Concept 20.1   Eukaryotes Acquired Features from Both Archaea and Bacteria

• Early events in the evolution of the eukaryotic cell probably included the loss of the firm cell wall and infolding of the cell membrane. Such infolding probably led to segregation of the genetic material in a membrane-enclosed nucleus. Review Figure 20.1
• Some organelles were acquired by endosymbiosis. 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 20.2 and ANIMATED TUTORIAL 20.1

Concept 20.2   Major Lineages of Eukaryotes Diversified in the Precambrian

• Most eukaryotes can be placed in one of eight major clades that originated in the Precambrian: alveolates, excavates, stramenopiles, rhizarians, amoebozoans, Plantae, fungi, and animals. The first five of these clades are collectively referred to as protists. Review Figure 20.3
• The term “protist” does not describe a formal taxonomic group, but rather is shorthand for “all eukaryotes that are not plants, animals, or fungi.” 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 Figure 20.6, ACTIVITY 20.1, and ANIMATED TUTORIAL 20.2
• 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 foraminiferans, whose shells have contributed to great limestone deposits; the radiolarians, which have thin, stiff pseudopods and glassy endoskeletons; and the cercozoans, which take many forms and live in diverse habitats.
• The excavates include a wide variety of symbiotic as well as free-living species. The diplomonads and parabasalids lack mitochondria, having apparently lost them during the course of their evolution. Heteroloboseans are amoebas with a two-stage life cycle. Euglenids are often photosynthetic and have anterior flagella and spiraling strips of protein that support their cell surface. The kinetoplastids, which include several human pathogens, have a single, large mitochondrion. Review Figure 20.13
• 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. Review Figure 20.17

Concept 20.3   Protists Reproduce Sexually and Asexually

• Asexual reproduction gives rise to clonal lineages of organisms.
• Conjugation in Paramecium is a sexual process but not a reproductive one. Review Figure 20.18
• Alternation of generations, which includes a multicellular diploid phase and a multicellular haploid phase, is a feature of the life cycle of many multicellular protists (as well as of some fungi and all land plants). The alternating generations may be heteromorphic or isomorphic.
• In alternation of generations, specialized cells of the diploid organism, called sporocytes, divide meiotically to produce haploid spores. Spores give rise to the multicellular, haploid, gamete-producing generation through mitosis. Gametes fuse and give rise to the diploid organism.

Concept 20.4   Protists Are Critical Components of Many Ecosystems

• The diatoms are responsible for about one-fifth of the photosynthetic carbon fixation on Earth. They and other members of the phytoplankton are the primary producers in the marine environment.
• Endosymbiotic relationships are common among microbial protists and typically benefit both the endosymbionts and their protist or animal partners. Review Figure 20.19
• Some protists are pathogens of humans and other vertebrates. Review ANIMATED TUTORIAL 20.3
• Ancient diatoms are the major source of today’s petroleum and natural gas deposits.