21.1 Protist Classification—The Saga Continues

Protists are eukaryotes that are not members of the plant, animal, or fungi kingdoms.

Today's protists include groups whose ancestors were among the very last to split from the organisms that gave rise to plants, animals, and fungi.

21.2 Protist Structure and Function

Some protists move by changing their cell shape, and some move by means of specialized organelles. Other protists do not move actively but are carried by wind, water, or other organisms.

Some protists reproduce asexually by mitosis. Others have life cycles that combine asexual and sexual forms of reproduction.

• pseudopod (606)

• cilium (607)

• flagellum (607)

• spore (607)

• conjugation (608)

• alternation of generations (608)

• sporangium (609)

21.3 The Ecology of Protists

The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible.

Some heterotrophic protists engulf and digest their food, while others live by absorbing molecules from the environment.

Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit.

Parasitic protists are responsible for some of the world's most deadly diseases, including several kinds of debilitating intestinal diseases, African sleeping sickness, and malaria.

• algal bloom (611)

• gullet (612)

• food vacuole (612)

• plasmodium (613)

21.4 Fungi

Fungi are heterotrophic eukaryotes with cell walls that contain chitin.

Fungi are champions of decomposition. Many species help ecosystems maintain homeostasis by breaking down dead organisms and recycling essential elements and nutrients.

Parasitic fungi can cause serious diseases in plants and animals by disrupting homeostasis.

Some fungi form mutualistic associations with photosynthetic organisms in which both partners benefit.

• chitin (618)

• mycelium (619)

• hypha (619)

• lichen (623)

• fruiting body (619)

• mycorrhiza (624)

Think Visually Make a concept map that illustrates the various ways that protists move, reproduce, and obtain food.