Cell Theory
Three principles. That is the whole theory. If you can remember a three-item list, you know cell theory.
- All living things are made of cells. From the tallest redwood tree to the smallest bacterium to the person reading this sentence - every living organism is built from cells.
- All cells come from pre-existing cells. No cell pops into existence from nothing. Every cell was made by another cell dividing. Rudolf Virchow summed this up in 1855 with the Latin phrase “Omnis cellula e cellula” - all cells from cells.
- The cell is the basic unit of life. It is the smallest thing that can independently carry out the functions of life - metabolism, growth, reproduction, response to stimuli.
A modern addition to cell theory is sometimes called the fourth principle: cells contain hereditary information (DNA) that is passed from parent cell to daughter cell during division. This principle was not part of the original theory because DNA had not yet been discovered, but it is now considered a foundational extension.
The Virus Problem
Here is an important wrinkle the MCAT loves to test: viruses challenge cell theory. Viruses carry genetic material (DNA or RNA) and can evolve, but they cannot reproduce on their own. They must hijack a host cell’s machinery to replicate. Because they lack the ability to independently carry out metabolic processes, viruses are not considered living organisms by most biologists. They exist in a gray zone - not quite alive, not quite dead. We will cover viruses in detail in Section 1.9.