Bauman Chapter 3 Answers to Critical Thinking Questions
p. 58
In your opinion, are viruses alive? What factors led you to your conclusion?
In one opinion, viruses are almost alive, derived from living things that have been
reduced to the bare basics, because viruses have genetic material, the code of life.
Alternative viewpoint: Viruses have no independent metabolic activity, therefore they are
non-living.
p. 69
After a man infected with the bacterium Escherichia coli was treated with the correct
antibiotic for this pathogen, the bacterium was no longer found in the man’s blood, but
his symptoms of fever and inflammation worsened. What caused the man’s response to
the treatment? Why was his condition worsened by the treatment?
The antibiotic killed the bacteria, but as the dead bacteria were degraded, the bacterial
components, including lipopolysaccharides (LPS) of the outer membrane, were
released. The lipid A component of LPS caused the body’s defense responses (fever
and inflammation) to intensify.
p. 73
Solutions hypertonic to bacteria and fungi are used for food preservation. For instance,
jams and jellies are hypertonic with sugar, and pickles are hypertonic with salt. How do
hypertonic solutions kill bacteria and fungi that would otherwise spoil these foods?
The hypertonic solutions have higher concentrations of solutes than the solute
concentrations in the bacterial and fungal cells. Since the cell membranes largely
prevent the solutes from entering the cells and achieving equilibrium, water tends to flow
out of the cells. The resulting dehydration brings metabolic activity to a halt, both
because metabolites would crystallize out of solution and because water is critical to
most metabolic reactions.
p. 86
Eukaryotic cells are almost always larger than prokaryotic cells. What structures might
allow for their larger size?
Exchange rates across the surface of a cell and surface-to-volume ratios are considered
major factors that put limits on cell size. Eukaryotic cells have extensive internal
membranes, particularly the endoplasmic reticulum, that function to vastly increase the
functional surface area of the cell. Many of the critical energy production functions (ATP
synthesis, photosynthesis) take place in the bacterial cell membrane, while in eukaryotes
the mitochondria and chloroplasts are sites of these critical functions, so surface-tovolume ratios are not limiting for these functions in eukaryotes.
p. 91
1. A scientist develops a chemical that prevents Golgi bodies from functioning.
Contrast the specific effects the chemical would have on human cells versus
bacterial cells.
A chemical that prevents Golgi function would cause a variety of problems in the human
body, all related to secretion. The respiratory tract would dry out, making breathing and
gas exchange more difficult. Digestive system problems would include loss of lubricating
mucous as well as maladsorption due to lack of digestive enzyme secretion. These
would be among the most immediate problems. Problems with hormone secretion would
soon develop. Bacteria, on the other hand, would be unaffected because they have no
Golgi bodies.
2.
Methylene blue binds to DNA. What structures in a yeast cell would be stained by
this dye?
The nucleus and mitochondria contain DNA, so methylene blue would stain the nucleus
and mitochondria.
3.
A new chemotherapeutic drug kills bacteria, but not humans. Discuss the possible
ways the drug may act selectively on bacterial cells.
Bacterial ribosomes differ from eukaryotic ribosomes in structure and some aspects of
function. Drugs targeting those biochemical differences may be toxic to bacteria but
harmless to humans. Bacteria possess metabolic pathways not present in humans (e.g.
folic acid synthesis); drugs may inhibit such processes in bacteria without harm to
humans. Bacteria have cell walls critical to their survival, while human cells have no
comparable structures, so bacterial cell wall synthesis may be targeted by drugs without
having an impact on human cellular function or structures.
4.
Some bacterial toxins cause cells lining the digestive tract to secrete ions, making
the contents of the tract hypertonic. What effect does this have on a patient’s water
balance?
When the digestive system is hypertonic as a result of salt secretion, water leaves the
digestive tract cells and enters the intestinal chamber. Water leaves the bloodstream to
replace the water lost from the digestive tract cells. Continued loss of water into the
intestines “pulls” more water from the blood, which then depletes the water stored in the
tissues, possibly leading to potentially dangerous dehydration.
5.
A researcher carefully inserts an electrode into a plant cell. He determines that the
electrical charge across the cytoplasmic membrane is -70 millivolts. Then he slips
the electrode deeper into the cell across another membrane and measures an
electrical charge of -90 millivolts compared to the outside. What relatively large
organelle is surrounded by the second membrane? Explain your answer.
The organelle the electrode penetrated was likely the central vacuole, a prominent
feature of many plant cells.
6.
Does Figure 3.2a on page 60 best represent a Gram-positive or a Gram-negative
cell? Defend your answer.
The drawing in Figure 3.2a appears to represent a Gram-positive bacterial cell. The cell
wall is relatively thick compared to the cell membrane, and there is no indication of a
lipid bilayer external to the cell wall. (Gram-negative bacteria have a thin cell wall with an
external lipid bilayer.)
7.
An electron micrograph of a newly discovered cell shows long projections with a
basal body in the cell wall. What kind of projections are these? How might this cell
behave in its environment because of the presence of this structure?
Long projections from basal bodies are flagella, so this cell is probably motile.
8.
An entry in a recent scientific journal reports positive phototaxis in a newly described
species. What condition could you create in the lab to encourage the growth of
these organisms?
Positive phototaxis suggests these organisms need light, most likely for photosynthesis.
They need to be incubated in the presence of light for at least part of the day. Dark
incubators would not be suitable.
9.
A medical microbiological lab report indicates that sample X contained a biofilm, and
that one species in the biofilm was identified as Neisseria gonorrhoeae. Is this strain
of Neisseria likely to be pathogenic? Why or why not?
A strain of Neisseria found in a biofilm may be pathogenic. Pathogens tend to have
stringent environmental requirements, and the interior conditions of a biofilm may meet
those needs, while other members of the biofilm community may provide nutritional
needs.