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Answers to Thinking Critically Questions
Mader: Inquiry into Life, Twelfth Edition
Chapter 28
1. In the chapter we mentioned that although a few large bacteria have been discovered, most
are very small. Why does there seem to be a limit on bacterial size? Would you expect the
largest bacteria to be able to grow and reproduce faster or slower than the smallest bacteria?
Explanation/Answer: As unicellular organisms, bacteria have to rely on diffusion or active
transport of nutrients across their plasma membrane. The bigger the cell, the higher the ratio
of the total cell volume to surface area, which means that a proportionately smaller amount of
plasma membrane to feed a larger amount of cell. The largest bacteria would be expected to
grow more slowly because of the nutritional requirements mentioned above. However, some
of the smallest bacteria do grow slowly, and some are even obligate parasites because they
lack certain genes needed for efficient metabolism.
2. Even though archaea and bacteria are both prokaryotic, archaea are more closely related to
eukaryotic cells (including plants, animals, fungi, and protists) than they are to bacteria.
Since archaea and bacteria are both prokaryotic, what kind of evidence suggests that they are
more closely related to humans than to, say, an E. coli bacterium? (See Table 28.1.)
Explanation/Answer: Table 28.1 shows that archaea and eukaryotes share certain structural
features that are not seen in bacteria, such as introns, histones, several types of RNA
polymerase, and methionine as the first amino acid in protein synthesis.
Comparison of nucleic acid sequences (especially areas of the genome that change very
slowly over time) shows greater similarity between archaea and eukaryotes.
3. Sickle cell disease occurs mostly in people of African ancestry. When a person inherits two
copies of the sickle cell gene, his or her hemoglobin is abnormal, causing red blood cells to
be more fragile and sticky than usual. This leads to anemia and circulatory problems. About
70,000 Americans have sickle cell disease, and about 2 million carry one copy of the gene.
Even though it can cause a serious disease, the sickle cell gene is thought to have been
favored by natural selection, because it gives some protection against malaria. Considering
the type of cells infected by Plasmodium species, how might this protection work? What do
you think might happen if an effective vaccine against malaria is ever developed and widely
used?
Explanation/Answer: The Plasmodium parasite infects red blood cells. The sickle cell gene
must change something about the cell to make it inhospitable to the parasite, whether that is
the shape, the metabolism, the pH, etc. Research has shown that red blood cells from people
with one copy of the gene (who don’t have sickle cell anemia) are resistant to the parasite, so
the change must be subtle. Even though it causes a serious disease, the benefits of having the
sickle cell gene in the population have long outweighed the risk of sickle cell anemia. If
malaria was significantly reduced due to vaccination, the sickle gene would no longer offer
an advantage. The number of people with the gene might then begin to decrease, especially
if the disease kills people before they reach a reproductive age.
4. Many viruses contain their own enzymes for replicating their genetic material, while others
use the host cell’s enzymes. Would DNA viruses or RNA viruses be more likely to produce
their own enzyme(s) for this purpose, and why?
Explanation/Answer: RNA viruses would be more likely to produce their own enzymes
because their genetic material is RNA. In order to reproduce, these viruses need to make
multiple copies of their RNA genome. The host cell does not normally make copies of RNA
from an RNA template, so RNA viruses always produce their own RNA-replicating enzyme.
Some DNA viruses also produce their own enzyme to replicate their DNA, but others use the
host cell’s DNA-copying enzymes.
5. Why is it easier to create drugs to treat bacterial infections than protozoal or fungal
infections? Specifically, why do drugs such as penicillin have a very low incidence of side
effects?
Explanation/Answer: Bacteria generally produce all their own enzymes, ribosomes, and
other cellular machinery needed for their reproduction, while viruses rely on host machinery.
Penicillin prevents the synthesis of peptidoglycan, which is part of the cell wall of most
bacteria, but is not found in animal cells.
Protozoa and fungi are both eukaryotes, so their metabolism bears many similarities with
ours. For example, it would be difficult to inhibit the ribosome function of protozoa or fungi,
because these structures are very similar to those found in human cells.