Mastering Concepts
17.1
1. What features define the protists?
Kingdom Protista contains eukaryotes that do not belong to any other kingdom of eukaryotes.
2. Describe examples of how protists are important.
Examples of how protists are important economically and ecologically include:
- they are the basis of most aquatic food webs;
- parasites cause disease in both plants and animals, including humans;
- protists can help people find oil reserves;
- they are used to make paints reflective and make chocolate smooth and creamy.
3. Why are evolutionary biologists interested in protists?
Studying protists reveals the evolutionary history of eukaryotes in general, especially the origins
of mitochondria, chloroplasts, multicellularity, and of the other eukaryotic kingdoms (plants,
fungi, and animals).
17.2
1. What mode of nutrition do the algae use?
Algae are photosynthetic organisms.
2. Describe several criteria for classifying the algae.
Criteria for classifying algae include the forms of chlorophyll and other photosynthetic pigments,
the structures of the cell wall, the storage carbohydrates that they use, the number of flagella they
possess, and whether they are unicellular, multicellular, filamentous, or colonial.
3. List and describe the characteristics of the major groups of algae.
The seven phyla of algae are:
Euglenoids: unicellular flagellates
Dinoflagellates: marine algae with cellulose plates and two flagella of different lengths; cause
red tides
Golden algae: yellowish accessory proteins
Diatoms: ornate shapes and 2-part silica shells
Brown algae: marine algae with complex body forms; kelps are in this group
Red algae: marine algae with reddish and bluish pigments other than chlorophyll a
Green algae: chlorophylls a and b; starch; cellulose cell walls; alternation of generations; closest
relatives of land plants
17.3
1. What mode of nutrition do the slime molds and water molds use?
Slime molds and water molds are heterotrophs.
2. Compare and contrast the plasmodial and cellular slime molds.
Both types of slime molds are heterotrophic protists living in damp habitats. Plasmodial slime
molds, however, form a mass of cells with a single membrane, while cellular slime molds retain
their cellular membranes in their sluglike masses.
3. What has been the role of water molds in the environment and history?
Water molds are decomposers; they are also parasites of plants and animals. They have ruined
crops, including the 1870 downy mildew of grapes that nearly destroyed the French wine
industry and the even worse Irish potato famine in the 1840s.
17.4
1. What mode of nutrition do protozoa use?
Most protozoa are heterotrophic, though a few are photoautotrophs.
2. What are the characteristics of each major group of protozoa?
The major groups of protozoa and their characteristics are:
- flagellated protozoans (euglenoids, dinoflagellates, and trypanosomes): move by flagella;
- amoeboid protozoans (amoebas, forams, and radiolarians): move by pseudopodia
- ciliates: propelled by movements of cilia
- apicomplexans: nonmotile protozoans that parasitize animals.
3. List three diseases caused by flagellated protozoa.
Flagellated protozoans cause vaginitis, hiker’s diarrhea, African sleeping sickness, and Chagas
disease.
4. Compare and contrast amoebas, foraminiferans, and radiolarians.
All three produce extensions of their cytoplasm known as pseudopodia. However, while the
amoebas are soft bodied, both the foraminiferans and the radiolarians have shells of calcium
carbonate or silica.
5. How do ciliates move and eat?
Ciliates move through the rowing action of their many cilia. They eat by swirling food items
down into the oral groove. A food vacuole forms around the food, and digestion occurs within
the food vacuole.
6. What are the distinguishing characteristics of apicomplexans?
The distinguishing characteristic of apicomplexans is the apical complex, a cluster of
microtubules and organelles at one end of the cell.
17.5
1. How have molecular sequences changed protist classification?
Molecular sequence data are helping biologists to reorganize the protist groups so that closely
related organisms are grouped together. Two new such classifications include the alveolates and
the stramenopiles.
2. What features unite some of the major lineages of protists?
The dinoflagellates, apicomplexans, radiolarians, ciliates, and foraminiferans all have alveoli
beneath their cell membranes. The water molds, diatoms, brown and golden algae all have a
flagellum with tubular hairs.
17.6
1. What were the researchers asking about Yellowstone Lake, and how did they answer their
question?
