Bauman Chapter 5 Answers to Critical Thinking Questions
Chapter 5
p. 132
Explain why thermophiles do not cause disease in humans.
Thermophiles (“heat-lovers”) grow best at temperatures well above human body
temperature. They do not cause disease in humans because they do not grow in
humans.
p. 133
In addition to extremes in temperature and pH, other chemical and physical agents,
including ionizing radiation, alcohol, enzymes, and heavy-metal ions, denature proteins.
For example, the first antimicrobial drug, salvarsan, contained the heavy metal arsenic
and was used to inhibit the enzymes of the bacterium Treponema pallidum, the
causative agent of syphilis.
Given that both human and bacterial enzymes are denatured by heavy metals,
how was salvarsan used to treat syphilis without poisoning the patient? Why is syphilis
no longer treated with arsenic-containing compounds?
Actually, patients who received salvarsan were poisoned by the treatment, and
treatment was risky. Bacteria have a faster growth rate than human cells, therefore the
arsenic more rapidly affected the bacterial cells and killed them before lasting harm was
done to the human patients cells when treatment was applied properly. In this century
we have available a large number of antibiotics that are affective on Treponema pallidum
and are not harmful to humans, the more dangerous arsenic-containing compounds are
no longer used to treat syphilis.
p. 141, top
Examine the biosynthetic pathway for the production of the amino acids tryptophan,
tyrosine, and phenylalanine in Figure 5.12b. Where do the initial reactants (PEP and
erythrose 4-phosphate) originate?
Both PEP and erythrose 4-phosphate are products of glucose metabolism. PEP is
produced by glycolysis of glucose, while erythrose 4-phosphate is produced when
glucose is metabolized in the pentose-phosphate pathway.
p. 141, bottom
Even though Pseudomonas aeruginosa and Enterococcus faecalis usually grow
harmlessly in the body, they can cause disease. Because these bacteria use the EntnerDoudoroff pathway instead of glycolysis to catabolize glucose, investigators sometimes
use clinical tests that provide evidence of the Entner-Doudoroff pathway to identify the
presence of these potential pathogens.
Suppose you were able to identify the presence of any specific organic
compound. Name a substrate molecule you would find in Pseudomonas and
Enterococcus cells, but not in human cells.
The substrate 6-phosphogluconic acid is produced in bacteria using the EntnerDoudoroff pathway but not in human cells, making it a good candidate as an indicator of
the presence of Pseudomonas and Enterococcus.
p. 142
We have examined the total ATP, NADH, and FADH2 production in the Krebs cycle for
each molecule of glucose coming through Embden-Meyerhof glycolysis. How many of
each of these molecules would be produced if the Entner-Doudoroff pathway were used
instead of glycolysis?
For each molecule of glucose metabolized by the Entner-Doudoroff pathway followed by
metabolism of pyruvic acid via the Krebs cycle, a net of three ATP, seven NADH, and
two FADH2 molecules are produced.
p. 147
Cyanide is a potent poison because it irreversibly blocks cytochrome a3. What effect
would its action have on the rest of the electron transport chain? What would be the
redox state (reduced or oxidized) of coenzyme Q in the presence of cyanide?
When cyanide blocks cytochrome a3 function, cytochrome a3 can no longer accept
electrons, and transport of electrons is blocked. Cytochrome a3 is the terminal electron
acceptor, therefore the other molecules of the chain, including coenzyme Q, are
reduced, unable to pass on the electrons they have received.
p. 148
Suppose you could insert a tiny pH probe into the space between mitochondrial
membranes. Would the pH be above or below 7.0? Why?
The pH of the intermembrane space of the mitochondrion would be below 7.0. Hydrogen
ions are being pumped into the space, and pH is inversely proportional to the hydrogen
ion concentration (as hydrogen ion increases, the pH becomes a smaller value).
p. 151
How and where do cells use the molecules of NADH and FADH2 produced during betaoxidation?
