The cell theory, based on the concept that higher organisms consist of
smaller units called cells, was formulated in 1838 by two German
biologists, Mathias-Jacob Schleiden, a botanist, and Theodor Schwann, an
anatomist. The theory had far-reaching effect upon the study of
biological phenomena. It suggested that living things had a common basis
of organization. Appreciation of its full significance, however, had to
await more precise knowledge of the structure and activities of cells.
Some organisms,[1] for instance, amoebae, consist of a single cell each
and are therefore called unicellular organisms. Higher animals are
multicellular, containing aggregations of cells grouped into tissues and
organs. A man, for instance, consists of millions of many different
cells performing a variety of different functions. Cells of higher
animals differ vastly from one another in size, shape, and function;
they are specialized cells.
[Illustration: Figure 1 _One of the earliest photographs of cells taken
with a microscope. This photomicrograph shows cells in the blood of a
pigeon. It was made by J. J. Woodward, U. S. Army surgeon, in 1871.
Woodward had made the first cell micrograph (a graphic reproduction of
the image of an object formed by a microscope) in 1866._]
There is a remarkable similarity, moreover, in the molecular composition
and metabolism[2] of all living things. This similarity has been taken
to mean that life could have originated only once in the past and had a
specific chemical composition on which its metabolic processes depended.
This structure and metabolism were handed down to subsequent living
things by reproduction, and all variations thereafter resulted from
occasional mutation, or changes in the nature of the
heredity-transmitting units. One of the most extraordinary of all the
attributes of life is its ordered complexity, both in function and
structure.
It is agreed among biologists that the functional manifestations of life
include movement, respiration, growth, irritability (reaction to
environmental changes), and reproduction and that these phenomena are
therefore possessed by all cells. The first four of these can be grouped
under a single word: metabolism. We can therefore say that living things
have two common properties: metabolism and reproduction. Therefore, when
we say we are studying life processes, we actually are studying the
metabolism and reproduction of cells. Since metabolism is the sum of the
biochemical reactions taking place in a living organism, it properly
belongs to the field of investigation of biochemists. Cell reproduction
is the concern of both biochemists and morphologists[3] since it can be
studied by either biochemical or morphological techniques.