Prokaryotic cells, which include all bacteria and archaea, represent the earliest and most fundamental form of life on Earth. These single-celled organisms are defined by their simple internal structure, primarily the absence of a membrane-bound nucleus and other complex organelles found in eukaryotic cells. Prokaryotes are remarkably adaptable, thriving in nearly every environment, from soil and water to the internal systems of other organisms. This highly efficient cellular architecture allows for rapid growth and response to environmental changes.
The Cell Envelope
The cell envelope serves as the protective, multi-layered boundary between the prokaryotic cell’s interior and the external environment. The innermost layer is the plasma membrane, a phospholipid bilayer that regulates the passage of substances into and out of the cell. This membrane is also the site for metabolic processes, such as cellular respiration, which are typically confined to organelles in more complex cells.
External to the plasma membrane is the cell wall, a rigid structure that provides physical protection and maintains the cell’s characteristic shape. In bacteria, the cell wall is largely composed of peptidoglycan, a unique polymer of sugars and amino acids that resists the high internal osmotic pressure of the cell, preventing it from bursting. An outermost layer, often called a capsule or slime layer, may be present, composed mostly of polysaccharides. This layer helps the cell adhere to surfaces, protects against desiccation, and can shield pathogenic bacteria from the host’s immune system.
Internal Cellular Machinery
The entire internal volume of the cell, enclosed by the plasma membrane, is the cytoplasm. This gel-like, aqueous substance is the site where all metabolic reactions occur, containing dissolved salts, nutrients, and enzymes necessary for life. Unlike eukaryotic cells, the cytoplasm of a prokaryote is not divided into compartments by internal membranes.
Suspended within this fluid matrix are ribosomes, the non-membrane-bound structures responsible for synthesizing proteins. Prokaryotic ribosomes are smaller than their eukaryotic counterparts, designated as 70S ribosomes, which is a distinction often targeted by certain antibiotics.
Genetic Organization
Prokaryotic genetic material is concentrated in the nucleoid, an irregularly shaped region of the cytoplasm. This area does not have a surrounding membrane, which is the defining difference from a eukaryotic nucleus. The main genome is typically a single, double-stranded, circular chromosome that is highly coiled to fit within the small cell volume.
Many prokaryotes also possess smaller, extra-chromosomal DNA molecules called plasmids. These are small, circular pieces of DNA that replicate independently of the main chromosome. Plasmids often carry genes that confer advantageous traits, such as resistance to antibiotics or the ability to metabolize unusual substances. The presence of plasmids allows for the rapid spread of beneficial traits through a bacterial population via genetic exchange.
External Appendages
Specialized appendages extend from the cell surface, facilitating movement, attachment, and genetic transfer. Flagella are long, whip-like structures composed of the protein flagellin that rotate like a propeller, providing motility. This movement allows the bacterium to propel itself through liquid environments in response to chemical signals, a process known as chemotaxis.
Shorter, hair-like structures known as fimbriae are primarily used for adhesion. Fimbriae allow the prokaryote to stick to surfaces, host tissues, or other cells, which is a crucial step for colonization and infection. A different type of appendage is the pilus (plural: pili), which is generally longer than fimbriae and fewer in number. The specialized sex pilus is responsible for bacterial conjugation, the transfer of plasmid DNA from one cell to another.