OVERVIEW Living Small - WALKER'S CLASS
28OVERVIEWLiving SmallKnowing that most prokaryotes are extremely small organ-Protists1 µm! Figure 28.1 Which of these organismsare prokaryotes and which are eukaryotes?EVOLUTIONKEY CONCEPTS28.1 Most eukaryotes are single-celled organisms28.2 Excavates include protists with modifiedmitochondria and protists with unique flagella28.3 Chromalveolates may have originated bysecondary endosymbiosis28.4 Rhizarians are a diverse group of protistsdefined by DNA similarities28.5 Red algae and green algae are the closestrelatives of land plants28.6 Unikonts include protists that are closely relatedto fungi and animals28.7 Protists play key roles in ecological communitiesisms, you might assume that Figure 28.1 depicts six prokaryotes and one much larger eukaryote. But in fact, the onlyprokaryote is the organism immediately above the scale bar.The other six organisms are members of diverse, mostly unicellular groups of eukaryotes informally known as protists.Very small eukaryotes have intrigued scientists for more than300 years, ever since the Dutch microscopist Antoni vanLeeuwenhoek first laid eyes on them. As he discovered, viewinga drop of pond water under a light microscope can reveal a fascinating world of unicellular protists and prokaryotes. Someprotists propel themselves with whipping flagella, while otherscreep along by means of blob-like appendages. Some areshaped like tiny trumpets; others resemble miniature jewelry.Recalling his observations, van Leeuwenhoek wrote, “No morepleasant sight has met my eye than this, of so many thousandsof living creatures in one small drop of water.”Until recently, biologists thought that 300 years of observation had uncovered a representative sample of living protistspecies. But in the last decade, genetic prospecting has turnedup a treasure trove of previously unknown protists within theworld of microscopic life. Many of these newly discoveredorganisms are just 0.5–2 µm in diameter—as small as manyprokaryotes.The surprising discovery of many species of minuscule protists followed close on the heels of recent findings regardingprotist phylogeny. All protists were once classified in a singlekingdom, Protista, but advances in eukaryotic systematicshave caused the kingdom to crumble. It has become clear thatthe kingdom Protista is in fact polyphyletic (see Figure 26.10):Some protists are more closely related to plants, fungi, or animals than they are to other protists. As a result, the kingdomProtista has been abandoned, and various lineages of protistsare now recognized as kingdoms in their own right. Most biologists still use the term protist, but only as a convenient way torefer to eukaryotes that are not plants, animals, or fungi.In this chapter, you will become acquainted with some ofthe most significant groups of protists. You will learn abouttheir structural and biochemical adaptations as well as theirenormous impact on ecosystems, agriculture, industry, andhuman health.CONCEPT28.1Most eukaryotes aresingle-celled organismsProtists, along with plants, animals, and fungi, are classified aseukaryotes; they are in domain Eukarya, one of the three domains of life. Unlike the cells of prokaryotes, eukaryotic cellsCHAPTER 28Protists575
have a nucleus and other membrane-bounded organelles, suchas mitochondria and the Golgi apparatus. Such organelles provide specific locations in which particular cellular functionsare accomplished, making the structure and organization ofeukaryotic cells more complex than those of prokaryotic cells.We’ll survey the diversity of eukaryotes throughout therest of this unit, beginning in this chapter with the protists.As you explore this material, bear in mind that the organisms in most eukaryotic lineages are protists, and most protists are unicellular.Thus, life differs greatly from how most of us commonlythink of it. The large, multicellular organisms that we knowbest (plants, animals, and fungi) are the tips of just a fewbranches on the great tree of life (see Figure 26.21).Structural and Functional Diversity in ProtistsGiven the polyphyletic nature of the group once called Protista, it isn’t surprising that few general characteristics of protists can be cited without exceptions. In fact, protists exhibitmore structural and functional diversity than any othergroup of eukaryotes.Most protists are unicellular, although there are some