Chapter 18
Protists
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Protists are the simplest eukaryotes
Section 18.1
Classifying protists is difficult – they are a paraphyletic group. Originally, protists were defined as eukaryotes that are not plants, fungi or animals.
Figure 18.1
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Protists are extremely diverse
Section 18.1
Each protist phylum is so different from the others that they may actually be different kingdoms. Evolutionary relationships among protists are being clarified by molecular sequence data.
Figure 18.1
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Protists are organized into three groups
Section 18.1
Biologists traditionally classify protists in terms of the kingdom they most closely resemble:
Figure 18.1
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Protists are ecologically important
Section 18.1
Figure 18.8
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Protists are medically important
Section 18.1
Disease-causing protists include Cryptosporidium, which is carried in feces and contaminates swimming water.
Swimmers who ingest this protist become ill with vomiting, diarrhea, cramps, and dehydration.
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Protists are the most ancient eukaryotes
Protists are helpful in piecing together the development of modern-day eukaryotic cells.
Figures 18.1, 3.8. 3.9
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Section 18.1
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Protists help trace endosymbiosis events
Section 18.1
Endosymbiosis explains the origin of mitochondria and chloroplasts, which developed from free-living bacteria.
Figure 18.2
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Protists show primary endosymbiosis
Section 18.1
Chloroplasts in red algae, green algae, and plant cells have two membranes, indicating they developed from a single endosymbiosis event.
Figure 18.2
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Protists show secondary endosymbiosis
Section 18.1
Chloroplasts in brown algae and euglena have three membranes, indicating they developed from two successive endosymbiosis events.
Figure 18.2
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Volvox colonies show clues to multicellularity
Section 18.1
This Volvox colony is somewhere between a group of individuals and a multicellular organism.
Figure 18.11
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Single-celled protists show clues to multicellularity
Section 18.1
Single-celled Chlamydomonas protists closely resemble certain Volvox cells.
Single-celled Choanoflagellate protists closely resemble cells of the simplest multicellular animals, sponges.
Figure 15.11
Unicellular organism
Chlamydomonas
Choanoflagellate
Multicellular relative
Volvox
Sponge
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Many protists are photosynthetic
Section 18.2
Algae are aquatic, photosynthetic protists.
Common types include euglenoids, dinoflagellates, diatoms,�golden algae, brown algae, red algae, and green algae.
Figure 18.A
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Euglenoids are both autotrophic and heterotrophic
Section 18.2
These protists use their flagella to swim around their fresh water habitats.
Euglenoids have triple-membrane, green chloroplasts for photosynthesis when there is light, and in the dark feed on organic matter.
Figure 18.3
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Dinoflagellates are crucial in ocean food webs
Dinoflagellates are characterized by two flagella. They use them to whirl around in the ocean.
Some are photosynthetic, some live inside animals such as jellyfish, and some are bioluminescent.
They can overgrow and produce toxins, causing red tides.
Figures 18.4, 18.5
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Section 18.2
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Brown and golden algae contain yellowish pigments for photosynthesis
Section 18.2
Golden algae are unicellular or colonial autotrophs in light and heterotrophs in the dark.
Brown algae are the largest and most complex protists. They form giant underwater kelp forests.
Figures 18.6, 18.8
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Diatoms have silica cell walls
Section 18.2
Diatoms have yellowish pigments for photosynthesis and are abundant in all moist habitats.
Their cell walls are very intricate and give them unique shapes.
Figure 18.7
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Red algae can live in deep water
Section 18.2
Red algae have photosynthetic pigments that absorb red and blue wavelengths of light. These wavelengths do not dissipate in deep water.
People eat red algae and use the agar they produce as a thickening agent in many things.
Figure 18.9
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Green algae are close relatives of plants
Section 18.2
Green algae are a diverse group – some are microscopic, others are large and multicellular
They share many features with plants, including using chlorophyll a and b for photosynthesis and producing starch.
Figure 18.11
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Green algae reproduce like plants
Section 18.2
In alternation of generations, haploid gametes and diploid zygotes can both grow into adult organisms.
This life cycle, alternating between haploid and diploid forms, is only found in green algae and plants.
Figure 18.10
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Some heterotrophic protists resemble fungi
Section 18.3
Slime molds and water molds are heterotrophic protists that have filamentous feeding structures.
However, they are only distantly related to fungi in terms of DNA sequence.
Figure 18.12
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Slime molds are unicellular and multicellular protists
Section 18.3
Slime molds can exist as single cells or as large masses that behave like a multicellular organism.
Figure 18.13
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Cellular slime molds keep their individual cells
Section 18.3
Cellular slime molds, like those shown here, live as haploid cells until resources become limited. They then aggregate into a mobile “slug” and then a fruiting body, which produces spores.
