A prokaryotic cell is a type of cell that does not have a nucleus enclosed by a membrane. Instead, its DNA is located in a region of the cell called the nucleoid. Prokaryotic cells also lack other membrane-bound organelles, such as mitochondria and the endoplasmic reticulum.
All known prokaryotes belong to one of two domains of life: Bacteria or Archaea. Although these cells are generally simpler in internal organization than eukaryotic cells—the type found in animals, plants, fungi, and protists—prokaryotes have sophisticated structures that allow them to obtain energy, reproduce, move, communicate, and adapt to remarkably diverse environments.
What makes a cell prokaryotic?
The defining feature of a prokaryotic cell is its lack of a membrane-bound nucleus. In eukaryotic cells, DNA is enclosed inside a nucleus surrounded by a membrane. In prokaryotes, the main chromosome occupies a region of the cytoplasm called the nucleoid.
Prokaryotes also lack membrane-bound organelles. Their genetic material, ribosomes, and other cellular components are found within the cell’s interior, although many of these components are organized into specialized regions.
Most prokaryotic cells are unicellular, meaning each organism consists of a single cell. Some can live in groups or form multicellular-looking colonies, but each individual cell remains structurally independent.
The absence of a nucleus does not mean that prokaryotes lack DNA or that their cells are poorly organized. Prokaryotic DNA contains the genetic instructions needed for cellular functions, while ribosomes and other molecular systems carry out those instructions.
The basic structure of a prokaryotic cell
Although bacterial and archaeal cells differ in important ways, many prokaryotic cells share several basic structures.
| Structure | Main function |
|---|---|
| Plasma membrane | Controls what enters and leaves the cell and provides a site for important chemical reactions |
| Cytoplasm | Jelly-like interior where many cellular reactions occur |
| Nucleoid | Region containing the cell’s main chromosome |
| Ribosomes | Build proteins from genetic instructions |
| Cell wall | Provides strength and helps maintain cell shape in many prokaryotes |
| Capsule or slime layer | Can protect the cell and help it attach to surfaces |
| Flagella | Allow some cells to move |
| Pili or fimbriae | Help some cells attach to surfaces or interact with other cells |
Not every prokaryote has every structure. Cell architecture varies substantially among bacteria and archaea.
Plasma membrane
The plasma membrane is a thin barrier surrounding the cytoplasm. It is made primarily of lipids and proteins and regulates the movement of substances into and out of the cell.
The membrane is also important for energy production. Because prokaryotes do not have mitochondria, many of the processes associated with cellular respiration occur at the plasma membrane or in association with it.
Archaea have distinctive membrane chemistry that differs from that of bacteria and eukaryotic cells. This is one of several molecular features that distinguish the two major groups of prokaryotes.
Cytoplasm
The cytoplasm is the cell’s internal environment. It contains water, dissolved molecules, ions, ribosomes, the nucleoid, and other cellular structures.
Many metabolic reactions take place in the cytoplasm. These reactions allow the cell to break down nutrients, manufacture molecules, process genetic information, and maintain itself.
Nucleoid and DNA
A prokaryotic cell’s main genetic material is found in the nucleoid, a region rather than a membrane-enclosed compartment.
Many prokaryotes have a single major, usually circular chromosome, although exceptions exist. The chromosome is highly organized and compacted so that it can fit inside the small cell.
Prokaryotes may also contain plasmids, which are small DNA molecules separate from the main chromosome. Plasmids can carry genes that provide useful traits under particular conditions. Some plasmids, for example, contain genes involved in antibiotic resistance or specialized metabolic abilities.
Ribosomes
Ribosomes are molecular machines that assemble proteins. Prokaryotic ribosomes are smaller in structure than the ribosomes found in the cytoplasm of eukaryotic cells, although their basic role is the same.
Because proteins perform much of a cell’s structural and chemical work, ribosomes are essential for growth, maintenance, and reproduction.
Cell wall
Many prokaryotes have a cell wall outside the plasma membrane. The wall helps maintain cell shape and protects the cell from mechanical stress and, in many environments, from bursting because of differences in water concentration.
Bacterial cell walls typically contain peptidoglycan, a strong network of sugars and amino acids. The structure of this wall differs between major bacterial groups.
Archaeal cell walls do not contain peptidoglycan. Their walls can be made from different materials, depending on the species.
Capsule and slime layers
Some prokaryotes produce an external layer made of secreted material. When it forms a relatively organized coating, it may be called a capsule; less organized material is often described as a slime layer.
These structures can help cells avoid environmental stresses, adhere to surfaces, and interact with their surroundings. Some bacteria use extracellular materials to help form biofilms, communities of microorganisms attached to a surface and embedded in a self-produced matrix.
Flagella
Some prokaryotes have flagella, structures used for movement. Bacterial and archaeal flagella are structurally and evolutionarily distinct, despite serving similar functions.
Movement can help a cell find favorable conditions, such as areas with suitable nutrients, or move away from harmful conditions.
