General · EdgepediaSeptember 17, 20267 min read

Cell biology

Cell biology is the branch of biology that studies cells, the smallest units of living things, including their structure, organelles, life cycle, division, and the molecular interactions that produce cellular function. The human body is composed of trillions of cells, which provide structure, take in nutrients, convert nutrients into energy, and carry out specialized functions.¹ The field rests on cell theory, the principle that all living things are made of cells, the cell is the basic unit of life, and new cells arise from existing cells.²

Key fact	Detail
Cell theory	All living things comprise one or more cells; the cell is the basic unit of life; new cells arise from existing cells, by binary fission in prokaryotes and mitosis and meiosis in eukaryotes²^•³
Naming of the cell	Robert Hooke coined the term "cell" in 1665 for the box-like structures he saw in cork tissue through a lens²
Two cell types	Every cell is either prokaryotic (bacteria and archaea, no membrane-bound nucleus) or eukaryotic (protists, fungi, plants, and animals, with a membrane-bound nucleus and organelles)⁴
Size scale	Prokaryotic cells are around 1 µm; eukaryotic cells are typically 10–100 µm⁵
Shared components	All cells have a plasma membrane, cytoplasm, DNA, and ribosomes⁶
Microscopy limits	Student light microscopes magnify up to about 400 times with roughly 200 nm resolution; electron microscopes reach 100,000x magnification and 50 picometer resolution²
Endosymbiosis	Mitochondria and chloroplasts are of prokaryotic origin, an idea proposed in 1905 and argued from microscopic, genetic, molecular, fossil, and geological data by Lynn Margulis in 1967⁷

Cell theory and its origins

The microscope made the field possible. Robert Hooke first used the term "cells" in 1665 to describe the small chambers within cork that he observed under a microscope of his own design, publishing his drawings in Micrographia. He did not know the cork cells were long dead and lacked the internal structures of living cells.⁷ In the 1670s, Antony van Leeuwenhoek, a Dutch lens maker, observed the movements of single-celled organisms, which he called "animalcules," and discovered bacteria and protozoa.²

Nearly 200 years after Hooke, the German botanist Matthias Schleiden described plant tissues as composed of cells in 1838, though he believed cells formed through crystallization rather than cell division. In 1839, after a conversation with Schleiden, the German physiologist Theodor Schwann realized that similarities existed between plant and animal tissues, laying the foundation for the idea that cells are the fundamental components of both.⁷

The principle of biogenesis. In 1852, Robert Remak, a neurologist and embryologist working in Germany, published convincing evidence that cells are derived from other cells as a result of cell division. Three years later, Rudolf Virchow, a well-respected pathologist, published an editorial essay entitled "Cellular Pathology," which popularized cell theory with the Latin phrase omnis cellula a cellula ("all cells arise from cells"). Because Virchow's essay closely resembled Remak's earlier work and did not credit it, there is controversy over which scientist should receive credit for articulating this tenet.⁷ Virchow's broader claim that all diseases are essentially disturbances of cells shaped the rise of pathology and histology in the nineteenth century.⁸

Cell structure

Cells fall into two broad categories. Prokaryotic cells, found in the domains Bacteria and Archaea, lack a nucleus and other membrane-bound organelles; their DNA sits in a region of the cytoplasm called the nucleoid, usually as a single circular chromosome. Eukaryotic cells, found in protists, fungi, plants, and animals, have a true nucleus in which DNA is surrounded by a membrane, plus other membrane-bound organelles that allow compartmentalization of functions.⁶^•⁴ Prokaryotic cells are around 1 µm across, compared with 10–100 µm for eukaryotic cells; some organelles, such as chloroplasts and mitochondria, are bigger than prokaryotic cells and would not fit inside one.⁵

Despite their differences, all cells share four common components: a plasma membrane that separates the cell's interior from its environment, a jelly-like cytoplasm, DNA as the genetic material, and ribosomes, the particles that synthesize proteins.⁶

