Protists –
The Simplest Eukaryotes
Chapter 20
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
20.1 Harmful Algal Blooms
 Protist
• Eukaryote that is not a fungus, animal, or plant
• Aquatic protist
• Single-celled and multicellular autotrophs
• Heterotrophs
• Photosynthetic protists
• Take up carbon dioxide and release oxygen
• Food source for aquatic animals
Harmful Algal Blooms
 Algal bloom
• Population explosion of tiny aquatic producers
• Extra nutrients are present and the cells multiply fast
• Threatening to the ecosystem
• “Red Tide”  algal bloom of reddish pigment protists
• Not all algal blooms are red
• Not all events are related to tidal changes
Harmful Dinoflagellate Blooms
 Dinoflagellate bloom – Karenia brevis
• Toxin  chemical that is made by one organism and
harms another
• Brevetoxin  interferes with animal nerve cells by
binding to a protein in their cell membrane
• Sickens and kills marine invertebrates, fish, sea turtles,
sea birds, dolphins, and manatees
• Human affects
• Tainted food  intestinal problems and nervous system
symptoms such as headache, loss of coordination, and
paralysis
• Inhaling toxin  constricted airways, difficulty breathing,
and irrigated nasal membranes, increase risk lung cancer
20.2 A Collection of Lineages
 Protists
• The simplest eukaryotes
• Most are single-celled
• Some are multicelled and large
Protist Structure
 Protist cells have a nucleus (eukaryotes)
 Most have one or more mitochondria
 Many have chloroplasts that evolved from
cyanobacteria or from another protist
 Dominant stage of life cycle: Haploid or diploid
Protist Life Cycle
 Most reproduce both sexually and asexually
 Malaria
• Haploid stage the divides in human blood and liver
cells
• Only diploid stage in the parasite’s life is the zygote
• Zygote undergoes meiosis within a few hours of
fertilization
 Silica-shelled cells – diatoms
• Diploid-dominant life cycles
• Diploid cells divide by mitosis and only the games
are haploid
Protist Life Cycles
Malaria
Multicelled
algae
Diatoms
Protists Nutritional Modes
 Heterotrophs, autotrophs, or switch between
nutritional modes
• Heterotrophic  decomposers, prey on smaller
organisms, or live inside larger organisms
• Autotrophs  have chloroplasts
Protist Diversity
 Gene sequencing shows that protists are not a
monophyletic group
• Collection of lineages, some only distantly related
 Evolutionary connections among groups are
gradually becoming clear
Protist Evolutionary Tree
**Text Figure 20.2 p.312**
Comparing Prokaryotes and Eukaryotes
Key Concepts:
SORTING OUT THE PROTISTS
 Protists include many lineages of single-celled
eukaryotic organisms and their closest
multicelled relatives
 Gene sequencing and other methods are
clarifying how protist lineages are related to one
another and to plants, fungi, and animals
20.3 Ancient Flagellates
 Flagellated protozoans
• Single-celled heterotrophs with one or more flagella
• No cell wall, pellicle retains shape
• Pellicle  layer of proteins that gives shape to many
unwalled, single-celled protists.
