A small world revealed by the microscope
In 1665 Robert Hooke looked at cork with his own microscope and called the little room-like spaces "cells" — in fact he was seeing the walls of dead cells. In the 1670s Antonie van Leeuwenhoek in the Netherlands used single-lens microscopes to see microbes in rainwater, sperm and red blood cells, reporting by letter to the Royal Society. In 1838–39 botanist Matthias Schleiden and zoologist Theodor Schwann proposed cell theory — all plants and animals are made of cells — and in 1855 Rudolf Virchow added that every cell comes from a cell, pushing aside spontaneous generation and founding cellular pathology, which sees disease as cells gone wrong.
Cell structure
There are two broad kinds of cells. Prokaryotes such as bacteria and archaea lack a nuclear membrane; eukaryotic cells of animals, plants and fungi have a nucleus holding their DNA. In 1977 Carl Woese compared ribosomal RNA and divided life into three domains: bacteria, archaea and eukaryotes. Inside eukaryotic cells are energy-producing mitochondria, ribosomes that make proteins (described with the electron microscope by George Palade in 1955), the endoplasmic reticulum and Golgi apparatus that process and ship proteins, and lysosomes that break things down; plants add chloroplasts and a cell wall. In 1967 Lynn Margulis (then Sagan) proposed endosymbiosis — that mitochondria and chloroplasts descend from bacteria that came to live inside other cells — an idea later widely accepted on genetic evidence. Enzymes speed the cell's countless reactions, and the membrane controls what enters and leaves.
Division, differentiation and ageing
Cells copy their DNA and split in two by mitosis to grow the body and heal wounds; sex cells are made by meiosis. Starting from a single fertilised egg, cells differentiate into hundreds of types, such as neurons, muscle cells and blood cells. In 1961 Leonard Hayflick found that normal human cells in culture divide roughly 40–60 times and then stop — the Hayflick limit — whereas cancer cells multiply endlessly because their controls have broken. In 2006 Shinya Yamanaka turned adult cells back into embryonic-like stem cells with a handful of genes, creating induced pluripotent stem (iPS) cells and sharing the 2012 Nobel Prize. The origin of cells remains a central question in the study of life's origin and evolution.
FAQ
- What are the largest and smallest cells?
- An ostrich egg yolk is often cited as the largest single cell, and the human egg is one of our largest cells. Among the smallest are bacteria such as mycoplasmas, about 0.2–0.3 μm across.
- Are viruses cells?
- No. Viruses cannot carry out metabolism or divide on their own and replicate only inside host cells, so they are not considered cells.
Sources reviewed: 2026-10-26
References
- Hooke, R. (1665). Micrographia. London: J. Martyn and J. Allestry.
- van Leeuwenhoek, A. (1677). Observations concerning little animals observed in rain-, well-, sea- and snow-water. Philosophical Transactions of the Royal Society of London, 12, 821–831.
- Schleiden, M. J. (1838). Beiträge zur Phytogenesis. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin, 137–176.
- Schwann, T. (1839). Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Berlin: Sander.
- Virchow, R. (1858). Die Cellularpathologie in ihrer Begründung auf physiologische und pathologische Gewebelehre. Berlin: Hirschwald.
- Palade, G. E. (1955). A small particulate component of the cytoplasm. Journal of Biophysical and Biochemical Cytology, 1(1), 59–68.
- Hayflick, L., & Moorhead, P. S. (1961). The serial cultivation of human diploid cell strains. Experimental Cell Research, 25(3), 585–621.
- Sagan, L. (1967). On the origin of mitosing cells. Journal of Theoretical Biology, 14(3), 225–274.
- Woese, C. R., & Fox, G. E. (1977). Phylogenetic structure of the prokaryotic domain: the primary kingdoms. PNAS, 74(11), 5088–5090.
- Takahashi, K., & Yamanaka, S. (2006). Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell, 126(4), 663–676.