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Biological Sciences
Mark Fitzpatrick

BIOA01H3 – Lecture 33 rd Chapter 22 22.1 The Evolution of Protists Involved Endosymbiosis  Likely evolved 1.5-2bya  We don’t understand how exactly they’ve evolved but know that it involves endosymbiosis  Protists contain mitochondria and many also contain chloroplasts 22.2 Characteristics of Protists  Protists have membrane-bound nucleus, with multiple, linear chromosomes  In addition to cytoplasmic organelles, incl. mitochondria and chloroplasts, also have microtubules and microfilaments, providing motility and cytoskeletal support  Share characteristics of transcription and translation  All organisms in eukaryotic lineages consist of protists except for animals, land plants, and fungi, which arose from protist ancestors  Although some protists have characteristics resembling those of fungi, plants, or animals, several features are distinctive in only protists  In contrast to fungi, most protists are motile or have motile stages in life cycle and cell walls made of cellulose not chitin  Unlike plants, may photoautotrophic protists can also live as heterotrophs & some regularly combine both modes of nutrition  Protists don’t retain developing embryos in parental tissue nor have highly differentiated structures such as roots, stems, leaves  Photosynthetic protists also sometimes referred to as algae; generally aquatic & often unicellular and microscopic o All animals are multicellular and have features i.e. internal digestive tract & complex developmental stages o Protists also lack nerve cells, highly differentiated structures i.e. limbs o Also lack a heart, collagen, which is an extracellular support protein  Most protists don’t share a common ancestor and are no more closely related to each other than they are to fungi, plants, or animals 22.3 Protists’ Diversity Is Reflected in Their Metabolism, Reproduction, Structure, and Habitat  Protists are highly diverse in metabolism, reproduction, structure, and habitat 1 Habitat  Live in aqueous habitats, incl. aquatic or moist terrestrial locations  In bodies of water, small photosynthetic protists called phytoplankton, the organisms that capture energy of sunlight in all aquatic habitats  Phototrophs provide organic substances and oxygen for heterotrophic bacteria, other protists, and small crustaceans and animal larvae that are the primary constituents of zooplankton  Protists living on terrestrial, play important roles among detritus feeders that recycle matter from organic back to inorganic Structure  Most protists single cells, others live in colonies in which individual cells show little or no differentiation  Within colonies, individuals use cell signaling to cooperate on tasks i.e. feeding or movements  Some protists are larger multicellular organisms; i.e. giant kelp of coastal waters  Have complex intracellular structures, some of which are found nowhere else among living organisms  These unique structures reflect key aspects of habitats in which protists live i.e. unicellular protists living in freshwater pond o Its cytoplasm hypertonic to water surrounding it, meaning water flows into cell by osmosis o Has specialized cytoplasmic organelle, contractile vacuole, which gradually fills w/ water o When vacuole reaches maximum size, moves to plasma membrane and forcibly contracts  Cells of some protists supported by external cell wall or by internal/external shell build up from organic or mineral matter  Instead of cell wall, other protists have pellicle, layer of supportive protein fibers located inside cell just under plasma membrane, providing strength and flexibility  Sometime during live, almost all protists move  Some move by amoeboid motion, in which cells extends one or more lobes of cytoplasm called pseudopodia  Rest of cytoplasm and nucleus flow into pseudopodium, completing movement  Others move by flagella or cilia Metabolism  Almost all protists are aerobic organisms, live either as heterotrophs or photoautotrophs 2  Some heterotrophic protists obtain organic molecules by engulfing part or all of other organisms (phagocytosis) and digesting internally  Others absorb small organic molecules from environment by diffusion  Some live as both heterotrophs and autotrophs Reproduction  May be asexual, by mitosis, or sexual, through meiotic cell division and formation of gametes  Those that reproduce by both mitosis and meiosis, two modes of cell division often combined into life cycle that is highly distinctive among different protists group 22.4a Excavates Lack “Typical” Mitochondria  Groups takes name from hollow (excavated) ventral feeding groove found in most members  All are unicellular animal parasites lacking mitochondria and move by flagella  b/c lacking mitochondria, limited production of ATP via glycolysis  lack of mitochondria led biologists to consider group as most ancient line of protists; now appears ancestor of group had mitochondria  nuclei of Excavates contain genes derived from mitochondria and also have organelles likely evolves from mitochondria  may’ve lost mitochondria as adaptation to parasitic way of life, in which oxygen in short supply; consider two subgroups, Diplomonadida and Parabasala o Diplomonadida  means double cell and organisms look like two cells together w/ two identical, functional nuclei and multiple flagella arranged symmetrically around cell’s longitudinal axis  Best known diplomonad is Giardia lamblia  Some are free living, many live in animal intestines; some don’t cause harm to hose, others do o Parabasala  STD trichomoniasis caused by parabasalid Trichomonasvaginalis  Infection usually symptomless in men, but in women can cause severe inflammation and irritation of vagina and vulva  If untreated, can cause infection in uterus and fallopian tubes resulting in infertility  Parabasalids take names from cytoplasmic structures associated w/ nucleus, parabasal bodies; some consider these as Golgi apparatus of cells 3  Also characterized by fin called undulating membrane  formed by flagella buried in fold of cytoplasm, in addition to freely beating flagella  Buried flagellum allows parabasalids to move through thick, viscous fluids 22.4b Discicristates Include the Euglenoids and Kinetoplastids: Highly Motile Protists  Sometimes referred to as protozoa (proto = first; zoon = animal) b/c they’re similar to animals in that they ingest food and move by themselves  Discicristates named for disk shaped mitochondrial cristae  Group includes about 1800 species, almost all unicellular and highly motile, swimming by means of flagella  Although most are photosynthetic, some can also live as heterotrophs and some even alternate between photosynthetic and heterotrophs Euglenoids  Important primary producers in freshwater ponds, streams, etc.  