28 protists

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28 protists

  1. 1. Chapter 28 Protists PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  2. 2. • Overview: A World in a Drop of Water • Even a low-power microscope – Can reveal an astonishing menagerie of organisms in a drop of pond water Figure 28.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 µm
  3. 3. • These amazing organisms – Belong to the diverse kingdoms of mostly single-celled eukaryotes informally known as protists • Advances in eukaryotic systematics – Have caused the classification of protists to change significantly Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  4. 4. • Concept 28.1: Protists are an extremely diverse assortment of eukaryotes • Protists are more diverse than all other eukaryotes – And are no longer classified in a single kingdom • Most protists are unicellular – And some are colonial or multicellular Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  5. 5. • Protists, the most nutritionally diverse of all eukaryotes, include – Photoautotrophs, which contain chloroplasts – Heterotrophs, which absorb organic molecules or ingest larger food particles – Mixotrophs, which combine photosynthesis and heterotrophic nutrition Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  6. 6. • Protist habitats are also diverse in habitat • And including freshwater and marine species (a) The freshwater ciliate Stentor, a unicellular protozoan (LM) 100 µm 100 µm 4 cm (b) Ceratium tripos, a unicellular marine dinoflagellate (LM) (c) Delesseria sanguinea, a multicellular marine red alga 500 µm Figure 28.2a–d (d) Spirogyra, a filamentous freshwater green alga (inset LM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  7. 7. • Reproduction and life cycles – Are also highly varied among protists, with both sexual and asexual species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  8. 8. • A sample of protist diversity Table 28.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  9. 9. Endosymbiosis in Eukaryotic Evolution • There is now considerable evidence – That much of protist diversity has its origins in endosymbiosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  10. 10. • The plastid-bearing lineage of protists – Evolved into red algae and green algae • On several occasions during eukaryotic evolution – Red algae and green algae underwent secondary endosymbiosis, in which they themselves were ingested Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  11. 11. • Diversity of plastids produced by secondary endosymbiosis Plastid Alveolates Dinoflagellates Apicomplexans Secondary endosymbiosis Cyanobacterium Ciliates Red algae Primary endosymbiosis Stramenopiles Heterotrophic eukaryote Plastid Euglenids Secondary endosymbiosis Green algae Figure 28.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorarachniophytes
  12. 12. Diplomonadida Figure 28.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cercozoa Ancestral eukaryote Plants Charophyceans (Opisthokonta) Chlorophytes Red algae Metazoans Choanoflagellates Amoebozoa Fungi Cellular slime molds Plasmodial slime molds Entamoebas Gymnamoebas Plantae Chlorophyta Rhodophyta Animalia Fungi Radiolarians Radiolaria Foraminiferans Stramenopila Chlorarachniophytes Brown algae Golden algae Diatoms Oomycetes Ciliates Apicomplexans Euglenozoa Alveolata Dinoflagellates Euglenids Kinetoplastids Parabasalids Parabasala Diplomonads • Concept 28.2: Diplomonads and parabasalids have modified mitochondria • A tentative phylogeny of eukaryotes – Divides eukaryotes into many clades (Viridiplantae)
  13. 13. • Diplomonads and parabasalids – Are adapted to anaerobic environments – Lack plastids – Have mitochondria that lack DNA, an electron transport chain, or citric-acid cycle enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  14. 14. Diplomonads • Diplomonads – Have two nuclei and multiple flagella Figure 28.5a (a) Giardia intestinalis, a diplomonad (colorized SEM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm
  15. 15. Parabasalids • Parabasalids include trichomonads – Which move by means of flagella and an undulating part of the plasma membrane Flagella Undulating membrane 5 µm Figure 28.5b (b) Trichomonas vaginalis, a parabasalid (colorized SEM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  16. 16. • Concept 28.3: Euglenozoans have flagella with a unique internal structure • Euglenozoa is a diverse clade that includes – Predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  17. 17. • The main feature that distinguishes protists in this clade – Is the presence of a spiral or crystalline rod of unknown function inside their flagella Flagella 0.2 µm Crystalline rod Figure 28.6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ring of microtubules
  18. 18. Kinetoplastids • Kinetoplastids – Have a single, large mitochondrion that contains an organized mass of DNA called a kinetoplast – Include free-living consumers of bacteria in freshwater, marine, and moist terrestrial ecosystems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  19. 19. • The parasitic kinetoplastid Trypanosoma – Causes sleeping sickness in humans Figure 28.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 9 µm
