Protists

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alternation of generations

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A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants and some algae.

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alveolate

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A protist with membrane-bounded sacs (alveoli) located just under the plasma membrane.

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amoeba

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A protist grade characterized by the presence of pseudopodia.

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amoebozoan

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A protist in a clade that includes many species with lobe- or tube-shaped pseudopodia.

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apicomplexan

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A protist in a clade that includes many species that parasitize animals. Some apicomplexans cause human disease.

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Archaeplastida

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One of five supergroups of eukaryotes proposed in a current hypothesis of the evolutionary history of eukaryotes. This monophyletic group, which includes red algae, green alage, and land plants, descended from an ancient protist ancestor that engulfed a cyanobacterium. See also Excavata, Chromalveolata, Rhizaria, and Unikonta.

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blade

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A leaflike structure of a seaweed that provides most of the surface area for photosynthesis.

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brown alga

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A multicellular, photosynthetic protist with a characteristic brown or olive color that results from carotenoids in its plastids. Most brown algae are marine, and some have a plantlike body (thallus).

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cellular slime mold

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A type of protist that has unicellular amoeboid cells and aggregated reproductive bodies in its life cycle.

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Chromalveolata

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One of five supergroups of eukaryotes proposed in a current hypothesis of the evolutionary history of eukaryotes. Chromalveolates may have originated by secondary endosymbiosis and include two large protist clades, the alveolates and the stramenopiles. See also Excavata, Rhizaria, Archaeplastida, and Unikonta.

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ciliate

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A type of protist that moves by means of cilia.

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coral reef

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Typically a warm-water, tropical ecosystem dominated by the hard skeletal structures secreted primarily by the resident cnidarians. Some reefs also exist in cold, deep waters.

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cytoplasmic streaming

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A circular flow of cytoplasm, involving myosin and actin filaments, that speeds the distribution of materials within cells.

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diatom

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A unicellular photosynthetic alga with a unique glassy cell wall containing silica.

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dinoflagellate

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Member of a group of mostly unicellular photosynthetic algae with two flagella situated in perpendicular grooves in cellulose plates covering the cell.

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diplomonad

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A protist that has modified mitochondria, two equal-sized nuclei, and multiple flagella.

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euglenid

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A protist, such as Euglena or its relatives, characterized by an anterior pocket from which one or two flagella emerge.

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euglenozoan

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Member of a diverse clade of flagellated protists that includes predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites.

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Excavata

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One of five supergroups of eukaryotes proposed in a current hypothesis of the evolutionary history of eukaryotes. Excavates have unique cytoskeletal features, and some species have an “excavated” feeding groove on one side of the cell body. See also Chromalveolata, Rhizaria, Archaeplastida, and Unikonta.

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foram (foraminiferan)

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An aquatic protist that secretes a hardened shell containing calcium carbonate and extends pseudopodia through pores in the shell.

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golden alga

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A biflagellated, photosynthetic protist named for its color, which results from its yellow and brown carotenoids.

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heteromorphic

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Referring to a condition in the life cycle of plants and certain algae in which the sporophyte and gametophyte generations differ in morphology.

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holdfast

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A rootlike structure that anchors a seaweed.

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isomorphic

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Referring to alternating generations in plants and certain algae in which the sporophytes and gametophytes look alike, although they differ in chromosome number.

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kinetoplastid

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A protist, such as a trypanosome, that has a single large mitochondrion that houses an organized mass of DNA.

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mixotroph

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An organism that is capable of both photosynthesis and heterotrophy.

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oomycete

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A protist with flagellated cells, such as a water mold, white rust, or downy mildew, that acquires nutrition mainly as a decomposer or plant parasite.

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opisthokont

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Member of the diverse clade Opisthokonta, organisms that descended from an ancestor with a posterior flagellum, including fungi, animals, and certain protists.

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parabasalid

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A protist, such as a trichomonad, with modified mitochondria.

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plasmodial slime mold

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A type of protist that has amoeboid cells, flagellated cells, and a plasmodial feeding stage in its life cycle.

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plasmodium

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A single mass of cytoplasm containing many diploid nuclei that forms during the life cycle of some slime molds.

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producer

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An organism that produces organic compounds from CO₂ by harnessing light energy (in photosynthesis) or by oxidizing inorganic chemicals (in chemosynthetic reactions carried out by some prokaryotes).

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protist

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An informal term applied to any eukaryote that is not a plant, animal, or fungus. Most protists are unicellular, though some are colonial or multicellular.

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pseudopodium (pl., pseudopodia)

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A cellular extension of amoeboid cells used in moving and feeding.

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radiolarian

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A protist, usually marine, with a shell generally made of silica and pseudopodia that radiate from the central body.

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red alga

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A photosynthetic protist, named for its color, which results from a red pigment that masks the green of chlorophyll. Most red algae are multicellular and marine.

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redox reaction

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A chemical reaction involving the complete or partial transfer of one or more electrons from one reactant to another; short for oxidation-reduction reaction.

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Rhizaria

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One of five supergroups of eukaryotes proposed in a current hypothesis of the evolutionary history of eukaryotes; a morphologically diverse protist clade that is defined by DNA similarities.

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secondary endosymbiosis

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A process in eukaryotic evolution in which a heterotrophic eukaryotic cell engulfed a photosynthetic eukaryotic cell, which survived in a symbiotic relationship inside the heterotrophic cell.

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stipe

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A stemlike structure of a seaweed.

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stramenopile

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A protist in which a “hairy” flagellum (one covered with fine, hairlike projections) is paired with a shorter, smooth flagellum.

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thallus (pl., thalli)

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A seaweed body that is plantlike, consisting of a holdfast, stipe, and blades, yet lacks true roots, stems, and leaves.

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Unikonta

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One of five supergroups of eukaryotes proposed in a current hypothesis of the evolutionary history of eukaryotes. This clade, which is supported by studies of myosin proteins and DNA, consists of amoebozoans and opisthokonts.