Cracking the AP Biology Exam

9

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)

A. Euglenophyta

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.

B. Dinoflagellata

Dinoflagellates are unicellular organisms that live in marine and fresh water. They have photosynthetic pigments, two flagella, and cell walls that contain cellulose.

C. Chrysophyta

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.

D. Chlorophyta

Green algae are unicellular and have photosynthetic pigments. Most have flagella at some stage of their life and store food as starch.

E. Phaeophyta

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.

F. Rhodophyta

Red algae are mostly multicellular, marine seaweeds. In addition to chlorophyll a and b, they have red photosynthetic pigments (phycobilins).

G. Bacillariophyta

Diatoms are usually nonmotile, unicellular organisms with cell walls made of silica.

ANIMAL-LIKE PROTISTS (NONPHOTOSYNTHETIC HETEROTROPHS)

H. Zoomastigina

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).

I. Rhizopoda

A type of amoeba that performs phagocytosis by surrounding and engulfing food using pseudopods (“false feet”).

J. Ciliophora

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.

K. Sporozoa

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.

L. Foraminifera

These unicellular protists produce calcareous tests (shells) with pores through which cytoplasmic projections extend.

FUNGUSLIKE PROTISTS

M. Myxomycota

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.