Chapter 28 - Chapter 28 - Protists Chapter 28 Protists...

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Chapter 28 - Protists Chapter 28 Protists Lecture Outline Overview: A World in a Drop of Water In the past, taxonomists classified all protists in a single kingdom, Protista. However, it is now clear that Protista is in fact paraphyletic. Some protists are more closely related to plants, fungi, or animals than they are to other protists. As a result, the kingdom Protista has been abandoned. o Various lineages are recognized as kingdoms in their own right. Scientists still use the convenient term protist informally to refer to eukaryotes that are not plants, animals, or fungi. Concept 28.1 Protists are an extremely diverse assortment of eukaryotes Protists exhibit more structural and functional diversity than any other group of organisms. Most protists are unicellular, although there are some colonial and multicellular ones. At the cellular level, many protists are very complex. o This is to be expected of a single cell that must carry out the basic functions performed by all the specialized cells in a multicellular organism. Protists are the most nutritionally diverse of all eukaryotes. o Some are photoautotrophs, containing chloroplasts. o Some are heterotrophs, absorbing organic molecules or ingesting food particles. o Some are mixotrophs, combining photosynthesis and heterotrophic nutrition. Protists can be divided into three groups, based on their roles in biological communities. o These groups are not monophyletic. o Protists include photosynthetic algal protists, ingestive protozoans, and absorptive protists. Protist habitats are also very diverse. The life cycles of protists vary greatly. o Some are exclusively asexual, while most have life cycles including meiosis and syngamy. Endosymbiosis has a place in eukaryotic evolution. Much of protist diversity is the result of endosymbiosis, a process in which unicellular organisms engulfed other cells that evolved into organelles in the host cell. The earliest eukaryotes acquired mitochondria by engulfing alpha proteobacteria.
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o The early origin of mitochondria is supported by the fact that all eukaryotes studied so far either have mitochondria or had them in the past. Later in eukaryotic history, one lineage of heterotrophic eukaryotes acquired an additional endosymbiont—a photosynthetic cyanobacterium—that evolved into plastids. o This lineage gave rise to red and green algae. o This hypothesis is supported by the observation that the DNA of plastids in red and green algae closely resembles the DNA of cyanobacteria. o Plastids in these algae are surrounded by two membranes, presumably derived from the cell membranes of host and endosymbiont. On several occasions during eukaryotic evolution, red and green algae underwent
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Chapter 28 - Chapter 28 - Protists Chapter 28 Protists...

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