Cells are the basic structural and functional components of every living organism. Scientists classify all life into two major superkingdoms based on cell structure: prokaryotes and eukaryotes. This division is based on distinct internal organization. Understanding the characteristics that define each cell type provides a framework for comprehending the vast diversity of biological life.
The Primary Distinction: Membrane-Bound Compartments
The defining characteristic separating the two cell types is the presence or absence of internal membrane-bound compartments, or compartmentalization. Eukaryotic cells possess a true nucleus, a specialized structure surrounded by the nuclear envelope, which houses the cell’s genetic material. This organization physically separates transcription and translation, enabling greater control over gene expression.
Prokaryotic cells, including all bacteria and archaea, lack a membrane-enclosed nucleus. Their genetic material is clustered in the cytoplasm in a region known as the nucleoid, which is not membrane-bound. This arrangement allows transcription and translation to occur almost simultaneously, contributing to rapid growth.
Eukaryotes feature a network of other membrane-bound internal structures, collectively called organelles, that perform specialized functions (e.g., mitochondria, endoplasmic reticulum, and Golgi apparatus). Prokaryotes perform all metabolic processes directly within the cytoplasm or on the plasma membrane, lacking these complex internal systems.
Comparative Scale and Internal Complexity
Size significantly influences the internal organization and complexity of the two cell types. Prokaryotic cells are typically much smaller, ranging from 0.1 to 5.0 micrometers (\(\mu\)m) in diameter. This small volume allows for quick diffusion of nutrients and waste products, eliminating the need for complex internal transport systems.
Eukaryotic cells are substantially larger, typically measuring between 10 and 100 \(\mu\)m in diameter. To manage this increased size, eukaryotes rely on the cytoskeleton, a complex internal scaffolding composed of protein filaments. The cytoskeleton provides structural support, maintains cell shape, and facilitates the movement of materials and organelles, overcoming the limitations of simple diffusion.
The outermost structural layers also vary, particularly the cell wall. Many prokaryotes, specifically bacteria, possess a rigid cell wall composed of peptidoglycan. Eukaryotic cell walls, when present, are chemically distinct; for example, plant walls are made of cellulose, and fungal walls are built from chitin.
How Genetic Material is Organized
The physical structure and packaging of the DNA molecule serve as a reliable distinguishing method. Eukaryotic cells store genetic information across multiple linear DNA molecules organized into distinct chromosomes. These linear chromosomes are intricately associated with specialized proteins known as histones.
DNA wraps tightly around histones, which function like spools to condense the lengthy molecule so it fits inside the nucleus. This combination of DNA and protein forms chromatin, which facilitates gene regulation and the orderly separation of chromosomes during cell division. The presence of multiple, linear chromosomes complexed with histones is a defining feature of the eukaryotic genome.
Prokaryotic cells typically contain their entire genome on a single, circular chromosome located in the nucleoid region. Although this DNA is supercoiled, it is generally not associated with histone proteins like eukaryotic DNA. Prokaryotes frequently harbor smaller, independent rings of extrachromosomal DNA called plasmids, which can carry beneficial traits like antibiotic resistance.
Methods of Cell Division
The process by which cells reproduce is a fundamental point of divergence, reflecting the complexity of their internal structures. Prokaryotes employ a simple, rapid division method called binary fission. In this process, the single circular chromosome is replicated, and the two copies attach to the plasma membrane.
The cell then elongates, and the membrane and cell wall grow inward, pinching off to create two genetically identical daughter cells. This method is straightforward and highly efficient, allowing prokaryotic populations to grow exponentially under favorable conditions.
Eukaryotic cells employ complex, multi-stage division processes to ensure the accurate segregation of their multiple, linear chromosomes. For growth and tissue repair, eukaryotes utilize mitosis, which ensures each daughter cell receives a complete, identical set of chromosomes. This requires the formation of a spindle apparatus made of microtubules to precisely separate the replicated chromosomes.
Sexual reproduction involves meiosis, a specialized division that reduces the chromosome number by half to create gametes. This adds a layer of reproductive complexity not found in prokaryotes.