.
How Prokaryotes Differ from Eukaryotic Cells
The differences between prokaryotic and eukaryotic cells define two distinct levels of cellular complexity. The most obvious distinction is the nucleus; eukaryotes possess a membrane-enclosed nucleus to house their linear DNA, while prokaryotes store their circular DNA in the non-membrane-bound nucleoid region. Eukaryotic cells also contain a range of membrane-bound organelles that compartmentalize functions, a feature absent in prokaryotes.
Prokaryotic cells are significantly smaller, allowing molecules and waste to diffuse rapidly throughout the cell, eliminating the need for complex internal transport systems. Prokaryotes reproduce asexually through a simple process called binary fission, which creates two identical clones. Eukaryotes divide through the more complex processes of mitosis and meiosis, often involving sexual reproduction which generates genetic variation.