Chapter28LO - Chapter 28: Protists Overview: Living Small *...

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Chapter 28: Protists Overview: Living Small * Even a low-power microscope can reveal a great variety of organisms in a drop of pond water * Protist is the informal name of the kingdom of mostly unicellular eukaryotes * Advances in eukaryotic systematics have caused the classification of protists to change significantly * Protists constitute a paraphyletic group, and Protista is no longer valid as a kingdom Concept 28.1: Most eukaryotes are single-celled organisms * Protists are eukaryotes and thus have organelles and are more complex than prokaryotes * Most protists are unicellular, but there are some colonial and multicellular species * Structural and Functional Diversity in Protists * Protists exhibit more structural and functional diversity than any other group of eukaryotes * Single-celled protists can be very complex, as all biological functions are carried out by organelles in each individual cell * Protists, the most nutritionally diverse of all eukaryotes, include: * Photoautotrophs, which contain chloroplasts * Heterotrophs, which absorb organic molecules or ingest larger food particles * Mixotrophs, which combine photosynthesis and heterotrophic nutrition * Protists can reproduce asexually or sexually, or by the sexual processes of meiosis and syngamy Endosymbiosis in Eukaryotic Evolution * There is now considerable evidence that much protist diversity has its origins in endosymbiosis * Mitochondria evolved by endosymbiosis of an aerobic prokaryote * Plastids evolved by endosymbiosis of a photosynthetic cyanobacterium * The plastid-bearing lineage of protists evolved into red algae and green algae * On several occasions during eukaryotic evolution, red and green algae underwent secondary endosymbiosis , in which they were ingested by a heterotrophic eukaryote Cladistic View of Endosymbiosis * Five Supergroups of Eukaryotes * It is no longer thought that amitochondriates (lacking mitochondria) are the oldest lineage of eukaryotes * Our understanding of the relationships among protist groups continues to change rapidly * One hypothesis divides all eukaryotes (including protists) into five supergroups I. Concept 28.2: Excavates include protists with modified mitochondria and protists with unique flagella * The clade Excavata is characterized by its cytoskeleton * Some members have a feeding groove * This controversial group includes the diplomonads, parabasalids, and euglenozoans * Diplomonads and Parabasalids
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* These 2 groups live in anaerobic environments, lack plastids, and have modified mitochondria Diplomonads * Have modified mitochondria called mitosomes * Derive energy anaerobically, for example, by glycolysis * Have two equal-sized nuclei and multiple flagella * Are often parasites, for example, Giardia intestinalis Parabasalids * Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically * Include Trichomonas vaginalis , the pathogen that causes yeast infections in human females Euglenozoans * Euglenozoa
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Chapter28LO - Chapter 28: Protists Overview: Living Small *...

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