Cell theory

Cell theory rests on three classical tenets: every living organism is composed of one or more cells; the cell is the smallest unit that exhibits the properties of life; and new cells arise only from the division of existing cells. Modern formulations add that cells carry hereditary information in DNA and pass it to daughter cells, that all cells share a fundamentally similar chemical composition and metabolism, and that an organism's activity depends on the collective activity of its cells.

The theory came together over two centuries of microscopy. Robert Hooke named the "cells" he saw in cork in 1665, and Antonie van Leeuwenhoek observed living single-celled organisms shortly after. In 1838 and 1839 Matthias Schleiden and Theodor Schwann proposed that plants and animals alike are built from cells, and in 1855 Rudolf Virchow added the crucial third tenet — omnis cellula e cellula, every cell from a cell — which displaced the older idea of spontaneous generation. Louis Pasteur's experiments later put that idea to rest experimentally.

Cell theory frames how biology is organized. It explains why prokaryotes and eukaryotes are compared at the level of shared features — plasma membrane, cytoplasm, ribosomes, and DNA — with eukaryotes distinguished by a nucleus and membrane-bound organelles. It underlies the continuity of life through cell division, makes disease intelligible as cellular dysfunction, as when cancer arises from uncontrolled division, and sets the boundary that excludes viruses, which are not cells and cannot reproduce without a host. Endosymbiotic theory extends the picture by proposing that mitochondria and chloroplasts descend from once free-living prokaryotes.

The MCAT expects fluency with these fundamentals in the biology and biochemistry section, particularly the prokaryote-versus-eukaryote comparison and the reasons viruses fall outside cell theory. USMLE Step 1 builds on the same base when it covers cell biology of cancer and the control of cell division.