All living cells share a basic set of features: a cell membrane, genetic material, cytoplasm, and machinery for making proteins. Beyond those fundamentals, however, cells are organized in two very different ways. Prokaryotic cells are generally simpler and lack a nucleus, while eukaryotic cells have a nucleus and a more compartmentalized internal structure.
This distinction is one of the most important organizing ideas in biology. It explains major differences between bacteria and archaea on one hand and animals, plants, fungi, and many single-celled organisms on the other. It also helps explain how cells store and use DNA, produce energy, build proteins, and organize complex biological processes.
The basic difference between prokaryotes and eukaryotes
The defining structural difference is the location of the cell’s DNA.
In a prokaryote, DNA is not enclosed within a membrane-bound nucleus. Instead, the main chromosome occupies a region of the cell called the nucleoid. Prokaryotes include bacteria and archaea.
In a eukaryote, DNA is enclosed inside a membrane-bound nucleus. Eukaryotes include animals, plants, fungi, and many other organisms, including diverse unicellular species.
The distinction goes beyond the nucleus. Eukaryotic cells contain numerous membrane-bound organelles, such as mitochondria and, in plants and algae, chloroplasts. These compartments allow different chemical processes to occur in specialized environments. Prokaryotic cells generally do not have membrane-bound organelles of this type, although they can have highly organized internal structures and specialized membrane systems.
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Nucleus | Absent | Present |
| Main DNA | Usually one circular chromosome | Multiple linear chromosomes in the nucleus |
| Membrane-bound organelles | Generally absent | Common |
| Typical cell size | Generally smaller | Generally larger |
| Ribosomes | Present; smaller type | Present; larger type in the cytoplasm |
| Cell division | Usually by binary fission | Mitosis; meiosis produces reproductive cells in organisms that use sexual reproduction |
| Examples | Bacteria, archaea | Animals, plants, fungi, many protists |
These are useful generalizations rather than absolute rules. Biology contains exceptions and unusual cell types, but the nucleus and overall cellular organization remain the central distinctions.
Prokaryotic cell structure
Prokaryotic cells are typically compact and structurally efficient. A bacterial or archaeal cell can carry out all of the processes necessary for life without the extensive internal compartmentalization found in eukaryotes.
The plasma membrane forms the cell’s boundary. It controls what enters and leaves the cell and provides a site for important biochemical processes. In many prokaryotes, processes associated with energy production take place at the plasma membrane because the cell lacks mitochondria.
Many prokaryotes also have a cell wall outside the plasma membrane. The composition of the wall differs between major groups. Bacterial cell walls commonly contain peptidoglycan, whereas archaeal cell walls have different chemical compositions.
Some prokaryotes possess an external capsule or other protective layer. Others have flagella, which can provide movement, or surface structures involved in attachment and interactions with other cells.
Inside the cell, the cytoplasm contains water, dissolved molecules, ions, ribosomes, and other cellular components. The DNA occupies the nucleoid region rather than a membrane-enclosed nucleus.
Prokaryotes can also contain plasmids. These are relatively small, usually circular DNA molecules separate from the main chromosome. Plasmids can carry genes that provide particular advantages under certain conditions and can sometimes be transferred between cells.
Prokaryotic ribosomes
Prokaryotes have ribosomes, the molecular machines that assemble proteins from amino acids. Their ribosomes are smaller than the ribosomes found in the cytoplasm of eukaryotic cells.
This difference has practical biological consequences. Some antibiotics work by interfering with bacterial ribosomes, disrupting bacterial protein production while having different effects on the ribosomes of human cells.
The presence of ribosomes in both cell types also illustrates an important point: prokaryotes are not simply “basic” versions of eukaryotic cells. They possess sophisticated molecular machinery and can perform remarkably complex biochemical activities.
Eukaryotic cell structure
Eukaryotic cells are characterized by compartmentalization. Instead of carrying out most processes in a relatively continuous interior, they distribute functions among specialized compartments.
The nucleus contains most of the cell’s DNA and is surrounded by the nuclear envelope, a double membrane containing nuclear pores. These pores regulate movement of molecules between the nucleus and the cytoplasm.
Surrounding the nucleus is the cytoplasm, which contains organelles suspended in a fluid called the cytosol. The term cytoplasm is often used broadly for the contents of the cell outside the nucleus, although terminology can vary depending on context.
Different eukaryotic organelles perform different functions. This organization allows processes that might interfere with one another to occur in separate compartments and lets cells regulate chemical conditions precisely.
Major eukaryotic organelles
The mitochondria are major sites of cellular respiration in eukaryotic cells. They use energy-rich molecules to help produce ATP, a principal energy carrier used by cells.
Chloroplasts, found in plants and many algae, carry out photosynthesis. They capture light energy and use it to drive the production of energy-rich organic molecules.
The endoplasmic reticulum (ER) is an extensive membrane network. Rough ER, which is associated with ribosomes, is involved in producing and processing many proteins destined for secretion, membranes, or certain cellular compartments. Smooth ER has roles that include lipid synthesis and other metabolic functions.
The Golgi apparatus modifies, sorts, and packages many proteins and lipids for delivery to different destinations within or outside the cell.
Lysosomes contain enzymes that break down various biological materials. They are particularly prominent in many animal cells. Plants and other eukaryotes also possess acidic, enzyme-containing compartments that perform overlapping degradative functions.
