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CHAPTER 29
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Protists
Chapter 29
Eukaryotic Origins
• Eukaryotic cells differ from prokaryotes
– Presence of a cytoskeleton
– Compartmentalization (nucleus and organelles)
• Appearance of eukaryotes in microfossils occurred
about 1.5 BYA
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• The nucleus and endoplasmic reticulum
arose from infoldings of prokaryotic cell
membrane
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• Many organelles
evolved via
endosymbiosis
between an
ancestral
eukaryote and a
bacterial cell
– Mitochondria
• Aerobic bacteria
engulfed by larger
bacteria
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• Endosymbiosis supported by
– DNA inside mitochondria and chloroplasts
• DNA similar to bacteria DNA in size and character
– Ribosomes inside mitochondria similar to
bacterial ribosomes
– Chloroplasts and mitochondria replicate by
binary fission – not mitosis
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Defining Protists
• Most diverse of the four eukaryotic
kingdoms
• United on the basis that they are not fungi,
plants, or animals
• Vary considerably in every other aspect
– Unicellular, colonial, and multicellular groups
– Most are microscopic but some are huge
– All symmetries
– All types of nutrition
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Working model for protists classification
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• Locomotion
– Flagella
• One or more
– Cilia
• Shorter and more numerous than flagella
– Pseudopodia (“false feet”)
• Nutrition
– Phototrophs
– Heterotrophs
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• Asexual reproduction
– Typical mode of reproduction
– Some species have an unusual mitosis
• Mitosis – equal size daughter cells
• Budding – one daughter cell smaller
• Sexual reproduction
– Meiosis is a major eukaryote innovation
– Union of haploid gametes which are produced by
meiosis
– Advantage in allowing frequent genetic recombination
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Diplomonads
•
•
•
•
Unicellular
Move with flagella
2 nuclei
Giardia
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Parabasalids
• Live in termite guts
– Host cellulose degrading bacteria
•
•
•
•
Trichomonas vaginalis – STD
Undulating membrane for locomotion
Use flagella
Lack mitochondria – derived trait
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Euglenozoa
• Among the earliest eukaryotes to possess
mitochondria
• 1/3rd have chloroplasts and are autotrophic
– May become heterotrophic in the dark
• Others lack chloroplasts and are heterotrophic
• All have a flexible pellicle
• No sexual reproduction
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reproduction or display.
• Euglena
– Two anterior (and unequal) flagella
– Contractile vacuoles – collect excess water
– Stigma – movement towards light
– Numerous small chloroplasts
– Concept of a single Euglena genus is now
being debated
a.
6.5 µm
© Andrew Syred/Photo Researchers, Inc.
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• Kinetoplastids
– 2nd major group in Euglenozoa
– Unique, single mitochondrion
– Trypanosomes cause human diseases
• African sleeping sickness – tsetse fly
• Chagas disease – skin contact with urine or blood
of infected wild animal
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• Difficult to control because organisms repeatedly change
their protective coat
– Release of sterilized flies
– Traps scented like cows but treated with insecticides
– Sequencing of genomes revealed core of common genes in all 3
– hope for single drug target
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Alveolata
• Flattened vesicles called
alveoli
• Dinoflagellates
• Apicomplexans
• Ciliates
• Common lineage despite
diverse modes of
locomotion
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Dinoflagellates
•
•
•
•
Photosynthetic, unicellular with flagella
Live in aquatic environments
Some are luminescent
Do not appear to be directly related to any other
phylum
• “Red tide” are “blooms” – fish, birds, and marine
mammals may die from toxins
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Apicomplexans
• Spore-forming animal parasites
• Apical complex is a unique arrangement of
organelles at one end of the cell
– Enables the cell to invade its host
• Plasmodium causes malaria
– Complex life cycle – sexual, asexual, different
hosts
– Eradication focused on eliminating mosquito
vector, drug development, vaccines
• DDT-resistant mosquitoes
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Other apicomplexans
• Toxoplasma gondii
– Causes infections in
humans with
immunosuppression
– Can cross placental barrier
to harm fetus
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Ciliates
• 3rd group of apicomplexans
• Feature large numbers of cilia arranged in
longitudinal rows or spirals around the cell
• Pellicle – tough but flexible outer covering
• 2 types of nuclei
– Micronucleus – without will reproduce asexually
– Macronucleus – essential for function
• Have two types of vacuoles
– Food vacuoles – digestion of food
– Contractile vacuoles – regulation of water balance
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Stramenopila
• Brown algae, diatoms, and oomycetes
• Very fine hairs on their flagella
– A few species have lost their hairs during
evolution
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Brown algae
• Conspicuous seaweeds
of northern regions
• Life cycle involves
alternation of generations
– Sporophyte – multicellular
and diploid
– Gametophyte –
multicellular and haploid
• Not plants
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• Diatoms
– Phylum Chrysophyta
– Photosynthetic, unicellular organisms
– Unique double shells made of silica
– Some move using raphes
• Two long grooves lined with vibrating fibrils
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Oomycetes
•
•
•
•
“Water molds”
Either parasites or saprobes
Were once considered fungi
Motile zoospores with two unequal flagella
– Produced asexually
• Undergo sexual reproduction
• Found in water or on land
• Phytophthora infestans
– Irish potato famine (1845–1847)
– 400,000 people died
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Rhodophyta
• Red algae range from microscopic to very large
• Lack flagella and centrioles
• Have accessory photosynthetic pigments within
phycobilisomes
• Origin has been a source of controversy
– Tentatively, treated as a sister clade of Chlorophyta
(green algae)
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Choanoflagellida
• Most like common ancestor of all animals
• Single emergent flagellum, surrounded by
funnel-shaped contractile collar
– Structure matched in sponges
• Use collar to feed on bacteria
• Have a surface tyrosine kinase receptor
found in sponges
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Protists Without
a Clade
• Amoebas are paraphyletic
– Rhizopoda (True amoebas)
• Move by means of cytoplasmic projections called
pseudopods
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– Actinopoda (Radiolarians)
• Glassy exoskeletons made of silica
• Needlelike pseudopods
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• Foraminifera
– Heterotrophic marine protists
– Pore-studded shells called tests, through
which thin podia emerge
– Use podia for swimming and feeding
– Limestones are rich in forams
• White Cliffs of Dover
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• Slime molds
– Were once considered fungi
– Include two lineages
– Plasmodial slime molds
• Huge, single-celled, multinucleate, oozing masses
– Cellular slime molds
• Single cells combine and differentiate, creating an
early model of multicellularity
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• Plasmodial slime molds
– Stream along as a plasmodium
• Nonwalled, multinucleate mass of cytoplasm
• Form called feeding phase
– Ingests bacteria and other organic material
– When food or moisture is scarce, organism
forms sporangia, where spores are produced
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• Cellular slime molds
– Important group for the study of cell
differentiation because of their relatively
simple developmental systems
– Individual organisms behave as separate
amoebas
– Move through soil ingesting bacteria
– When food is scarce, organisms aggregate to
form a slug
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