Protists
Protists are single celled
eukaryotes. A few forms are
multi-cellular.
• Protists often have a very
complicated internal structure:
a single cell must do all the
functions that we have many
different cell types to do.
• Protists can be divided into
plant-like, fungus-like, and
animal-like forms. This
division probably has little to
do with their evolutionary
history.
Protist Phylogeny
• Protists are very
diverse, and
probably
represent
several different
evolutionary
lineages, more
distinct from
each other than
plants, animals,
and fungi are.
Fungus-like Protists
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Water molds and slime molds
Like fungi, they are heterotrophs: they
get food and energy from other
organisms. Many are decomposers of
dead organisms.
Unlike fungi, the fungus-like protists
produce motile (moving) cells during
part of their like cycle. Also, these
protists surround and engulf bacteria
as food.
Slime molds live as separate cells
most of their lives, feeding on bacteria.
When conditions get harsh, they
aggregate into a multicellular slug,
which migrates to a new location. The
slug then forms a fruiting body that
generates spores. The spores from the
fruiting body are very hardy.
Slime Molds
• Slime molds
can be rather
large and
colorful.
More Fungus-like Protists
• The water molds include some
of the worst plant diseases.
• Phytphthora infestans causes
rot in plants. In the 1840’s,
Phytophthora caused the
potato blight in Ireland. The
Irish were poor and
overcrowded, and lived mostly
on potatoes. The blight
caused crop failures in several
years, resulting in the deaths
of of the population. Many
Irish came to the US at that
time.
Animal-like Protists
• The animal-like protists are
classified according to how
they move: amoeba-like,
ciliated, or flagellated.
• Amoeba-like protists move by
extending pseudopods, part of
their cytoplasm, then pulling
the rest of the cell along
behind. The can surround and
engulf their food this way.
• Some amoeba-like protists
have hard shells. The
foraminiferans are covered in
calcium carbonate—chalk,
which we use on chalk boards.
Amoebas in Action
Ciliated Protists
• Cilia are small hairs
surrounding the protist’s body.
The cilia beat in a
synchronized pattern to cause
movement.
• Paramecium is a typical ciliate.
It has a gullet to swallow food,
and a contractile vacuole to get
rid of excess water.
• Genetics: the DNA used for
sexual reproduction is stored
in the small micronucleus. A
copy of this information is used
to run the cell: the copy is kept
in the much larger
macronucleus.
Flagellates
• Flagellates have a small
number of long flagella, long
whiplike hairs that beats to
propel the cell.
• Some nasty parasites are
flagellates, including Giardia
lamblia, which causes diarrhea
and which is found in most of
the surface waters of the US.
Another is Trypanosoma
brucei, which causes sleeping
sickness in Africa. Also
Trichmonas vaginalis, a
sexually transmitted disease.
Sporozoans
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Sporozoans are animal-like protists
that have part of their life cycle inside
the cells of their hosts.
The most important example is
Plasmodium, the parasite that causes
malaria. Malaria kills 1-2 million
people each year.
Mosquitoes are part of the life cycle.
They suck blood from infected
humans, ingesting the sporozoans.
The sporozoans undergo sexual
reproduction in the mosquito’s gut.
When the mosquito bites another
person, the sporozoans infect the
blood and liver of the host. The
parasites multiply asexually inside the
red blood cells, destroying them as
they leave.
Algae
• The plant-like protists are
called algae. Most are
single-celled, but a few
form large multicellular
seaweed.
• The plant-like protists
have chloroplasts. Some,
like the Euglena, also
have flagella for
movement.
• Some, such as the
diatoms, have calcium
carbonate (chalk) or silica
shells.
Algae Blooms
• Algae blooms are the
sudden growth of a large
population of single
celled algae. They occur
near the coastlines, with
the algae feed on fertilizer
runoff and sewage.
Algae blooms deplete the
oxygen in the water,
killing fish and other
organisms. Some of the
algae also secrete toxins.
Red tide and Pfiesteria
are examples of this.
Multicellular Algae
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The multicellular algae are commonly
called seaweed. They can be
classified into red, brown, and green
algae. All have chloroplasts, but the
pigments in the chloroplasts vary,
giving the different colors.
Some, like kelp, are very large and
contain several different types of cells
and tissues to do specialized work.
These include leaves for
photosynthesis, gas-filled bladders for
buoyancy, the root-like holdfast, and
tubes to transport nutrients throughout
the body.
Algae contain the polysaccharide
“agar”, which is tasteless and is used
to thicken foods such as soft ice
cream.
Green Algae
• The green algae are the
ancestors of plants. The term
“plant” implies living on the
land or derived from plants
living on the land. Green
algae and plants have very
similar chloroplasts.
• Green algae have some
unicellular species (such as
Chlamydomonas), some
simple colonies (such as
Volvox), and some relatively
complex multicellular
organisms (such as
Acetabularia).
• Green algae are haploid most
of their live cycle, with only a
very short diploid phase.