Cell theory

Three statements that every living thing obeys.

The idea

Cell theory, assembled by Schleiden and Schwann in 1838–39 and completed by Virchow in 1855, makes three claims: all living organisms are made of one or more cells, the cell is the smallest unit that shows every property of life, and new cells arise only when existing cells divide. The modern version adds that cells pass hereditary information to their daughters as DNA, that energy flow (metabolism) happens inside cells, and that all cells share the same basic chemistry of membranes, proteins, and nucleic acids. The third statement rules out spontaneous generation, which Pasteur’s swan-neck flask experiments of 1859–61 finally put to rest. Everything from a 0.3 µm Mycoplasma to a blue whale is built on this one plan.

Work through an example

An adult human body contains roughly 30 trillion (3 × 10¹³) human cells, every one descended from a single fertilized egg by repeated mitosis. Because each division doubles the count, reaching 3 × 10¹³ takes at least 45 successive doublings, since 2⁴⁵ ≈ 3.5 × 10¹³; in reality far more divisions occur, because cells also die and are replaced. Your red blood cells alone are renewed at about 2–3 million per second.

What to watch for

Viruses are not an exception to cell theory — they are not cells at all. A virus has no cytoplasm, no ribosomes, and no metabolism of its own, so it cannot grow or divide; it only multiplies by hijacking a living host cell. Likewise, "all cells are microscopic" is false: a chicken egg yolk is a single cell, and some human neurons run more than a meter from the spinal cord to the toe.