General · EdgepediaSeptember 17, 20268 min read

Cell (biology)

The cell is the basic structural and functional unit of living organisms: a bounded mass of cytoplasm containing genetic material, capable of metabolism, growth, and reproduction. Cell biology is the study of the structure and function of this unit, including the cell life cycle (cell death and cancer), cell differentiation and stem cells, cell motility, and the roles of organelles and cellular structures in membrane transport and signaling.¹ MeSH, the controlled vocabulary of subject headings maintained by the National Library of Medicine, gives the field a wider definition, covering how cells are structured, how they behave, grow, reproduce, and become diseased, as well as the chemistry and workings of their parts.²

Key fact	Detail
Definition	The cell is the basic unit of living organisms; cell biology studies its structure and function, including the cell cycle, differentiation, motility, and organelles¹
Cell theory	All organisms are made of one or more cells; all life functions occur within cells; all cells come from preexisting cells³
Two classes	Prokaryotic cells lack a nuclear envelope and cytoplasmic organelles, but possess cytoskeletal structures, including FtsZ and MreB homologues; eukaryotic cells have a nucleus separating the genetic material from the cytoplasm⁴^•¹⁶
Size range	Living prokaryotes span cell volumes from 0.02 to 400 μm³; eukaryotic cells are frequently at least a thousandfold greater in volume than prokaryotic cells⁵^•⁴
Organelle origin	Mitochondria and chloroplasts evolved from bacteria that established a symbiotic relationship inside a eukaryotic host cell⁶
Known exceptions	Viruses have no cell membrane, cytoplasm, organelles, or chromosomes and do not fit the cell theory⁷

Cell theory

Early microscopy. In 1665 the English botanist Robert Hooke (1635–1702), then first secretary of the Royal Society in London, coined the term "cell" for the microscopic units he saw in cork. With only thirty-times magnification he observed a pattern of tiny rectangular holes in a thin slice of cork from oak bark, and a similar pattern in bones and plants; he described the small chambers as resembling "Honey-comb" or "small Boxes or Bladders of Air." Hooke did not know that the cork cells were long dead and lacked the internal structures of living cells.³^•⁶

Schleiden and Schwann. In 1838 the German botanist Matthias Schleiden (1804–1881), professor of botany at Jena, described plant tissues as composed of cells, a conclusion drawn from extensive microscopic observation; plant cells were comparatively easy to see because thick cell walls separate them. In 1839 the German physiologist Theodor Schwann (1810–1882), after a conversation with Schleiden, recognized similarities between plant and animal tissues and generalized the idea that cells are the fundamental components of both. Schleiden and Schwann are considered the founders of the cell theory, though they disagreed about how new cells arise: Schleiden believed cells formed through crystallization rather than division, and their views on cell formation fell within the realm of spontaneous generation, a popular idea at the time.⁶^•³^•⁸

Remak and Virchow. In 1852 Robert Remak (1815–1865), a neurologist and embryologist living in Germany, published convincing evidence that cells are derived from other cells as a result of cell division, an idea initially questioned by many. Three years later, in 1855, the pathologist Rudolf Virchow (1821–1902) published an editorial essay, "Cellular Pathology," which popularized the concept using the Latin phrase omnis cellula a cellula ("all cells arise from cells"), essentially the second tenet of modern cell theory. Because Virchow's essay closely resembled Remak's earlier publication and did not credit it, there is controversy over which scientist should receive credit for articulating this principle.⁶ Taken together, the findings of Schleiden, Schwann, and Virchow constitute the cell theory: every organism consists of a single cell or several cells, every life function of an organism takes place inside cells, and each cell arises from a cell that already exists.³

Prokaryotic and eukaryotic cells

Cells are divided into two main classes, initially defined by whether they contain a nucleus. In bacteria, the prokaryotic type of cell, no membrane encloses the nucleus, indeed, prokaryotes have none, whereas eukaryotic cells possess a nucleus, within which the DNA is kept apart from the surrounding cytoplasm. Prokaryotic cells are generally smaller and simpler: in addition to lacking a nucleus, their genomes are less complex and they lack membrane-bound organelles, but they do possess cytoskeletal structures, including homologues of FtsZ and MreB.¹⁶ In prokaryotes the plasma membrane subserves many of the specialized functions carried out by eukaryotic organelles.⁴^•⁹