colonial and multicellular species. Single-celled protists are justifiably considered the simplest eukaryotes, but at the cellularlevel, many protists are very complex—the most elaborate ofall cells. In multicellular organisms, essential biological functions are carried out by organs. Unicellular protists carry outthe same essential functions, but they do so using subcellular organelles, not multicellular organs. The organelles thatprotists use are mostly those discussed in Chapter 6, including the nucleus, endoplasmic reticulum, Golgi apparatus,and lysosomes. Certain protists also rely on organelles notfound in most other eukaryotic cells, such as contractile vacuoles that pump excess water from the protistan cell (seeFigure 7.16).Protists are more nutritionally diverse than other eukaryote groups. Some protists are photoautotrophs and containchloroplasts. Some are heterotrophs, absorbing organicmolecules or ingesting larger food particles. Still other protists, called mixotrophs, combine photosynthesis andheterotrophic nutrition. Photoautotrophy, heterotrophy, andmixotrophy have all arisen independently in many protist lineages.Reproduction and life cycles also are highly varied amongprotists. Some protists are only known to reproduce asexually; others can also reproduce sexually or at least employ thesexual processes of meiosis and fertilization. All three basictypes of sexual life cycles (see Figure 13.6) are representedamong protists, along with some variations that do not quitefit any of these types. We will examine the life cycles of several protist groups later in this chapter.576UNIT FIVEThe Evolutionary History of Biological DiversityEndosymbiosis in Eukaryotic EvolutionWhat gave rise to the enormous diversity of protists that existtoday? There is abundant evidence that much of protist diversity has its origins in endosymbiosis, the process inwhich certain unicellular organisms engulf other cells, whichbecome endosymbionts and ultimately organelles in the hostcell. For example, as we discussed in Chapter 25, structural,biochemical, and DNA sequence data indicate that the firsteukaryotes acquired mitochondria by engulfing an aerobicprokaryote (specifically, an alpha proteobacterium). The earlyorigin of mitochondria is supported by the fact that all eukaryotes studied so far have either mitochondria or modifiedversions of them.Much evidence also indicates that later in eukaryotic history,a lineage of heterotrophic eukaryotes acquired an additionalendosymbiont—a photosynthetic cyanobacterium—that thenevolved into plastids. As the hypothesis illustrated inFigure 28.2 shows, this plastid-bearing lineage gave rise to twolineages of photosynthetic protists, or algae: red algae andgreen algae. This hypothesis is supported by the observationthat the DNA of plastid genes in red algae and green algaeclosely resembles the DNA of cyanobacteria. In addition, plastids in red algae and green algae are surrounded by two membranes. Transport proteins in these membranes are homologousto proteins in the inner and outer membranes of cyanobacterialendosymbionts, providing further support for the hypothesis.On several occasions during eukaryotic evolution, red algaeand green algae underwent secondary endosymbiosis: Theywere ingested in the food vacuoles of heterotrophic eukaryotesand became endosymbionts themselves. For example, protistsknown as chlorarachniophytes likely evolved when a heterotrophic eukaryote engulfed a green alga. Evidence for thisprocess can be found within the engulfed cell, which contains atiny vestigial nucleus, called a nucleomorph. Genes from the nucleomorph are still transcribed, and their DNA sequences indicate the engulfed cell was a green alga. Also consistent with thehypothesis that chlorarachniophytes evolved from a eukaryotethat engulfed another eukaryote, their plastids are surroundedby four membranes. The two inner membranes originated asthe inner and outer membranes of the ancient cyanobacterium.The third membrane is derived from the engulfed alga’s plasmamembrane, and the outermost membrane is derived from theheterotrophic eukaryote’s food vacuole. In some other protists,plastids acquired by secondary endosymbiosis are surroundedby three membranes, indicating that one of the original fourmembranes was lost during the course of evolution.Five Supergroups of EukaryotesOur understanding of the evolutionary history of protists hasbeen in a state of flux in recent years. Not only has kingdomProtista been abandoned, but a variety of other hypotheseshave been discarded as well. For example, in the early 1990s,