Most of the NADH and FADH2 produced during beta-oxidation are used in the electron
transport chain to produce ATP, while small amounts are used in other metabolic
reactions (e.g. to produce ethanol from pyruvic acid).
p. 152
Photosynthetic organisms are rarely pathogenic. Why?
Photosynthetic organisms have minimal nutritional requirements, and obtain most of
their requirements from inorganic sources. Pathogenic organisms usually need to obtain
nutrients from the host species. Since living sources of nutrients are not required by
photosynthetic organisms, they are rarely pathogenic.
p. 155
We have seen that of the two ways that ATP is generated via chemiosmosis—
photophosphorylation and oxidative phosphorylation—the former can be cyclical, but the
latter is never cyclical. Why can’t oxidative phosphorylation be cyclical; that is, why aren’t
electrons passed back to the molecules that donated them?
The energy used to drive oxidative phosphorylation is derived from catabolic reactions,
which release the energy stored in complex molecules, but the energy capture is not
highly efficient, much energy is lost, and it requires more energy to re-form the
metabolites than is captured from their catabolism. Photophosphorylation is ultimately
driven by light energy, which provides enough energy to re-generate metabolic
intermediates.
p. 160
What class of enzyme is involved in amination reactions? What class of enzyme
catalyzes transaminations?
Amination reactions are synthetic reactions catalyzed by anabolic polymerase enzymes.
Transaminations are exchange reactions catalyzed by transferases.
p. 166
1. Why does an organism that uses glycolysis and the Krebs cycle also need the
pentose phosphate pathway?
The pentose phosphate pathway is an important source of metabolic intermediates
required for amino acid and nucleotide synthesis.
2.
Describe how bacterial fermentation causes milk to sour.
Bacteria sour milk by metabolizing lactose sugar to lactic acid. The monosaccharides of
lactose (a disaccharide) are catabolized by glycolysis, and the resulting pyruvic acid is
reduced to lactic acid.
3.
Giardia lamblia and Entamoeba histolytica are protozoa that live in the colons of
mammals and can cause life-threatening diarrhea. Interestingly, these microbes lack
mitochondria. What kind of pathway must they have for carbohydrate catabolism?
Organisms lacking mitochondria can metabolize carbohydrates using glycolysis, but the
energy yield is much lower since the pyruvic acid is not further metabolized by aerobic
respiration to produce energy.
4.
Two cultures of a facultative anaerobe are grown in the same type of medium, but
one is exposed to air and the other is maintained under anaerobic conditions. Which
of the two cultures will contain more cells at the end of a week? Why?
The culture grown with exposure to air contains more cells at the end of the week. The
availability of oxygen makes more complete metabolism of compounds possible, thus
more energy is available for synthesis and growth than is available when oxygen is
absent.
5.
What is the maximum number of molecules of ATP that can be generated by a
bacterium after the complete aerobic oxidation of a fat molecule containing three 12carbon chains? (Assume that all the available energy released during catabolism
goes to ATP production.)
Complete oxidation of a fat molecule containing three 12-carbon fatty acids will produce
305 ATP molecules.
The glycerol molecule enters the glycolytic pathway at step 5 and then enters the Krebs
cycle: [2 ATP + 4NADH (3 ATP)] = 14 ATP net (after subtracting the ATP needed to
activate the glycerol molecule).
The beta-oxidation of three 12-carbon fatty acids produces 3 x [5NADH(3 ATP) +
5FADH(2 ATP)] = 75 ATP molecules.
Oxidation of the acetyl-CoA molecules produced by the beta-oxidation (3 fa x 6
acetylCoA/chain) x [ 1 ATP + 3 NADH(3 ATP) + FADH(2 ATP)] = 216 ATP molecules.
Total = (14 + 75 + 216) = 305.
6.
In terms of its effects on metabolism, why is a fever over 40oC often life threatening?
Heat denatures proteins, and denatured proteins do not function, so a high fever can
cause loss of protein function and thereby disrupt metabolic activities.
