• Eukaryotes that are not plants, animals, or fungi have traditionally been called protists.

• The term does not describe a formal taxonomic group, but is a convenience term.

• Eukaryotes are monophyletic.• They are thought to be more closely related to

Archaea than to Bacteria.• But mitochondria and chloroplasts are clearly

derived from bacterial lineages.

Events in the origin of the eukaryotic cell:• Cell wall was lost• Cytoskeleton formed• Nuclear envelope developed• Digestive vacuoles appeared• Mitochondria formed by endosymbiosis

•Eukaryotes evolved as “symbiotic consortiums” ofprokaryotic cells•Have observed similar behavior in amoebas containing symbiotic bacteria•DNA evidence: mitochondria, cholorplast, centriole found to have own DNA- different from eukaryotic cell- more like endosymbiotic bacteria

• Development of chloroplasts occurred in a series of endosymbioses.

• Primary endosymbiosis: a cyanobacterium was engulfed. Chloroplast has two membranes. Remnants of peptidoglycan cell wall can be found in glaucophytes.

• Primary endosymbiosis also gave rise to chloroplasts of red algae, green algae, and land plants.

• Secondary endosymbiosis: a eukaryote engulfed a green alga cell which became a chloroplast.

• Chloroplast has three membranes.• Tertiary endosymbiosis: a dinoflagellate lost its

chloroplast and took up another protist that had acquired its chloroplast through secondary endosymbiosis.

There are five major clades of protistan eukaryotes.• There is enormous diversity within these groups.• Most are unicellular and microscopic; but some are

multicellular and some are quite large (e.g., giant kelp).

• Protists were traditionally classified on the basis of life histories and reproductive features.

• Today, electron microscopy and gene sequencing reveal more evolutionary patterns.

• Lateral gene transfer may be a complicating factor.

Alveolates• Sacs called alveoli lie just beneath plasma

membrane. • All unicellular, most are photosynthetic.

• Dinoflagellates• Apicomplexans• Ciliates

Dinoflagellates: mostly marine; photosynthetic; important primary producers in the oceans.

• Some species cause red tides.

• Some are endosymbionts with invertebrates (e.g., corals).

Apicomplexans: obligate parasites• Apical complex—organelles at the tip of the cell;

help it invade host tissue.• Elaborate life cycles featuring asexual and sexual

reproduction and life stages in different hosts.• Plasmodium is the causative agent of malaria.

Ciliates: numerous hairlike cilia (identical to eukaryotic flagella).

• Heterotrophic; some have photosynthetic endosymbionts.

• Complex body form.

• The ciliate Paramecium is covered by a flexible pellicle with trichocysts—defensive organelles that can explode as sharp darts.

• Cilia provide precise locomotion.• Lives in fresh water: contractile vacuoles excrete

excess water taken in by osmosis.• Also has digestive vacuoles.

• Excavates

• Diplomonads

• Parabasalids

• Heteroloboseans

• Euglenids

• Kinetoplastids

• Diplomonads and parabasalids: unicellular and lack mitochondria (a derived condition).

• Giardia lamblia causes the intestinal disease giardiasis.

Heteroloboseans: amoeboid body form• Naegleria has two stages, one with amoeboid cells

and the other with flagellated cells.• Some species can enter the human body and cause

a fatal disease of the nervous system.

Euglenids and kinetoplastids: have flagella; mitochondria with disc-shaped cristae.

• Some euglenids are always heterotrophic; some are photosynthetic but can loose their pigments and feed on organic matter.

Stramenopiles• Rows of tubular hairs on the longer of their two

flagella• Some lack flagella but are descended from

ancestors that possessed them.

• Diatoms

• Brown algae

• Oomycetes

Diatoms: unicellular; some species associate in filaments

• Lack flagella except in male gametes.• Deposit silicon dioxide in two-piece cell walls;

intricate patterns are unique to each species.• Reproduce both sexually and asexually.• Abundant in oceans and fresh waters and are major

photosynthetic producers.

Brown algae: brown color from the carotenoid fucoxanthin.

• Multicellular, marine.• Attached forms develop holdfasts with alginic acid

to glue them to rocks. Alginic acid is used by humans as an emulsifier in ice cream, cosmetics, and other products.

• Giant kelps may be up to 60 meters long.

