All life on Earth is constructed from cells, the basic structural and functional units for every organism. Although all cells share common features like a plasma membrane, cytoplasm, and genetic material, they are broadly divided into two distinct organizational blueprints. These two cellular categories, prokaryotes and eukaryotes, represent different strategies for organizing the necessary functions of life.
The Defining Difference Internal Structure and Compartmentalization
The primary distinction between these two cell types lies in their internal architecture. The terms reflect this difference: “prokaryote” means “before kernel,” and “eukaryote” means “true kernel,” referring to the nucleus.
Eukaryotic cells possess a true nucleus, a specialized compartment enclosed by a double membrane that stores the cell’s DNA. They also feature numerous membrane-bound organelles, such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus. These internal membranes create distinct, specialized micro-environments, known as compartmentalization, allowing complex biochemical reactions to occur simultaneously with increased efficiency.
Prokaryotic cells lack a nucleus and all other membrane-bound organelles, resulting in a simpler internal structure. Their genetic material is concentrated in an irregularly shaped region of the cytoplasm called the nucleoid. All metabolic functions, including energy production and protein synthesis, occur directly within the general cytoplasm.
Genetic Material and Replication
The organization and duplication of genetic material vary considerably between the two cell types. Prokaryotic DNA is structured as a single, circular chromosome residing freely within the nucleoid region. This chromosome is less tightly organized and has few associated proteins.
Eukaryotic cells store their DNA in the nucleus as multiple, linear chromosomes. These long DNA molecules are extensively packaged, tightly wound around proteins called histones to form a compact structure. Replication often employs hundreds or thousands of starting points, known as origins of replication, along each chromosome due to the genome’s size.
For duplication, prokaryotes rely on binary fission, a simple and rapid process where the circular chromosome is duplicated, and the cell divides into two identical daughter cells. Eukaryotic cell division involves mitosis or meiosis, a multi-step process. Mitosis ensures the precise separation of the multiple linear chromosomes through the formation of a spindle apparatus.
Cellular Components Beyond the Nucleus
Structural differences extend to other cellular components, including size. Prokaryotic cells are much smaller, typically ranging from 0.1 to 5.0 micrometers (µm) in diameter. Eukaryotic cells are substantially larger, generally measuring between 10 and 100 µm, often representing a thousand-fold volume difference.
Both cell types use ribosomes to synthesize proteins, but their size differs. Prokaryotes possess smaller 70S ribosomes suspended throughout the cytoplasm. Eukaryotes contain larger 80S ribosomes, which may be free-floating or bound to the endoplasmic reticulum.
A cell wall is present in many organisms from both groups, but its chemical composition is unique. Most bacteria have a rigid cell wall composed primarily of peptidoglycan. Eukaryotic cell walls, found in plants and fungi, are constructed from different materials, such as cellulose or chitin.
Ecological Roles and Classification
Structural differences correlate directly with classification and ecological roles. All organisms are grouped into three domains of life: Bacteria, Archaea, and Eukarya. Prokaryotes encompass the Domains Bacteria and Archaea, while eukaryotes constitute the Domain Eukarya.
Organisms in the Domain Eukarya include all animals, plants, fungi, and protists, and they can be single-celled or multicellular. Prokaryotes are almost always unicellular, far more numerous, and metabolically diverse. Bacteria act as decomposers, photosynthesizers, and essential members of microbiota. Archaea are specialized organisms often found in extreme environments, such as hot springs or highly saline waters.