Cracking the AP Biology Exam
Diversity of Organisms
III. KINGDOM PROTISTA
Protists are eukaryotes. They have a nucleus and membrane-bound organelles. Although most protists are unicellular, some are multicellular or form colonies. Protists differ in cellular structure, mode of nutrition, and type of reproduction. Protists may be plantlike, animal-like, or funguslike. They have an alternating, two-part life cycle made up of diploid, spore-forming sporophytes and haploid, gamete-forming gametophytes. Most scientists consider phylum Protista obsolete because it consists of eukaryotes that did not fit the definition of plants, animals, or fungi. Many suggest this kingdom should be split into newly designated kingdoms.
PLANTLIKE PROTISTS (PHOTOSYNTHETIC)
These unicellular organisms have photosynthetic pigments, chlorophyll a and b. Euglenas have flagella that protrude from a gullet and an eyespot that helps them respond to light (phototaxis). They live in freshwater rich with organic material.
Dinoflagellates are unicellular organisms that live in marine and fresh water. They have photosynthetic pigments, two flagella, and cell walls that contain cellulose.
This group includes the golden algae. Although most are unicellular, some are multicellular. They have photosynthetic pigments and are golden in color. Their cells are covered by tiny scales of either silica or calcium carbonate.
Green algae are unicellular and have photosynthetic pigments. Most have flagella at some stage of their life and store food as starch.
Brown algae are mostly multicellular, photosynthetic, and possess chlorophyll a, chlorophyll c, and carotenoids. Their reproductive cells, both asexual zoospores and sexual gametes, are usually biflagellated.
Red algae are mostly multicellular, marine seaweeds. In addition to chlorophyll a and b, they have red photosynthetic pigments (phycobilins).
Diatoms are usually nonmotile, unicellular organisms with cell walls made of silica.
ANIMAL-LIKE PROTISTS (NONPHOTOSYNTHETIC HETEROTROPHS)
The zooflagellates are unicellular protozoans that move by means of a flagellum. Some live in the gut of termites (Trichonympha); others are parasitic and cause disease such as African sleeping sickness (Trypanosoma).
A type of amoeba that performs phagocytosis by surrounding and engulfing food using pseudopods (“false feet”).
They are unicellular protozoans that move around and feed using tiny hairs (cilia). The best known is the Paramecium. It possesses two nuclei, an oral groove, and contractile vacuoles.
Sporozoans are nonmotile, parasitic spore-formers. They are characterized by their lack of flagella and an amoeboid body form. They include the Plasmodium, which causes malaria.
These unicellular protists produce calcareous tests (shells) with pores through which cytoplasmic projections extend.
Slime molds produce large multinucleated masses (plasmodium). Sometimes slime molds have stalks that grow upward, and form spores (fruiting bodies). Other times, they produce gametes, which fuse and produce a diploid zygote to form a multinucleated mass. They are found in moist soil, decaying leaves, or logs in a damp forest.