{"id":145,"date":"2017-09-04T05:06:56","date_gmt":"2017-09-04T05:06:56","guid":{"rendered":"http:\/\/americanboard.org\/Subjects\/general-science\/?page_id=145"},"modified":"2017-09-22T21:05:08","modified_gmt":"2017-09-22T21:05:08","slug":"eukaryotes","status":"publish","type":"page","link":"https:\/\/americanboard.org\/Subjects\/general-science\/eukaryotes\/","title":{"rendered":"Eukaryotes"},"content":{"rendered":"
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\u2b05 Previous Lesson<\/a>\u00a0Workshop Index<\/a>\u00a0Next Lesson \u27a1<\/a><\/div>\n

<\/p>\n

Eukaryotes<\/h1>\n

Objective<\/h4>\n

Now we will focus on some of the diversity of living eukaryotic organisms in order to understand how life has evolved. We will start small and then move on to bigger and more familiar organisms later.<\/p>\n

Previously Covered<\/h4>\n

We reviewed the basics of cell form and function, including the differences and similarities among prokaryotes, eukaryotes, plants and animals.<\/p>\n

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Enter the Algae<\/h3>\n

Algae<\/strong> (singular alga<\/em>) are unicellular or simple multicellular organisms that photosynthesize. It is likely that the six taxonomic divisions of algae known today arose from a common ancestral eukaryotic cell. Three of the six divisions are made up mostly of unicellular organisms; the other three are mainly multicellular, and the vast majority of algae are photosynthetic. Algae range in complexity from a single cell to a filament of cells to a solid body of cells that somewhat resembles a plant. The divisions of algae are based on the presence, number, and position of flagella; their pigmentation; and other factors, like how they store food. The components of the algal cell wall, which often feels slimy due to its polysaccharides, also distinguish different algae.<\/p>\n

Most single-celled algae occur on the surface of the ocean or freshwater bodies, where they are known as plankton. Unicellular algae are free-living autotrophs, though some may live in colonies. Multicellular algae occur primarily in shallow waters along shorelines. Divisions of algae have been named for the algae\u2019s dominant pigment, which may hide the greening of the chloroplasts. Each Division is discussed and described below.<\/p>\n

Staying Single<\/h3>\n

Division Euglenophyta<\/strong> consists of about 1,000 species of the ubiquitous Euglena<\/em>. These unicellular freshwater algae contain chloroplasts encased in a three-layer membrane, suggesting that Euglenas<\/em> evolved by ingesting and incorporating green algae, which have a two-layer chloroplast membrane.<\/p>\n

Euglenas<\/em> are generally elongated, two-flagellum cells, with many chloroplasts that give the cell its striking emerald-green color. Euglenas<\/em> have a photoreceptor and are remarkable in that they be either heterotrophs and\/or autotrophs. In sunlight, Euglenas<\/em> are green and photosynthetic autotrophs, but put them in a dark closet and they lose their pigmentation and become heterotrophs.<\/p>\n

\"Generalized<\/center><\/p>\n

Generalized structure of Euglena<\/p>\n

Division Chrysophyta<\/strong> includes the diatoms (approximately 10,000 species), the golden-brown algae<\/strong> (approximately 1,500 species), and the smaller group (approximately 600 species) of yellow-green algae<\/strong>. Chrysophyta are characterized by the presence of chlorophylls a<\/strong> and c<\/strong>; fucoxanthin, a yellow-brown pigment present in all except the yellow-greens; silicon-impregnated cell walls (lacking cellulose); and their storage of food as oil rather than starch.<\/p>\n

Diatoms<\/strong> are famous for their beautiful, lacey double shells made of silica. The lacey look of the shells are actually pores that allow passage of materials into and out of the alga. Diatoms, like most plankton, are an important part of the marine food chain. Unlike their Divisional relatives, which reproduce via binary fission, diatoms sometimes reproduce sexually via a fusion of gametes, called syngamy<\/strong>.<\/p>\n

Division Pyrrophyta<\/strong>, or fire algae, is made up of about 1,000 species of single-celled marine dinoflagellates<\/strong>. Most species are photoautotrophs, but some are heterotrophs; most have two flagella. Dinoflagellates come in a variety of bewilderingly weird forms, kept in shape by rigid cellulose walls. A few are naked, and most of these are parasitic. As their name suggests, most fire algae are red. They are often the culprits in occurrences of toxic red tides. Some dinoflagellates (especially Gessnerium catenellum<\/em>) that form red tides release neurotoxins, which have caused massive fish kills and occasional poisoning of humans who ingest it in filter-feeding mussels and other shellfish.<\/p>\n

\"Diatoms\"<\/center><\/p>\n

Diatoms<\/p>\n

\"Example<\/center><\/p>\n

Example of a typical dinoflagellate<\/p>\n

Getting Together<\/h3>\n

Division Chlorophyta<\/strong> contains the green algae<\/strong>, which may be either unicellular or multicellular. Chlorophyta are characterized by their chlorophylls a<\/strong> and b<\/strong> (as in plants), the presence of beta-carotene photosynthetic pigments, and their storage of food as starch. Most have cellulose in their cell walls. They are believed to be the ancestors of plants. Most green algae are freshwater organisms, although some are marine organisms; some of the 7,000 species in this Division live on tree trunks or in symbiosis with other organisms. The Chlorophyta are unique in that some species in this Division are multicellular. Scientists speculate that multicellularity may have arisen from this Division.<\/p>\n