7.
Using the categories defined in the text on page 129, classify each of the enzymes
of glycolysis (see page 138), the Krebs cycle (see page 143), fat biosynthesis (see
Figure 5.31 on page 158), and transaminations (see Figure 5.32 on page 159).
Glycolysis enzymes: Step 1 enzyme is a transferase; steps 2 and 3 enzymes are
isomerases; step 4 enzyme is a lyase; step 5 enzyme is an isomerase; step 6
enzyme is an oxidoreductase; step 7 enzyme is a transferase; steps 8 and 9
enzymes are an isomerase and a lyase; step 9 enzyme is a transferase.
Krebs cycle enzymes: Step 1 enzyme is a transferase; step 2 enzyme is an isomerase;
steps 3 and 4 enzymes are oxidoreductases; step 5 enzyme is a
transferase/hydrolase; step 6 enzyme is another oxidoreductase; step 7 enzyme is
an isomerase; step 8 enzyme is an oxidoreductase.
Fat biosynthesis enzymes: G3P to DHAP is catalyzed by an isomerase; DHAP to
glycerol is catalyzed by a transferase; fatty acids are synthesized from acetyl-CoA
by oxidoreductases; formation of the fat molecule from glycerol and fatty acids is
catalyzed by polymerases.
Transaminations are carried out by transferases.
8.
How are photophosphorylation and oxidative phosphorylation similar? How are they
different?
Both photophosphorylation and oxidative phosphorylation use energy to pump hydrogen
ions across a membrane, creating a concentration differential which is then used to drive
ATP synthesis. Photophosphorylation uses light energy to pump the ions, and is
essentially a cyclic process as long as light energy is available. The energy of oxidative
phosphorylation is derived from the catabolism of energy-containing biomolecules and is
a linear process.
9.
Members of the pathogenic bacterial genus Haemophilus require NAD+ and heme
from their environment. For what purpose does Haemophilus use these growth
factors?
These factors are needed for ATP production: NAD+ is required as an electron
acceptor/donor and heme molecules are needed to form cytochromes, components of
the electron transport chain.
10. Compare and contrast aerobic respiration, anaerobic respiration, and fermentation.
Aerobic respiration, anaerobic respiration and fermentation are all metabolic activities
that capture the chemical energy in biomolecules and use it to produce ATP. Aerobic
respiration is the most efficient of these processes, realizing the greatest energy capture,
and is dependent on the availability of oxygen as the final electron acceptor. Anaerobic
respiration is somewhat less efficient in terms of energy recovery than aerobic
respiration, and utilizes final electron acceptors other than oxygen (nitrates, sulfates,
etc.). Fermentation is the least efficient of these catabolic pathways, using high-energy
organic molecules (e.g. pyruvic acid) as final electron acceptors.
11. Scientists estimate that up to one-third of Earth’s biomass is composed of
methanogenic prokaryotes in ocean sediments (Science 295:2067-2070). Describe
the metabolism of these organisms.
The methanogenic prokaryotes in ocean sediments are anaerobic organisms that
metabolize hydrogen and carbon dioxide by anaerobic respiration to produce energy and
methane.
12. A young student was troubled by the idea that a bacterium is able to control its
diverse and complex metabolic activities, even though it lacks a brain. How would
you explain their metabolic control?
Metabolic activities are controlled by a variety of mechanisms, none of which require
conscious control, and are largely driven by internal and external conditions. The
concentrations of reactants and products (e.g. availability of nutrients), the
concentrations of proteins, the presence or absence of repressor or activator molecules,
which are themselves produced in response to environmental conditions, are all ways in
which metabolic activities are controlled.
13. If a bacterium uses beta-oxidation to catabolize a molecule of the fatty acid
arachidonic acid, which contains 20 carbon atoms, how many acetyl-CoA molecules
will be generated?
Ten molecules of acetyl-CoA are produced by the complete beta-oxidation of
arachidonic acid.