Pili and fimbriae
Many bacteria have thin surface structures called pili or fimbriae. Depending on the structure, these can help bacteria attach to surfaces, interact with other cells, or exchange genetic material.
One important example is the sex pilus, which can participate in the transfer of DNA between bacterial cells through a process called conjugation.
How prokaryotic cells differ from eukaryotic cells
The most important difference is the organization of genetic material. Prokaryotes lack a membrane-bound nucleus, whereas eukaryotes store their chromosomes inside a nucleus.
Prokaryotes also lack membrane-bound organelles such as mitochondria, chloroplasts, the endoplasmic reticulum, and Golgi apparatus. Eukaryotic cells use these compartments to separate different cellular processes.
| Feature | Prokaryotic cells | Eukaryotic cells |
|---|---|---|
| Nucleus | Absent | Present |
| Membrane-bound organelles | Absent | Present |
| Main DNA location | Nucleoid | Nucleus |
| Typical organization | Usually unicellular | Unicellular or multicellular |
| Ribosomes | Present; generally smaller | Present; generally larger in the cytoplasm |
| Cell wall | Common, but composition varies | Present in plants, fungi, and some other groups; absent in animal cells |
| Examples | Bacteria and archaea | Animals, plants, fungi, and protists |
The distinction is fundamentally about cellular organization, not simply size. Many prokaryotic cells are small, but size alone does not define a prokaryote.
Bacteria and archaea are both prokaryotes
The term prokaryote groups bacteria and archaea according to their shared cellular organization. However, bacteria and archaea are not simply two versions of the same type of organism.
They differ in their cell membranes, cell walls, genetic machinery, and other molecular characteristics. At the level of evolutionary classification, Archaea and Bacteria are separate domains of life.
Archaea are particularly well known for species that live in environments such as highly salty waters, acidic habitats, or oxygen-free sediments, although archaea are not restricted to extreme environments. They are also found in ordinary environments, including soils, oceans, and the digestive systems of animals.
Bacteria occupy an enormous range of habitats and perform diverse roles in ecosystems, including decomposition, nutrient cycling, and relationships with plants and animals.
How prokaryotic cells obtain energy
Prokaryotes display extraordinary metabolic diversity. Different species can use very different sources of energy and carbon.
Some obtain energy by breaking down organic molecules. Others use light. Still others obtain energy from inorganic substances such as hydrogen, sulfur compounds, iron, ammonia, or hydrogen ions.
This diversity allows prokaryotes to thrive in environments ranging from ordinary soil and freshwater to deep sediments and chemically extreme habitats.
Because prokaryotes lack mitochondria and chloroplasts, their energy-related processes occur in the cytoplasm, across the plasma membrane, or in specialized internal membrane systems found in some species.
How prokaryotic cells reproduce
Most prokaryotes reproduce through binary fission. In this process, the cell copies its DNA, grows, and divides into two daughter cells.
Binary fission is not the same as sexual reproduction. However, prokaryotes can exchange genetic material through processes such as conjugation, transformation, and transduction.
These processes can introduce new DNA into a cell without producing offspring through sexual reproduction. They are important sources of genetic variation and can allow useful traits to spread through microbial populations.
For example, genes associated with antibiotic resistance can sometimes move between bacterial cells through horizontal gene transfer.
Examples of prokaryotic cells
Escherichia coli
Escherichia coli, commonly called E. coli, is a bacterium frequently found in the intestines of humans and other animals. Many strains are harmless and form part of the normal intestinal microbial community, while some strains can cause disease.
E. coli is also widely used in biological research because it grows readily and its genetics are well understood.
Cyanobacteria
Cyanobacteria are bacteria capable of oxygen-producing photosynthesis. They use light energy to drive the production of organic molecules and release oxygen as a byproduct.
Although they were historically called blue-green algae, cyanobacteria are bacteria, not eukaryotic algae.
Methanogenic archaea
Methanogens are archaea that produce methane as part of their metabolism. They commonly live in oxygen-free environments, including certain sediments and the digestive systems of some animals.
Their metabolism illustrates the biochemical diversity found among archaea.
Halobacterium
Despite its name, Halobacterium is an archaeon rather than a bacterium. Members of this group are adapted to very salty environments and use specialized molecular systems that allow them to function under high-salt conditions.
Why prokaryotic cells matter
Prokaryotes are fundamental to ecosystems and to the functioning of the biosphere. They decompose organic matter, recycle elements such as carbon and nitrogen, participate in photosynthesis, and form relationships with plants and animals.
Many bacteria live harmlessly or beneficially on and inside organisms. Others cause infectious diseases. Prokaryotes are also important in agriculture, food production, biotechnology, environmental cleanup, and industrial processes.
Their small size and simple cellular organization can be misleading. Prokaryotic cells contain highly coordinated molecular systems capable of sensing their environment, regulating genes, producing energy, moving, communicating, exchanging DNA, and adapting to changing conditions.
A prokaryotic cell is therefore best understood not as an incomplete version of a eukaryotic cell, but as a distinct and highly successful form of cellular organization.