" Figure 28.2 Diversity of plastids produced by endosymbiosis. Studies of plastid-bearingeukaryotes suggest that plastids evolved from a gram-negative cyanobacterium that was engulfed by anancestral heterotrophic eukaryote (primary endosymbiosis). That ancestor then diversified into red algae andgreen algae, some of which were subsequently engulfed by other eukaryotes (secondary symbiosisMembranesare representedas dark lines inthe cell.Cyanobacterium1 2ApicomplexansRed amenopilesPlastidSecondaryendosymbiosisOne of thesemembranes waslost in red andgreen een algaChlorarachniophytesmany biologists thought that the oldest lineage of livingeukaryotes consisted of the amitochondriate protists, organismswithout conventional mitochondria and with fewer membranebounded organelles than other protist groups. But recent structural and DNA data have undermined this hypothesis. Many ofthe so-called amitochondriate protists have been shown to havemitochondria—though reduced ones—and some of these organisms are now classified in entirely different groups. For example,microsporidians, once considered amitochondriate protists, arenow classified as fungi.The ongoing changes in our understanding of the phylogeny of protists pose challenges to students and instructorsalike. Hypotheses about these relationships are a focus of scientific activity, changing rapidly as new data cause previousideas to be modified or discarded. In this chapter, our discussion is organized around one current hypothesis: the fivesupergroups of eukaryotes shown in Figure 28.3, on the nexttwo pages. Because the root of the eukaryotic tree is notknown, all five supergroups are shown as diverging simultaneously from a common ancestor. We know that is not correct, but we do not know which organisms were the first todiverge from the others. In addition, while some of the groupsin Figure 28.3 are well supported by morphological and DNAdata, others are more controversial. As you read this chapter,it may be helpful to focus less on the specific names of groupsof organisms and more on why the organisms are importantand how ongoing research is elucidating their evolutionaryrelationships.CONCEPT CHECK28.11. Cite at least four examples of structural and functional diversity among protists.2. Summarize the role of endosymbiosis in eukaryoticevolution.3. WHAT IF? After studying Figure 28.3, on the nexttwo pages, draw a simplified version of the phylogenetic tree that shows only the five supergroups ofeukaryotes. Now sketch how the tree would look ifthe unikonts were the first group of eukaryotes to diverge from other eukaryotes.For suggested answers, see Appendix A.CHAPTER 28Protists577
" Figure 28.3Exploring Protistan DiversityThe tree below represents a phylogenetic hypothesis for the relationships among all theeukaryotes on Earth today. The eukaryotic groups at the branch tips are related in larger“supergroups,” labeled vertically at the far right of the tree. The kingdoms Plantae (landplants), Fungi, and Animalia (animals) have survived from the five-kingdom system ofclassification. Groups that were formerly classified in the kingdom Protista are listed inbeige boxes. Dotted lines indicate evolutionary relationships that are uncertain andproposed clades that are under active llatesApicomplexansStramenopilesGolden algae! ExcavataSome members of this supergroup havean “excavated” groove on one side ofthe cell body. Two major clades (theparabasalids and diplomonads) havemodified mitochondria; others (theeuglenozoans) have flagella that differin structure from those of other organisms. Excavates include parasites suchas Giardia, as well as many predatoryand photosynthetic species.5 µmBrown ytesCharophytesLand plantsSlime ungiChoanoflagellatesAnimalsUnikontaAmoebozoansThe Evolutionary History of Biological DiversityArchaeplastidaRed algaeRhizariaCercozoansUNIT onadsGiardia intestinalis, a diplomonadparasite. This diplomonad (colorizedSEM), which lacks the characteristicsurface groove of the Excavata, caninfect people when they drink watercontaminated with feces containingGiardia cysts. Drinking such water—even from a seemingly pristine stream—can cause severe diarrhea. Boiling thewater kills the parasite.
