Chapter 26: Protists

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Diversity in the Protists

• Protists have various means of locomotion, including pseudopodia, flagella, and cilia; a few are nonmotile.
• Protists obtain their nutrients autotrophically or heterotrophically.
  • Protists are free-living or symbiotic, with symbiotic relationships ranging from mutualism to parasitism.
• Most protists live in the ocean or in freshwater ponds, lakes, and streams.
  • Parasitic protists live in the body fluids or cells of their hosts.
• Many protists reproduce both sexually and asexually; others reproduce only asexually.

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How Did Eukaryotes Evolve?

• According to the hypothesis of serial endosymbiosis, mitochondria and chloroplasts arose from symbiotic relationships between larger cells and the smaller bacteria that were incorporated and lived within them.
• Chloroplasts of red algae, green algae, and plants probably arose in a single primary endosymbiotic event in which a cyanobacterium was incorporated into a cell.
  • Multiple secondary endosymbiosis led to chloroplasts in euglenoids, dinoflagellates, diatoms, golden algae, and brown algae and to the non-functional chloroplasts in apicomplexans.
• Relationships among protists are determined largely by ultrastructure, which is the fine details of cell structure revealed by electron microscopy, and by comparative molecular data.
  • Biologists have compared nuclear genes, many of which code for proteins, in different protist taxa.

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Excavates

• Excavates are a diverse group of unicellular protists with flagella, an excavated oral groove, and atypical, greatly modified mitochondria.
  • The inclusion of diplomonads, parabasalids, and euglenoids in the excavate superfamily is controversial.
• Diplomonads are excavates with one or two nuclei, no functional mitochondria, no Golgi complex, and up to eight flagella.
• Parabasilids are anaerobic, flagellated excavates that often live in animals.
  • Trichonymphs and trichomonads are examples of parabasilids
• Euglenoids are unicellular and flagellate. Some euglenoids are photosynthetic.

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Chromalveolates

• Chromalveolates probably originated as a result of secondary endosymbiosis in which an ancestral cell engulfed a red alga.
  • Some DNA sequence data suggest that the chromalveolates are not monophyletic.
• Alveolates have similar ribosomal DNA sequences and alveoli, flattened vesicles located just inside the plasma membrane.
  • Most stramenopiles have motile cells with two flagella, one of which has tiny hairlike projections off the shaft.
• Dinoflagellates are mostly unicellular, biflagellate, photosynthetic alveolates of great ecological importance as producers in marine ecosystems.
  • Their alveoli, flattened vesicles under the plasma membrane, often contain cellulose plates impregnated with silicates.
  • Some dinoflagellates produce toxic blooms known as red tides.
• Apicomplexans are parasites that produce sporozoites and are nonmotile.
  • An apical complex of microtubules attaches the apicomplexan to its host cell.
  • The apicomplexan Plasmodium causes malaria.
• Ciliates are alveolates that move by hairlike cilia, have micronuclei (for sexual reproduction) and macronuclei (for controlling cell metabolism and growth), and undergo a sexual process called conjugation.
• Water molds have a coenocytic mycelium.
  • They reproduce asexually by forming biflagellate zoospores and sexually by forming oospores.
• Diatoms are mostly unicellular, with shells containing silica.
  • Some diatoms are part of floating plankton, and others live on rocks and sediments where they move by gliding.
• Brown algae are multicellular stramenopiles that are ecologically important in cooler ocean waters.
  • The largest brown algae (kelps) possess leaflike blades, stemlike stipes, anchoring holdfasts, and gas-filled bladders for buoyancy.
• Golden algae are mostly unicellular, biflagellate freshwater and marine stramenopiles that are of ecological importance as a component of the ocean’s extremely minute nanoplankton.

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Rhizarians

• Rhizarians are amoeboid cells that often have hard outer shells, called tests, through which cytoplasmic projections extend; molecular evidence indicates that this group is monophyletic.
• Forams secrete many-chambered tests with pores through which cytoplasmic projections extend to move and obtain food.
• Actinopods are mostly marine plankton that obtain food by means of axopods, slender cytoplasmic projections that extend through pores in their shells.

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Archaeplastids

• Archaeplastids, are considered a monophyletic group based on molecular data and on the presence of chloroplasts bounded by outer and inner membranes.
• Red algae, which are mostly multicellular seaweeds, are ecologically important in warm tropical ocean waters.
• Green algae exhibit a wide diversity in size, structural complexity, and reproduction. Botanists hypothesize that ancestral green algae gave rise to land plants.

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Unikonts

• Unikonts have a single posterior flagellum in flagellate cells.
  • Amoebas move and obtain food using cytoplasmic extensions called pseudopodia.
• The feeding stage of plasmodial slime molds is a multinucleate plasmodium.
  • Reproduction is by haploid spores produced within sporangia.
• Cellular slime molds feed as individual amoeboid cells.
  • They reproduce by aggregating into an aggregate (slug) and then forming asexual spores.
• Choanoflagellates are unikonts that are probably the closest living nonanimal relative of animals.
  • A collar of microvilli surrounds their single flagellum at the base.
• Choanoflagellates are included with animals in the opisthokont clade, which also includes fungi.

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