The researchers wanted to know whether the evolution of the diatoms was related to
environmental change. To answer this question they took a sediment core from the bottom of the
lake and examined diatoms at regular intervals. They correlated the diatom features with carbon14 dating and environmental data from pollen samples. Their research revealed an evolutionary
change in the diatoms that correlated with climatic change.
2. Spines increase a cell’s surface area, so the spiniest diatoms should be the most buoyant. What
selective force might explain the observed shift in spine number? How might you test your
hypothesis in a lab?
[Answers will vary]
Write It Out
1. Explain why evolutionary biologists are interested in choanoflagellates, green algae, and
organisms with mitochondria whose genomes resemble those of bacteria.
Biologists are interested in these organisms because they can help in understanding the origin of
mitochondria and the evolutionary history of the eukaryotes.
2. Name an organism that uses each form of locomotion: cilia; flagella; amoeboid motion.
Cilia – Paramecium caudatum; flagella – Trypanosoma brucei; amoeboid motion – Amoeba
proteus;
3. Name at least one type of organism that has each of the following structures: cyst; pseudopod;
plasmodium; silica cell wall; apical complex
Cyst – Giardia intestinalis; pseudopod - Amoeba; plasmodium – plasmodial slime mold; silica
cell wall – diatom; apical complex – Toxoplasma
4. Name the protist that causes each of the following diseases: malaria; toxoplasmosis; grape
downy mildew; paralytic shellfish poisoning; amoebic dysentery; leishmaniasis; late blight of
potato; hiker’s diarrhea; African sleeping sickness; Chagas disease
Malaria - Plasmodium falciparum; toxoplasmosis – Toxoplasma gondii; grape downy mildew –
Plasmopara viticola; paralytic shellfish poisoning - Gonyaulax catenella; amoebic dysentery –
Entamoeba histolytica; leishmaniasis – Leishmania; late blight of potato - Phytophthora
infestans; hiker’s diarrhea – Giardia intestinalis; African sleeping sickness – Trypanosoma
brucei; Chagas disease – Trypanosoma cruzi
5. Describe the relationship between nutrient pollution and harmful algal blooms. Why might
harmful algal blooms be more frequent in summer? What steps could coastal communities take
to prevent nutrient pollution?
Because algae thrive in water with abundant light and nutrients, pollution with excess nutrients
leads to an increase in harmful algal blooms. Such blooms are more frequent in summer, when
temperatures are warmer and people fertilize their lawns. To prevent nutrient pollution, coastal
communities could pass laws that limit the amounts or types of fertilizers that are used. They
could also provide better drainage and runoff with proper water collection and treatment.
6. Explain why the fossil record for diatoms is much more complete than that of other protists,
such as amoebae and slime molds.
The silica-rich shells of the diatoms are much more easily preserved in sediments.
7. A diatom’s silica cell wall is rigid and somewhat resembles a shoebox with a lid. How might
these protists divide? Draw a diagram of the process.
The diatoms could separate their cell wall into bottom and top, undergo mitosis and reform a
matching top and bottom.
8. How is it adaptive for a red alga to have pigments other than chlorophyll?
Red algae also have reddish and bluish photosynthetic pigments that allow the algae to live at
depths that may exceed 200 m, using wavelengths of light that chlorophyll cannot capture.
9. Study the spherical Volvox colony in figure 17.11. The cells in the colony have eyespots and
flagella, and they are connected to one another by thin strands of cytoplasm (these features are
difficult to see in the photo). Propose some ways that Volvox’s colonial lifestyle might be
adaptive in its freshwater habitat.
The eyespots in the colony may enable it to detect light sources in all directions. The thin strands
of cytoplasm that connect them may enable them to communicate through chemicals that pass
along the cytoplasmic strands. The flagella would allow the colony to move in any direction. By
having different cells in the colony perform different functions the colony is able to specialize
without being truly multicellular. In addition, the relatively large size of Volvox may protect it
from single-celled predators.
10. What adaptations enable cellular slime molds to survive a temporarily harsh environment?
When food becomes scarce, the cellular slime molds secrete chemical attractants, which
stimulate the neighboring amoebae to aggregate into a sluglike structure. The “slug” moves
toward light, stops, and forms a stalk topped by a fruiting body that produces spores. Water, soil,
animals, or birds spread the spores to new habitats, where food may be more abundant.
11. Develop a testable hypothesis to explain why the plasmodium of Physarum polycephalum is
bright yellow. How would you test your hypothesis?