! Chromalveolata! ArchaeplastidaThis group may have originated by an ancient secondary endosymbiosis event. Chromalveolates include some of the most importantphotosynthetic organisms on Earth, such as the diatoms shownhere. The group also includes the brown algae that form underwaterkelp “forests,” as well as important pathogens, such as Plasmodium,which causes malaria, and Phytophthora, which caused the devastating potato famine in 19th-century Ireland.50 µmThis group of eukaryotes includes red algae and green algae, alongwith land plants (kingdom Plantae, discussed in Chapters 29 and30). Red algae and green algae include unicellular species, colonialspecies (such as the green alga Volvox), and multicellular species.Many of the large algae known informally as “seaweeds” are multicellular red or green algae. Protists in Archaeplastida include keyphotosynthetic species that form the base of the food web in someaquatic communities.20 µm50 µmDiatom diversity. These beautiful single-celled protists are importantphotosynthetic organisms in aquatic communities (LM).! RhizariaThis group contains many species of amoebas, most of which havepseudopodia that are threadlike in shape. Pseudopodia are extensions that can bulge from any portion ofthe cell; they are used in movement andin the capture of prey. Several recentmolecular phylogenetic studies havesuggested that Rhizaria should benested within Chromalveolata;this hypothesis is currently beingtested by other research groups.100 µmVolvox, a colonial freshwater green alga. The colony is a hollowball whose wall is composed of hundreds of biflagellated cells (see insetLM) embedded in a gelatinous matrix. The cells are usually connected bycytoplasmic strands; if isolated, these cells cannot reproduce. The largecolonies seen here will eventually release the small “daughter” colonieswithin them (LM).! UnikontaThis group of eukaryotes includes amoebas that have lobe- or tubeshaped pseudopodia, as well as animals, fungi, and non-amoebaprotists that are closely related to animals or fungi. According toone current hypothesis, the unikonts may have been the firstgroup of eukaryotes to diverge from other eukaryotes (see Figure28.23); however, this hypothesis has yet to be widely accepted.100 µmGlobigerina, a foram in the supergroup Rhizaria. Threadlike pseudopodia extend through pores in the shell, or test (LM). The inset SEMshows a foram test, which is hardened by calcium carbonate.A unikont amoeba. This amoeba (Amoeba proteus) is usingits pseudopodia to move.CHAPTER 28Protists579
28.2Flagella5 µmCONCEPTExcavates include protistswith modified mitochondriaand protists with unique aeplastidaUnikontaNow that we have examined some of the broad patterns ineukaryotic evolution, we will look more closely at the fivemain groups of protists shown in Figure 28.3.We begin this tour with Excavata (the excavates), a claderecently proposed based on morphological studies of thecytoskeleton. Some members of this diverse group also havean “excavated” feeding groove on one side of the cell body.The excavates include the diplomonads, the parabasalids,and the euglenozoans. Molecular data indicate that each ofthese three groups is monophyletic, but the data have neither confirmed nor strongly refuted the monophyly of theexcavate supergroup. Although many excavates share certainunique cytoskeletal features, we cannot yet tell whether thatis because the excavates are monophyletic or because thecommon ancestor of eukaryotes had those features. Overall,support for the excavate clade is relatively weak, making itone of the more controversial of the five supergroups.Diplomonads and ParabasalidsThe protists in these two groups lack plastids and have modified mitochondria (until recently, they were thought to lackmitochondria altogether). Most diplomonads and parabasalidsare found in anaerobic environments.Diplomonads have modified mitochondria called mitosomes. These organelles lack functional electron transportchains and hence cannot use oxygen to help extract energyfrom carbohydrates and other organic molecules. Instead,diplomonads get the energy they need from anaerobic biochemical pathways.Structurally, diplomonads have two equal-sized nuclei andmultiple flagella. Recall that eukaryotic flagella are extensionsof the cytoplasm, consisting of bundles of microtubules covered by the cell’s plasma membrane (see