The five-kingdom system recognizes the two fundamentally different types of cell, prokaryotic and eukaryotic. A common feature of the five-kingdom system and other classification schemes is the recognition of three kingdoms of multicellular eukaryotes, the Plantae, Fungi, and Animalia. Plants, fungi, and animals generally differ in structure and life cycles and in their modes of nutrition, the criterion Whittaker used to define these kingdoms. Plants are autotrophic in nutritional mode, making their food by photosynthesis. Fungi are heterotrophic organisms that are absorptive in nutritional mode.
Most animals live by ingesting food and digesting it within specialized cavities.
In the five-kingdom system, Protista became a grab bag containing all eukaryotes that did not fit the definitions of plants, fungi, or animals. Most protists are unicellular forms, but Whittaker s Kingdom Protista also includes relatively simple multicellular organisms. These relatively simple multicellular organisms are believed to be direct descendants of unicellular protists. Like any classification scheme, the five-kingdom system is not a natural fact, but a human construct. During the past two decades, systematists using comparisons of nucleic acids and proteins to probe the relationships between different groups of organisms have been pointing out problems with the traditional five-kingdom scheme.