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Chapter 17 Answers
Evolution of Microbial Life
Visual Understanding
1. Figure 17.6
Viruses have dramatically different forms and shapes. What are the consistent features of
all of the viruses shown?
They all have a strand of nucleic acid, DNA or RNA, inside a protein shell.
2. Figure 17.9
Are the Volvox shown here multicellular organisms or numerous single-celled organisms
working together?
Volvox are colonial protists, which are single-celled organisms working together. The
bodies of all multicellular organisms exhibit two qualities: cell specialization, which is
lacking in Volvox, and intercellular coordination. Volvox are capable of some
coordinated colonial activity, but they don't exhibit a full level of intercellular
coordination seen in multicellular organisms.
Challenge Questions
1. Why can’t we prove how life began on earth?
The tests that we can do give indications of how life might have begun, but are not
definitive. Scientists think that they have a pretty good idea, but it is not proven. Since
we have not discovered life elsewhere yet, we have nothing to compare, either.
2. How do prokaryotes obtain the energy for life?
There are a variety of methods that prokaryotes use. The autotrophic prokaryotes use
sunlight, or chemicals such as sulfur or hydrogen sulfide. The heterotrophic prokaryotes
use sunlight or decompose dead tissue or attack living tissue.
3. Why do we say that bacteria are alive but viruses are not?
All living organisms exhibit two basic characteristics that are not present in viruses:
cellular organization and the ability to grow and reproduce independently. Viruses are
molecular entities, DNA or RNA wrapped in a protein coat, but not cellular entities.
Also, viruses cannot reproduce on their own; they require the cellular mechanisms of
their host.
4. Your friend, Tomiko, complains that she doesn’t understand how anyone can believe
in the endosymbiotic theory. She asks you to explain what evidence there is for this
theory. Do so.
Mitochondria are about the same size as bacteria, and are surrounded by two
membranes. The outer could represent a vesicle formed by endocytosis, while the inner
could be the original cell membrane of a bacterium. Further, mitochondria maintain
some DNA which resembles bacterial DNA in that it is a circular molecule which
contains genes for oxidative metabolism. Mitochondria are also able to divide by fission.
Chloroplasts are another type of organelle that are similar to bacteria for many of the
same reasons.
5. Compare the methods of movement found in five major groups of protists.
There are heterotrophs that use pseudopodia – projections of the cytoplasm. There are
heterotrophs with flagella. The third heterotrophic group has some mobility: the slime
molds and water molds that can do certain types of streaming movements. The
photosynthetic protists include groups with flagella and several nonmotile groups that
either float freely in water or are fixed in place.
6. Describe three ways that fungi cooperate with other organisms.
Fungi form symbiotic relationships with photosynthesizers, typically plants, algae, and
cyanobacteria. One example is mycorrhizae which is an association between certain soil
fungi and the roots of many plants. Another example is a lichen composed of algae and
fungi working in harmony to live on rocks and other places where neither one could live
alone. A third example is another type of lichen composed of cyanobacteria and fungi
that also lives on bare rocks and other harsh environments.