All living organisms are made of cells and cell products. This simple statement, called the cell the-ory, was first proposed more than 150 years ago. You may think of a theory as a guess or hypothesis, and sometimes this is so. A scientific theory, however, is actually the best explanation of all available evidence. All of the evidence science has gathered so far sup-ports the validity of the cell theory.
Cells are the smallest living subunits of a multicel-lular organism such as a human being. A cell is a com-plex arrangement of the chemicals, is living, and carries out specific activities. Microorganisms, such as amoebas and bac-teria, are single cells that function independently. Human cells, however, must work together and func-tion interdependently. Homeostasis depends upon the contributions of all of the different kinds of cells.
Human cells vary in size, shape, and function. Most human cells are so small they can only be seen with the aid of a microscope and are measured in units called micrometers (formerly called microns). One micrometer 5 1/1,000,000 of a meter or 1/25,000 of an inch (see Appendix A: Units of Measure). One exception is the human ovum or egg cell, which is about 1 millimeter in diameter, just visible to the unaided eye. Some nerve cells, although microscopic in diameter, may be quite long. Those in our arms and legs, for example, are at least 2 feet (60 cm) long.
With respect to shape, human cells vary greatly. Some are round or spherical, others rectangular, still others irregular. White blood cells even change shape as they move.
Cell functions also vary, and since our cells do not act independently, we will cover specialized cell func-tions as part of tissue functions. Based on function, there are more than 200 different kinds of human cells.