  20. 20. Euglenids • Euglenids – Have one or two flagella that emerge from a pocket at one end of the cell – Store the glucose polymer paramylon Long flagellum Eyespot: pigmented organelle that functions as a light shield, allowing light from only a certain direction to strike the light detector Light detector: swelling near the base of the long flagellum; detects light that is not blocked by the eyespot; as a result, Euglena moves toward light of appropriate intensity, an important adaptation that enhances photosynthesis Short flagellum Euglena (LM) Nucleus Contractile vacuole 5 µm Plasma membrane Figure 28.8 Pellicle: protein bands beneath the plasma membrane that provide strength and flexibility (Euglena lacks a cell wall) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplast Paramylon granule
  21. 21. • Concept 28.4: Alveolates have sacs beneath the plasma membrane • Members of the clade Alveolata – Have membrane-bounded sacs (alveoli) just under the plasma membrane 0.2 µm Figure 28.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flagellum Alveoli
  22. 22. Dinoflagellates • Dinoflagellates – Are a diverse group of aquatic photoautotrophs and heterotrophs – Are abundant components of both marine and freshwater phytoplankton Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  23. 23. • Each has a characteristic shape – That in many species is reinforced by internal plates of cellulose • Two flagella – Make them spin as they move through the water Figure 28.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 µm Flagella
  24. 24. • Rapid growth of some dinoflagellates – Is responsible for causing “red tides,” which can be toxic to humans Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  25. 25. Apicomplexans • Apicomplexans – Are parasites of animals and some cause serious human diseases – Are so named because one end, the apex, contains a complex of organelles specialized for penetrating host cells and tissues – Have a nonphotosynthetic plastid, the apicoplast Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  26. 26. • Most apicomplexans have intricate life cycles – With both sexual and asexual stages that often require two or more different host species for completion 2 The sporozoites enter the person’s liver cells. After several days, the sporozoites undergo multiple divisions and become merozoites, which use their apical complex to penetrate red blood cells (see TEM below). 1 An infected Anopheles mosquito bites a person, injecting Plasmodium sporozoites in its saliva. Inside mosquito Inside human Sporozoites (n) 7 An oocyst develops from the zygote in the wall of the mosquito’s gut. The oocyst releases thousands of sporozoites, which migrate to the mosquito’s salivary gland. Merozoite Liver Liver cell Apex Oocyst MEIOSIS Zygote (2n) Red blood cell Merozoite (n) Red blood cells FERTILIZATION Gametes Key 3 The merozoites divide asexually inside the red blood cells. At intervals of 48 or 72 hours (depending on the species), large numbers of merozoites break out of the blood cells, causing periodic chills and fever. Some of the merozoites infect new red blood cells. Gametocytes (n) Haploid (n) Diploid (2n) Figure 28.11 0.5 µm 4 Some merozoites form gametocytes. 6 Gametes form from gametocytes. Fertilization occurs in the mosquito’s digestive tract, and a zygote forms. The zygote is the only diploid stage in the life cycle. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 Another Anopheles mosquito bites the infected person and picks up Plasmodium gametocytes along with blood.
  27. 27. Ciliates • Ciliates, a large varied group of protists – Are named for their use of cilia to move and feed – Have large macronuclei and small micronuclei Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  28. 28. • The micronuclei – Function during conjugation, a sexual process that produces genetic variation • Conjugation is separate from reproduction – Which generally occurs by binary fission Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  29. 29. • Exploring structure and function in a ciliate FEEDING, WASTE REMOVAL, AND WATER BALANCE Paramecium, like other freshwater protists, constantly takes in water by osmosis from the hypotonic environment. Bladderlike contractile vacuoles accumulate excess water from radial canals and periodically expel it through the plasma membrane. Contractile Vacuole Paramecium feeds mainly on bacteria. Rows of cilia along a funnel-shaped oral groove move food into the cell mouth, where the food is engulfed into food vacuoles by phagocytosis. Oral groove Cell mouth 50 µm Thousands of cilia cover the surface of Paramecium. Micronucleus Food vacuoles combine with lysosomes. As the food is digested, the vacuoles follow a looping path through the cell. Macronucleus Figure 28.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The undigested contents of food vacuoles are released when the vacuoles fuse with a specialized region of the plasma membrane that functions as an anal pore.
