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Answers to In-Chapter Critical Thinking Questions Chapter 1 A Brief History of Microbiology p. 5 A few bacteria produce disease because they derive nutrition from human cells and produce toxic wastes. Algae do not cause disease. Why not? Algae have simple nutritional requirements (carbon dioxide, water, light, a few salts) that are easily obtained from the environment; therefore, they do not need to obtain nutrients from other living things. p. 11 How might the debate over spontaneous generation have been different if Buchner had conducted his experiments in 1857 instead of 1897? Buchner’s experiments demonstrated that intact living cells were not required for fermentation of sugars. Had the experiments been done in the 1850s, prior to Pasteur’s experiments on spontaneous generation, one argument for spontaneous generation (the appearance of yeast cells in fermenting sugar solutions) would have been weakened. Spallanzani’s results might have been more widely accepted as demonstrating that spontaneous generation does not occur. When scientists accepted that animals do not arise from spontaneous generation, they might have been less likely to exempt microorganisms. p. 14 French microbiologists, led by Pasteur, tried to isolate a single bacterium by diluting liquid media until only a single type of bacterium could be microscopically observed in a sample of the diluted medium. What advantages does Koch’s method have over the French method? Many bacteria are very small and may be easily overlooked during examination with a light microscope. In contrast, Koch’s method allows a single microbe to reproduce into a much more visible population of cells—a colony—which is less likely to be overlooked. In addition, many microorganisms, especially pathogens, have stringent nutritional requirements; dilution in standard growth medium might be harmful to the microbes and therefore prevent their detection. p. 14 Why aren’t Koch’s postulates always useful in proving the cause of a given disease? Consider a variety of diseases, such as cholera, pneumonia, Alzheimer’s, AIDS, Down syndrome, and lung cancer. Koch’s postulates are not useful in determining the cause of diseases that are not infections (e.g., Down syndrome, lung cancer). Some diseases (e.g., cholera) occur only under specific conditions (high numbers, specific genetic component), so the microbes may be present in asymptomatic persons, and Koch’s first postulate is not met. Disease caused by microorganisms that are infectious only in humans is difficult to prove by Koch’s postulates because ethical considerations prohibit deliberate exposure of humans to potential harm (e.g., HIV/AIDS), preventing the third postulate from being applied. Some syndromes are common to a variety of microbes (e.g., pneumonia), so identifying a single causative agent is not possible (first postulate again). Alzheimer’s disease may be a physiologic/genetic disorder and not suitable for the application of Koch’s postulates, or it may be a syndrome caused by a nonliving agent (prion), which also precludes application of the postulates. p. 19 Albert Kluyver said, “From elephant to . . . bacterium—it is all the same!” What did he mean? Kluyver was referring to the metabolism/biochemistry that is common to all cellular life on the planet, regardless of the number of cells per organism or size. The statement can be extended to shared cellular structures as well. p. 20 The ability of farmers around the world to produce crops such as corn, wheat, and rice is often limited by the lack of nitrogen-based fertilizer. How might scientists use Beijerinck’s discovery to increase world supplies of grain? Nitrogen compounds (NO3, NO2, NH3, etc.) are often a limited resource for growing plants, but nitrogen gas (N2) is abundant in the air. Beijerinck’s discovery that some bacteria can convert nitrogen gas to nitrogen compounds is important to the cultivation of these grains. It may be possible to (1) introduce into the soil bacteria capable of converting N2 to organic forms of nitrogen, (2) create soil conditions that promote the growth of such bacteria, and/or (3) genetically modify crop plants with bacterial genes to convert nitrogen gas for themselves. Chapter 2 The Chemistry of Microbiology p. 29 Neon (atomic mass 10) and argon (atomic mass 18) are inert elements, which means that they very rarely form chemical bonds. Give the electron configurations of their atoms and explain why these elements are inert. Neon has two electrons in the inner shell and eight electrons in the outer cloud/shell. Argon has two in the innermost cloud/shell, eight electrons in the middle cloud, and eight electrons