The kingdom Protista is a grab-bag category for an enormously diverse array of eukaryotic organisms, most of which are unicellular. The ancestors of all eukaryotes would be classified as protists if they lived today. The revolution that led to the first eukaryotes was the development of intracellular compartments separated by membranes. The evolution of early eukaryotes also depended on the process of endosymbiosis, in which prokaryotic cells became part of eukaryotic cells and formed organelles like mitochondria and chloroplasts.

Protists often have complex structures and life cycles. Two important protist cell forms are the flagellate and the amoeba. These forms occur in many unrelated taxonomic groups.

Protist phylogeny is still an unsolved problem and is the subject of intense research among biologists today. For convenience, protists are often split into three categories based on their superficial similarities to animals, plants, or fungi. These divisions don’t have phylogenetic significance.

The animal-like protists are called protozoans. Some protozoans capture and ingest food with the use of pseudopodia; others swim to pursue prey, or waft prey toward them, using flagella or cilia; still other protozoans are parasites.

Many distinct groups of protists are considered plantlike because they carry out photosynthesis. They range in size from single cells to seaweeds. Green algae are the closest relatives of the plants. The chromists are a diverse group of mostly plantlike protists, although the water molds and their relatives, which are funguslike protists, are also chromists.

The other category of funguslike protists we saw was the slime molds. Slime molds form organized masses similar in appearance to those formed by fungi; and like fungi, they produce and release spores from fruiting bodies.

Protists can be found in almost any environment where there is water. With a simple microscope and a drop of pond water, anyone can rediscover a fascinating hidden world of protist diversity.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education