Unicellular eukaryotes
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Unicellular Eukaryotes. Chapter 11. Emergence of Eukaryotes . First evidence of life dates to 3.5 billion years ago. First cells were bacteria-like. Emergence of Eukaryotes . Origin of complex eukaryote cells Most likely symbiosis among prokaryotic cells.

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Unicellular Eukaryotes

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Unicellular Eukaryotes

Chapter 11

Emergence of Eukaryotes

  • First evidence of life dates to 3.5 billion years ago.

    • First cells were bacteria-like.

Emergence of Eukaryotes

  • Origin of complex eukaryote cells

    • Most likely symbiosis among prokaryotic cells.

      • Modification of engulfed prokaryote into an organelle: Primary endosymbiosis.

      • Aerobic bacteria engulfed by bacteria.

      • May have become mitochondria found in most modern eukaryotic cells.

      • Engulfed photosynthetic bacteria evolved into chloroplasts.

      • Descendants in green algae lineage gave rise to multicellular plants.

Emergence of Eukaryotes

  • Other groups apparently originated by secondary endosymbiosis.

    • One eukaryotic cell engulfed another eukaryotic cell and the latter became transformed into an organelle.


  • Protozoans are an extremely diverse assortment of unicellular eukaryotes.


  • Protozoa

    • Lack a cell wall

    • Have at least one motile stage in life cycle

    • Most ingest their food

    • Carry on all life activities within a single cell.

    • Can survive only within narrow environmental ranges.

    • Very important ecologically.

    • At least 10,000 species of protozoa are symbiotic in or on other plants or animals.

      • Relationships may be mutualistic, commensalistic, or parasitic.


  • Protozoans are more diverse than all other eukaryotes.

    • No longer classified in a single kingdom.

    • Recently shown that there are at least seven or more clades.

      • May be more than 60 monophyletic eukaryotic clades.

  • “Protozoa” is now used informally without implying phyletic relationship.

Cladogram of the Major Divisions of Organisms

Nutrition in Protozoans

  • Autotrophs - contain chloroplasts.

  • Heterotrophs - absorb organic molecules or ingest larger food particles.

    • Phagocytosis

  • Mixotrophs - combine photosynthesis and heterotrophic nutrition.

Protozoans Occur in Diverse Habitats

  • Freshwater and marine species.

    • Require moisture

  • Free-living and symbiotic species.

    • Mutualistic – benefits both.

    • Commensalistic – one benefits, the other is neutral.

    • Parasitic – one benefits at a cost to the other.

Locomotion in Protozoans

  • Protists have three main methods of locomotion.

  • Some protists, like this Euglena, have one or two long flagella that they can whip around an propel themselves through the water.

Locomotion in Protozoans


Locomotion in Protozoans

  • Some, like this Tetrahymena, are covered with numerous, but shorter, cilia that facilitate movement and/or feeding.

Locomotion in Protozoans


Locomotion in Protozoans

  • Others use ameboid movement to get around.

    • A pseudopod is extended forward, followed by the rest of the organism.

Locomotion in Protozoans


Form & Function - Locomotion

  • Cilia and flagella share an internal structure of the axoneme (extending beyond the cell) consisting of 9 pairs of microtubules surrounding a central pair.

  • Inside the cell, the kinetosome consists of 9 triplets of microtubules just like centrioles.

Form & Function - Locomotion

  • Pseudopodia are extensions of the cell cytoplasm used for locomotion.

Form & Function - Locomotion

  • Lobopodia – large blunt extensions of the cell body.

  • Filopodia – are thin extensions, usually branching.

  • Reticulopodia – repeatedly rejoin to form a netlike mesh.

Form & Function - Locomotion

  • Axopodia – long thin pseudopodia supported by axial rods of microtubules.

Functional Components – Specialized Organelles

  • Nucleus – contains DNA.

  • Mitochondria – organelle used in energy production.

  • Golgi – part of the secretory system of the ER.

  • Plastids – organelles containing photosynthetic pigments.

  • Extrusomes – organelles that extrude something from the cell.


  • Holozoic feeders, or phagotrophs, ingest particles of food.

    • Food vacuole – the membrane-bound vesicle containing the food.

    • Food vacuoles fuse with lysosomes containing digestive enzymes.


  • Often, the site of phagocytosis is a definite mouth structure, the cytostome.


  • Saprozoic feeding (ingestion of soluble food) may be by pinocytosis or by transport of solutes across the cell membrane.

Excretion and Osmoregulation

  • Contractile vacuoles fill with fluid and then expel the fluid outside the cell.

    • Function in osmoregulation.

    • More common in freshwater species.


  • Fission is the cell multiplication process in protozoa.

    • Binary fission – one individual splits into two equal sized individuals.

    • Budding – progeny cell much smaller than parent.

    • Multiple fission – multiple nuclear divisions followed by multiple cytoplasmic divisions producing several offspring.


  • All of above accompanied by some form of mitosis.

    • Mitosis in protozoa divisions varies from metazoan mitosis.

      • Nuclear membrane often persists.

      • Spindle may form within the nuclear membrane.

      • Centrioles not observed in ciliates.

      • Macronucleus of ciliates elongates, constricts, and divides without mitosis (amitosis).


  • Many types of protists reproduce sexually as well as asexually.

    • Isogametes – all look alike.

    • Anisogametes – two different types.

  • Syngamy – gametes from two individuals fuse to form the zygote.

  • Autogamy – gametes from one individual fuse.

  • Conjugation – gametic nuclei are exchanged.


  • Many protists are able to survive harsh conditions through the formation of cysts, dormant forms with resistant outer coverings and a shutdown of metabolism.

