Classification of Protists

Classification of Protists - with these groups has yet to...

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Classification of Protists The protists include heterotrophs , autotrophs , and some organisms that can vary their nutritional mode depending on environmental conditions. Protists occur in freshwater, saltwater, soil, and as symbionts within other organisms. Due to this tremendous diversity, classification of the Protista is difficult. Historically the group has been subdivied based on the mode of nutrition, photosynthestic pigments, and the type of organelles used for locomotion. For example, the organisms using cilia to propel themselves were all placed in the Phylum Ciliata; those using pseudopodia were all in the Phylum Sarcodina. This is an example of form classification, and worked well enough until scientists began to examine the protists both biochemically and ultratsructurally (with electron microscopes). They discovered the form classification mentioned above did not support the existence of monophyletic groups, and thus should be abandoned. Several new kingdoms have been proposed for the old protista, although consensus amongst systematists working
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Unformatted text preview: with these groups has yet to fully emerge. Several organisms once placed in the protists have been moved to other Kingdoms, while others have moved from the Kingdom Fungi to the protists. Kingdom Archaezoa Organisms placed in this proposed kingdom lack mitochondria. Scientists interpret this as an indication of the divergence of this group from other "protists" prior to the endosymbiosis event that led to the development of the mitochondrion. However, some recent studies seem to indicate that some of the organisms placed in this group are secondarily mitochondrialess: their ancestors had mitochondria but lost them over time. This casts doubt on the monophyletic nature of this proposed kingdom. Members of the diplomonad subgroup of archaezoans have two flagella, two nuclei, and no mitochondria. Giardia lamblia , an intestinal parasite that causes giardiasis, is a member of this group. A colorized scanning electron migrograph of this organism is shown in Figure 4....
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