18.1 Studying Viruses and Prokaryotes

QUICK LABOBSERVINGExamining Bacteria in YogurtSome types of bacteria can ferment milk, producing lactic acid in the process.Yogurt is a product of fermentation. It is acidic and stays fresh longer thanmilk, and it is also digested more easily. In this exercise, you will prepare amicroscope slide of yogurt.PROBLEM What types of bacteria can you observe in yogurt?PROCEDURE1. Using a toothpick, place a dab of yogurt ona microscope slide. Caution: Do not eat inthe laboratory.2. Mix the yogurt in a drop of water and carefullyadd a coverslip.3. Examine the slide with a compound microscope.4. Record your observations by drawing a pictureof what you see through the microscope.ANALYZE AND CONCLUDE1. Identify Recall the terms bacillus, coccus, and spirillafrom the previous section. Which type or types ofbacteria did you observe in your slides?2. Analyze Many people do not produce lactase,which is an enzyme that breaks down the milksugar lactose. As a result, lactose-intolerant peoplehave trouble digesting dairy products. Why mightthey have fewer problems eating yogurt?MATERIALS• toothpick• dab of yogurt• microscope slide• drop of water• coverslip• microscopeMAIN IDEAProkaryotes play important roles in ecosystems.Even though you can’t easily see them, prokaryotes play important roles inevery ecosystem they occupy. Some, such as cyanobacteria, produce oxygenthrough photosynthesis. Others help recycle carbon, nitrogen, hydrogen, andsulfur through the ecosystem. The absence of prokaryotes in the environmentcan disrupt an ecosystem, since other organisms rely on them for survival.Photosynthesizing prokaryotes include purple and green photosyntheticbacteria and cyanobacteria. Whereas purple and green bacteria use light tomake carbohydrates, they do not produce oxygen. Cyanobacteria, however, aresimilar to plants in how they produce oxygen as a byproduct of photosynthesis.Cyanobacteria are named for their greenish blue (cyan) color. Recall fromChapter 12 that cyanobacteria played an important part on early Earth,supporting the life forms we are familiar with today. Fossil evidence suggeststhere was very little oxygen on Earth prior to the appearance of cyanobacteria.Some colonies of photosynthesizing cyanobacteria, as well as other bacteria,are also able to fix nitrogen. Although much of the atmosphere is made upof nitrogen gas (N 2 ), this is not in a form that plants or animals can use tomake amino acids or proteins.560 Unit 6: Classification and Diversity