Scientists believe that there are three pathways that lead from unicellularity to multicellularity. The first arose from colonies of single-celled organisms. A colony is a group of independently functioning, single-celled organisms connected to each other via strands of cytoplasm. There are four genera of multicellular Chlorophytes. In order of complexity they are: Chlamydomonas<\/em>, Gonium<\/em>, Pandorina<\/em>, and Volvox<\/em>. Volvox is a hollow sphere whose single outer layer is comprised of as many as 60,000 minute flagellated cells.<\/p>\n

\"Volvox<\/center><\/p>\n

Volvox are the prototypical colonial alga<\/p>\n

The second path toward multicellularity is really an intermediate form of organization, referred to as coenocytic<\/strong>. In this case, a single organism consists of many nuclei in a common cytoplasm. Nuclear divisions are not accompanied by separation of the cytoplasm or the formation of cell walls. Coenocytic algae include the large Valonia<\/em> and the filamentous Cladophora<\/em>.<\/p>\n

The third road to multicellularity is represented by algae that undergo nuclear division as well as division of the cytoplasm and the formation of cell walls. Because of this type of cell division, these algae may occur in sheets, filaments, or even a three-dimensional form. One green algae species that evinces this type of multicellularity is Spirogyra<\/em>.<\/p>\n

\"Example<\/center><\/p>\n

Example of a mat of Spirogyra<\/p>\n

Living Together<\/h3>\n

There are two Divisions that contain only multicellular algae, phaeophyta and rhodophyta. Division Phaeophyta <\/strong>consists of brown algae<\/strong>\u2014the seaweeds that thrive in cold marine waters. Kelps<\/strong> are members of this Division and occur, sometimes in huge water-forests, offshore in cooler climes. Many species occur along rocky shorelines, where their holdfasts<\/strong>, the kelp equivalent of roots, keep them anchored to the rocks and prevent them from being washed away by battering waves. The body of a multicellular alga is called a thallus<\/strong>, which consists of the holdfast, the stem-like stipe<\/strong>, and the leaf-like blade<\/strong>. Though these structures are likened to similar structures in plants, their internal organization is very different, and they resemble plant structures only superficially. The rockweed, Fucus<\/em>, is a typical species in this Division.<\/p>\n

The Division Rhodophyta<\/strong>, the red algae, consists of about 4,000 species of mostly tropical and subtropical marine seaweeds (very few species are freshwater). Their red color is conferred by the presence of phycobilins, and they also contain chlorophylls<\/strong> a<\/strong> and d<\/strong>, as well carotenoids. Because of these pigments, Rhodophyta can survive at great depths, up to 175 meters below the surface. Some red algae have a slimy, mucilaginous outer layer, though others\u2014the coralline algae\u2014deposit calcium carbonate in their cell walls and are important in reef building.<\/p>\n

\"Example<\/center><\/p>\n

Example of kelp forest<\/p>\n

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Question<\/h4>\n

Eukaryotes are characterized by the presence of<\/p>\n

    \n
  1. a plasma membrane.<\/li>\n
  2. cilia and flagella.<\/li>\n
  3. chromosomes.<\/li>\n
  4. DNA.<\/li>\n<\/ol>\n

    Reveal Answer <\/a><\/p>\n

    The correct answer is C. Prokaryotes have naked strands of DNA, while eukaryotes have their DNA on chromosomes in the nucleus. Both prokaryotes and eukaryotes have plasma membranes. Eukaryotes are not characterized by cilia or flagella, and all cells have DNA.<\/p>\n<\/section>\n

    \n

    Question<\/h4>\n

    Which of the following is a form of multicellular algae?<\/p>\n

      \n
    1. Diatoms<\/li>\n
    2. Volvox<\/li>\n
    3. Chlorophyta<\/li>\n
    4. Kelp<\/li>\n<\/ol>\n

      Reveal Answer <\/a><\/p>\n

      The correct answer is D. Kelp are members of Division Phaeophyta, which contains only multicellular algae. Diatoms are unicellular algae with rigid silicon shells, so A is incorrect. Volvox are colonies of independent unicellular algae, so B is not correct. Chlorophyta is a Division of algae that includes both unicellular and multicellular algae, so C is not correct.<\/p>\n<\/section>\n

      Micro-Critters<\/h3>\n

      The word protozoa<\/strong> means “first animals” and it refers to the animal-like organisms in various phyla of the Kingdom Protista<\/strong>. Protozoa are unicellular heterotrophs, mainly free-living or parasitic. They reproduce asexually via binary fission, though some have sexual cycles and others undergo conjugation, or nucleus exchange between cells. Protozoa are organized into phyla based primarily on their mode of locomotion:<\/p>\n