Chapter 29 summary

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Christoph Richter

Freeman,Biological Science, 4e, Chapter 29 Chapter 29 - Protists Learning Objectives: Students should be able to ... • Define protist, and explain why the protists are considered a paraphyletic group. • Give several examples illustrating the medical and ecological importance of protists. • Describe these key innovations of the protists, explain why they were important, and outline the major hypotheses for how they evolved: nuclear envelope, multicellularity, structures for support and protection, mitochondria, chloroplasts. • Describe the diversity of feeding, locomotion, and reproduction seen in protists. Lecture Outline • Protists are eukaryotes. • Fundamental features of eukaryotes: o The nuclear envelope is a synapomorphy that defines the domain Eukarya. o Most eukaryotic cells are large, have many organelles, and have an extensive system of structural proteins called the cytoskeleton. o Multicellularity has evolved multiple times in eukaryotes. o Eukaryotes reproduce either asexually via mitosis or sexually via meiosis. • Protists are all the eukaryotes that are not fungi, green plants, or animals. (Fig. 29.1) • As a group, protists are paraphyletic: They include some, but not all, descendants of the common ancestor of eukaryotes. • Protists tend to live in or near water. (Fig. 29.2) I. Why Do Biologists Study Protists? A. Impacts on human health and welfare 1. The Irish potato famine in 1845 was caused by a protist, Phytophthora infestans. a. One million people (out of a population of 9 million) died, and millions of others emigrated to other countries. 2. Malaria a. Malaria is the world's worst chronic health problem, affecting 300 million people worldwide each year and killing 1 million per year. b. It is caused by four species of the protist species Plasmodium. c. Plasmodium cells enter the bloodstream via a mosquito bite, infect liver cells and (later) red blood cells, and destroy the cells. (Fig. 29.3) © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 29 d. Plasmodium is probably the best studied of all the protists, but we still do not have effective measures to control it. (1) Mosquitoes have continually evolved resistance to insecticides. (2) Plasmodium has evolved resistance to many drugs. (3) Efforts to develop a vaccine have not been successful. e. Students should be able to explain why public health workers are promoting the use of insecticide-treated sleeping nets as a way of reducing the incidence of malaria. 3. Many other protists also cause human disease. (Table 29.1) 4. Harmful algal blooms a. Algal blooms occur when a unicellular aquatic species experiences rapid population growth. b. Many harmful algal blooms are caused by photosynthetic protists called dinoflagellates, which contain toxins. (Fig. 29.4) (1) High levels of toxins build up in shellfish that filter the protists out of the water, and people who eat the shellfish can then get sick or die. B. Ecological importance of protists 1. Protists are extremely abundant in their habitats. For example, there are 1000 flagellated protists per teaspoon of pond water and 60 million dinoflagellates per liter of seawater. 2. Protists play a key role in aquatic food chains as primary producers. 3. Could protists help reduce global warming? a. Phytoplankton in the ocean take up large quantities of carbon dioxide and often carry it to the bottom of the ocean when they die. (Fig. 29.5) b. Fertilizing the ocean with iron speeds up the carbon cycle and might increase the amount of carbon taken into the marine carbon sink. c. Students should be able to draw a flowchart showing the chain of events that starts with massive iron fertilization and ends with large deposits of carbon-containing compounds on the ocean floor. II. How Do Biologists Study Protists? A. Microscopy: studying cell structure 1. Detailed studies of cell structure revealed that many protists have a characteristic overall form, organelles with distinctive features, or both. a. For example, special flagella with tiny hairlike projections, known as stramenopiles, have been identified on the protist that caused the potato famine and on some types of brown algae. (Fig. 29.6) 2. Seven major groups of eukaryotes have been identified in this way, based on diagnostic morphological features (synapomorphies) that either protect or support the cell, or aid in reproduction or feeding. (Table 29.2) © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 29 B. Evaluating molecular phylogenies 1. DNA sequence analysis has demonstrated that the seven major eukaryotic groups are indeed monophyletic. (Fig. 29.7) 2. Current data suggest that the first major split in the eukaryotic tree was between the Unikonta (one flagellum) and the Bikonta (two flagella). C. Discovering new lineages via direct sequencing 1. Direct sequencing is based on collecting samples from a habitat and isolating the DNA in those samples⎯that is, without knowing which organism the DNA comes from. 2. A vast array of new protist lineages have been discovered in this way, including many protists that turn out to be as tiny as bacteria. 3. Students should be able to explain why direct sequencing studies enable researchers to characterize species that have never been seen before. III. What Themes Occur in the Diversification of Protists? A. What morphological innovations evolved in protists? 1. Early eukaryotes had a nuclear envelope, an endomembrane system, mitochondria, a cytoskeleton, a flagellum, and no cell wall. a. Students should be able to map the origin of the nuclear envelope and the eukaryotic flagellum on Figure 29.7. 