A prokaryotic cell represents a fundamental unit of life, characterized by its simple internal organization. Unlike more complex cells, prokaryotes do not possess a true nucleus to house their genetic material. They also lack membrane-bound organelles. This simpler structure allows them to carry out all life functions within a single, open cellular space.
Typical Prokaryotic Cell Dimensions
Prokaryotic cells measure between 0.1 to 5.0 micrometers (µm) in diameter. A human hair is approximately 70-100 micrometers wide, meaning dozens to hundreds of prokaryotes could fit across its width. This microscopic size makes them invisible to the naked eye, requiring powerful microscopes for observation. Their compact nature contributes to their widespread presence across diverse environments.
Comparing Prokaryotic and Eukaryotic Cells
Prokaryotic and eukaryotic cells differ significantly in size. Eukaryotic cells are larger, ranging from 10 to 100 micrometers or more in diameter. This means a typical eukaryotic cell can be 10 to 1,000 times larger in volume than a prokaryotic cell. For instance, if a prokaryotic cell were the size of a grain of sand, a eukaryotic cell might be comparable to a small pebble or even a small house.
Eukaryotic cells’ larger size correlates with increased internal complexity, including a membrane-bound nucleus and various organelles like mitochondria and endoplasmic reticulum. While prokaryotic cells are single, open compartments, eukaryotic cells are highly compartmentalized. This allows eukaryotic cells to perform specialized functions within different regions, a feature not found in prokaryotes. The absence of these internal structures contributes to the prokaryotic cell’s smaller overall volume, yet their simplicity contributes to their success.
Why Small Size Benefits Prokaryotes
Small prokaryotic cell size offers functional advantages, particularly regarding their surface area-to-volume ratio. As a cell’s size decreases, its surface area relative to its volume increases. This high surface area-to-volume ratio efficiently supports nutrient uptake and waste removal. Nutrients and waste products can quickly diffuse across the cell membrane to all parts of the cell, as the distance they need to travel internally is minimal.
This efficient exchange of materials supports a high metabolic rate, allowing prokaryotes to process nutrients and reproduce rapidly. Their rapid resource uptake and waste expulsion enable them to thrive in various environments. The rapid growth and division facilitated by their small size contribute to their evolutionary success and ecological dominance. Without the need for complex internal transport systems, the small scale of prokaryotes optimizes their basic cellular functions.
Unusual Prokaryotic Cell Sizes
While most prokaryotes fall within the 0.1 to 5.0 micrometer range, exceptions exist. Among the smallest known prokaryotes are Mycoplasma species, which can measure as little as 0.1 to 0.5 micrometers in diameter. These organisms are the smallest self-replicating cells and lack a cell wall, contributing to their diminutive size.
Conversely, some prokaryotes grow unusually large, becoming visible to the naked eye. Thiomargarita namibiensis, for example, can reach diameters of 0.1 to 0.3 millimeters (100-300 micrometers), with some specimens growing up to 0.75 millimeters (750 micrometers). Even larger, Thiomargarita magnifica, discovered more recently, can average 9,000 micrometers (0.9 cm) in length, with the largest recorded reaching 2 centimeters. These larger prokaryotes often achieve their size by having a large central vacuole, pushing their cellular contents to the periphery and maintaining an efficient surface area for exchange.