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.