Prokaryotic cells are small, relatively simple cells that lack a membrane-bound nucleus. They range from 0.1 to 5.0 micrometers in diameter, making them roughly 10 to 100 times smaller than typical animal or plant cells. Despite their size, prokaryotic cells pack a surprising amount of structural complexity into a tight space, and recent research has challenged the old assumption that they’re little more than bags of DNA and enzymes.
Two domains of life, bacteria and archaea, are prokaryotic. They share a basic blueprint but differ in important chemical details, particularly in their membranes and cell walls.
The Genetic Core: The Nucleoid
Instead of storing DNA inside a membrane-wrapped nucleus like your own cells do, prokaryotic cells keep their genetic material in a region called the nucleoid. This is not a true organelle but rather a concentrated zone within the cell’s interior where a single, circular chromosome resides. The DNA is tightly coiled and organized at both short-range and long-range levels using specialized proteins that help compress what would otherwise be an unwieldy molecule. If stretched out, a typical bacterial chromosome would be about a thousand times longer than the cell itself.
Many prokaryotic cells also carry plasmids, which are small, circular pieces of DNA that exist separately from the main chromosome. Plasmids replicate independently and often carry genes that give the cell an edge in harsh environments. A well-studied example is the R1 plasmid, which carries resistance genes for five different antibiotics, including ampicillin and chloramphenicol. Plasmids can also be passed between cells, which is one of the main ways antibiotic resistance spreads through bacterial populations. Cells typically keep plasmid numbers low because maintaining extra DNA costs energy.
Cell Membrane and Cytoplasm
Like all cells, prokaryotes are enclosed by a plasma membrane, a thin lipid bilayer that controls what enters and exits the cell. In bacteria, fatty acid chains are attached to a glycerol backbone through ester bonds, which is the same basic chemistry found in human cell membranes. Archaea use a fundamentally different approach: their membrane lipids have branching hydrocarbon chains (called isoprenoids) connected through ether bonds to a mirror-image version of the glycerol backbone. These ether-linked lipids make archaeal membranes exceptionally stable, which helps explain why many archaea thrive in extreme environments like hot springs and salt flats.
The cytoplasm fills the interior of the cell and contains everything needed for metabolism: enzymes, nutrients, ions, and the molecular machinery for reading DNA and building proteins. It’s a dense, gel-like environment where thousands of chemical reactions happen simultaneously.
The Cell Wall
Most prokaryotic cells are wrapped in a rigid cell wall that sits outside the plasma membrane and gives the cell its shape. In bacteria, this wall is built from peptidoglycan, a mesh-like material made of long sugar chains cross-linked by short chains of amino acids. The sugar strands are polymers of two alternating sugar molecules connected by chemical bonds, and the amino acid bridges stitch these strands together into a net that completely surrounds the cell.
Bacteria fall into two broad categories based on wall thickness. Gram-positive bacteria have a thick, multilayered peptidoglycan wall exposed directly to the outside environment, often studded with molecules called teichoic acids. Gram-negative bacteria have a much thinner peptidoglycan layer, sometimes only one molecule thick, but they compensate with an additional outer membrane, a second lipid bilayer that acts as an extra barrier. This distinction matters practically because it affects how bacteria respond to antibiotics and how they interact with the immune system.
Archaeal cell walls never contain peptidoglycan. Some archaea use a protein lattice called an S-layer, while others have different polysaccharide-based walls. This is one of the clearest structural differences between the two prokaryotic domains.
Ribosomes: The Protein Factories
Prokaryotic cells build proteins using 70S ribosomes, which are smaller than the 80S ribosomes found in eukaryotic cells. Each 70S ribosome consists of two pieces: a large 50S subunit containing about 27 proteins, and a small 30S subunit with around 19 proteins. (The “S” stands for Svedberg units, a measure of how fast a particle settles in a centrifuge, not a direct measure of size.) These ribosomes float freely in the cytoplasm, often clustered along strands of messenger RNA, translating genetic instructions into proteins. The structural differences between prokaryotic and eukaryotic ribosomes are the reason certain antibiotics can target bacteria without harming human cells.
Capsules and Protective Layers
Many prokaryotes secrete a sticky outer coating called a capsule or slime layer, collectively known as the glycocalyx. These layers are made primarily of polysaccharides, long sugar-based polymers that form a gel-like shield around the cell. Capsules serve several survival functions: they help bacteria stick to surfaces and host tissues, they promote biofilm formation (the slimy colonies that coat everything from medical devices to river rocks), and they help bacteria evade the immune system by making it harder for white blood cells to engulf them. Species like Streptococcus pneumoniae and Klebsiella pneumoniae depend on their capsules to cause disease.
Flagella, Pili, and Fimbriae
Prokaryotic cells use several types of surface appendages to move and interact with their surroundings. Flagella are long, whip-like structures that rotate like propellers to drive the cell through liquid. Not all prokaryotes have them, but those that do can swim toward nutrients or away from toxins.
Fimbriae are much shorter and finer, measuring just 2 to 10 nanometers in diameter, and a single cell can be covered in hundreds of them. Their primary job is adhesion. Bacteria use fimbriae to cling to surfaces, other cells, and host tissues. This is a critical first step in infection for many pathogens; E. coli uses fimbriae to latch onto cells in the urinary tract, and Neisseria gonorrhoeae uses them to anchor to the lining of the reproductive tract.
Pili are longer and fewer in number than fimbriae. Some pili, called conjugative or sex pili, form a bridge between two bacterial cells and allow one to transfer a copy of a plasmid to the other, spreading genes (including antibiotic resistance) horizontally. Type IV pili give bacteria a form of movement called twitching motility, a jerky crawling across solid surfaces. Pathogens like Pseudomonas aeruginosa and Vibrio cholerae use type IV pili for both movement and colonization. Spirochetes, a group that includes the bacteria behind Lyme disease and syphilis, have a unique variation: internal filaments that wind through the space between their membranes, producing a distinctive corkscrew motion that lets them bore through thick, viscous fluids like mucus.
Internal Storage and Compartments
Prokaryotic cells store surplus nutrients in granules and inclusions scattered throughout the cytoplasm. Many bacteria stockpile excess carbon as glycogen or as a biodegradable plastic-like polymer. Some store elemental sulfur in granules, particularly species that use hydrogen sulfide as an energy source. Others pack away nitrate in small vacuoles.
Beyond simple storage, some prokaryotes have genuine internal compartments that challenge the old textbook image of a structureless interior. Carboxysomes are protein-shelled compartments that concentrate carbon-fixing enzymes, improving the efficiency of photosynthesis in cyanobacteria. Gas vesicles are hollow, protein-walled structures that let aquatic bacteria control their buoyancy and float to the depth where light or nutrients are optimal. Magnetotactic bacteria build chains of magnetosomes, tiny membrane-bound crystals of iron minerals that function as a built-in compass, orienting the cell along Earth’s magnetic field. Cyanobacteria contain internal membrane systems called thylakoids where photosynthesis takes place, and certain members of the Planctomycetes group even have membrane-enclosed compartments that, in at least one species, surround the cell’s DNA in a structure strikingly reminiscent of a eukaryotic nucleus.
These discoveries have reshaped how biologists think about prokaryotic cells. While they remain simpler than eukaryotic cells in overall organization, the gap is narrower than anyone assumed a few decades ago.