18.1 Studying Viruses and Prokaryotes

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Recall from Chapter 13 that nitrogen fixation is the processof converting atmospheric nitrogen into ammonia (NH 3 ) andother nitrogen compounds that plants can then use. Prokaryotessupply usable nitrogen to ecosystems ranging fromgrasslands and forests to the arctic tundra.Some types of nitrogen-fixing bacteria are free-living, whileothers live along with other organisms. Legumes, a group ofplants including peas, beans, alfalfa, and clover, have a mutualisticrelationship with nitrogen-fixing bacteria. These bacterialive in the plant’s nodules, small rounded lumps that form theroots, as shown in FIGURE 18.16. The bacteria provide usablenitrogen to the plant by capturing nitrogen gas from airtrapped in the soil. They combine the nitrogen with hydrogento produce ammonia. In return, the plant supplies food andshelter to the bacteria.Scientists have found many ways to use prokaryotes to benefit industry andthe environment. One important use of prokaryotes is in bioremediation(by-oh-rih-mee-dee-AY-shuhn), a process that uses microbes and other livingthings to break down pollutants. For example, some types of bacteria candigest oil, which is helpful for cleaning up oil spills and other industrialaccidents. Workers spray oil-polluted beaches with a fertilizer that helps thebacteria grow.Bacteria can digest almost any product that humans can make, includingpoisons. Therefore, they play an important role in recycling and composting.When you hear the term biodegradable, it often refers to the ability of bacteriato break down a material. Some of the only materials made by humans thatcannot be biodegraded are certain types of plastics.Apply When there is a toxic chemical spill, sometimes workers will spray bacteriaover the contaminated area. Why might they do this?FIGURE 18.16 Root nodules ofthis white clover contain nitrogenfixingbacteria. The symbiotic bacteriaconvert nitrogen from theatmosphere (N 2 ) into a formusable by the clover. In return, theplant produces carbohydratesthrough photosynthesis that thebacteria can consume. (inset coloredSEM; magnification 90)18.5 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. Describe two ways bacteria providenutrients to humans.2. What are two roles prokaryotes playin the cycling of elements in anecosytem?CRITICAL THINKING3. Connect Think of an examplein which the use of bioremedia-tion either has improved the environmentor has the potential todo so.4. Synthesize How do prokaryoteslend stability to an ecosystem?Connecting CONCEPTS5. Ecology Prokaryotes in cowintestines produce moremethane if the cow is fed adiet high in grains rather thangrass. Some scientists proposethat overfeeding grain to cowscontributes to global warming.How did these scientists arriveat this hypothesis, and howcould it be tested?Chapter 18: Viruses and Prokaryotes 561
CHAPTER 18MATERIALS• petri dish containing aselective medium• petri dish containing anonselective medium• permanent marker• 2 leaves from the sameplant• pencil with eraser• transparent tapePROCESS SKILLS• Observing• Evaluating OutcomesINVESTIGATIONLeaf Print BacteriaPPFM are pink-pigmented bacteria that grow on plant surfaces. They help plants bystimulating seed germination and plant growth. A unique feature of PPFM bacteriais that they use methanol as their sole source of carbon. Therefore, a simplenutrient mixture that contains only methanol as a carbon source will “select” forPPFM bacteria. This type of mixture is called a selective medium. A mixture thatcontains many nutrients is called a nonselective medium. In this lab, you will makeleaf prints on selective and nonselective media to study the growth of PPFM andother bacteria.PROBLEM How do populations of bacteria grown on various media differ?PROCEDURE1. Obtain two petri dishes, one containing a selective medium (methanol carbonsource only) and the other containing a nonselective, nutrient-rich medium.2. Label the petri dishes with your name, the date, and “selective” or “nonselective.”3. To make the first leaf print, lay one of the leaves on the surface of the selectivemedium. Use the eraser end of a pencil to gently press the leaf into the medium,as shown below. After making the impression, carefully lift the leaf away from themedium and discard. Close the petri dish.4. Repeat step 3 using your other leaf and the nonselectivemedium.5. Seal the petri dishes with tape. Store them upside down atroom temperature.Caution: Once the dishes are sealed, they should not beopened again. Follow your teacher’s directions for disposal ofthe petri dishes at the end of the lab.6. Write a prediction based on what you know about PPFMbacteria and the selective and nonselective media.7. Observe the dishes over a period of one or two weeks. Record your observationsdaily. These should be both qualitative (shape, color, and size of colonies) andquantitative (number of colonies). To count colonies, it may help to create a gridon the underside of each petri dish.8. At the end of the observation period, compare your results with those from therest of the class.ANALYZE AND CONCLUDE1. Analyze How many types of bacteria were present on the nonselective medium?the selective medium? What might account for differences between the two?2. Analyze Is PPFM bacteria growth the same on both types of media? Whichobservations support this conclusion?3. Analyze Do all bacteria grow at the same rate? Which observations supportthis conclusion?4. Infer Why might bacteria grow faster on the nonselective medium?EXTEND YOUR INVESTIGATIONDesign an experimental procedure to test the hypothesis that different types ofPPFM bacteria are found on different plants.562 Unit 6: Classification and Diversity
Page 2 and 3: How are bacteriahelpful to humans?c
Page 4 and 5: FIGURE 18.2 Relative Sizes of Cells
Page 6 and 7: 18.2Viral Structure andReproduction
Page 8 and 9: In some viruses, capsids form a 20-
Page 10 and 11: Lytic InfectionA lytic infection (L
Page 12 and 13: Influenza Winter usually causes con
Page 14 and 15: 18.4Bacteria and ArchaeaKEY CONCEPT
Page 16 and 17: FIGURE 18.13 Prokaryote StructureTh
Page 18 and 19: 18.5Beneficial Rolesof ProkaryotesK
Page 22 and 23: 18.6Bacterial Diseasesand Antibioti
Page 24 and 25: MAIN IDEABacteria can evolve resist
Page 26 and 27: INVESTIGATIONModeling VirusesThe st
Page 28 and 29: Chapter AssessmentChapter Vocabular
Page 30: For more test practice,go to ClassZ

18.1 Studying Viruses and Prokaryotes

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