Major Protozoan Taxa

  • After the eukaryotic cell evolved, diversification followed, resulting in many clades.

    • Opisthokonta is a very large clade characterized by a combination of flattened mitochondrial cristae and one posterior flagellum on flagellated cells.

      • Includes animals, fungi, chaonoflagellates and microsporidians.


  • The clade Stramenopila includes several groups of heterotrophs as well as certain groups of algae.

    • Most stramenopiles have a “hairy” flagellum paired with a “smooth” flagellum.


  • Water molds, diatoms, golden algae, brown algae.


  • The clade Viridiplantae contains unicellular and multicellular green algae, bryophytes and vascular plants.

    • Phylum Chlorophyta – single celled algae colonial forms.


  • Volvox is a colonial organism that shows a division of labor where most cells are somatic cells concerned with nutrition & locomotion, but a few germ cells are responsible for reproduction.




  • Phylum Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites. Kinetoplastids & Euglenids.

    • Persistence of nucleoli during mitosis.

    • Cell membrane contains microtubules to stiffen it into a pellicle.


  • Subphylum Euglenida have one or two flagella that emerge from a pocket at one end of the cell.

    • Contain chloroplasts surrounded by a double membrane – may have arisen by secondary endosymbiosis.


  • Subphylum Kinetoplastahave 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.

    • Others are parasitic.

      • Trypanosoma


  • Phylum Retortamonada includes commensal and parasitic unicells.

    • Lack mitochondria & Golgi


  • Phylum Retortamonada: Diplomonads:

    • Are adapted to anaerobic environments.

    • Lack plastids.

    • Lack mitochondria but may have mitochondrial genes in the nucleus.

  • Diplomonads have two nuclei and multiple flagella.

    • Giardia


  • Parabasalids move by means of flagella and an undulating part of the plasma membrane.

    • This clade may have diverged from the main eukaryotic clade very early.

    • Trichomonas


  • Members of the clade (superphylum) Alveolata have membrane-bounded sacs (alveoli) just under the plasma membrane.

    • Dinoflagellates, apicomplexans, ciliates.

Alveolata - Ciliates

  • Members of the phylum Ciliophora use cilia to move and feed.

  • Ciliates have large macronuclei and small micronuclei.

Alveolata - Ciliates

  • Ciliates are a large, varied group of protists.



Alveolata - Ciliates

  • Ciliates are structurally complex.

  • All ciliates have a kinety system made up of the cilia, kinetosomes and other fibrils.

  • Many have structures that can be expelled such as trichocysts and toxicysts.

    • Defensive function

Alveolata – Types of Ciliates

  • Suctorians – ciliates that lose cilia as adults, grow a stalk and become sessile.

    • Use tubelike tentacles for feeding.

  • Symbiotic ciliates – some commensal, others parasitic.

  • Free-living ciliates – may be swimmers, or sessile.

    • Stentor, Vorticella, Paramecium

Alveolata – Reproduction in Paramecium

  • Paramecium, as well as many other protists, reproduce asexually by binary fission.

Alveolata – Reproduction in Paramecium

  • Conjugation is a sexual process that produces genetic variation.

  • Conjugation is separate from reproduction which generally occurs by binary fission.

Alveolata – Reproduction in Paramecium


Alveolata – Dinoflagellates

  • Phylum Dinoflagellata is a diverse group of aquatic photoautotrophs and heterotrophs.

    • Abundant in both marine and freshwater phytoplankton.

Alveolata – Dinoflagellates

  • 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.

Alveolata – Dinoflagellates

  • Rapid growth of some dinoflagellates is responsible for causing “red tides,” which can be toxic to humans.

California Noctiluca Bloom


Alveolata – Dinoflagellates

  • Some dinoflagellates are bioluminescent.

  • Others live symbiotically with corals (zooxanthellae).

Alveolata – Apicomplexans

  • Apicomplexans are parasites of animals and some cause serious human diseases.

    • Named because one end, the apex, contains a complex of organelles specialized for penetrating host cells and tissues.

    • Have a non-photosynthetic plastid, the apicoplast.

Alveolata – Apicomplexans

  • Most apicomplexans have intricate life cycles with both sexual and asexual stages that often require two or more different host species for completion.


  • Amebas are found in fresh and salt water as well as moist soil.

  • An ameba feeds by wrapping a pseudopod around its food – phagocytosis.


  • Entamoebas are parasites of vertebrates and some invertebrates.

    • Entamoeba histolytica causes amebic dysentery in humans.


  • This Phylum has slender pseudopodia that extend through openings in the test, then branch and run together forming a net (reticulopodia).

  • Foraminiferans, or forams are named for their porous, generally multichambered shells, called tests.


  • Pseudopodia extend through the pores in the test.

  • Foram tests in marine sediments form an extensive fossil record.


  • Radiolaria refers to marine testate ameba with intricate skeletons.

    • They have very diverse and beautiful forms.

    • Useful for determining the age of rock strata.


  • The pseudopodia of radiolarians, known as axopodia radiate from the central body.

Phylogeny and Adaptive Diversification

  • Phylum Retortamonada

  • Phylum Diplomonadea

  • Phylum Parabasala

    • Order Trichomonadida

  • Phylum Euglenozoa

    • Subphylum Euglenida

      • Class Euglenoidea

    • Subphylum Kinetoplasta

      • Class Trypanosomatidea

  • Phylum Stramenopiles

  • Phylum Ciliophora

    Phylum Dinoflagellata

    Phylum Apicomplexa

    Class Gregarinea

    Class Coccidea

    Phylum Foraminifera

    Phylum Radiololaria

    Phylum Viridiplantae

    Phylum Amoebozoa

    Phylum Opisthokonta

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