  30. 30. CONJUGATION AND REPRODUCTION 1 Two cells of compatible mating strains align side by side and partially fuse. 2 Meiosis of micronuclei produces four haploid micronuclei in each cell. 3 Three micronuclei in each cell disintegrate. The remaining micronucleus in each cell divides by mitosis. MEIOSIS 4 The cells swap one micronucleus. Macronucleus Compatible mates Haploid micronucleus Diploid micronucleus Diploid micronucleus MICRONUCLEAR FUSION 5 9 Two rounds of cytokinesis partition one macronucleus and one micronucleus into each of four daughter cells. 8 The original macronucleus disintegrates. Four micronuclei become macronuclei, while the other four remain micronuclei. 7 Three rounds of mitosis without cytokinesis produce eight micronuclei. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6 Micronuclei fuse, forming a diploid micronucleus. The cells separate. Key Conjugation Reproduction
  31. 31. • Concept 28.5: Stramenopiles have “hairy” and smooth flagella • The clade Stramenopila – Includes several groups of heterotrophs as well as certain groups of algae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  32. 32. • Most stramenopiles – Have a “hairy” flagellum paired with a “smooth” flagellum Hairy flagellum Smooth flagellum Figure 28.13 5 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  33. 33. Oomycetes (Water Molds and Their Relatives) • Oomycetes – Include water molds, white rusts, and downy mildews – Were once considered fungi based on morphological studies Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  34. 34. • Most oomycetes – Are decomposers or parasites – Have filaments (hyphae) that facilitate nutrient uptake Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  35. 35. • The life cycle of a water mold 1 Encysted zoospores land on a substrate and germinate, growing into a tufted body of hyphae. 2 Several days later, the hyphae begin to form sexual structures. 3 Meiosis produces eggs within oogonia (singular, oogonium). Germ tube Cyst 9 Each zoosporangium produces about 30 biflagellated zoospores asexually. MEIOSIS ASEXUAL REPRODUCTION Zoospore (2n) 8 The ends of hyphae form tubular zoosporangia. Zoosporangium (2n) 4 On separate branches of the same or different individuals, meiosis produces several haploid sperm nuclei contained within antheridial hyphae. Oogonium Egg nucleus (n) Antheridial hypha with sperm nuclei (n) SEXUAL FERTILIZATION Zygote REPRODUCTION germination Zygotes (oospores) (2n) Key Haploid (n) Diploid (2n) 7 The zygotes germinate and form hyphae, and the cycle is completed. Figure 28.14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6 A dormant period follows, during which the oogonium wall usually disintegrates. 5 Antheridial hyphae grow like hooks around the oogonium and deposit their nuclei through fertilization tubes that lead to the eggs. Following fertilization, the zygotes (oospores) may develop resistant walls but are also protected within the wall of the oogonium.
  36. 36. • The ecological impact of oomycetes can be significant – Phytophthora infestans causes late blight of potatoes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  37. 37. Diatoms • Diatoms are unicellular algae Figure 28.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 µm – With a unique two-part, glass-like wall of hydrated silica
  38. 38. • Diatoms are a major component of phytoplankton – And are highly diverse Figure 28.16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 µm
  39. 39. • Accumulations of fossilized diatom walls – Compose much of the sediments known as diatomaceous earth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  40. 40. Golden Algae • Golden algae, or chrysophytes – Are named for their color, which results from their yellow and brown carotenoids • The cells of golden algae – Are typically biflagellated, with both flagella attached near one end of the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  41. 41. • Most golden algae are unicellular – But some are colonial 25 µm Figure 28.17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  42. 42. Brown Algae • Brown algae, or phaeophytes – Are the largest and most complex algae – Are all multicellular, and most are marine Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  43. 43. • Brown algae – Include many of the species commonly called seaweeds • Seaweeds – Have the most complex multicellular anatomy of all algae Blade Stipe Figure 28.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Holdfast
  44. 44. • Kelps, or giant seaweeds – Live in deep parts of the ocean Figure 28.19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  45. 45. Human Uses of Seaweeds • Many seaweeds – Are important commodities for humans – Are harvested for food (a) The seaweed is grown on nets in shallow coastal waters. (b) A worker spreads the harvested seaweed on bamboo screens to dry. Figure 28.20a–c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) Paper-thin, glossy sheets of nori make a mineral-rich wrap for rice, seafood, and vegetables in sushi.
