CLASSIFYING LIFE FORMS
Exactly what is a plant and how is it different from other life forms? This may
initially seem like a simple question. Everyone knows that an elm tree is a plant,
whereas a dog is not. Nevertheless, the precise definition of plants is still a matter of
debate among some scientists. All living things are made up of protoplasm, a
complex material composed of organic substances such as sugars, proteins and
fats. Protoplasm is arranged in tiny units called cells. All living things are composed
of cells. As recently as the late 1960s, scientists believed that all organisms could be
classified as members of either the plant or the animal kingdom. Life forms that are
green and that can synthesize their own food using light energy were put in the plant
kingdom. Those organisms that lack green pigment and are able to move about
were considered to be animals. Researchers now agree that living things are more
properly divided into two groups-prokaryotes and eukaryotes. These major groups
comprise five kingdoms. Major differences between cells are used to distinguish
between these groups and kingdoms.
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