Figure 6.24). Theyare quite different from prokaryotic flagella, which are filaments composed of the globular protein flagellin attached tothe cell surface (see Figure 27.6).Many diplomonads are parasites. An infamous exampleis Giardia intestinalis (also known as Giardia lamblia; seeFigure 28.3), which inhabits the intestines of mammals.580UNIT FIVEThe Evolutionary History of Biological DiversityUndulatingmembrane! Figure 28.4 The parabasalid Trichomonas vaginalis(colorized SEM).Parabasalids also have reduced mitochondria; calledhydrogenosomes, these organelles generate some energy anaerobically, releasing hydrogen gas as a by-product. The best-knownparabasalid is Trichomonas vaginalis, a sexually transmitted parasite that infects some 5 million people each year. T. vaginalistravels along the mucus-coated lining of the human reproductive and urinary tracts by moving its flagella and by undulatingpart of its plasma membrane (Figure 28.4). In females, if thevagina’s normal acidity is disturbed, T. vaginalis can outcompete beneficial microorganisms there and infect the vagina.(Trichomonas infections also can occur in the urethra of males,though often without symptoms.) T. vaginalis has a gene thatallows it to feed on the vaginal lining, promoting infection.Studies suggest that the protist acquired this gene by horizontalgene transfer from bacterial parasites in the vagina.EuglenozoansProtists called euglenozoans belong to a diverse clade thatincludes predatory heterotrophs, photosynthetic autotrophs,and parasites. The main morphological feature that distinguishes protists in this clade is the presence of a rod with either a spiral or a crystalline structure inside each of theirflagella (Figure 28.5). The two best-studied groups of euglenozoans are the kinetoplastids and the euglenids.KinetoplastidsProtists called kinetoplastids have a single, large mitochondrion that contains an organized mass of DNA called akinetoplast. These protists include species that feed on prokaryotes in freshwater, marine, and moist terrestrial ecosystems, aswell as species that parasitize animals, plants, and other protists. For example, kinetoplastids in the genus Trypanosoma infect humans and cause sleeping sickness, a neurological diseasethat is invariably fatal if not treated. The infection occurs viathe bite of a vector (carrier) organism, the African tsetse fly(Figure 28.6). Trypanosomes also cause Chagas’ disease, which
Flagella0.2 µm8 µmCrystalline rod(cross section)Ring of microtubules(cross section)! Figure 28.5 Euglenozoan flagellum. Most euglenozoanshave a crystalline rod inside one of their flagella (the TEM is a flagellumshown in cross section). The rod lies alongside the 9 ! 2 ring ofmicrotubules found in all eukaryotic flagella (compare with Figure 6.24).is transmitted by bloodsucking insects and can lead to congestive heart failure.Trypanosomes evade immune responses with an effective“bait-and-switch” defense. The surface of a trypanosome iscoated with millions of copies of a single protein. However,before the host’s immune system can recognize the proteinand mount an attack, new generations of the parasite switchto another surface protein with a different molecular structure. Frequent changes in the surface protein prevent thehost from developing immunity (see Figure 43.24). About athird of Trypanosoma’s genome is dedicated to producingthese surface proteins.EuglenidsA euglenid has a pocket at one end of the cell from whichone or two flagella emerge (Figure 28.7). Many species of theeuglenid Euglena are mixotrophs: In sunlight they are autotrophic, but when sunlight is unavailable, they can becomeheterotrophic, absorbing organic nutrients from their environment. Many other euglenids engulf prey by phagocytosis.CONCEPT CHECK9 µm! Figure 28.6 Trypanosoma, the kinetoplastid that causessleeping sickness. The purple, ribbon-shaped cells among these redblood cells are the trypanosomes (colorized SEM).Long flagellumEyespot: pigmentedorganelle thatfunctions as a lightshield, allowing lightfrom only a certaindirection to strikethe light detectorShort flagellumContractile vacuole28.21. Why do some biologists describe the mitochondria ofdiplomonads and parabasalids as “highly reduced”?2. WHAT IF? DNA sequence data for a diplomonad, aeuglenid, a plant, and an unidentified protist suggestthat the unidentified species is most closely