Chapter Summary

• The term protist does not describe a formal taxonomic group. It is shorthand for “all eukaryotes that are not plants, animals, or fungi.”

• Early events in the evolution of the eukaryotic cell included the loss of the firm cell wall and infolding of the cell membrane. Such infolding led to segregation of the genetic material in a membrane-enclosed nucleus. Review Focus: Key Figure 26.1

• Mitochondria evolved by endosymbiosis with a proteobacterium.

• Primary endosymbiosis of a eukaryote and a cyanobacterium gave rise to the first chloroplasts. Secondary endosymbiosis and tertiary endosymbiosis between chloroplast-containing eukaryotes and other eukaryotes gave rise to the distinctive chloroplasts of euglenids, dinoflagellates, and other groups. Review Figure 26.2, Animation 26.1

• Most eukaryotes can be placed in one of eight major clades that diverged about 1.5 billion years ago: alveolates, stramenopiles, rhizarians, excavates, plants, amoebozoans, fungi, and animals. Review Figure 26.3

• Most, but not all, protists are unicellular.

• Alveolates are unicellular organisms with sacs (alveoli) beneath their cell membranes. Alveolate clades include the marine dinoflagellates, the parasitic apicomplexans, and the diverse, highly motile ciliates. Review Activity 26.1, Animation 26.2

• Stramenopiles typically have two flagella of unequal length, the longer one bearing rows of tubular hairs. Among the stramenopiles are the unicellular diatoms; the multicellular brown algae; and the nonphotosynthetic oomycetes, many of which are saprobic.

• Rhizarians are unicellular and aquatic. They include the cercozoans; the foraminiferans, which secrete shells of calcium carbonate; and the radiolarians, which have thin, stiff pseudopods and glassy endoskeletons.

• The excavates include parasitic as well as free-living species. The diplomonads and parabasalids lack typical mitochondria. Heteroloboseans have an amoeboid body form and a two-stage life cycle. Euglenids have anterior flagella; some are photosynthetic. The kinetoplastids, which include several human pathogens, have a single, large mitochondrion.

• The amoebozoans move by means of lobe-shaped pseudopods. A lobosean consists of a single amoeboid cell. Plasmodial slime molds are amoebozoans whose vegetative stage is a coenocyte that moves by cytoplasmic streaming. In cellular slime molds, the individual cells maintain their identity at all times but aggregate to form fruiting structures.

• Asexual reproduction gives rise to clonal lineages of organisms.

• Conjugation in Paramecium is a sexual process but not a reproductive one. Review Figure 26.19

• Alternation of generations, which includes a multicellular diploid stage and a multicellular haploid stage, is a feature of many multicellular protist life cycles (as well as those of some fungi and all land plants). The alternating generations may be heteromorphic or isomorphic.

• The diatoms are responsible for about one-fifth of the photosynthetic carbon fixation on Earth. They and other members of the phytoplankton are important primary producers in the marine environment. Ancient diatoms are a major source of today’s petroleum and natural gas deposits.

• Some protists are pathogens of humans and other vertebrates. Review Figure 26.20, Animation 26.3

• Endosymbiotic relationships are common among microbial protists and often benefit both the endosymbionts and their protist or animal partners. Review Investigating Life: Can Corals Reacquire Dinoflagellate Endosymbionts Lost to Bleaching?