in the outer cloud/shell. An eight-electron cloud is a highly stable (low-energy) state and has no more room to add electrons. Losing an electron greatly decreases the stability, so not sharing electrons with other atoms is a more stable (lower-energy) state than “giving away” or “accepting” electrons. Because electron sharing (or loss) is the basis for chemical interactions, these elements do not interact with other atoms. p. 31 An article in the local newspaper about gangrene states that the tissue-destroying toxin, lecithinase, is an “organic compound.” But many people consider “organic” chemicals to mean something is good. Explain the apparent contradiction. Chemists use the term organic to mean compounds that contain carbon and hydrogen, which includes all biomolecules (“life molecules”). Nonscientists use the term organic to mean “from nature,” which is perceived as good or better than synthetic. There is therefore some overlap: both groups use the term organic to mean biomolecules, but the latter position overlooks the fact that many natural processes produce toxins (e.g., lecithinase, botulism toxin, rattlesnake venom). p. 32 The deadly poison cyanide has the chemical formula H–CN. Describe the bonds between carbon and hydrogen, and between carbon and nitrogen, in terms of the number of electrons involved. Triple covalent bonds are stronger and more difficult to break than single covalent bonds. Explain the reason why by referring to the stability of a valence shell that contains eight electrons. Hydrogen shares its single electron with carbon. The carbon shares one electron with the hydrogen and three electrons with nitrogen. Both atoms in a triple bond have a more stable state when the valence shell is “full” with eight electrons. Removing three electrons simultaneously (breaking the triple bond) requires much more energy than removing a single electron. p. 33 According to the chart in Figure 2.6, what type of bond (nonpolar covalent, polar covalent, or ionic) would you expect between chlorine and potassium? Between carbon and nitrogen? Between phosphorous and oxygen? Explain your reasoning in each case. The bond between K and Cl is an ionic bond. Chlorine is much more electronegative than potassium and “steals” an electron from potassium. The bond between C and N is a nonpolar covalent bond. The electronegativity of carbon and nitrogen is nearly equal, so electrons are shared essentially equally. The bond between P and O is a polar covalent bond. Oxygen is somewhat more electronegative than phosphorous, so an electron is shared between the atoms, although unequally—the electron spends more time in the vicinity of the O nucleus than of the P nucleus. p. 36 How can hydrogen bonding between water molecules help explain water’s ability to absorb large amounts of energy before evaporating? The polar nature of the bonds between H and O and the nonlinear structure of the water molecule mean that each H2O molecule is usually participating in three or four hydrogen bond pairs. Energy is required to break these bonds, and three to four bonds need to be broken for each water molecule to escape the liquid and evaporate. p. 38 How can a single molecule of magnesium hydroxide neutralize two molecules of hydrochloric acid? Magnesium hydroxide dissociates to release two OH–, whereas each molecule of HCl dissociates to release only one H+. p. 40 We have seen that it is important that biological membranes remain flexible. Most bacteria lack sterols in their membranes and instead incorporate unsaturated phospholipids in the membranes to resist tight packing. Reexamine Table 2.4. Which fatty acid might best protect the membranes of an ice-dwelling bacterium? Linoleic acid is the least linear of the fatty acids in Table 2.4 and will best interfere with tight packing of the fatty acids in bacterial membranes that would otherwise occur at cold temperatures. p. 44 Why isn’t there a stereoisomer of glycine? Glycine’s R group is hydrogen. Glycine therefore has two hydrogens attached to the central carbon, and its “mirror images” are identical; that is, there are no stereoisomers. p. 47 A textbook states that only five nucleotide bases are found in cells, but a laboratory worker correctly reports that she has isolated eight different nucleotides. Explain why both are correct. Living organisms use five nucleotide bases (adenine, cytosine, guanine, thymine, and uracil). Cells combine A, C, or T nucleotide bases with either of two different sugars (deoxyribose and ribose) to form nucleotides (for DNA and RNA, respectively). Thymine is combined only with deoxyribose, and uracil is combined only with ribose; therefore, there are eight nucleotides [(3 2) + 1 + 1 = 8].