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Unformatted text preview: 1 2009 BIO153: Lecture 8 The Origin of the Eukaryotes February 4, 2008 The eukaryotic cell is extremely complex relative to the prokaryotic cell: membrane bound organelles endoplasmic reticulum cytoskeleton often does not have a cell wall nucleus; DNA in chromosomes complex flagellum sex common How did eukaryotes arise? from prokaryotic cells loss of cell wall; infolding of cell membrane to increase SA vesicles; some studded with ribosomes infolding enclosed a nucleus; cytoskeleton developed some acquired a flagellum for locomotion The loss of the prokaryotic cell wall & infolding of cell membrane (enclosing nucleus ER, Golgi apparatus, lysosomes) was a critical step the infolding of the cell membrane creates a large area for biochemical reactions to occur The evolution of mitochondria: likely occurred through the endosymbiosis of a bacterium performing respiration by an anaerobic eukaryote The advantage: a higher yield of ATP (respiration yields more ATP than fermentation, the process occurring in anaerobic organisms) The evolution of chloroplasts likely occurred in the same fashion: engulfed cyanobacteria that were capable of oxygenic photosynthesis evidence: the lamellar structure (thylakoids) that is found in 2 free-living cyanobacteria is similar to that found in chloroplasts Evidence for the endosymbiosis hypothesis: It is thought then, that chloroplasts and mitochondria were once free-living bacteria engulfed by early eukaryotes: 1. mitochondria & chloroplasts are about the right size to be bacterial cells 2. both mitochondria and chloroplasts replicate by fission 3. mitochondria and chloroplasts have their own genomes (circular chromosome), ribosomes ; and make their own proteins 4. mitochondria and chloroplasts have double membranes 5. mitochondria and chloroplasts are affected by antibiotics (which dont affect eukaryotic cells) 2 endosymbiosis: In some lineages, a protist engulfed another protist! evidence: in some organisms, the chloroplasts have 4 membranes in some, 3 membranes (1 lost) The first eukaryotes were anaerobes Although the vast majority of contemporary eukaryotes are aerobes, it makes sense that the earliest eukaryotes were anaerobes until they acquired mitochondria. A few anaerobic eukaryotes exist today: contemporary anaerobic eukaryotes that are well known are mostly pathogens (e.g....
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