Prokaryotic cells have four core components: a plasma membrane, cytoplasm, DNA, and ribosomes. Beyond those essentials, most prokaryotes also carry a rigid cell wall, and many sport external appendages like flagella or pili. What they lack is equally defining: there’s no nucleus, no membrane-bound organelles, and no mitotic spindle. Everything happens in a single open compartment, which turns out to be a remarkably efficient design.
The Four Universal Components
Every prokaryotic cell, whether it’s a bacterium in your gut or an archaeon in a deep-sea vent, shares these four structures:
- Plasma membrane: A thin lipid bilayer that separates the cell’s interior from its environment. It controls what enters and exits.
- Cytoplasm: The jelly-like fluid filling the cell, where chemical reactions take place and other components float.
- DNA: The genetic instructions for the cell, typically in the form of a single circular chromosome. Unlike in your cells, this DNA isn’t enclosed in a nucleus. It sits in a region of the cytoplasm called the nucleoid.
- Ribosomes: Tiny molecular machines that read genetic instructions and build proteins. Prokaryotic ribosomes are classified as 70S, made of a smaller 30S subunit (containing one strand of structural RNA and about 21 proteins) and a larger 50S subunit (containing two strands of structural RNA and 31 proteins). These are smaller and structurally different from the 80S ribosomes in human cells, which is why certain antibiotics can target bacterial ribosomes without harming yours.
DNA Without a Nucleus
The most distinctive thing about prokaryotic genetic material is its location. Instead of being sealed inside a membrane-bound nucleus, the main chromosome occupies an irregularly shaped zone called the nucleoid. The chromosome is typically circular, and it’s tightly coiled through a process called supercoiling to fit inside such a small cell.
Many prokaryotes also carry plasmids, which are small, separate loops of circular DNA. Plasmids aren’t essential for basic survival, but they often carry useful extras like genes for antibiotic resistance or the ability to break down unusual food sources. Prokaryotes can share plasmids with each other, which is one reason antibiotic resistance spreads so quickly through bacterial populations.
The Cell Wall
Almost all prokaryotes have a rigid cell wall outside the plasma membrane. It prevents the cell from bursting under internal pressure, maintains the cell’s shape, and protects against harsh conditions. But the wall’s composition differs dramatically between the two major groups of prokaryotes.
In bacteria, the wall is built from peptidoglycan, a mesh-like polymer made of sugar chains cross-linked by short chains of amino acids. The sugar strands form the backbone; the amino acid bridges hold everything together into a sturdy net. How thick that net is divides bacteria into two broad categories. Gram-positive bacteria have a thick, multilayered peptidoglycan wall exposed to the outside, often decorated with additional sugar-based polymers called teichoic acids. Gram-negative bacteria have a much thinner peptidoglycan layer, but they compensate with an extra outer membrane that serves as a second barrier.
Archaea take a completely different approach. Their cell walls lack peptidoglycan entirely. Instead, nearly all known archaea use an S-layer, a crystalline coat made of protein or glycoprotein that tiles the cell surface in a repeating pattern. Some archaeal species use other polymers, such as pseudomurein (structurally similar to peptidoglycan but chemically distinct), though these appear only in specific groups. The absence of peptidoglycan is one key reason archaea are naturally resistant to antibiotics that target bacterial cell walls.
External Appendages
Many prokaryotic cells have hair-like or whip-like structures extending from their surface. These aren’t decorative. Each type serves a specific mechanical purpose.
Flagella are long, helical filaments that spin like propellers. The base of each flagellum rotates counterclockwise, whipping the filament around and generating thrust. This lets bacteria swim toward nutrients or away from toxins, a behavior called chemotaxis. Some species have a single flagellum; others have dozens distributed across their surface.
Pili (sometimes called fimbriae) are thinner, shorter, and far more numerous than flagella. Their primary job is adhesion. In disease-causing bacteria, pili latch onto host tissues, which is often the first step in establishing an infection. Pili on gut bacteria, for example, can bind to the lining of the intestine, to red blood cells, and even to fungal cells. A specialized type called sex pili has a different role entirely: it connects two bacterial cells during conjugation, the process by which one cell transfers plasmid DNA to another.
Protective Outer Layers
Some prokaryotes add yet another layer outside the cell wall. A capsule is a thick, organized coating usually made of sugars. It serves several purposes: it shields the cell from dehydration, helps it stick to surfaces, and in pathogenic bacteria, it can prevent immune cells from engulfing and destroying the bacterium. A looser, less structured version of this coating is sometimes called a slime layer. Not every prokaryote has one, but those that do gain a significant survival advantage in hostile environments.
Why So Small
Prokaryotic cells range from 0.1 to 5.0 micrometers in diameter, making them roughly 10 to 100 times smaller than a typical human cell. This isn’t a limitation. It’s a feature. A small cell has a high surface-area-to-volume ratio, meaning nutrients can diffuse quickly from the membrane to every corner of the interior, and waste can exit just as fast. There’s no need for the internal transport systems that larger eukaryotic cells require. If a prokaryotic cell grew too large, its membrane wouldn’t have enough surface area to keep up with the metabolic demands of its increased volume.
Internal Membrane Structures
Prokaryotic cells are often described as lacking internal membranes, but that’s an oversimplification. While they don’t have the membrane-bound organelles found in eukaryotes (no mitochondria, no endoplasmic reticulum, no Golgi apparatus), some prokaryotes do have specialized internal membrane systems.
Cyanobacteria are the best-known example. They contain thylakoid membranes, internal membrane sheets that house the protein complexes responsible for photosynthesis. These membranes convert sunlight into chemical energy, functioning much like the thylakoids inside plant chloroplasts. In some cyanobacterial species, these membranes form concentric, multilayered arrangements that maximize the surface area available for capturing light. Other prokaryotes fold their plasma membrane inward to create specialized zones for energy production or other metabolic tasks.
How Prokaryotic Cells Reproduce
Prokaryotes reproduce through binary fission, a process far simpler than the mitosis used by eukaryotic cells. It begins when the cell copies its circular chromosome starting from a specific point called the origin of replication. As copying proceeds, the two new chromosomes migrate toward opposite ends of the cell, and the cell elongates to help separate them. Once replication is complete, the membrane pinches inward at the middle, and a new dividing wall (called a septum) forms between the two halves. The septum splits, and two independent daughter cells are released. No mitotic spindle forms at any point in this process, which is one reason bacteria can divide so rapidly, sometimes in as little as 20 minutes under ideal conditions.
What Prokaryotic Cells Lack
Understanding what’s absent helps clarify the picture. Prokaryotic cells have no true nucleus, so their DNA is exposed directly to the cytoplasm. They have no mitochondria, no chloroplasts (though cyanobacteria perform photosynthesis using internal membranes), no endoplasmic reticulum, and no Golgi apparatus. They also lack the complex internal skeleton (cytoskeleton) that gives eukaryotic cells their shape and internal transport highways, though recent research has identified simpler protein filaments in some bacteria that play structural roles. The trade-off for this simplicity is speed: fewer internal compartments means faster growth, faster reproduction, and the ability to thrive in an astonishing range of environments.