Mastering Concepts
18.1
1. What features define the protists?
Protists are eukaryotes that are not plants, fungi, or animals.
2. Describe examples of how protists are important.
Protists form the base of many aquatic food webs; are parasites that cause disease in plants and animals, including humans; can help people find oil reserves; and have multiple practical uses, including making paints reflective and making chocolate smooth and creamy.
3. Why are evolutionary biologists interested in protists?
Studying protists reveals the evolutionary history of mitochondria, chloroplasts, multicellularity, and the other eukaryotic kingdoms (plants, fungi, and animals).
18.2
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 types of photosynthetic pigments, the composition of the cell wall, the storage carbohydrates that they use, the number of flagella they possess, and whether they are unicellular, colonial, or multicellular.
3. List and describe the characteristics of the major groups of algae.
The major groups of algae are: (1) Euglenoids: unicellular flagellates; (2) Dinoflagellates: marine algae with cellulose plates and two flagella of different lengths; (3) Golden algae: yellowish accessory pigments; (4) Diatoms: ornate shapes and 2-part silica shells; (5) Brown algae: marine algae with complex body forms; (5) Red algae: marine algae with reddish and bluish pigments; (6) Green algae: chlorophylls a and b, starch, cellulose cell walls, alternation of generations
18.3
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 form a mass of nuclei with a single membrane, whereas cellular slime molds retain their individual membranes. Cellular slime molds can exist as one-celled or multicellular organisms.
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 Irish potato famine in the 1840s.
18.4
1. What mode of nutrition do protozoa use?
Most protozoa are heterotrophic, but some are photoautotrophs.
2. What are the characteristics of each major group of protozoa?
The major groups of protozoa are: (1) flagellated protozoa (euglenoids, dinoflagellates, and trypanosomes): move by flagella; (2) amoeboid protozoa (amoebae, forams, and radiolarians): move by pseudopodia; (3) ciliates: propelled by cilia; (4) apicomplexans: nonmotile protozoans that parasitize animals.
3. List three diseases caused by flagellated protozoa.
Examples of diseases caused by flagellated protozoa might include vaginitis, hiker’s diarrhea, African sleeping sickness, and Chagas disease.
4. Compare and contrast amoebae, foraminiferans, and radiolarians.
Amoebae, foraminiferans, and radiolarians produce extensions of their cytoplasm known as pseudopodia. However, whereas the amoebae lack cell walls, the foraminiferans have shells of calcium carbonate, and the radiolarians have shells of silica.
5. How do ciliates move and eat?
Ciliates move via the rowing action of their many cilia. They eat by swirling food items 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?
Apicomplexans are nonmotile, spore-forming, internal parasites of animals. Their cells contain an apical complex, a cluster of microtubules and organelles at one end of the cell.
18.5
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.
2. What features unite some of the major lineages of protists?
The red algae, green algae, and green plants all have chloroplasts derived from a primary endosymbiosis event. 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.
18.6
1. Use the food chain in figure 18.21a to explain why bioluminescence is adaptive to dinoflagellates.
The food chain shows that sticklebacks consume copepods, which consume dinoflagellates. The dinoflagellates emit light when disturbed; the light attracts sticklebacks and therefore indirectly protects the dinoflagellates.
2. What selects for a copepod’s light-avoidance response?
Conflicting selective pressures are acting on the copepods. On the one hand, light reveals the presence of dinoflagellates, which the copepods eat. On the other hand, the light also attracts sticklebacks, which prey on the copepods. However, copepods that avoid light evidently have greater reproductive success than those that do not.
Write It Out
1. Explain why evolutionary biologists are interested in choanoflagellates, green algae, and organisms with mitochondria whose genomes resemble those of bacteria.
Choanoflagellates are the closest living relatives to the common ancestor of all animals. Likewise, green algae are the closest living relatives to the common ancestor of all plants. Organisms with mitochondria resembling bacteria help researchers understand the endosymbiosis event that gave rise to mitochondria.
2. List some examples of protist diversity. How do different protist groups obtain energy? How do they move? Describe the differences in protist appearance.
Protists include a huge variety of eukaryotes, most of which are unicellular. They range from algae to slime molds and water molds to protozoa. Algae obtain energy by photosynthesis; heterotrophic protists obtain energy by consuming other organisms. Protists move via flagella, cilia, or pseudopodia. Protists may be uni- or multicellular; may or may not have cell walls; may or may not have photosynthetic pigments; and may or may not have flagella, cilia, or pseudopodia.
3. 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, farmers fertilize their crops, and homeowners 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, water collection, and treatment.
4. 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 easily preserved in sediments; in contrast, the cells of amoebae and slime molds lack shells.
5. 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.
6. Explain at least one line of evidence that green algae are the closest relatives of plants.
Lines of evidence that green algae are the closest relatives of plants include similarities in life cycles and photosynthetic pigments. Both groups also have cellulose cell walls and use starch as a storage polysaccharide. DNA sequence similarities provide additional evidence.
7. Natural selection favors stalk formation in cellular slime molds even though the cells of the stalk die. Explain this observation.
Even though the stalk cells die, the stalk increases the overall fitness of the organism by increasing the probability that the spores will be dispersed to a new habitat and food source.
8. Why might overwatering your plants make them more susceptible to infection by some kinds of heterotrophic protists?
Water molds use swimming cells to spread from plant to plant or from one part of a plant to another. Overwatering a plant increases the chance that a water mold will infect its roots or leaves.
9. 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 two flagella, one of which has tubular hairs. Photosynthetic species produce a yellowish pigment. The dinoflagellates, ciliates, apicomplexans, foraminiferans, and radiolarians are now all classified as alveolates because of the flattened sacs beneath their cell membranes.
10. 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. Name at least one unique characteristic next to each protist group in figure 18.22.
Euglenoids: pellicle; dinoflagellates: cellulose plates, two flagella; golden algae: yellow pigments, may be autotrophs or heterotrophs; diatoms: silica cell walls; brown algae: large, multicellular bodies; red algae: reddish or bluish pigments; green algae: starch, alternation of generations; slime molds: individuals may be uni- or multicellular; water molds: funguslike filaments; flagellates: heterotrophs with one or more flagella; amoeboid protozoa: pseudopodia; ciliates: cells fringed with cilia; apicomplexans: apical complex.
2. Molecular data have changed protist classification. Use a red pen to circle the protist groups classified as stramenopiles; use a blue pen to circle the groups classified as alveolates.
Stramenopiles include water molds, diatoms, brown algae, and golden algae. Alveolates include dinoflagellates, apicomplexans, and ciliates.
3. Name examples of disease-causing organisms in each of the four groups of protozoa.
Flagellated protozoa: Giardia, trypanosomes; amoeboid protozoa: Entamoeba histolytica; ciliates: the fish parasite, “ich”; apicomplexans: Plasmodium, Toxoplasma