related tothe diplomonad. Further studies reveal that the unknown species has fully functional mitochondria. Basedon these data, at what point on the phylogenetic tree inFigure 28.3 did the mystery protist’s lineage probablydiverge from other eukaryote lineages? Explain.For suggested answers, see Appendix A." Figure 28.7 Euglena, a euglenidcommonly found in pond water.Light detector: swelling near thebase of the long flagellum; detectslight that is not blocked by theeyespot. As a result, Euglena movestoward light of appropriateintensity, an important adaptationthat enhances photosynthesis.NucleusChloroplastPlasma membraneEuglena (LM)5 µmPellicle: protein bands beneaththe plasma membrane thatprovide strength and flexibility(Euglena lacks a cell wall.)CHAPTER 28Protists581
Chromalveolates may haveoriginated by secondaryendosymbiosisExcavataDiatomsGolden algaeBrown llatesApicomplexans e supergroup Chromalveolata (the chromalveolates), alarge, extremely diverse clade of protists, has recently beenproposed based on two lines of evidence. First, some (thoughnot all) DNA sequence data suggest that the chromalveolatesform a monophyletic group. Second, some data support thehypothesis that the chromalveolates originated more than abillion years ago, when a common ancestor of the group engulfed a single-celled, photosynthetic red alga. Because redalgae are thought to have originated by primary endosymbiosis (see Figure 28.2), such an origin for the chromalveolates is referred to as secondary endosymbiosis.How strong is the evidence that the chromalveolates originated by secondary endosymbiosis? Many species in the cladehave plastids whose structure and DNA indicate that they areof red algal origin. Others have reduced plastids that seem to bederived from a red algal endosymbiont. Still other species lackplastids altogether, yet some of these species have plastid genesin their nuclear DNA. Such data have led researchers to suggestthat the common ancestor of the chromalveolates had plastidsof red algal origin, but that later, some evolutionary lineageswithin the group lost the plastids. Others question this idea,based on the absence of plastid genes in the genomes of severalchromalveolates that lack plastids. Overall, the endosymbioticorigin of the chromalveolates is an interesting idea, but like anyscientific hypothesis, new data may show it to be incorrect.The chromalveolates are perhaps the most controversial ofthe five supergroups we describe in this chapter. Even so, formany scientists, this supergroup represents the best currenthypothesis for the phylogeny of the two large protist cladesto which we now turn: the alveolates and the stramenopiles.AlveolatesThe alveolates are a group of protists whose monophyly iswell supported by molecular systematics. Structurally, speciesin this group have membrane-bounded sacs (alveoli) justunder the plasma membrane (Figure 28.8). The function of582UNIT FIVEThe Evolutionary History of Biological Diversitythe alveoli is unknown; researchers hypothesize that theymay help stabilize the cell surface or regulate the cell’s waterand ion content.The alveolates include three subgroups: a group of flagellates(the dinoflagellates), a group of parasites (the apicomplexans),and a group of protists that move using cilia (the ciliates).DinoflagellatesThe dinoflagellates are characterized by cells that are reinforced by cellulose plates. Two flagella located in grooves inthis “armor” make dinoflagellates (from the Greek dinos,whirling) spin as they move through the water (Figure 28.9).Dinoflagellates are abundant components of both marine andfreshwater plankton, communities of mostly microscopic organisms that drift in currents near the water’s surface. Thesedinoflagellates include some of the most important species ofphytoplankton (photosynthetic plankton, which include photosynthetic bacteria as well as algae). However, many photosynthetic dinoflagellates are mixotrophic, and roughly half ofall dinoflagellates are purely heterotrophic.FlagellumAlveoliAlveolate! Figure 28.8 Alveoli. These sacs under the plasma membrane are acharacteristic that distinguishes alveolates from other eukaryotes (TEM).Flagella3 µm28.30.2 µmCONCEPT! Figure 28.9 Pfiesteria shumwayae, a dinoflagellate. Beatingof the spiral flagellum, which lies in a groove that encircles the cell,makes this alveolate spin (colorized SEM).
