Prokaryotic cells are the simplest and oldest form of life, encompassing organisms in the domains Bacteria and Archaea. These single-celled organisms thrive in nearly every environment, demonstrating remarkable metabolic and structural diversity. Prokaryotes are defined by their structural simplicity; they are typically much smaller and lack a true nucleus, meaning their genetic material is not enclosed within a membrane. This simple architecture allows for rapid growth and division through specialized, efficient structures that enable all necessary life functions.
Defining Characteristics and Organization
The overall design of a prokaryotic cell lacks internal compartmentalization. Prokaryotes do not possess membrane-bound organelles, such as mitochondria, the endoplasmic reticulum, or Golgi apparatus. Consequently, many biochemical processes, including energy production and metabolism, occur directly within the cell’s internal fluid.
The genetic information is stored in a single, typically circular chromosome of DNA. This chromosome is located in the cytoplasm in a specific area called the nucleoid region. The small size of prokaryotes, generally ranging from 0.1 to 5.0 micrometers in diameter, enables rapid diffusion of nutrients and waste products. This high surface-area-to-volume ratio contributes to their fast metabolic and growth rates.
Structures of the Protective Exterior
The external structures form a protective boundary that mediates interaction with the surrounding environment, handling defense, adhesion, and transport. The cell wall lies outside the plasma membrane and provides a rigid framework that maintains the cell’s characteristic shape. It also prevents the cell from bursting due to osmotic pressure. The main component of the bacterial cell wall is peptidoglycan, a unique polymer of linked sugars and amino acids.
The thickness and organization of the peptidoglycan layer form the basis for the Gram stain classification, differentiating bacteria into two major groups. Gram-positive bacteria possess a thick layer of peptidoglycan (often 20 to 80 nm thick), which is threaded with teichoic acids for substantial structural support. Gram-negative bacteria have a much thinner peptidoglycan layer (2 to 7 nm thick). This thin layer is situated between the plasma membrane and an outer membrane containing lipopolysaccharides (LPS).
Beneath the cell wall is the plasma membrane, a lipid bilayer that functions as a selective barrier, regulating the passage of substances into and out of the cell. Since prokaryotes lack mitochondria, the plasma membrane takes on the function of energy generation. It hosts the protein complexes required for the electron transport chain and ATP synthesis. This process creates an electrochemical gradient across the membrane that powers the cell.
Some bacteria secrete a sticky, outermost layer called the glycocalyx, organized as a dense capsule or a looser slime layer. The capsule primarily serves a protective role, helping the cell resist dehydration and shielding it from being engulfed by immune cells (phagocytosis). The glycocalyx also facilitates adhesion, allowing bacteria to stick to surfaces and to each other. This adhesion is a precursor to forming protective biofilms.
The prokaryotic cell may possess various protein-based extensions, or appendages, that project from the cell surface. Flagella are long, whip-like structures that function as rotating propellers, providing motility and allowing it to move through liquid environments. Shorter, hair-like appendages known as fimbriae are numerous and primarily enable the cell to adhere to surfaces or host tissues. Pili are typically longer and fewer than fimbriae; specialized types, such as the sex pilus, facilitate the transfer of genetic material between two bacterial cells during conjugation.
Internal Machinery and Genetic Material
The structures contained within the plasma membrane are responsible for the cell’s metabolic activity, synthesis, and genetic regulation. The cytoplasm is the entire internal volume, a gel-like substance composed mostly of water, dissolved salts, enzymes, and nutrients. This fluid environment is the location where most of the cell’s metabolic reactions, including glycolysis and fermentation, take place.
The nucleoid is the irregularly shaped region containing the single, circular chromosome, which serves as the cell’s genetic blueprint. This main chromosome contains the genes necessary for the cell’s survival, growth, and reproduction. Many prokaryotes also carry smaller, extra-chromosomal DNA molecules called plasmids, which are circular and replicate independently. Plasmids often contain genes that provide a selective advantage, such as antibiotic resistance or the ability to metabolize unusual compounds.
Scattered throughout the cytoplasm are ribosomes, which are complex molecular machines responsible for protein synthesis (translation). Prokaryotic ribosomes are smaller than those found in complex cells but perform the universal function of creating all required proteins and enzymes. The cytoplasm may also house inclusion bodies, which are dense granules that serve as storage depots for reserve materials. These inclusions store nutrients, such as glycogen, sulfur, or polyphosphate, allowing the cell to survive when resources become scarce.