  46. 46. Alternation of Generations • A variety of life cycles – Have evolved among the multicellular algae • The most complex life cycles include an alternation of generations – The alternation of multicellular haploid and diploid forms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  47. 47. • The life cycle of the brown alga Laminaria 1 The sporophytes of this seaweed are usually found in water just below the line of the lowest tides, attached to rocks by branching holdfasts. 2 In early spring, at the end of the main growing season, cells on the surface of the blade develop into sporangia. Sporangia 3 Sporangia produce zoospores by meiosis. Sporophyte (2n) 7 The zygotes grow into new sporophytes, starting life attached to the remains of the female gametophyte. Developing sporophyte MEIOSIS Zoospores Female Gametophytes (n) Zygote (2n) Egg Mature female gametophyte (n) Key Figure 28.21 Male FERTILIZATION Haploid (n) Diploid (2n) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sperm 6 Sperm fertilize the eggs. 4 The zoospores are all structurally alike, but about half of them develop into male gametophytes and half into female gametophytes. The gametophytes look nothing like the sporophytes, being short, branched filaments that grow on the surface of subtidal rocks. 5 Male gametophytes release sperm, and female gametophytes produce eggs, which remain attached to the female gametophyte. Eggs secrete a chemical signal that attracts sperm of the same species, thereby increasing the probability of fertilization in the ocean.
  48. 48. • Concept 28.6: Cercozoans and radiolarians have threadlike pseudopodia • A newly recognized clade, Cercozoa – Contains a diversity of species that are among the organisms referred to as amoebas • Amoebas were formerly defined as protists – That move and feed by means of pseudopodia • Cercozoans are distinguished from most other amoebas – By their threadlike pseudopodia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  49. 49. Foraminiferans (Forams) • Foraminiferans, or forams – Are named for their porous, generally multichambered shells, called tests 20 µm Figure 28.22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  50. 50. • Pseudopodia extend through the pores in the test • Foram tests in marine sediments – Form an extensive fossil record Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  51. 51. Radiolarians • Radiolarians are marine protists – Whose tests are fused into one delicate piece, which is generally made of silica – That phagocytose microorganisms with their pseudopodia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  52. 52. • The pseudopodia of radiolarians, known as axopodia – Radiate from the central body Axopodia Figure 28.23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 200 µm
  53. 53. • Concept 28.7: Amoebozoans have lobeshaped pseudopodia • Amoebozoans – Are amoeba that have lobe-shaped, rather than threadlike, pseudopodia – Include gymnamoebas, entamoebas, and slime molds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  54. 54. Gymnamoebas • Gymnamoebas – Are common unicellular amoebozoans in soil as well as freshwater and marine environments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  55. 55. • Most gymnamoebas are heterotrophic – And actively seek and consume bacteria and other protists Pseudopodia 40 µm Figure 28.24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  56. 56. Entamoebas • Entamoebas – Are parasites of vertebrates and some invertebrates • Entamoeba histolytica – Causes amebic dysentery in humans Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  57. 57. Slime Molds • Slime molds, or mycetozoans – Were once thought to be fungi • Molecular systematics – Places slime molds in the clade Amoebozoa Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  58. 58. Plasmodial Slime Molds • Many species of plasmodial slime molds – Are brightly pigmented, usually yellow or orange 4 cm Figure 28.25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  59. 59. • At one point in the life cycle – They form a mass called a plasmodium 1 The feeding stage is a multinucleate plasmodium that lives on organic refuse. 2 The plasmodium takes a weblike form. Feeding plasmodium Zygote (2n) 3 The plasmodium erects stalked fruiting bodies (sporangia) when conditions become harsh. Mature plasmodium (preparing to fruit) Young sporangium SYNGAMY 1 mm Mature sporangium Amoeboid cells (n) Flagellated cells (n) Figure 28.26 7 The cells unite in pairs (flagellated with flagellated and amoeboid with amoeboid), forming diploid zygotes. 6 These cells are either amoeboid or flagellated; the two forms readily convert from one to the other. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Germinating spore Spores (n) Key MEIOSIS Haploid (n) Diploid (2n) Stalk 5 The resistant spores disperse through the air to new locations and germinate, becoming active haploid cells when conditions are favorable. 4 Within the bulbous tips of the sporangia, meiosis produces haploid spores.
