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Chapter 29

Chapter 29 Notes - Protists

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University of Toronto Mississauga
Christoph Richter

Notes From Reading CHAPTER 29:P ROTISTS (PGS. 623-656) Key Concepts  Protists are paraphyletic grouping that includes all eukaryotes except the green plants, fungi, and animals. Biologists study protists to understand how eukaryotes evolved, because they are important in freshwater and marine ecosystems and global arming, and because some species cause debilitating diseases in plants, humans and other organisms  Protists are diverse morphologically. They vary in the types of organelles they contains; they may be unicellular or multicellular, and they may have a cell wall other external covering, or no such covering  Protists vary widely in terms of how the find food. Many species are photosynthetic, while others obtain carbon compounds by ingesting food or parasitizing other organisms  Protists vary widely in terms of how they reproduce. Sexual reproduction evolved in protists, and many protists species can reproduce both sexually and asexually Introduction  Eukarya – Range from single-celled organisms that are the size of bacteria to sequoia trees and blue whales  The largest and most morphologically complex organisms on the tree of life are eukaryotes (algae, plants, fungi and animals)  Eukaryotes are defined by the presence of the shared, derived character called the nuclear envelope and prokaryotes  Unlike bacteria and archaea, which reproduce by fission, eukaryotes undergo cell division can also undergo meiosis  Protists – all eukaryotes that are not green plants, fungi, or animals  Protists do not make up a monophyletic group  Protists constitute a paraphyletic group – meaning that they represent some, but not all, of the descendants of a single common ancestor  There is no trait that is found in protists but no other organisms  Protists are found in wet soils, aquatic habitats, or the bodies of other organisms 29.1 Why Do Biologists Study Protists?  Biologists study protists because they are intrinsically interesting as they are so important medically and ecologically  Because they are critical to understanding the evolution of plants, fungi and animals Impacts on Human Health and Welfare  The most spectacular crop failure in history, the Irish potato famine  Was caused by a protist: Phytophthora Malaria  Malaria, the world’s most chronic public health problem, is caused by Plasmodium  Plasmodium cells enters a person’s bloodstream during a mosquito bite  Plasmodium initially infects liver cells; later, some cells change into a distinctive cell type that infects red blood cells  Plasmodium cells multiply inside the host cells and kill them as they exit to infect additional liver cells or red blood cells  Because each Plasmodium species spends part of its life cycle inside mosquitoes, most antimalarial campaigns have focused on controlling these insects Harmful Algal Bloom  Harmful alga blooms occur when toxin-producing protists reach high densities in an aquatic environment  Algal blooms of dinoflagellates are known as red tides  Biologists prevent poisonings by carefully monitoring protist populations in regions where shellfish are harvested for food  If harmful protist species begin to bloom, the shellfish beds are immediately closed to harvest until toxin are at lower levels Notes From Reading CHAPTER 29:P ROTISTS (PGS. 623-656) Ecological Importance of Protists  Protists represent just 10% of the total number of named eukaryotic species and have relatively low species diversity but are extraordinarily abundant  Species that produce chemical energy by photosynthesis are called primary producers  Diatoms, for example, are photosynthetic protists that rank among the leading primary producers in the oceans because they are so abundant  Production of organic molecules in the world’s oceans, in turn, is responsible for almost half of the total carbon that is fixed on Earth Protists Play a Key Role in Aquatic Food Chains  Small organisms that live near the surface of oceans or lakes and that drift along or swim only short distances are called plankton  The organic compounds produced by phytopakton (photysynthetic plankton) are the basis of food chains in fresh water and marine environments  A food chain described nutritional relationships among organisms Could Protists Help Reduce Global Warming?  The movement of carbon atoms from carbon dioxide molecules in the atmosphere to organisms in the soil or the ocean and then back to the atmosphere is called global carbon cycle  Protists play a key role in the global carbon cycle and act as carbon skinks that could help reduce global warming  A carbon sink is a long-lived carbon reservoir 29.2 How Do Biologists Study Protists?  Although protists have been the focus of intense study, there are so diverse that it has been difficult to find any overall patterns in their evolution and diversification  Recently, researchers have made dramatic progress in understanding protist diversity by combining data on the morphology of key groups and phylogenetic analyses of DNA sequence data Microscopy: Studying Cell Structure  Many protists have a characteristic overall form with synapomorphies – shared, derived traits that are used to distinguish major monophyletic groups  Eight major groups of eukaryotes came to be identified on the basis of diagnostic morphological characteristics  In almost every case, the synapomorphoes listed in the table represent changes in structure that protect and support the cell or that influence the organism’s ability to move or feed Evaluating Molecular Phylogenies  The current phylogenetic tree based on sequence data has identified eight major lineages of eukaryotes Discovering New Lineages via Direct Sequencing  Direct Sequencing is based on collecting organisms from a habitat and analyzing their DNA without growing larger populations of individuals in laboratory culture  This had led to the discovery of several new lineages of eukaryotes, including protists  Eukaryotic cells are much more variable in size than previously imagined  A whole new world of tiny protists has just been discovered 29.3 What Themes Occur in the Diversification of Protists?  Because protists are a paraphyletic group, they do not share derived characteristics that set them apart from all other lineages on the tree of life  The key to understanding the protists is to recognize the important innovations that occurred as they evolved What Morphological Innovations Evolved in Protists?  The earliest eukaryotes were probably single-celled organisms with a nucleus and endomembrane system, mitochondria and a cytoskeleton, but no cell wall  It is also likely that these cells swam using a novel type of flagellum Notes From Reading CHAPTER 29:P ROTISTS (PGS. 623-656) The Nuclear Envelope  The leading hypothesis for the origination of the nuclear envelope is that it is derived from the infoldings of the plasma membrane  The endoplasmic reticulum may have also originated this way The Mitochondrion  Mitochondria are organelles that generate ATP using pyruvate as an election donor and oxygen as the ultimate electron acceptor  The Endosymbiosis Theory proposes that mitochondria originated when a bacterial cell took up residence inside a eukaryote about 2 billion years ago  Symbiosis occurs when individuals of two different species live in physical contact  Endosymbiosis occurs when an organism of one species lives inside an organism of another species  The endosymbiosis theory proposed that mitochondria evolved through a series of steps  The process began when eukaryotic cells started to used their cytoskeletal elements to surround and engulf smaller prey, which then began to live symbiotically within its eukaryotic host  The engulfed cell survived by absorbing carbon molecules with high potential energy from its host and oxidizing them, using oxygen as a final electron acceptor  The host cell, in contrast, is proposed to be a predator capable only of anaerobic fermentation  Observations consistent with the endosymbiosis theory include the following:  Mitochondria are about the size of an average bacterium and replicate by fission as bacteria do  Mitochondria have their own ribosomes to manufacture their own proteins  Mitochondria have double membranes, consistent with the engulfing mechanism  Mitochondria have their own genomes with genes that code for the enzymes needed to replicate and transcribe their own genomes  Phylogenetic data support the endosymbiosis theory  The mitochondrial gene sequences turned out to be much more closely related to the sequences from alpha proteobacteria that to sequences from the nuclear DNA of eukaryotes Multicellularity  In some lineages of protists, the key morphological innovation was multicellularity  Multicellularity is a synapomorphy shared by all of the brown algae and all of the plamodial
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