Peroxisomes carry out several metabolic reactions, including reactions involving fatty acids and the breakdown of certain potentially harmful compounds.
The cytoskeleton is not a membrane-bound organelle, but it is an essential part of eukaryotic organization. Networks of protein filaments help maintain cell shape, position organelles, enable movement, and assist in transporting materials within the cell.
Plant cells have additional structures that distinguish them from animal cells, including a cellulose-rich cell wall, chloroplasts in photosynthetic tissues, and a prominent central vacuole that contributes to storage and cellular water balance.
How DNA is organized differently
DNA exists in both prokaryotes and eukaryotes, but its organization is substantially different.
Most prokaryotes have a main chromosome that is circular and located in the nucleoid. The chromosome is not simply loose inside the cell; it is compacted and organized by interactions with DNA-associated proteins and by the way the DNA molecule is folded.
Eukaryotic chromosomes are generally linear and located inside the nucleus. Each chromosome consists of a very long DNA molecule associated with proteins called histones. DNA wrapped around histones forms chromatin, which helps package the genetic material and also contributes to regulation of gene activity.
Eukaryotic cells typically have multiple nuclear chromosomes. Humans, for example, have 23 types of chromosomes in their somatic cells, with two copies of each type.
Eukaryotic cells also contain DNA outside the nucleus. Mitochondria have their own DNA, and chloroplasts also contain DNA. These genomes are much smaller than nuclear genomes.
How prokaryotes and eukaryotes use DNA
The basic information flow is shared across cellular life: DNA provides instructions used to produce RNA, and RNA can direct protein synthesis.
In prokaryotes, transcription—the production of RNA from DNA—and translation—the production of protein from RNA—can occur in closely coupled ways because there is no nuclear membrane separating the chromosome from ribosomes in the cytoplasm.
In eukaryotes, transcription occurs primarily inside the nucleus, while translation occurs on ribosomes in the cytoplasm or associated with the endoplasmic reticulum. Before many eukaryotic messenger RNAs leave the nucleus, they undergo processing that can include modification of their ends and removal of introns, noncoding segments within many genes.
This physical separation gives eukaryotic cells additional opportunities to regulate gene expression. It also means that newly produced RNA generally must be processed and transported before it can be translated.
Organelles and the origin of mitochondria and chloroplasts
One of the most significant differences between prokaryotic and eukaryotic cells is the presence of mitochondria and chloroplasts in many eukaryotes. These organelles have several characteristics that distinguish them from most other eukaryotic cellular structures.
Mitochondria and chloroplasts contain their own DNA and ribosomes, and both are surrounded by membranes. Their genetic and structural features support the endosymbiotic theory, which proposes that these organelles originated from ancient bacteria that entered into long-term symbiotic relationships with ancestral eukaryotic cells.
Over evolutionary time, many genes associated with these organisms were transferred to the host cell’s nuclear genome, while the bacterial descendants became increasingly integrated into the cell.
The result is not simply a prokaryotic cell living inside a eukaryotic cell. Modern mitochondria and chloroplasts are highly integrated organelles whose activities depend on cooperation between their own genomes and the nuclear genome.
Cell division and reproduction
Prokaryotic cells commonly reproduce through binary fission. The chromosome is replicated, the copies become separated, and the cell divides into two daughter cells. The process is simpler than eukaryotic nuclear division, although it still requires carefully coordinated molecular machinery.
Eukaryotic cells use mitosis to distribute duplicated chromosomes during ordinary cell division. Mitosis helps ensure that daughter cells receive appropriate sets of chromosomes.
Cells involved in sexual reproduction undergo meiosis, a specialized form of cell division that reduces chromosome number and generates genetically varied reproductive cells or their precursors. Meiosis is fundamentally different from binary fission and from ordinary mitotic division because it involves two successive rounds of chromosome segregation after a single round of DNA replication.
Why eukaryotic cells can be so complex
The larger size and compartmentalization of eukaryotic cells create both opportunities and challenges.
Membrane-bound organelles allow chemical reactions to be separated and regulated. The nucleus protects and organizes a large genome while controlling access to genetic information. The endomembrane system allows proteins and lipids to be manufactured, modified, sorted, and transported through specialized pathways.
Eukaryotic cells can therefore support elaborate patterns of specialization. In multicellular organisms, cells with essentially the same genome can develop into very different cell types because they activate different sets of genes and respond differently to their surroundings.
Prokaryotes achieve complexity differently. Their small size, rapid reproduction in many species, diverse metabolic capabilities, and ability to exchange genetic material make them highly adaptable. A simpler cellular architecture does not mean a simpler ecological or evolutionary role.
The evolutionary relationship between the two cell types
Prokaryotes and eukaryotes are not best understood as two stages on a ladder from “simple” to “advanced.” Both represent successful and diverse forms of cellular life.
Bacteria and archaea have their own deep evolutionary histories. Eukaryotes arose later in Earth’s history, and their cells appear to have emerged through a series of evolutionary events that included symbiotic relationships between different kinds of cells.
Modern eukaryotic cells retain clues to these relationships. Mitochondria and chloroplasts, in particular, preserve features associated with their bacterial ancestry.
The fundamental distinction remains straightforward: prokaryotic cells lack a membrane-bound nucleus and generally lack membrane-bound organelles, whereas eukaryotic cells have a nucleus and extensive internal compartmentalization. Their differences in DNA organization, ribosomes, cell division, and energy-processing structures follow from this broader difference in cellular architecture.
