Prokaryotes are the simplest and most ancient forms of life on Earth. A prokaryote is defined as a unicellular organism, meaning its entire existence is contained within a single, self-sufficient cell. The organisms belonging to this group, primarily the domains Bacteria and Archaea, are microorganisms that do not organize into multicellular structures. This single-celled structure is a defining characteristic that separates them from most visible life.
The Defining Feature: Unicellularity
The individual prokaryotic cell is an autonomous entity, capable of metabolism, reproduction, and response to its environment. This single-celled structure is a universal trait for all prokaryotes, including both Bacteria and Archaea.
Even when prokaryotes are observed in clusters, chains, or complex layered formations like microbial mats, they are still considered unicellular organisms. These formations are colonies or aggregates where the cells often benefit from each other. Unlike cells in a multicellular organism, individual cells within a prokaryotic colony maintain separate existence, do not depend on the whole for survival, and do not differentiate into specialized tissues or organs.
Internal Organization of the Single Prokaryotic Cell
The organization of the single prokaryotic cell is characterized by its relative simplicity and lack of internal compartments. A defining feature is the absence of a true, membrane-enclosed nucleus, which distinguishes it from other cell types.
Instead of a nucleus, the genetic material is localized in an irregular region of the cytoplasm called the nucleoid. This genetic material is typically a single, circular chromosome of DNA, and it is not separated from the rest of the cell contents by a membrane. Prokaryotic cells also lack other membrane-bound internal structures, such as mitochondria, the endoplasmic reticulum, and Golgi bodies.
The cell’s interior is filled with cytoplasm, a gel-like substance containing salts, enzymes, and other molecules. Ribosomes are present within the cytoplasm, functioning as the machinery for protein synthesis. The entire cell content is enclosed by a plasma membrane, which regulates the passage of substances, and is typically surrounded by a rigid cell wall that provides physical protection and shape.
Distinguishing Prokaryotes from Eukaryotic Life
The simple internal structure of a prokaryote stands in contrast to the organization of eukaryotic life forms. Eukaryotic cells, which make up animals, plants, fungi, and protists, are characterized by the presence of a true nucleus and numerous membrane-bound compartments. These internal structures, known as organelles, allow for the compartmentalization of functions, making the eukaryotic cell much larger and more complex.
Eukaryotic cells are typically ten to a hundred times larger in diameter than prokaryotic cells, which usually measure between 0.1 and 5.0 micrometers. While some eukaryotes, such as amoebas and yeast, are single-celled, the vast majority of visible life forms are multicellular eukaryotes. In these multicellular organisms, individual eukaryotic cells work together and differentiate to form tissues, organs, and organ systems.
The distinction between prokaryotes and eukaryotes is based on this difference in cellular architecture and complexity. Since prokaryotes lack internal membranes, their small size allows for rapid diffusion of nutrients and waste throughout the entire cell. This efficient transport system supports their rapid metabolism and reproduction through binary fission.