Emergence of Eukaryotes Cellular Symbiosis Chapter 11 n First evidence of life n n n Protozoan Groups Dates to 3.5 billion years ago First cells were bacteria­like Origin of complex eukaryote cells n Most likely symbiosis among prokaryotic cells n Modification of engulfed prokaryote into an organelle: n n Primary endosymbiosis Aerobic bacteria engulfed by bacteria May have become mitochondria found in most modern n eukaryotic cells Engulfed photosynthetic bacteria evolved into chloroplasts n Protozoans Emergence of Eukaryotes n Protozoans n Carry on all life activities within a single cell n Can survive only within narrow environmental ranges n Very important ecologically n At least 10,000 species of protozoa are symbiotic in or on other plants or animals n Relationships may be mutualistic, commensalistic, or parasitic n Trichomonas Descendants in green algae lineage gave rise to multicellular plants 11­2 Copyright © The McGraw­Hill Companies, Inc. Permission required for reproduction or display. Protozoa are a diverse assemblage with mixed affinities. Unicellular No organs or tissues but specialized organelles n At least one motile stage in the life cycle n Most ingest their food n Free­living and symbiotic n n 11­3 Biological Contributions n Form and Function Protozoa have intracellular specialization or organization of organelles in cells. Reproduction is asexual by fission, budding or cysts; or sexual by conjugation or syngamy of gametes. n Volvox Stentor n They move by pseudopodia, flagella, cilia and they can direct cell movements. n Most are naked, but some have a simple endoskeleton or exoskeleton. Pediastrum
1 Locomotor Organelles n n Cilia and flagella are morphologically the same Each contains a “9 + 2" pattern of paired microtubules, the axoneme, covered by cell membrane The Axoneme n n n The center pair of tubules end at a small plate. The kinetosome, a short tube of nine triplet microtubules joins at the base of the axoneme. The kinetosome has the same structure as the centriole and therefore they may give rise to each other Play Amoeba Movie Pseudopodia n n n Fig. 11.12 large blunt extensions of the cell body containing both endoplasm and ectoplasm. chief means of locomotion in amebas Actin filaments interact with cross­linking proteins to change endoplasm from liquid to gel Pseudopodia n In the limax form, the whole body moves rather than sending out arms. n Axopodia – long thin pseudopodia
2 Contractile Vacuole Excretion and Osmoregulation Excretion of metabolic wastes is by diffusion. n The main end product of nitrogen metabolism is ammonia that diffuses out of small protozoa. n Contractile vacuoles fill and empty to maintain osmotic balance. n n Paramecium has complex contractile vacuoles with an “excretory” pore surrounded by ampullae of six feeder canals. Nutrition Autotrophs synthesize their own food and heterotrophs obtain organic molecules made by others. n Phagotrophs or holozoic feeders ingest food particles while osmotrophs or saprozoic feeders ingest soluble food. n Few use any one strategy exclusively (e.g. Euglena). n Fig. 11.16 n n There is no known lipid bilayer that retains water against a gradient. A proton pump may actively transport H+ ions and cotransport bicarbonate; this would draw across water. Nutrition Variety of Phagocytosis (Fig. 11.2)
Holozoic nutrition uses phagocytosis; the membrane invaginates around a food particle. n n n n n food vacuole = phagosome. Lysomes fuse with phagosomes and dump enzymes to digest the contents. As digested food is absorbed, the phagosome becomes smaller. Saprozoic feeding may be by pinocytosis; diffusion is of little importance in protozoan nutrition. 3 Reproduction n Fig. 11.10 Asexual Processes n n n n Fission most common Budding occurs when a small progeny cell pinches off from a parent cell, as seen in some ciliates. Multiple fission, or schizogony, undergoes several nuclear divisions followed by cytokinesis, causing many simultaneous individuals to form at once If union of gametes precedes multiple fission, it is called sporogony. Sexual Processes Form and Function n All of above accompanied by some form of n mitosis n Mitosis in protozoa divisions varies from n metazoan mitosis Nuclear membrane often persists Spindle may form within the nuclear membrane n Centrioles not observed in ciliates n Macronucleus of ciliates elongates, constricts, and divides without mitosis (amitosis) n n 11­21 Form and Function n Although some protozoa are exclusively asexual, sex is a widespread and important source of genetic variation. Form and Function
