Animal-Like Protista

Introduction Protists are eukaryotes and include unicellular (e.g., amoeba) and multicellular forms (e.g., algae) Filamentous algae Eukaryotic cell The word protista (Gr. protos, very first, ktistos, to establish) implies great antiquity Protista represent a polyphyletic group Two interesting scenarios regarding the history of life on earth emerged during the evolution of protists: the origin of the eukaryotic cell and the subsequent emergence of multicellular eukaryotes

The Emergence of the Eukaryotic Cell The small size and simpler construction of the prokaryotic cell has many advantages but also imposes a number of limitations: Number of metabolic activities that can occur at any one time is smaller Smaller size of the prokaryotic genome limits the number of genes which code for enzymes controlling activities Natural selection resulted in increasing complexity in some groups of prokaryotes; two major trends were apparent: 1. Toward multicellular forms such as cyanobacteria; different cell types with specialized functions Anabaena 2. The compartmentalization of different functions within cells; the first eukaryotes resulted from this solution

The Evolution of Eukaryotes The evolution of the compartmentalized nature of eukaryotic cells may have resulted from two processes 1. Specialization of plasma membrane invaginations.

The Evolution of Eukaryotes cont. 2. Endosymbiotic associations of prokaryotes may have resulted in the appearance of some organelles. Mitochondria, chloroplasts, and some other organelles evolved from prokaryotes living within other prokaryotic cells

The Endosymbiotic Theory The endosymbiotic theory was developed by Lynn Margulis It proposes that certain prokaryotic species, called endosymbionts lived within larger prokaryotes Chloroplasts are believed to have descended from endosymbiotic photosynthesizng prokaryotes, such as cyanobacteria, living in larger cells Mitochondria are postulated to be descendents of prokaryotic areobic heterotrophs. Perhaps they gained entry as parasites or undigested prey of larger prokaryotes. The association may then have progressed from parasitism or predation to mutualism. And, as the host and endosymbiont became more interdependent, they integrated into a single organism

Evidence for the Endosymbiotic Theory Chloroplasts and mitochondria are of the appropriate size to be descendents of eubacteria Their inner membranes contain several enzymes and transport systems similar to those of prokaryotic plasma membranes They replicate by splitting processes similar to binary fission present in prokaryotes They have DNA which is circular and not associated with histones or other proteins, as in prokaryotes They contain their own tRNA, ribosomes and other components for DNA transcription and translation into proteins Chloroplasts have ribosomes more similar to prokaryotic ribosomes (with regards to size, biochemical characters, etc) than to eukaryotic ribosomes; mitochondrial ribosomes vary, but are also more similar to prokaryotic ribosomes rRNA of chloroplasts is more similar in basic sequence to that of certain photosynthetic eubacteria than to rRNA in eukaryotic cytoplasm; also, chlorplast rRNA is transcribed from genes in the chloropast while eukaryotic rRNA is transcribed from nuclear DNA

Archezoans and Early Evolution of Eukaryotes An ancient lineage of eukaryotes branched away from the eukaryotic tree very early, perhaps as long as 2 billion years ago The group is referred to as the archezoa and contains only a few phyla Lack mitochondria and plastids, and have relatively simple cytoskeletons Their ribosomes have some characteristics more closely aligned with prokaryotes than with eukaryote Giardia intestinalis is a modern representative of an archezoan If organisms like Giardia diverged from the eukaryotic lineage before the process of nuclear fusion and meiosis evolved, their dual nuclei may be a clue to the past This coupled with the absence of mitochondria in this group and other archezoan is consistent with an origin occurring before the endosymbiotic relations that gave rise to the mitochondria in aerobic species

Overview of the ‘Tree of Life’

Introduction to Protozoan Protists Protozoans (Gr. proto = first; zoa = animal) are the single-celled animal-like members of the kingdom Protista They are clearly eukaryotes, e,g., distinct nuclei, membrane bound organelles, etc.; unlike animals, never develop from a blastula Remarkably diverse in terms of size, morphology, mode of nutrition, locomotory mechanism, and reproductive biology Protozoans are regarded as being a polyphyletic group

General Characteristics of Protozoan Protists Entire organism is bounded by the plasmalemma (cell membrane) The cytoplasm is often differentiated into a clear, outer gelatinous region, the ectoplasm, and an inner, more fluid region fluid or sol state, the endoplasm Many organelles are typical of most multicellular metazoan cells However, many protozoans contain organelles not generally found among the metazoa, e.g., contractile vacuoles and trichocysts