Episodes of explosive population growth, or blooms, in dinoflagellates sometimes cause a phenomenon called “redtide.” The blooms make coastal waters appear brownish red orpink because of the presence of carotenoids, the most common pigments in dinoflagellate plastids. Toxins produced bycertain dinoflagellates (such as Karenia brevis, which inhabitsthe Gulf of Mexico) have caused massive kills of invertebratesand fishes. Humans who eat molluscs that have accumulatedthe toxins are affected as well, sometimes fatally.the sporozoite cell contains a complex of organelles specializedfor penetrating host cells and tissues. Although apicomplexans are not photosynthetic, recent data show that they retaina modified plastid (apicoplast), most likely of red algal origin.Most apicomplexans have intricate life cycles with bothsexual and asexual stages. Those life cycles often require two ormore host species for completion. For example, Plasmodium,the parasite that causes malaria, lives in both mosquitoes andhumans (Figure 28.10).Historically, malaria has rivaled tuberculosis as the leadingcause of human death by infectious disease. The incidence ofmalaria was greatly diminished in the 1960s by insecticidesthat reduced carrier populations of Anopheles mosquitoes andby drugs that killed Plasmodium in humans. But the emergenceof resistant varieties of both Anopheles and Plasmodium has ledApicomplexansNearly all apicomplexans are parasites of animals, andsome cause serious human diseases. The parasites spreadthrough their host as tiny infectious cells called sporozoites.Apicomplexans are so named because one end (the apex) of" Figure 28.10 The two-host life cycle ofPlasmodium, the apicomplexan that causesmalaria.1 An infected Anophelesmosquito bites a person,injecting Plasmodiumsporozoites in its saliva.Are morphological differences between sporozoites,? merozoites, and gametocytes caused by differentgenomes or by differences in gene expression? Explain.Inside mosquito2 The sporozoites enter the person’sliver cells. After several days, the sporozoitesundergo multiple divisions and becomemerozoites, which use their apical complexto penetrate red blood cells (see TEM below).Inside humanMerozoiteSporozoites(n)8 An oocyst developsfrom the zygote in the wallof the mosquito’s gut. Theoocyst releases thousandsof sporozoites, whichmigrate to the mosquito’ssalivary gland.LiverLiver cellOocystMEIOSISApexZygote(2n)7 Fertilization occursin the mosquito’sdigestive tract, and azygote forms.Red bloodcellMerozoite(n)Red bloodcells3 The merozoites divideasexually inside the redblood cells. At intervals of48 or 72 hours (dependingon the species), largenumbers of merozoitesbreak out of the bloodcells, causing periodic chillsand fever. Some of themerozoites infect otherred blood cells.FERTILIZATIONGametesKeyHaploid (n)Diploid (2n)6 Gametes form from gametocytes; each malegametocyte produces several slender male gametes.0.5 µmGametocytes(n)4 Some merozoitesform gametocytes.5 Another Anopheles mosquito bi
protist phylogeny. All protists were once classified in a single kingdom, Protista, but advances in eukaryotic systematics have caused the kingdom to crumble. It has become clear that the kingdom Protista is in fact polyphyletic (see Figure 26.10): Some protists are more closely related to plants, fungi, or ani-
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the Walker Line [Billingsley and Locke, 1941], the Walker Belt [Stewart,1980],andtheWalkerLaneBelt[Carr, 1984]. For convenience, I use the term Walker Lane to encompass the zone of right-lateral faults that extends northward from the Garlock fault. Stewart [1980] brought to attention that the pattern of faulting in the Walker
Walker Lake, a terminal saline lake fed by the Walker River in Walker River Basin, Nevada, was once part of the ancient Pleistocene Lake Lahontan. Lake Lahontan covered much of Northwestern Nevada and began to recede from its last highstand, or peak 12,500ya, leaving Walker Lake and Pyramid Lake today as the main remnants today.
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LIVING AND NONLIVING THINGS. What is a LIVING thing? If something is LIVING - it is ALIVE ! x A plant is a living thing. x A n animal is a living thing. x YOU are a living thing. These are . living things: a tree . a cat a person . What can living th ings
WALKER HONEY FARM AND DANCING BEE WINERY, SOCIAL MEDIA STRATEGY OVERVIEW The purpose of social media for Walker Honey Farm and Dancing Bee Winery is to market the honey, mead and various products made at Walker Honey Farm. The farm is located in Rogers, Texas and has products sold in Brookshire, HEB and other retail outlets. The
UNIT 1 The pupils will be able to: LIVING AND NON-LIVING THINGS 1.1.1 group materials into living and non -living things . Note 1.1.2 state the characteristics of living things. 1.1.3 group living things into plants and an imals . 1.1.4 describe some external appearance of plants . Living and Non -living things
Unit 2A 1.3 Living and growing 1Bp1 Know animals or plants are living things. pages 33–36 pages 34–38 pages 21–25 3.1 Living and non-living 3.2 Living and non-living 3.5 Living things around me 6.2 Parts of a plant 6.7 What plants need 6.4 Where do plants come from? 1Bp2 Know that there are living things