Figure 18.13
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Plasmodial slime molds are one huge cell
Section 18.3
During their feeding stage, plasmodial slime molds form a plasmodium, which is a large cell containing thousands of diploid nuclei.
Figure 18.12
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Water molds are decomposers and parasites
Section 18.3
Water molds are decomposers and parasites.
They secrete digestive enzymes into their surroundings and absorb the nutrients.
Figure 18.14
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Water molds feed on plants and animals
Section 18.3
Some water molds ruin food crops, including potatoes, grapes and lettuce. Others grow on weak, dead, or dying aquatic organisms like fish.
Figure 18.14
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Protozoa are diverse heterotrophs
Section 18.4
Most protozoa are one-celled, heterotrophic, and motile. They are grouped together based on morphology and locomotion but are only distantly related to each other.
18.15, 18.16, 18.18
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Flagellated protozoa can cause disease
Section 18.4
Flagellated protozoa have one or more flagella, which they use to move around. They live in soil, oceans, and fresh water. Some are parasites that live in our bodies.
Figure 18.15
Trichomonas causes the STD trichomoniasis.
Trypanosoma causes African sleeping sickness.
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Termites rely on flagellated protozoa
Section 18.4
Trichonympha lives in the gut of termites. Bacteria that break down wood live in each of these unicellular protists. It is because of these bacteria within a protist that termites are able to digest wood.
Figure 18.15
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Amoeboid protozoa produce pseudopodia
Section 18.4
Amoeboid protozoa produce extensions known as pseudopodia, which are important in locomotion and capturing food. The amoeba shown here is consuming a ciliate
Figures 18.1, 18.16
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Some amoeboid protozoa cause disease
Section 18.4
Entamoeba species invade the human digestive tract and cause fever and severe diarrhea in humans. Infection often occurs through lake water.
Figures 18.1, 18.16
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Forams are marine amoeboid protozoa
Section 18.4
Foraminiferans (Forams) have calcium carbonate shells, which are used to date layers of rock.
Huge populations of forams live at the bottom of oceans.
Figure 18.17
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Radiolarians are close relatives of forams
Section 18.4
Radiolarians are among the oldest protozoans.
They have intricate shells made of silica.
Figure 18.17
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Ciliates are complex protozoa
Section 18.4
Ciliates are mostly unicellular protozoa characterized
by abundant hairlike cilia, which propel the organism and sweep food into the cell.
They have specialized cell structures such as food vacuoles, contractile vacuoles, and an anal pore. Some have 2 types of nuclei.
Figure 18.18
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Ciliates are diverse protozoa
Section 18.4
Some are symbionts that live in cattle or marine animals and help them digest food.
Some are parasites, such as the species that causes white spots on fish.
Others, such as Paramecium and Stentor, are free-living.
Figure 18.18
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Apicomplexans are nonmotile parasites
Section 18.4
The apicomplexans have a special cell structure that helps them attach to and invade host cells.
Cryptosporidium, the species shown here, causes waterborne diseases.
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Malaria is caused by an apicomplexan protist
Section 18.4
Plasmodium is a protist carried by mosquitoes. When transmitted to humans it infects the red blood cells.
Figure 18.19
SEXUAL CYCLE
ASEXUAL CYCLE
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Protist classification is changing
Section 18.5
New research based on genetic sequences is helping to assign each protist species into a lineage with its closest relatives.
Table 18.1
TABLE 18.1 Proposed Eukaryotic “Supergroups”: A Summary
Supergroup | Distinguishing Features | Examples |
Archaeplastida (a) | Photosynthetic eukaryotes with chloroplasts derived from primary endosymbiosis | Red algae, green algae, land plants |
Opisthokonta (b) | Motile cells with one flagellum | Choanoflagellates, animals, fungi |
Chromalveolata (c) | Chloroplasts (if present) are derived from secondary endosymbiosis | |
Alveolates | Flattened sacs (alveoli) beneath cell membrane | Dinoflagellates, apicomplexans, cillates (including Paramecium) |
Stramenopiles | Motile cells with two flagella, one of which has tabular hairs; fucoxanthin is an accessory pigment in photosynthetic forms | Water molds, diatoms, brown algae, golden algae |
Amoebozoa (d) | Amoeboid movement via pseudopodia; feed by phagocytosis; slime molds form spores | Amoeba, many slime molds (including Physarum and Dictyostelium) |
Excavata (e) | Unicellular, flagellated protists; may lack mitochondria; photosynthetic or parasitic; chloroplasts (when present) are derived from secondary endosymbiosis | Trichomonas, Trichonympha, Giardia, Euglena, trypanosomes |
Rhizaria (f) | Amoeboid movement; many produce shells | Radiolarians, foraminiferans |
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Investigating life: �Shining a spotlight on danger
Section 18.6
Some algae emit a beautiful blue light when agitated.
How did this bioluminescent display evolve?
Figure 18.20
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