• Include the most ancient eukaryotic lineages:
diplomonads and parabasalids
• Both groups lack mitochondria
• Both include species that infect humans
• Example: Trichomonas vaginalis
• Parabasalid that infects the reproductive tract 
disease trichomoniasis (6 million infected)
Euglenoids and Kinetoplastids
 Most euglenoids live in freshwater - Euglena
• Flagellated protozoan with multiple mitochondria
• Some have chloroplasts that arose by secondary
endosymbiosis from a green alga
• May be heterotrophic
• Contractile vacuoles expel excess water
• organelle that collects and expels excess water
 Most Trypanosomes are parasites
• Have a single giant mitochondrion
• Membrane-encased flagellum
• Biting insects are vectors
Disease-Causing Flagellates
 Trichomonas vaginalis
 Trypanosoma brucei
 Trypanosomes act
as vectors for
human disease
Euglena
20.4 Mineral-Shelled Protozoans
 Foraminiferans and radiolarians
• Single-celled heterotrophs with a secreted shell
• Many openings for pseudopods
20.4 Mineral-Shelled Protozoans
 Foraminiferans and radiolarians
• Foraminiferans
• Heterotrophic single-celled protists with a porous
calcium carbonate shell and long cytoplasmic
extensions
• Ex. Component of plankton
• Community of tiny drifting or swimming organisms
with small photosynthetic protists inside them
• Radiolarians
• Heterotrophic single-celled protist with a porous
shell or silica and long cytoplasmic extensions
• Ex. Component of plankton in tropical water
Key Concepts:
GROUPS WITH ANCIENT ROOTS
 Two of the earliest lineages of eukaryotic cells
are known informally as flagellated protozoans
 Foraminiferans and radiolarians are another
ancient lineage
20.5 Alveolates
 All alveolates have tiny sacs (alveoli) beneath
the plasma membrane
• All single-celled
 Examples:
• Ciliates, dinoflagellates, and apicomplexans
alveolus
plasma
membrane
(blue)
p.322
Ciliates
 Aquatic predators and parasites with many cilia
• Example: Paramecium
• Food goes in gullet  endocytosis
• Digestive waste  exocytosis
• Contractile vacuoles that collect and squirt out
excess water
• Macronucleus  control daily activities
• Micronucleus  function in sexual reproduction
• Sexual reproduction  swap of micronuclei
between two cells
• Mostly reproduce ASEXUALLY
Fig. 20.7a, p.323
contractile vacuole
pellicle
Fig. 20.7b, p.323
Dinoflagellates
 Aquatic heterotrophs and autotrophs with a
cellulose covering, two flagella, and Single-cell
• Photosynthetic protists cause algal blooms in
nutrient-rich water
 Vast majority are marine plankton
Apicomplexans
 Heterotrophs: Parasites living in animal cells
•
•
•
•
Cell-piercing structure made of microtubules
Reproduce sexually and asexually in host cells
Only gametes have flagella
Example: Plasmodium (malaria)
20.6 Malaria
 Plasmodium species cause malaria
 Leading cause of death – kills 1.3 million/year
sporozoites
g Plasmodium zygotes
develop inside the gut of
female mosquitoes. They
become sporozoites,
a Mosquito bites human,
which migrate to the
bloodstream carries the
insect’s salivary
sporozoites to liver.
glands.
d Some of the
merozoites enter
liver, cause more
malaria episodes.
f Female mosquito
bites, sucks blood
from infected human.
Gametocytes in blood
enter her gut, mature
into gametes, which
fuse to form zygotes.
e Others develop
into male, female
gametocytes that are
released
intobloodstream.
male gametocyte in
red blood cell
sporozoites
b Sporozoites
asexually reproduce
in liver cells.
merozoite
c Offspring
(merozoites) enter
blood, invade red
blood cells,
reproduce
asexually. They can
do so often, over a
prolonged period.
Disease symptoms
(fever, chills,
shaking) get more
and more severe.
Fig. 20.9a, p.324
sporozoites
merozoites about
to rupture a
red blood cell
Fig. 20.9b, p.324
Key Concepts:
THE ALVEOLATES
 Ciliated protozoans, dinoflagellates, and
apicomplexans are single-celled
photoautotrophs, predators, and parasites with a
unique layer of tiny sacs under their plasma
membrane
20.7 Stramenopiles
 “straw-haired”, shaggy flagellum that occurs
during the life cycle
 Defined by genetic similarities
 Types
• Water molds
• Diatoms
• Brown algae
Water Molds




Grows as nutrient-absorbing filaments
Heterotrophs
Most decompose organic debris in aquatic habitats
Few are parasites with economic effects  “plant
destroyers”
 Closest relatives
• Diatoms and brown algae
• Both have chloroplasts with a brown accessory
pigment (fucoxanthin) that tints them olive, golden,
or dark brown.
• Chloroplast evolved form red algae
Diatoms
 Single cell, Photosynthetic cells
 Brown accessory pigments in its chloroplasts
and a two-part silica shell
 Live in cool waters, damp soil, lakes, seas
 Hard parts accumulate as mineral deposits
• Diatoms (silica shells): Diatomaceous earth
• Used in filters, abrasive cleaners, and as
insecticides that do not harm vertebrates
Brown Algae
 Multicelled, photosynthetic stramenopiles
• Brown accessory pigment in its chloroplast
• Live in temperate or cool seas
• Include microscopic strands and giant kelps
• Kelp  the largest protists; pacific northwest,
shelter organisms
• Commercial uses
• Thickeners, emulsifiers, and suspension agents
• Ice cream, pudding, jelly beans, toothpaste,
cosmetics.