  60. 60. • The plasmodium – Is undivided by membranes and contains many diploid nuclei – Extends pseudopodia through decomposing material, engulfing food by phagocytosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  61. 61. Cellular Slime Molds • Cellular slime molds form multicellular aggregates – In which the cells remain separated by their membranes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  62. 62. • The life cycle of Dictyostelium, a cellular slime mold 1 In the feeding 9 In a favorable environment, amoebas stage of the life emerge from the spore cycle, solitary haploid coats and begin feeding. amoebas engulf bacteria. 8 Spores are released. 2 During sexual reproduction, two haploid amoebas fuse and form a zygote. SYNGAMY 7 Other cells crawl up the stalk and develop into spores. Emerging Spores amoeba (n) Solitary amoebas (feeding stage) 600 µm Zygote (2n) SEXUAL REPRODUCTION MEIOSIS Amoebas Fruiting bodies ASEXUAL REPRODUCTION 4 The resistant wall ruptures, releasing new haploid amoebas. Aggregated amoebas 5 When food is depleted, hundreds of amoebas congregate in response to a chemical attractant and form a sluglike aggregate (photo below left). Aggregate formation is the beginning of asexual reproduction. Migrating aggregate 6 The aggregate migrates for a while and then stops. Some of the cells dry up after forming a stalk that supports an asexual fruiting body. Figure 28.27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 The zygote becomes a giant cell (not shown) by consuming haploid amoebas. After developing a resistant wall, the giant cell undergoes meiosis followed by several mitotic divisions. Key 200 µm Haploid (n) Diploid (2n)
  63. 63. • Dictyostelium discoideum – Has become an experimental model for studying the evolution of multicellularity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  64. 64. • Concept 28.8: Red algae and green algae are the closest relatives of land plants • Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont – And the photosynthetic descendants of this ancient protist evolved into red algae and green algae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  65. 65. Red Algae • Red algae are reddish in color – Due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  66. 66. • Red algae – Are usually multicellular; the largest are seaweeds – Are the most abundant large algae in coastal waters of the tropics (b) Dulse (Palmaria palmata). This edible species has a “leafy” form. (c) A coralline alga. The cell walls of coralline algae are hardened by calcium carbonate. Some coralline algae are members of the biological communities around coral reefs. (a) Bonnemaisonia hamifera. This red alga Figure 28.28a–c has a filamentous form. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  67. 67. Green Algae • Green algae – Are named for their grass-green chloroplasts – Are divided into two main groups: chlorophytes and charophyceans – Are closely related to land plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  68. 68. • Most chlorophytes – Live in fresh water, although many are marine • Other chlorophytes – Live in damp soil, as symbionts in lichens, or in snow Figure 28.29 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  69. 69. • Chlorophytes include – Unicellular, colonial, and multicellular forms 20 µm 50 µm (a) Volvox, a colonial freshwater chlorophyte. The colony is a hollow ball whose wall is composed of hundreds or thousands of biflagellated cells (see inset LM) embedded in a gelatinous matrix. The cells are usually connected by strands of cytoplasm; if isolated, these cells cannot reproduce. The large colonies seen here will eventually release the small “daughter” colonies within them (LM). (b) Caulerpa, an intertidal chlorophyte. The branched filaments lack cross-walls and thus are multinucleate. In effect, the thallus is one huge “supercell.” Figure 28.30a–c (c) Ulva, or sea lettuce. This edible seaweed has a multicellular thallus differentiated into leaflike blades and a rootlike holdfast that anchors the alga against turbulent waves and tides. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  70. 70. • Most chlorophytes have complex life cycles – With both sexual and asexual reproductive stages 7 These daughter cells develop flagella and cell walls and then emerge as swimming zoospores from the wall of the parent cell that had enclosed them. The zoospores grow into mature haploid cells, completing the asexual life cycle. Flagella 1 In Chlamydomonas, mature cells are haploid and contain a single cup-shaped chloroplast (see TEM at left). 2 In response to a shortage of nutrients, drying of the pond, or some other stress, cells develop into gametes. 3 Gametes of opposite mating types (designated + and –) pair off and cling together. Fusion of the gametes (syngamy) forms a diploid zygote. − 1 µm Cell wall + Nucleus + Zoospores ASEXUAL REPRODUCTION Regions of single chloroplast Mature cell (n) SEXUAL REPRODUCTION + Key Haploid (n) Diploid (2n) + 6 When a mature cell reproduces asexually, it resorbs its flagella and then undergoes two rounds of mitosis, forming four cells (more in some species). Figure 28.31 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings − SYNGAMY Zygote (2n) − MEIOSIS 4 The zygote secretes a durable coat that protects the cell against harsh conditions. 5 After a dormant period, meiosis produces four haploid individuals (two of each mating type) that emerge from the coat and develop into mature cells.

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