Meiosis May occur during or just before gamete formation n In other groups, meiosis occurs after fertilization (zygotic meiosis) n Fertilization of one gamete by another n Some sexual phenomena do not involve syngamy n n n May precede phases of asexual reproduction Isogametes look alike; anisogametes, the egg and sperm, are dissimilar – most common. n Like metazoa, some protozoa undergo gametic meiosis: meiosis occurs before gamete formation. n n Syngamy Autogamy n n All individuals produced asexually in life cycle up to next zygote are haploid n Conjugation n 11­23 Gamete nuclei form by meiosis Fuse to form a zygote inside the parent organism Gamete nuclei exchanged between paired organisms 11­24 4 Phylum Chlorophyta Classification of Prozoans n Divided into Clades n n n n n Stramenophiles Opisthokonta n n Phylum Chlorophyta Phylum Euglenozoa n Subphylum Kinetoplasta n n n n n Some amebas Viridiplantae Alveolata Phylum Apicomplexa Phylum Ciliophora Phylum Dinoflagellata n Parabasalids n Amebas n Trichonympha Trypanosoma Phylum Retortamonada n Giardia (a diplomonad) Phylum Apicomplexa n All are endoparasites; hosts are in many animal phyla (Coccidia, Toxoplasma, Plasmodium). n
n An apical complex is a feature of this phylum; it is present only in certain stages. n The life cycle usually includes both sexual and asexual stages. 11­27 Plasmodium: The Malarial Organism Fig. 11.17
Malaria is the most important infectious disease of humans n Anopheles mosquitoes carry Plasmodium; the female injects the Plasmodium in her saliva. n n 5 Phylum Ciliophora n Ciliates are the most diverse and specialized of the protozoans n Macronuclei are responsible for metabolic and developmental functions. n Micronuclei participate in sexual reproduction and give rise to macronuclei afterwards. The pellicle varies from a simple membrane to thickened armor. Paramecium ­ Structures n n n Fecal material is discharged from the cytoproct. Two contractile vacuoles have radiating canals and serve in osmoregulation. A kidney­shaped macronucleus has a smaller micronucleus alongside; some species have up to seven micronuclei. Phylum Dinoflagellata n About half are photoautotrophic Chloroplasts possibly acquired by endosymbiosis Some among the most important primary producers in marine environments Commonly have two flagella Body naked or covered by cellulose plates Many have a mouth region through which they can ingest prey Many are bioluminescent n Zooxanthellae n n n n n n n They are always multinucleate with at least one macronucleus and a micronucleus. n n Paramecium: A Representative Ciliate n Live in mutualistic association with corals and other invertebrates n Only corals with symbiotic zooxanthellae form coral reefs 11­35 n Paramecium caudatum is common; it demonstrates the slipper­shape of paramecia. Its asymmetrical appearance is caused by the oral groove (cytostome) ; leads to a tubular cytopharynx. Major Protozoan Taxa n Pellicle varies from a simple membrane to thickened armor n Cilia Arranged in rows Propel food to the cytopharynx n Fused cilia (cirri) used in locomotion n Most are holozoic n n Dinoflagellates Source: http://www.geocities.com/oceanking_uk_2003/Plank.gif Source: http://www.microbeworld.org/news/articles/images/dinoflagellates.jpg
11­36 6 Major Protozoan Taxa Major Protozoan Taxa n n Amebas Found in fresh and salt water, and moist soils n Some planktonic, some require a substratum n Most reproduce by binary fission n Sporulation and budding are also seen n Nutrition is holozoic Entamoebidae n n n n Branched pseudopodia make them rhizopod amebas Lack mitochondria Entamoeba histolytica most important n Lives in the large intestine n n n n Slender pseudopodia extending through openings in test Most are foraminiferans n Ancient group of shelled amebas found in all oceans n n n 11­37 Can lead to amebic dysentery Granuloreticulosa Most live on the ocean floor Perhaps largest biomass of any animal group Most tests are many­chambered and made of CaCO 3 11­38 Major Protozoan Taxa
n Radiolarian n n Refers to marine testate ameba with intricate skeletons Oldest known protozoa Pelagic and live in shallow water Shell surface fused with spines n Cytoplasm around the capsule extends axopodia to catch prey n Reproduce by binary fission, budding,and sporulation n n n Useful for determining the age of rock strata 11­39 7