What Is a Cell? The Cell as the Basic Unit of Life

Every living organism, whether a single free-floating bacterium or a multi-tonne blue whale, is built from the same basic unit of construction: the cell. The word itself comes from the Latin cellula, meaning "a small room," and it was coined in 1665 by the English scientist Robert Hooke. Examining a thin slice of cork under a primitive compound microscope of his own design, Hooke saw a honeycomb of tiny, empty box-like compartments and called them "cells" because they reminded him of the small rooms monks lived in. What Hooke actually observed were the dead, empty cell walls of cork tissue -- he had no way of seeing the living contents inside -- but the name stuck and has been used for the fundamental unit of life ever since.

A little over a decade later, the Dutch tradesman Anton van Leeuwenhoek, grinding his own remarkably powerful lenses, became the first person to observe living, free-moving cells -- bacteria, protozoa, red blood cells, and sperm cells -- in pond water and other everyday samples. Where Hooke had seen only lifeless walls, Leeuwenhoek's observations hinted for the first time that cells were not empty rooms but were themselves alive. In 1831, Robert Brown discovered a dense, roughly spherical body within the cell that he named the nucleus, and a few years later the physiologist Purkinje introduced the term "protoplasm" for the living, jelly-like material filling a cell -- the substance in which all of a cell's chemical and physical activity actually takes place.

Today, the cell is understood as the basic structural and functional unit of every living organism. This means two things at once. Structurally, no living thing exists that is not made up of one or more cells -- there is no smaller unit of life below the cell that can carry out all the processes we associate with being alive. Functionally, everything an organism does -- moving, digesting food, responding to a stimulus, reproducing, growing -- ultimately happens inside its cells, and the properties of the whole organism emerge from the properties and the cooperative behaviour of the cells that build it up. A human being's ability to think, run, and heal a wound is, at bottom, the sum of what billions of individual cells are doing and coordinating with one another.

Cells vary enormously across the living world in size, shape and number. In terms of size, the smallest cells known are those of Mycoplasma (also called PPLO, pleuropneumonia-like organisms), which may be as small as 0.3 micrometres in length -- barely visible even with a good light microscope. Bacterial cells are typically larger, ranging from about 3 to 5 micrometres, while a typical eukaryotic cell measures anywhere from 5 to 100 micrometres. At the other extreme, some cells are visible to the unaided eye: the egg of an ostrich is a single giant cell, and the alga Acetabularia can grow a single-celled stalk several centimetres tall. Cell shape is equally varied and is often closely related to the cell's function -- red blood cells are biconcave discs suited to carrying oxygen efficiently, nerve cells are drawn out into extremely long, thin processes suited to carrying electrical signals over distance, and many free-living single-celled organisms are simply spherical, oval or irregular in outline.

Finally, organisms differ enormously in how many cells make them up. Unicellular organisms, such as Amoeba, Paramecium, and many bacteria, consist of just a single cell that must independently carry out every function needed for life -- nutrition, respiration, excretion, reproduction and response to the environment all happen within that one cell. Multicellular organisms, by contrast, are built from enormous numbers of cells -- from a few hundred in some simple organisms to trillions in a large animal or plant -- and these cells typically specialise, grouping together into tissues, organs and organ systems that divide the work of keeping the whole organism alive between them. Whether unicellular or multicellular, however, every one of these cells is built and organised according to the same basic architectural plan that the rest of this chapter examines in detail.