Oomycetes: water molds, downy mildews• Once classed as fungi• All are absorptive heterotrophs—secrete enzymes

that digest large food molecules into smaller molecules that they can absorb.

• Water molds—all aquatic and saprobic (feed on dead organic matter).

Rhizaria• Unicellular and mostly aquatic; have long, thin

pseudopods.• Make up a large component of ocean sediments.

• Cercozoans

• Foraminiferans

• RadiolariansCercozoans: soil and aquatic organisms• One group has chloroplasts derived from a green

alga by secondary endosymbiosis—and that chloroplast contains a trace of the alga’s nucleus.

• Foraminiferans: external shells of calcium carbonate

• Threadlike, branched pseudopods extend through microscopic holes in the shell and form a sticky net, used to catch smaller plankton.

• Accumulations of shells have produced much of the world’s limestone.

Radiolarians: radial symmetry; thin, stiff pseudopods reinforced by microtubules.

• Pseudopods increase surface area of the cell, and help it stay afloat.

• Secrete glassy endoskeletons

Amoebozoans• Amoeboid body form; lobe-shaped pseudopods

• Loboseans

• Plasmodial slime molds

• Cellular slime molds

Loboseans: feed by phagocytosis, engulfing smaller organisms and particles with pseudopods.

• Many are adapted to living on the bottoms of lakes and ponds.

• Testate amoebas live in shells made from sand grains or secreted by the organism.

Plasmodial slime molds: • Vegetative state—plasmodium—a mass of

cytoplasm with no cell walls and many diploid nuclei (a coenocyte).

• Moves by cytoplasmic streaming and engulfs food particles by endocytosis.

Asexual reproduction among the protists:• Binary fission—equal splitting by mitosis

followed by cytokinesis.• Multiple fission—splitting of one cell into

more than two cells.• Budding—outgrowth of a new cell from the

surface of an old one.• Sporulation—formation of specialized cells

that can develop into new individuals.

Reproduction in Paramecium:• Two types of nuclei—one macronucleus and one to

several micronuclei.• Asexual reproduction—all nuclei are copied before

the cell divides. • Conjugation—two individuals fuse and exchange

genetic material; a sexual process, but not reproductive.

Some protists have alternation of generations.

• A multicellular, diploid, spore-producing organism gives rise to a multicellular, haploid, gamete-producing organism.

• When two haploid gametes fuse, a diploid organism is produced.

• The haploid organism, the diploid organism, or both may also reproduce asexually.

In the diploid organism, specialized cells called sporocytes divide meiotically to produce haploid spores.

• Spores develop into the haploid organism.• The haploid organism produces gametes by

mitosis and cytokinesis.• Gametes fuse to produce the diploid

organism.

Phytoplankton are important primary producers.• The diatoms perform about 1/5 of all carbon

fixation on Earth—about the same amount as the rainforests.

Some microbial eukaryotes are pathogens.• Plasmodium is a parasite in human red blood cells

and causes malaria, one of the world’s most serious diseases.

• Plasmodium has a complex life cycle that includes mosquitoes as an alternate host.

• Plasmodium is an extracellular parasite in the mosquito and an intracellular parasite in the human host.

Many microbial eukaryotes live as endosymbionts.• Some photosynthetic dinoflagellates live as

endosymbionts in corals. If the dinoflagellates die or are expelled, the coral is said to be bleached.

• If the corals don’t acquire new endosymbionts, they usually die or are damaged due to reduced food supply.

• Diatoms store energy as oil. Over millions of years, diatoms have died and sunk to the ocean floor, ultimately becoming petroleum and natural gas.

• Diatomaceous earth is sedimentary rock composed mostly of the silica cell walls of diatoms; used for insulation, filtration, metal polishing, and to kill insects (it clogs their breathing tubes).

• Foraminiferan shells make up extensive limestone deposits, and some sandy beaches.

• The shells are preserved as fossils in marine sediments and are used to estimate past environmental parameters such as sea temperature.

• Fossil shells in rocks are used in stratigraphy and dating of the rocks.

Dinoflagellates are important components of marine ecosystems.

• Corals and other species depend on symbiotic dinoflagellates for photosynthesis.

• They are important primary producers, providing food for many species and much of the atmospheric oxygen.

• Some species produce beautiful bioluminescence in the oceans.