Chapter 28 Protists 1 Chapter 28 Protists Living Small kingdom to crumble Protists are diverse unicellular group of eukaryotes All protists were once classified in a single kingdom Protista but advances in eukaryotic systematics have caused the The kingdom Protista is in fact polyphyletic some protists are more closely related to plants fungi or other animals than they are to other protists Most biologists use the term protists only to refer to eukaryotes that are not plants animals or fungi Concept 28 1 Most eukaryotes are single celled organisms Unlike the cells of prokaryotes eukaryotic cells have a nucleus and other membrane bounded organelles such as mitochondria and the Golgi apparatus Structural and functional diversity in protists are very complex The organisms in most eukaryotic lineages are protists and most protists are unicellular Single celled protists are justifiably considered the simplest eukaryotes but at the cellular level many protists Unicellular protists carry out the same essential biological functions such as those carried out by organs but they do so using subcellular organelles not multicellular organs The organelles that protists most likely use are the nucleus ER Golgi apparatus and lysosomes Certain protists rely on organelles not found in most other eukaryotic cells such as contractile vacuoles that pump excess water from the protistan cell Some protists are photoautotrophs and contain chloroplast Some are heterotrophs absorbing organic molecules or ingesting larger food particles Other protists are mixotrophs combining photosynthesis and heterotrophic nutrition Some protists are only known to reproduce asexually others can also reproduce sexually or at least employ the sexual processes of meiosis and fertilization Endosymbiosis in Eukaryotic Evolution There is abundant evidence that much of protist diversity has its origin in endosymbiosis the process in which certain unicellular organisms engulf other cells which become endosymbionts and ultimately organelles in the Structural biochemical and DNA sequence data indicate that the first eukaryotes acquired mitochondria by engulfing an aerobic prokaryote Evidence also indicates that later in eukaryotic history a lineage of heterotrophic eukaryotes acquired an additional Endosymbiont a photosynthetic cyanobacterium that then evolved into plastids Plastids a major organelle found in cells of plants that manufacture and store important chemical compounds used by the cell This plastid bearing lineage gave rise to two lineages of photosynthetic protists or algae red algae and green host cell algae DNA of plastid genes in red algae and green algae closely resembles the DNA of cyanobacteria Chapter 28 Protists 2 Plastids in red algae and green algae are surrounded by two membranes Transport proteins in these membranes are homologous to proteins in the inner and outer membranes of cyanobacterial endosymbionts On several occasions during eukaryotic evolution red and green algae underwent secondary endosymbiosis they were ingested in the food vacuoles of heterotrophic eukaryotes and became endosymbionts themselves Five Supergroups of Eukaryotes Many of the so called amitochondriate organisms without conventional mitochondria and with few membrane bounded organelles than other protist groups are now classified in entirely different groups Because the root of the eukaryotic tree is not known all five supergroups are shown as diverging simultaneously from a common ancestor Excavata Some members of this supergroup have an Excavated groove on one side of the cell body Diplomonads and parabasalids have modified mitochondria Euglenozoans have flagella that differ in structure from those of other organsisms Chromalveolata Archaeplastida Rhizaria Unikonata This group includes some of the most important photosynthetic organisms on Earth such as diatoms and also includes the brown algae that form underwater kelp forests This group includes red and green algae along with land plants This group contains many species of amoebas most of which have pseudopodia extensions that can bulge from any portion of the cell used in movements and in the capture of prey Includes amoebas that have lobe or tube shaped pseudopodia as well as animals fungi and non amoeba protists that are closly related to animals or fungi According to one hypothesis unikons may have been the first group of eukaryotes to diverge from other eukaryotes Concept 28 1 Summary Most eukaryotes are single celled organisms Protists are more diverse than all other eukaryotes and are no longer classified in a single kingdom Most are unicellular Protists include photoautotrophs heterotrophs and mixotrophs Protists are characterized by a wide diversity of life cycles Mitochondria and plastids are thought to be descendants of bacteria that were engulfed by other cells and became endosymbionts The plastid bearing lineage eventually evolved into red algae and green algae Other protist groups evolved from secondary endosymbiosis events in which red algae or green algae were themselves engulfed In one hypothesis eukaryotes are grouped into five supergroups each a monophyletic clade Excavata Chromalveolata Rhizaria Arachaeplastidia and Unikonta Concept 28 2 Excavates include protists with modified mitochondria and protists with unique flagella Excavata a clade recently proposed based on morphological studies of the cytoskeleton Some members of this group also have an excavated feeding groove on one side of the cell body Molecular data indicate that the diplomonads the parabasalids and the euglenozoans are monophyletic Diplomonads and Parbasalids The protists in these two groups lack plastids and have modified mitochondria and are found in anaerobic Chapter 28 Protists 3 environments Diplomonads have modified mitochondria called mitosomes They lack electron transport chains and instead get their energy they need from anaerobic biochemical pathways Structurally diplomonads have two equal sized nuclei and multiple flagella Eukaryotic flagella are extensions of the cytoplasm consisting of bundles of microtubules covered by the cell s plasma membranes surface Prokaryotic flagella are filaments composed of the globular protein flagellin attached to the cell Parabaslaids also have reduced mitochondria called hydrogenosomes these organelles generate some energy anaerobically releasing hydrogen as a by product Euglenozoans Protists called euglenozoans belong to a diverse clade that