Fig. 20.12, p.326
Key Concepts:
THE STRAMENOPILES
 Chrysophytes, diatoms, and brown algae are
stramenopiles, most of which are
photoautotrophs
20.8 Red Algae
 Photosynthetic protist that deposit cellulose in its
cell wall
 Stores sugars as starch
 Has chloroplast containing chlorophyll a (absorb
red/violet light) and red pigments called
phycobilins (absorb green light)
• More phycobilins in deeper water red algae
 Can live at greater depths than brown and green
algae
Red Algae
 Can survive in deep water (phycobilins)
• Chloroplasts evolved from cyanobacteria
 Red algae-like plastids occur in dinoflagellates
and apicomplexans
 Most red algae are multicelled
• Cultivated for commercial products
Red Algae
 Commercial use
• Agar  Keep baked goods and cosmetics moist,
vegetarian substitutes for gelatin
• Carrageenan  added to soy milk and dairy products
• Nori  dry sheets of red alga Porphyra, wraps kinds
of sushi
• Alternation of generations
• Of land plants and some protists, a life cycle in
which haploid and diploid multicelled bodies
form
• Nori is the haploid body or gametophyte
Life Cycle:
Porphyra
Gametophyte
• gameteproducing
haploid body
Sporophyte
• sporeforming
diploid body
20.8 Green Algae





Photosynthetic protists
Deposits cellulose in its cell wall
Have chloroplasts with chlorophylls a and b
Store carbohydrates as starch grains
Single-cell green algae partner with fungi to form
lichens
 Studies of green algae helped biologist
understand mechanisms of photosynthesis
• Determine the most effective wavelengths of light
• Clarify the Calvin-Benson cycle
Green Algae
Chlamydomonas
• Flagellated single-celled green alga in fresh water
Chlamydomonas Life Cycle
1.
Haploid cells reproduce asexually when conditions
favor growth
2. When nutrients are scares  gametes form by
mitosis
3. Fusion of two gametes produces a diploid zygotes
4. When conditions are favorable  zygote undergoes
meiosis and produces four haploid, flagellated cells
Life Cycle: Chlamydomonas
Single celled green alga
Key Concepts:
CLOSEST RELATIVES OF LAND PLANTS
 Red algae, green algae, and land plants all have
a cell wall made of cellulose, store sugars as
starch, and have chloroplasts that evolved from
a cyanobacterial ancestor.
• Thought to be descents form a common ancestor
 Red algae and green algae are photosynthetic
single cells and multicelled forms
 One lineage of multicelled green algae is the
closest living relatives of land plants
20.9 Amoebozoans
 Few have a cell wall, shell, or pellicle
 Most undergo dynamic changes in shape
• Quickly send out pseudopods, move about, and
capture food
 Amoebas (single cells) and slime molds (“social
amoebas”)
• Heterotrophic, free-living
 Fungal and animal signaling mechanisms may
have started in amoebozoan ancestors
20.9 Amoebozoans
 Amoebas (single cells)
• Extend pseudopods to move and to capture prey
• Some aid in their host’s digestive process
• Some cause disease
• 50 million people/year are affect by amebic
dysentery after drinking water contaminated
with pathogenic amoebas
Slime Molds
 Plasmodial slime molds
• Feed as a multinucleated mass
• Can be as big as a dinner plate
• Forms a spore-bearing structure when
environmental conditions become unfavorable
 Cellular slime molds
• Feed as a single cell
• Individual amoeba-like cells aggregate when food
is scarce (mobile “slug”)
Life Cycle: Cellular Slime Mold
a Spores give rise
to amoeboid cells.
e A fruiting
body forms
with resting
spores atop
a stalk.
Mature
fruiting
body
b Cells feed
and multiply
by mitosis
c When food
is scarce, cells
aggregate.
Migrating
slug
stage
d The cells form a
slug. It may start to
develop as a fruiting
body right away, or
migrate about. In the
slug, cells become
prestalk ( red) and
prespore ( tan) cells.
Fig. 20.18, p.329
Amoebozoans
Key Concepts: DISTANT RELATIVES OF
FUNGI AND ANIMALS
 A great variety of amoeboid species formerly
classified as separate lineages are now united
as the amoebozoans
 They are the closest living protistan relatives of
fungi and animals
Animation: Amoeboid motion
Animation: Body plan of Euglena
Animation: Cellular slime mold life cycle
Animation: Ciliate conjugation
Animation: Green alga life cycle
Animation: Paramecium body plan
Animation: Red alga life cycle