The size difference is substantial. Eukaryotic cells are frequently at least a thousandfold greater in cell volume than prokaryotic cells.⁴ Among prokaryotes themselves, living Bacteria and Archaea span cell volumes from 0.02 to 400 μm³. The smallest size for a free-living organism is thought to be set largely by the catalytic efficiency of enzymes and the protein-synthetic machinery, while the largest cells are limited by the ability of diffusion to bring nutrients in and dispose of waste.⁵ Size also matters physiologically because it establishes the surface-to-volume ratio, which limits the rate and type of nutrient uptake.¹⁰

Yet beyond those distinctions, prokaryotes and eukaryotes alike rely on the same fundamental molecular machinery to function, a sign that every cell alive today traces back to one common primordial forebear.⁴

Organelles and endosymbiosis

Eukaryotic cells are compartmentalized: the nucleus is surrounded by the cytoplasm, which in animal cells usually accounts for nine-tenths of the cell's volume, and the cytoplasm is enclosed by a cell membrane (plasmalemma).⁹ The origins of mitochondria, chloroplasts, the endoplasmic reticulum, and the nucleus explain the topological relationships among these intracellular compartments, and the nature of organelle genomes, together with the close resemblance of their proteins to those of present-day bacteria, indicates that mitochondria and plastids evolved from bacteria engulfed by other cells with which they initially lived in symbiosis.¹¹

The endosymbiotic theory states that mitochondria and chloroplasts arose when prokaryotic cells established a symbiotic relationship within a eukaryotic host. The Scottish botanist Robert Brown first described nuclei in plant cells in 1831. In 1905 the Russian botanist Konstantin Mereschkowski (1855–1921), noting that chloroplasts divide independently of the nucleus, suggested that chloroplasts originated from ancestral photosynthetic bacteria living symbiotically inside a eukaryotic cell. The American anatomist Ivan Wallin (1883–1969) tested similarities between mitochondria, chloroplasts, and bacteria in the 1920s, but his ideas were largely ignored for the next 50 years because scientists did not know that these organelles contain their own DNA. With the discovery of mitochondrial and chloroplast DNA in the 1960s the hypothesis was revived, and in 1967 the American geneticist Lynn Margulis (1938–2011) argued from microscopic, genetic, molecular, fossil, and geological data that these organelles are of prokaryotic origin.⁶

Supporting evidence includes the close relationship of mitochondrial and chloroplast DNA to bacterial counterparts in both sequence and chromosome structure, the structural similarity of organelle ribosomes to bacterial rather than eukaryotic ribosomes, and organelle division by binary fission resembling that of bacteria. Many genes originally present in these bacteria became incorporated into the nuclear genome, so only a few components of mitochondria and chloroplasts are still encoded by the organelle genomes.⁶^•⁴

Limits and exceptions of the cell theory

Viruses do not fit the cell theory: they have no cell membrane, no cytoplasm, no organelles, and no chromosomes, consisting instead of a protein coat or capsid surrounding DNA, RNA, or a few genes (the HIV virus has only 9 genes).⁷ Multinucleate coenocytes and syncytia found throughout the eukaryote superkingdom also pose problems for the current version of cell theory, prompting proposals to treat the eukaryotic cell as composed of an elemental "Cell Body" smaller than the cell itself and endowed with the basic attributes of a living entity.¹²

Cell size as a regulator

Cells in the human body differ widely in size depending on cell type, environmental inputs, and disease. Recent work shows that cell size is a major regulator of cell function: changes in size cause broad reorganization of the cellular transcriptome and proteome, and misregulated cell size is associated with pathophysiological phenotypes such as senescence, aging, and cancer, where unusual and heterogeneous cell size is typical. Size control is linked to biomolecular scaling, organelle homeostasis, and cell cycle progression; in budding yeast, for example, global transcript amounts scale with cell size through a size-dependent increase in transcription by limiting RNA polymerase II combined with a size-dependent decrease in mRNA degradation.¹³

Studying cells

Research in cell biology depends on laboratory methods for studying cell structure and function. Light microscopy is limited by the wavelength of visible light and the light-gathering power of the lens (numerical aperture); fluorescent dyes can label molecules of interest in fixed or living cells; and electron microscopy allows detailed visualization of cell structure, though microscopy alone cannot define the functions of organelles, which must be isolated for biochemical study.¹⁴ Single-cell analysis techniques have more recently enabled studies such as single-cell multiomics connecting 3D genome and transcriptome alterations in Alzheimer's disease.¹⁵

References

Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026