Essential Cell Biology 5th edition

A:34 Answers
where most of the energy is captured. In contrast, all carbon
atoms of a fatty acid are converted into acetyl CoA (see
Figure 13−11).
ANSWER 13–8
A. False. If this were the case, the reaction would be
useless for the cell. No chemical energy would be
harvested in a useful form (e.g., ATP) to be used for
metabolic processes. (The cells would certainly be warm,
though!)
B. False. No energy-conversion process can be 100%
efficient. Recall that entropy in the universe always
has to increase, and for most reactions this occurs by
releasing heat.
C. True. The carbon atoms in glucose are in a reduced state
compared with those in CO 2 , in which they are fully
oxidized.
D. False. The final steps of oxidative phosphorylation do
indeed produce some water (see Figure 13−3). But
water is so abundant in the biosphere that this is no
more than “a drop in the ocean.”
E. True. If it had occurred in only one step, then all the
energy would be released at once and it would be
impossible to harness it efficiently to drive other
reactions, such as the synthesis of ATP.
F. False. Molecular oxygen (O 2 ) is used only in the very last
step of the reaction (see Figure 13−3).
G. True. Plants convert CO 2 into sugars by harvesting the
energy of light in photosynthesis. O 2 is produced in the
process and released into the atmosphere by plant cells.
H. True. Anaerobically growing cells use glycolysis to
oxidize sugars to pyruvate: animal cells convert the
pyruvate into lactate, and no CO 2 is produced; yeast
cells, however, convert the pyruvate into ethanol and
CO 2 . It is this CO 2 gas released from yeast cells during
fermentation that makes bread dough rise and that
carbonates beer and champagne.
ANSWER 13–9 Darwin exhaled the carbon atom, which
therefore must be the carbon atom of a CO 2 molecule.
After spending some time in the atmosphere, the CO 2
molecule must have entered a plant cell, where it became
“fixed” by photosynthesis and converted into part of a
sugar molecule. While it is certain that these early steps
must have happened this way, there are many different
paths from there that the carbon atom could have taken.
The sugar could have been broken down by the plant cell
into pyruvate or acetyl CoA, for example, which then could
have entered biosynthetic reactions to build an amino acid.
The amino acid might have been incorporated into a plant
protein. You might have eaten the delicious leaves of the
plant in your salad, and digested the protein in your gut
to produce amino acids again. After circulating in your
bloodstream, the amino acid might have been taken up by
a developing red blood cell to make its own protein, such
as the hemoglobin in question. If we wish, of course, we can
make our food-chain scenario more complicated. The plant,
for example, might have been eaten by an animal that in
turn was consumed by you during a lunch break. Moreover,
because Darwin died more than 100 years ago, the carbon
atom could have traveled such a route many times. In
each round, however, it would have started again as fully
oxidized CO 2 gas and entered the living world through
photosynthesis in a plant.
ANSWER 13–10 Yeast cells proliferate much better
aerobically. Under anaerobic conditions they cannot perform
oxidative phosphorylation and therefore have to produce all
their ATP by glycolysis, which is less efficient. Whereas one
glucose molecule yields a net gain of two ATP molecules
by glycolysis, the additional use of the citric acid cycle and
oxidative phosphorylation boosts the energy yield up to
about 30 ATP molecules. The citric acid cycle depends on
O 2 because it needs NAD + to continue running.
ANSWER 13–11 The amount of free energy stored in
the phosphate bond in creatine phosphate is larger than
that of the anhydride bonds in ATP. Hydrolysis of creatine
phosphate can therefore be directly coupled to the
production of ATP.
creatine phosphate + ADP → creatine + ATP
The ΔGº for this reaction is –12.6 kJ/mole, indicating that it
proceeds rapidly to the right, as written.
ANSWER 13–12 The extreme conservation of glycolysis
is some of the evidence that all present-day cells are
derived from a single founder cell, as discussed in Chapter
1. The elegant reactions of glycolysis would therefore
have evolved only once, and then they would have been
inherited as organisms evolved. The later invention of
oxidative phosphorylation allowed organisms to capture
15 times more energy from fuel molecules than is possible
by glycolysis alone. This remarkable efficiency is close
to the theoretical limit and hence virtually eliminates the
opportunity for further improvements. Thus, the generation
of alternative pathways would result in no obvious
reproductive advantage that would have been selected in
evolution.
ANSWER 13–13 If one glucose molecule produces 30 ATPs,
then to generate 10 9 ATP molecules will require 1 × 10 9 /30 =
3.3 × 10 7 glucose molecules and 6 × 3.3 × 10 7 = 2 × 10 8
molecules of oxygen. Thus, in one minute, the cell will
consume 2 × 10 8 /(6 × 10 23 ) or 3.3 × 10 –16 moles of oxygen,
which would occupy 3.3 × 10 –16 × 22.4 = 7.4 × 10 –15 liters in
gaseous form. The volume of the cell is 10 –15 cubic meters
[= (10 –5 ) 3 ], which is 10 –12 liter. The cell therefore consumes
an amount of O 2 gas equivalent to about 0.7% of the cell
volume every minute, or an amount of O 2 gas equivalent to
the cell volume in 2 hours and 15 minutes.
ANSWER 13–14 The reactions each have negative ΔG
values and are therefore energetically favorable (see
Figure A13–14 for energy diagrams).
ANSWER 13–15
A. Pyruvate is converted to acetyl CoA, and the labeled 14 C
atom is released as 14 CO 2 gas (see Figure 13–10).
B. By following the 14 C-labeled atom through every
reaction in the citric acid cycle, shown in Panel 13–2 (pp.
442–443), you find that the added 14 C label would be
quantitatively recovered in oxaloacetate. The analysis
also reveals, however, that it is no longer in the keto
group but in the methylene group of oxaloacetate
(Figure A13–15).
ANSWER 13–16 In the presence of molecular oxygen,
oxidative phosphorylation converts most of the cellular
NADH to NAD + (see Figure 13−19). Since fermentation
requires NADH (see Figure 13−6), it is severely inhibited by
the availability of oxygen gas.