Most are autotrophs that carry out photosynthesis w/ same mechanisms like plants  If light not available, many euglenoids also live as heterotrophs  Others lack chloroplasts and live entirely as heterotrophs  Name Euglena roughly translates as “eyeball organism”, reference to large eyespot which is feature of photosynthetic euglenoids o Contains pigment granules in association w/ light sensitive structure and part of sensory mechanism stimulating cells to move toward moderately bright light or away from intense ones so organisms find optimal photosynthetic conditions  Also contains numerous organelles incl. contractile vacuole  Rather than external cell wall, have spirally shaped pellicle formed from strips of transparent, protein-rich material underneath membrane  Some euglenoids, strips arranged in spiral allowing cell to change shape in wriggling sort of motion that allows cell to change direction  Can also swim by flagella; has two; one rudimentary and short, other long Kinetoplastids  Sleeping sickness fatal disease, caused by various subspecies of Trypanosoma brucei that are transmitted from one host to another by bites of tsetse fly  Early symptoms incl. fever, headaches, rashes, and anemia  Untreaed, disease damages central nervous system, leading to sleeplike coma and eventual death 4  Disease proven difficult to control b/c present in wild mammals, providing inexhaustible reservoir for parasite  Other kinetoplastids are heterotrophs and are characterized by single mitochondrion that contains large DNA-protein deposit called kinetoplast  Most kinetoplastids also have leading and trailing flagellum 22.4c Alveolates Have Complex Cytoplasmic Structures and Move via Flagella or Cilia  Groups named for small, membrane-bound vesicles called alveoli in layer just under plasma membrane  Includes two motile, primarily free-living groups: Ciliophora and Dinoflagellata, and a nonmotile, parasitic group, Apicomplexa Ciliophora  Ciliophora large group, w/ nearly 10k known species of primarily unicellular but highly complex heterotrophic organisms that swim using cilia  Some live individually; some colonial  Some animal parasites; others live and reproduce in hosts as mutually beneficial symbionts  Have many highly developed organelles, incl. mouthlike gullet lined w/ cilia, structures that exude toxins and other defense materials from cell surface, contractile vacuoles, and complex system of food vacuoles  Pellicle reinforces cell’s shape  Complex cytoskeleton anchors cilia just below pellicle and coordinates ciliary beating  Cilia can stop, reverse beating in synchrony, allowing ciliates to stop, back up, and turn if encounter negative stimuli  Ciliates are only eukaryotes that have two types of nuclei in each cell: one or more small nuclei called micronuclei and single larger macronucleus Micronucleus is diploid nucleus contains complete complement genes. o Fncs primarily in cellular reproduction, which can be asexual or sexual o Number present depends on species Macronucleus develops from micronucleus but loses all genes except those required for basic functions of cell and for synthesis of ribosomal RNA. o Contains numerous copies of these genes, allowing it to synthesize larger quantities of proteins and rRNA  Ciliates abound in freshwater and marine habitats, where they feed on bacteria, algae, and each other 5 Dinoflagellata: The Dinoflagellates  Red tides caused by certain dinoflagellates that make up large proportion of marine phytoplankton  Typically have shell formed from cellulose plates  Beating of flagella, which fit into grooves in plates, makes dinoflagellates spin like a top as they swim  Most are unicellular  Live as heterotrophs or auto  Some lives as symbionts in tissues of other marine organisms i.e. jellyfish Apicomplexa  Nonmotile parasites  Take name from apical complex, group of organelles at one end of cell which helps cells attach to and invade host cells  Absorb nutrients through plasma membranes and lack food vacuoles  Can reproduce asexually and sexually forming gametes that fuse to produce cysts  As in Giardia, when host organism engulfs cysts, they divide to produce infective cells 22.4d Heterokonts Have Two Dissimilar Flagella at Some Stage of Their Life Cycles  Named for two different flagella: one smooth and a second covered w/ bristles  in most heterokonts, flagella occur only on reproductive cells  heterokonts include Oomycota (water moulds), Bacillariophyta (diatoms), and Chrysophyta (golden algae), and Phaeophyta (brown algae) Oomycota  organism causing blight was water mould  water moulds not fungi; they’re oomycetes but do share some features w/ fungi  like fungi, oomycetes grow as microscopic, nonmotile filaments called hyphae forming network called mycelium  also like fungi, heterotrophs, secreting enzymes that digest complex molecules of surrounding organic matter or living tissue into simpler molecules that are small enough to be absorbed into cells  other features not similar to fungi are differences in nucleotide sequence, clearly indicating close evolutionary relationships to other heterokonts than fungi  live almost exclusively in freshwater lakes, streams, etc. where they’re key decomposers
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