Plant Differentiation From Other Kingdoms

Plants are multicellular eukaryotes—that is, their cells contain membrane-bound structures called organelles. Plants differ from other eukaryotes because their cells are enclosed by more or less rigid cell walls composed primarily of cellulose. The most important characteristic of plants is their ability to photosynthesize. During photosynthesis, plants make their own food by converting light energy into chemical energy—a process carried out in the green cellular organelles called chloroplasts (see Chlorophyll; Chloroplast). A few plants have lost their chlorophyll and have become saprophytes or parasites—that is, they absorb their food from dead organic matter or living organic matter, respectively—but details of their structure show that they are evolved plant forms.

Fungi, also eukaryotic and long considered members of the plant kingdom, have now been placed in a separate kingdom because they lack chlorophyll and plastids and because their rigid cell walls contain chitin rather than cellulose. Unlike the majority of plants, fungi do not manufacture their own food; instead they are saprophytic, absorbing their food from either dead or living organic matter.

The various groups of algae were also formerly placed in the plant kingdom because many are eukaryotic and because most have rigid cell walls and carry out photosynthesis. Nonetheless, because of the variety of pigment types, cell wall types, and physical attributes found in the algae, they are now recognized as part of two separate kingdoms, containing a diversity of plantlike and other organisms that are not necessarily closely related. One of the phyla of algae, the green algae, is believed to have given rise to the plant kingdom, because its chlorophylls, cell walls, and other details of cellular structure are similar to those of plants.

The animal kingdom is also multicellular and eukaryotic, but its members differ from the plants in deriving nutrition from other organic matter; by ingesting food rather than absorbing it, as in the fungi; by lacking rigid cell walls; and, usually, by having sensory capabilities and being motile, at least at some stage. See Classification.