Prokaryotic cells contain four structures that are universal: a plasma membrane, cytoplasm, DNA in a region called the nucleoid, and ribosomes. Beyond these essentials, most prokaryotes also have a cell wall, and many carry additional structures like capsules, flagella, pili, and plasmids. Prokaryotic cells are typically 0.1 to 5 micrometers across, much smaller than eukaryotic cells, and they lack a membrane-bound nucleus or internal organelles.
The Four Structures Every Prokaryote Has
Every prokaryotic cell, whether bacterium or archaeon, shares the same four core components. The plasma membrane is the outer boundary that separates the cell’s interior from the environment. In bacteria, this membrane is composed primarily of protein and phospholipid in roughly a 3:1 ratio. It controls what enters and exits the cell and serves as an anchor point for many other structures.
Inside the membrane sits the cytoplasm, a gel-like fluid (sometimes called cytosol) that fills the cell and holds everything else in place. Suspended in that cytoplasm is the cell’s DNA, concentrated in a region called the nucleoid. Unlike eukaryotic cells, the nucleoid has no surrounding membrane. The DNA fibrils sit directly in the cytoplasm, giving the cell open access to its genetic instructions at all times.
The fourth universal structure is the ribosome. Prokaryotic cells are densely packed with 70S ribosomes, which are the molecular machines that read genetic instructions and build proteins. Each 70S ribosome is made of two subunits: a smaller 30S subunit (containing one strand of ribosomal RNA and about 21 proteins) and a larger 50S subunit (containing two strands of ribosomal RNA and 31 proteins). These ribosomes are slightly smaller than the ones found in eukaryotic cells, a difference that turns out to be medically useful since many antibiotics target bacterial ribosomes without affecting human ones.
The Cell Wall
Nearly all bacteria are surrounded by a rigid cell wall just outside the plasma membrane. This wall gives the cell its shape, prevents it from bursting in dilute environments, and provides structural support. The primary building material is peptidoglycan, a mesh-like polymer made of two repeating sugar units linked together in long chains, with short chains of amino acids (usually two to five residues) cross-linking those sugar strands into a sturdy lattice.
The thickness and arrangement of this peptidoglycan layer differs between two major groups of bacteria. Gram-positive bacteria have a thick, multilayered peptidoglycan wall exposed on the cell surface, often with additional sugar-based polymers called teichoic acids woven through it. Gram-negative bacteria have a much thinner peptidoglycan layer, typically just one layer thick, but compensate with an additional outer membrane made of lipids. This outer membrane contains lipopolysaccharide, a molecule that plays a major role in how these bacteria interact with immune systems.
Archaea, the other major group of prokaryotes, take a different approach. Most archaeal cell walls lack peptidoglycan entirely. Some use a protein-based surface layer instead. Their membranes also differ fundamentally from bacterial membranes. While bacteria build their membrane lipids with ester bonds linking fatty acid chains to a glycerol backbone, archaea use ether bonds connecting branched hydrocarbon chains to a mirror-image form of glycerol. This structural difference, sometimes called the “lipid divide,” makes archaeal membranes exceptionally heat-resistant and stable under extreme conditions.
Capsules and Slime Layers
Many bacteria surround themselves with an additional layer outside the cell wall, collectively called the glycocalyx. When this layer is thick and well-organized, it’s called a capsule. When it’s thinner and more loosely attached, it’s called a slime layer. Both are typically made of polysaccharides, though some capsules contain proteins.
Capsules serve as a form of armor. They help bacteria evade detection by the immune system, either by blocking immune factors from reaching the cell surface or by mimicking molecules found on host cells. Capsular polysaccharides also help bacteria stick to surfaces and form biofilms, the dense communities of microbes that colonize everything from medical implants to river rocks. Species like Streptococcus pneumoniae and Klebsiella pneumoniae rely heavily on their capsules for both adhesion and immune evasion.
Flagella, Pili, and Fimbriae
Prokaryotic cells can have several types of external appendages, each with a distinct job.
Flagella are long, whip-like structures that rotate to propel the cell through liquid environments. A bacterium may have one flagellum, a few, or many, depending on the species. The rotation is powered by a molecular motor embedded in the cell membrane.
Fimbriae are short, hair-like protein tubes that cover the surface of many Gram-negative bacteria. They function as adhesion tools, allowing the cell to cling to environmental surfaces or host cells and resist being flushed away. A single bacterium can have hundreds of fimbriae.
Pili are similar to fimbriae in basic structure (both are made of a protein called pilin) but are typically longer and fewer in number. Some pili are used for transferring DNA between bacterial cells during a process called conjugation. A specialized class called type IV pili can actually extend and retract, dragging the bacterium across a solid surface in a “twitching” or crawling motion. These pili are located at the cell’s poles and can even slingshot the bacterium forward over cellular surfaces.
Plasmids
In addition to the main chromosome in the nucleoid, many prokaryotic cells carry plasmids. These are small, circular DNA molecules that replicate independently from the chromosome. A single cell can harbor multiple copies of a plasmid, and different plasmids can coexist in the same cell.
Plasmids often carry genes that provide a survival advantage in specific situations. Antibiotic resistance genes are a common example. Others carry genes for toxin production, the ability to break down unusual nutrients, or resistance to heavy metals. Because plasmids can be transferred between cells, they allow bacterial populations to share useful traits rapidly, which is one reason antibiotic resistance can spread so quickly through a bacterial community.
Inclusion Bodies and Storage Granules
Prokaryotic cells often stockpile nutrients or useful molecules in dense deposits called inclusion bodies. These are not membrane-bound organelles. They sit directly in the cytoplasm as concentrated granules. Common examples include granules of glycogen (stored sugar), polyphosphate (stored phosphorus), and sulfur globules in bacteria that metabolize sulfur compounds. Some aquatic bacteria contain gas vesicles, small protein-shelled pockets of gas that let the cell control its buoyancy and float to the depth where light or nutrients are optimal.
Endospores
A few bacterial genera, most notably Bacillus and Clostridium, can produce endospores when conditions turn hostile. An endospore is not a reproductive structure. It’s a dormant, heavily armored version of the cell designed purely for survival. The formation process involves the cell replicating its DNA, then pinching off a section of membrane around one copy to form a forespore. Layers of protective material build up around this forespore: an inner membrane, a thick cortex reinforced with calcium and a chemical called dipicolinic acid, and finally a tough outer protein coat.
The result is one of the most resilient biological structures known. Endospores resist ultraviolet radiation, extreme pH, drought, chemical disinfectants, and boiling temperatures. They can remain viable for decades, possibly centuries. When conditions improve, the endospore germinates and returns to normal cell growth.