Prokaryotic cells are single-celled organisms, including bacteria and archaea, found in nearly every environment. They are characterized by their relatively simple internal organization, lacking a membrane-bound nucleus or other specialized compartments. Their widespread presence and adaptability highlight an efficient cellular design.
Fundamental Internal Structures
The innermost boundary of a prokaryotic cell is the cell membrane, a thin layer composed of phospholipids and proteins. This membrane regulates the passage of substances into and out of the cell, acting as a selective barrier. It also serves as the site for various metabolic reactions, including cellular respiration and, in some prokaryotes, photosynthesis.
The cytoplasm, a gel-like substance, fills the cell’s interior where most cellular processes occur. This aqueous environment contains water, enzymes, salts, and various organic molecules. Within this fluid, the cell’s genetic material is organized into a region known as the nucleoid.
The nucleoid is an irregularly shaped area housing the cell’s primary genetic information, typically a single, circular DNA molecule. Unlike the nucleus in more complex cells, it is not enclosed by a membrane. This genetic material is highly compacted by associated proteins, allowing it to fit within the cell. Ribosomes, small particles of ribosomal RNA and protein, are scattered throughout the cytoplasm. They are responsible for protein synthesis, translating genetic instructions into functional proteins.
Outer Protective and Structural Features
Most prokaryotic cells have a rigid cell wall outside the cell membrane. It provides structural support, maintains cell shape, and protects the cell from osmotic lysis, preventing bursting in varying water concentrations. In most bacteria, the cell wall is primarily composed of peptidoglycan, a unique polymer of sugars and amino acids.
Some prokaryotic cells possess an additional outermost layer, the capsule, which surrounds the cell wall. This optional layer, typically made of polysaccharides, offers further protection against environmental stresses like desiccation and helps the cell evade the host’s immune system by preventing phagocytosis. The capsule also facilitates adhesion, allowing prokaryotes to stick to surfaces and other cells, contributing to biofilm formation.
Appendages for Movement and Attachment
Prokaryotic cells often exhibit external structures for movement or attachment. Flagella are long, whip-like appendages that extend from the cell surface and rotate to propel the cell through liquid environments. This rotary motion, powered by a proton or sodium motive force, allows bacteria to move efficiently.
Other common appendages include pili and fimbriae, which are shorter and more numerous than flagella. Fimbriae are hair-like structures that primarily mediate adhesion, allowing the cell to attach to various surfaces, including host tissues. Pili, generally longer and fewer in number, also contribute to attachment and can facilitate twitching motility. A specialized sex pilus plays a role in bacterial conjugation, a process of genetic exchange between cells.
Specialized Internal Components
Beyond the fundamental structures, some prokaryotic cells contain additional internal components that provide adaptive advantages. Plasmids are small, circular DNA molecules that exist separately from the cell’s main chromosome. These extrachromosomal DNA molecules can replicate independently and often carry genes for beneficial traits, such as antibiotic resistance or the ability to degrade unusual substances. Plasmids are important for horizontal gene transfer, allowing genetic information to be shared between prokaryotes.
Another type of specialized internal component includes inclusion bodies, which serve as storage granules within the cytoplasm. These structures store various nutrients, such as glycogen or polyhydroxybutyrate, which the cell can utilize when external resources become scarce. These specialized components, while not universally present in all prokaryotes, contribute to the diversity and adaptability of these organisms.