All living organisms are made of cells and cell products. This simple statement, called the cell theory, 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 supports the validity of the cell theory.

Cells are the smallest living subunits of a multicellular organism such as a human being. Microorganisms, such as amoebas and bacteria, are single cells that function independently. Human cells, however, must work together and function 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 = 1/1,000,000 of a meter or 1/25,000 of an inch. 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. Based on function, there are more than 200 different kinds of human cells.

Buy the Book that holds this excerpt: Essentials of Anatomy And Physiology

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