Organelles of eukaryotic cells. Within the cell, specialized structures called organelles perform particular tasks. The nucleus serves as the cell's command center, sending directions for the cell to grow, mature, divide, or die, and houses the DNA inside a protective nuclear envelope. Mitochondria convert energy from food into a form the cell can use; they have their own genetic material, separate from the nuclear DNA, and can make copies of themselves. The endoplasmic reticulum processes molecules created by the cell and transports them to their destinations, and the Golgi apparatus packages those molecules for transport out of the cell. Lysosomes and peroxisomes act as the cell's recycling center, digesting invading bacteria, ridding the cell of toxic substances, and recycling worn-out components. The cytoskeleton, a network of long fibers, determines cell shape, participates in cell division, allows movement, and provides tracks that direct the movement of organelles.¹

The cell membrane

The cell membrane is not only the boundary of the unit of life; it is also a compartment that harbors essential functions including communication with the environment, transport of molecules, and certain metabolic functions. When cell theory was formulated around 1839, the reality of the membrane was unknown, and its universal character was not generally acknowledged until well into the twentieth century. Even after cells were assumed to be bounded by some kind of membrane, the fluid mosaic model of membrane structure was not accepted until as late as the 1970s.⁹

How cells are studied

Because individual cells are generally too small and transparent to see unaided, biologists rely on microscopy. A typical human red blood cell is about eight micrometers in diameter, meaning about 250 red blood cells could fit on a pinhead. Light microscopes, which can view living organisms, magnify up to roughly 400 times in student laboratories, or 1,000 times with oil immersion lenses, but transparent cell components must be colored with special stains, which usually kills the cells. Electron microscopes use a beam of electrons instead of light, providing far higher magnification and resolving power, but specimen preparation kills the sample and living cells cannot be viewed because electrons move best in a vacuum. Scanning electron microscopes detail the cell surface, while transmission electron microscopes reveal internal structures.²

Instrumental milestones. Each advance in instrumentation opened new levels of cellular detail: Kölliker described mitochondria in muscle in 1857, Golgi stained cells with silver nitrate and discovered the Golgi apparatus in 1898, Ruska built the first transmission electron microscope in 1931, and Porter and colleagues, pioneering electron microscopy of cells, identified the endoplasmic reticulum and many elements of the cytoskeleton in 1945.¹⁰^•¹¹ The Zeiss firm's first compound microscope, first sold in 1857, and Ernst Abbe's optical theory helped drive a nineteenth-century revolution in microscopy.¹²

Endosymbiotic theory

The origin of eukaryotic cells is explained by endosymbiotic theory. Robert Brown first described nuclei in plant cells in 1831, and in the early 1880s the German botanist Andreas Schimper described chloroplasts, noting their role in starch formation during photosynthesis and that they divided independently of the nucleus. Based on this independent reproduction, the Russian botanist Konstantin Mereschkowski suggested in 1905 that chloroplasts may have originated from ancestral photosynthetic bacteria living symbiotically inside a eukaryotic cell, the first articulation of the endosymbiotic hypothesis.⁷

The American anatomist Ivan Wallin experimentally examined similarities between mitochondria, chloroplasts, and bacteria in the 1920s, claiming he could culture mitochondria outside their host cells; many scientists dismissed his cultures as bacterial contamination. His ideas were largely ignored for the next 50 years because scientists did not know these organelles contained 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 argued from microscopic, genetic, molecular biology, fossil, and geological data that organelles such as mitochondria and chloroplasts are of prokaryotic origin. Modern genome sequencing supports the view that much of the mitochondrial genome was transferred to the host cell's nucleus, preventing mitochondria from living on their own.⁷

Modern directions

Cell biology today underpins biotechnology and medicine. Biotechnology research applies biochemistry, cellular biology, biophysics, and molecular biology techniques to issues related to humans and the environment, and gene therapy, nanotechnology, and regenerative medicine are active research areas funded by the National Institutes of Health.¹³ Genome editing methods such as CRISPR allow researchers to edit genomes precisely inside living cells without first removing the DNA.¹⁴ A central open problem in structural cell biology is how cellular functions spontaneously emerge from the interactions of the billions of molecules in a cell, a question of cellular self-organization that includes local confinement, molecular rulers, and the self-organizing behavior of membranes.¹⁵

References

Topic: Encyclopedia › Life and health › Biological foundations › Cell biology › Cell biology overview › Cell theory and outlines

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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