2. The nuclear envelope a. The nuclear envelope likely arose as infoldings of the plasma membrane that also gave rise to the endoplasmic reticulum. (Fig. 29.8) (1) Students should be able to explain why these observations support evolution of the nuclear envelope. b. The nuclear envelope allowed for the separation of transcription and translation. (1) This enabled alternative splicing and gave early eukaryotes more ways to control gene expression. c. Different major lineages of protists have different types of nuclei. 3. Endosymbiosis and the origin of the mitochondrion a. The endosymbiosis theory (Lynn Margulis, 1981) proposes that mitochondria evolved from an aerobic bacterium that was engulfed by an anaerobic eukaryotic cell. (Fig. 29.9) (1) This became a mutually beneficial symbiosis: The host supplied the bacterium with protection and carbon compounds, and the bacterium produced much more ATP than the host could produce on its own. b. Many lines of evidence now support this theory. (1) Mitochondria are similar in size to ˜-proteobacteria. (2) They divide independently of the host cell, and they divide by fission, as bacteria do. (3) They have their own ribosomes and synthesize their own proteins, and the ribosomes are similar in many ways to bacterial ribosomes. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 29 (4) They have double membranes, as would be expected if they were engulfed by another cell. (5) They have their own chromosomes, which are circular and similar to bacterial chromosomes. (6) The most conclusive evidence is that mitochondrial genes are very closely related to the genes from ˜-proteobacteria. (Fig. 29.10) c. Students should be able to describe how the chloroplast arose by endosymbiosis. They should also be able to list the types of evidence that support an endosymbiotic origin for the chloroplast. 4. Structures for support and protection a. The diversification of protists has been associated with the evolution of innovative structures for support and protection: (Fig. 29.11) (1) Glass-like shells in a box-and-lid arrangement (diatoms) (2) A cell wall made of cellulose plates (dinoflagellates) (3) Chambers of calcium carbonate (Foraminifera) (4) Coverings of tiny pebbles (other Foraminifera and some amoebae) (5) An internal support rod (parabasalids) (6) A collection of protein strips under the plasma membrane (euglenids) (7) Sac-like structures (alveolates) 5. Multicellula rity a. Multicellular organisms contain more than one cell and have cells that are specialized for different functions and express different genes. b. The vast majority of multicellular species are eukaryotes. c. Multicellularity evolved independently in several eukaryotic lineages: green plants, fungi, animals, brown algae, slime molds, and red algae. B. How do protists obtain food? 1. Protists have three ways of feeding: ingestive feeding, absorption of molecules directly from the environment, and photosynthesis. a. Some species of protists use multiple feeding methods, and all three methods may occur within a single clade. Example: alveolates. 2. Ingestive feeding a. The large cell size of protists enabled some of them to develop a unique feeding strategy: ingesting other organisms (ingestive feeding). b. Ingestion can occur by eating live or dead organisms or scavenging loose bits of organic debris. © 2011 Pearson Education, Inc. Freeman,Biological Science, 4e, Chapter 29 c. Protists that feed by engulfing typically lack a cell wall and can move their plasma membranes around their prey using pseudopodia. (Fig. 29.12a) d. Some ingestive feeders sit and wait for prey to come by or have cilia that move the environment by them. (Fig. 29.12b) 3. Absorptive feeding a. Some absorptive-feeding protists are decomposers and feed on detritus (dead organic matter). b. Others live insideother organisms. (1) Absorptive feeders that damage their hosts are called parasites. 4. Photosynthesis ⎯endosymbiosis and the origin of the chloroplast a. The eukaryotic chloroplast is thought to have originated when a protist engulfed a photosynthetic cyanobacterium (endosymbiosis theory). b. Evidence for the endosymbiosis theory of chloroplasts: (1) Chloroplasts have the same bacterial characteristics that mitochondria have (bacterial ribosomes, reproduction by fission, etc.). (2) Many endosymbiotic cyanobacteria live inside protists or animals today. (3) Chloroplast genes are very similar to cyanobacterial genes. (4) Photosynthetic organelles of some protists have the same cell wall component (peptidoglycan) found in the cell walls of cyanobacteria. (5) Chloroplasts have the same chlorophylls and the same system of internal membranes found in certain cyanobacteria. c. Students should be able to add a label indicating the origin of chloroplast genes to the phylogenetic tree in Figure 29.7. 5. Photosynthesis ⎯primary versus secondary endosymbiosis a. Several lineages of protists have chloroplasts that are surrounded by four membranes instead of two. b. This may have occurred when one eukaryotic species engulfed a second eukaryotic species that already contained an engulfed cyanobacterium. (Figs. 29.13 and 29.14) c. Students should be able to explain why the primary and secondary endosymbiosis events represent the most massive lateral gene transfers in the history of life. 6. Photosynthesis ⎯diversification in pigments a. Most photosynthetic lineages contain a unique collection of photosynthetic pigments. b. The presence of different pigments means that different species absorb different wavelengths and therefore avoid competition with one another. (Table 29.3
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