Cilia and Flagella Locomotor appendages that protrude from the protozoan cell Cilia are shorter and more numerous, whereas, flagella are longer and less less numerous Cilia and flagella are similar structurally; microtubules are arranged in a ring of 9 microtubule doublets surrounding a central pair of microtubles (9+2 arrangement); microtubules are covered by an extension of the plasma membrane; they are anchored to the cell by a basal body Cilia and flagella differ in their beating patterns

Pseudopodia When organisms like amoeba are feeding and moving, they form temporary cell extensions called pseudopodia The most familiar form are called lobopodia: contain ectoplasm and endoplasm; used for locomotion and engulfing food When a lobopodium forms, an extension of the ectoplasm called the hyaline cap appears and endoplasm flows into this cap As the endoplasm moves into the cap it fountains out and it changes from the fluid state to the gel state (endoplasm to ectoplasm) Pseudopodium anchors to the substrate and the cell is drawn forward

Nutrition and Digestion Ingested food particles generally become surrounded by a membrane, forming a distinct food vacuole; digestion is entirely intracellular Vacuoles move about in the fluid cytoplasm and the contents are broken down by enzymes The contents of the vacuoles can change, e.g., go from acidic to basic This is important because digestion for these organisms requires exposing the food to a series of enzymes, each of which has a specific role that operates under a narrow range of pH Controlled changes of pH that occur within the food vacuoles allow for the sequential disassembly of foods Once solubilized, nutrients move across the vacuole wall and into the endoplasm of the cell Indigestible solid wastes are commonly discharged to the outside through an opening in the plasma membrane

Excretion and Osmoregulation Contractile vacuoles are organelles involved in expelling water from the cytoplasm Fluid is collected from the cytoplasm by a system of membranous vesicles and tubules called spongiome tubules The collected fluid is transferred to a contractile vacuole and is subsequently discharged to the outside through a pore in the cell membrane Vacuoles are most commonly found among freshwater species pore Spongiome tubules ampulla Vacuole

Reproduction Asexual reproduction is commonly encountered among protozoans Some reproduce asexually through fission, a controlled mitotic replication of chromosomes and splitting of the parent into two or more parts Binary fission - protozoan splits into two individuals Multiple fission many nuclear divisions precede the rapid differentiation of the cytoplasm into many distinct individuals Budding a portion of the parent breaks off and differentiates into a new individual fission

Reproduction cont. Many protozoans possess the capacity for regeneration For example, encystment and excystment exhibited by freshwater and parasitic species During encystment, substantial dedifferentiation of the organism occurs, forming a cyst: compact, expels excess water, forms a gelatinous covering is secreted The cyst can withstand long periods of exposure to what would otherwise be intolerable conditions of acidity, thermal stress, dryness, etc. Once conditions improve excystment ensues with the regeneration of all former internal and external structures

Classification Phylum Sarcomastigophora Move by means of flagella and/or pseudopodia; possess a single type of nucleus. Subphylum Mastigophora Locomotion is by means of one of more flagella. Class Phytomastigophorea or phytoflagellates; autotrophic forms containing chlorophyll; one or two flagella, e.g., dinoflagellates; Euglena; Volvox Class Zoomastigophorea or zooflagellates: heterotrophic forms, e.g., trypanosmes that parasitize humans and cause “sleeping sickness”; tsetse flies serve as vectors Blood cell

Subphylum Sarcodina Mostly marine, but some inhabit freshwater and soil; some are parasitic Use pseudopodia for feeding and locomotion. Feed by a process known as phagocytosis. A number of species of sarcodines possess a protective outer shell or test, e.g. the radiolarians (silica) and foraminiferans (calcium carbonate) Both the radiolarians and the forams feed by extending their pseuopodia through openings in the shell. Foraminiferan Amoeba Radiolarian

Phylum Ciliophora Exclusive to freshwater Cilia or ciliary organelles present in at least one stage of the life cycle. The ciliates are unique in that they possess 2 kinds of nuclei: a large macronucleus and one or more smaller micronuclei. The macronucleus controls the normal metabolism of the cell, while the micronuclei are concerned with sexual reproduction.

Ciliophoran Reproduction Asexually via binary fission; sexually via conjugation. 2 individual align and partially fuse; all but one micronucleus in each cell disintegrates. The partners swap one micronucleus; this micronucleus then fuses to another micronucleus, forming a diploid organism with genetic material from the 2 individuals. conjugation

Phylum Apicomplexa This is an exclusively parasitic group of protozoans that lack locomotory organelles, except during certain reproductive stages. They possess a characteristic set or organelles called the apical complex, which aids in penetrating host cells. Includes parasites that cause malaria (e.g., Plasmodium) to humans; mosquitoes serve as vectors apical complex