[Answers may vary].
12. Search the Internet for a news story about Physarum polycephalum using a simple maze to
find the shortest distance between two food sources. Why would this behavior be adaptive? Do
you think this experiment shows that slime molds are intelligent? Explain.
[Answers may vary].
13. How does rainy weather foster the spread of Phytophthora, the protist that causes late blight
of potato?
Phytophthora is a water mold; it produces swimming spores. Abundant rain allows spores to
spread rapidly from host to host.
14. Use the Internet to learn about the sexually transmitted disease trichomoniasis. Describe how
the pathogen spreads and the symptoms, treatment, and prevention of the disease.
Trichomoniasis is caused by a sexually transmitted protozoan. Men are often asymptomatic, but
when present, their symptoms can include mild urethral burning or discharge. Symptoms in
women are more pronounced, with a frothy green vaginal discharge and strong odor.
Trichomoniasis is treated with prescription drugs, and like all STDs is best prevented by
abstaining from sex or having sex only in a committed monogamous relationship. The use of
latex condoms is also a preventative measure.
15. The flagellated protist Giardia does not have true mitochondria, but in 2005, researchers
reported that it does have tiny organelles called mitosomes. Chapter 6 describes the structure and
function of the mitochondrion. What features would you look for to test the hypothesis that
mitosomes are derived from mitochondria?
Mitochondria have a double membrane and contain their own DNA, and they undergo reactions
that do not occur elsewhere in the cell. Searching for these features in mitosomes might help
reveal a possible relationship with mitochondria.
16. Insecticides such as DDT kill mosquitoes that transmit the protist that causes malaria. The
use of DDT, however, causes serious reproductive problems in eagles, ospreys, and other fisheating birds. The insecticide is therefore banned for use in the United States. In countries where
malaria still rages, what information would you need to help you decide whether to allow the use
of DDT in the fight against malaria?
DDT kills mosquitoes, but it would be important to know what other species DDT harms as well.
The incidence and mortality rate from malaria in the region would also be important, so that the
costs can be balanced against the benefits.
17. How might global climate change affect the distribution of malaria? Explain your answer.
Because the malarial parasite requires mosquitoes for the fertilization stage of its life cycle, any
climate change that increased the area of the earth characterized by warm moist climate would
very likely result in a spread of malaria and increase in infection rates.
18. Give three examples of protists for which the classifications have recently changed. In each
case, what was the justification for the old category, and what is the justification for the change?
The water molds, diatoms, brown algae and golden algae are now classified as stramenopiles
because at some point in their life cycle they have a second flagellum with tube like hairs on it.
They also all produce a yellowish photosynthetic pigment. The dinoflagellates, ciliates,
apicomplexans, foraminiferans, and radiolarians are now all classified as alveolates because of
the flattened sacs beneath their cell membrane. Any of these examples could be used.
19. Suppose someone hands you a microscope and a single-celled organism. Create a flowchart
that you could use to identify the specimen.
Answers will vary, but one approach would be to create a flowchart that takes into account the
presence or absence of chloroplasts with specific photosynthetic pigments; flagella; pseudopodia;
cilia; cell walls of cellulose, calcium carbonate, or silica; apical complex.
Pull It Together
1. What features do all protists have in common?
All protists are eukaryotic organisms that are not classified as fungi, plants, or animals. Most are
unicellular.
2. How have molecular data changed the classification system depicted in this concept map?
Molecular biology has allowed scientists to classify protists by the degree of similarity in their
DNA, which has yielded different relations between protists than were made by morphological
comparison.
3. What are the two groups of slime molds?
One is the plasmodial slime molds, which consist of thousands of nuclei joined into a single
large cell. The other is the cellular slime molds, which may be unicellular or multicellular
structure.
4. Name examples of disease-causing organisms in each group of protozoans.
- flagellated protozoans: Giardia, trypanosomes
- amoeboid protozoans: Entamoeba histolytica
- ciliates: the external fish parasite, “ich”
- apicomplexans: Plasmodium, Toxoplasma
5. Which organisms in this concept map are heterotrophs, and which are autotrophs?
Funguslike and animal-like protists are heterotrophs, and plantlike protists are autotrophs. Some
plantlike protists, like euglena and golden algae, are also heterotrophs.