Essential Cell Biology 5th edition

Answers A:33
ANSWER 12–21 The membrane potential and the high
extracellular Na + concentration provide a large inward
electrochemical driving force and a large reservoir of Na +
ions, so that mostly Na + ions enter the cell as acetylcholine
receptors open. Ca 2+ ions will also enter the cell, but
their influx is much more limited because of their lower
extracellular concentration. (Most of the Ca 2+ that enters
the cytosol to stimulate muscle contraction is released from
intracellular stores, as we discuss in Chapter 17). Because
of the high intracellular K + concentration and the opposing
direction of the membrane potential, there will be little if
any movement of K + ions upon opening of a cation channel.
ANSWER 12–22 The diversity of neurotransmitter-gated ion
channels raises the hope of developing new drugs specific
for each channel type. Each of the diverse subtypes of
these channels is expressed in a narrow subset of neurons.
This narrow range of expression should make it possible, in
principle, to discover or design drugs that affect particular
receptor subtypes present in a selected set of neurons, thus
targeting particular brain functions with greater specificity.
Chapter 13
ANSWER 13–1 To keep glycolysis going, cells need
to regenerate NAD + from NADH. In the absence of
oxygen, there is no efficient way to do this without
fermentation. Without regenerated NAD + , step 6 of
glycolysis [the oxidation of glyceraldehyde 3-phosphate to
1,3-bisphosphoglycerate (Panel 13–1, pp. 436–437)] could
not occur, and the product glyceraldehyde 3-phosphate
would accumulate. The same thing would happen in cells
unable to make either lactate or ethanol: neither would
be able to regenerate NAD + , and so glycolysis would be
blocked at the same step.
ANSWER 13–2 Arsenate instead of phosphate becomes
attached in step 6 of glycolysis to form 1-arseno-3-
phosphoglycerate (Figure A13–2). Because of its sensitivity
to hydrolysis in water, the high-energy bond is destroyed
before the molecule that contains it can diffuse to
reach the next enzyme. The product of the hydrolysis,
3-phosphoglycerate, is the same product normally formed
in step 7 by the action of phosphoglycerate kinase. But
because hydrolysis occurs nonenzymatically, the energy
liberated by breaking the high-energy bond cannot be
captured to generate ATP. In Figure 13–7, therefore, the
reaction corresponding to the downward-pointing arrow
in step 7 would still occur, but the wheel that provides
the coupling to ATP synthesis is missing. Arsenate wastes
metabolic energy by uncoupling many phosphotransfer
reactions by the same mechanism, which is why it is so
poisonous.
ANSWER 13–3 The oxidation of fatty acids breaks the
carbon chain down into two-carbon units at a time (acetyl
groups that had become attached to CoA). Conversely,
during biogenesis, fatty acids are constructed by linking
together acetyl groups. Most fatty acids therefore have an
even number of carbon atoms.
ANSWER 13–4 Because the function of the citric acid cycle
is to harvest the energy released during the oxidation, it
is advantageous to break the overall reaction into as many
steps as possible (see Figure 13–1). Using a two-carbon
compound (acetyl CoA), the available chemistry would be
much more limited, and it would be impossible to generate
as many intermediates.
ANSWER 13–5 It is true that oxygen atoms are returned
to the atmosphere as part of CO 2 during the oxidative
degradation of glucose (see Figure 13−3). The CO 2
released from the cells, however, does not contain the
specific oxygen atoms consumed as part of the oxidative
phosphorylation process; these are converted into water.
One can show this directly by incubating living cells in an
atmosphere that includes molecular oxygen containing the
18 O isotope of oxygen instead of the naturally abundant
isotope, 16 O. In such an experiment, one finds that all the
CO 2 released from cells contains only 16 O. Therefore, the
oxygen atoms in the released CO 2 molecules do not come
directly from the atmosphere but from organic molecules
that the cell has first made and then oxidized as fuel (see
top of first page of Panel 13–2, pp. 442–443).
ANSWER 13–6 The cycle continues because intermediates
are replenished as necessary by reactions leading into
the citric acid cycle (instead of away from it). One of the
most important reactions of this kind is the conversion
of pyruvate to oxaloacetate by the enzyme pyruvate
carboxylase:
pyruvate + CO 2 + ATP + H 2 O → oxaloacetate + ADP + P i + 2H +
This reaction feeds oxaloacetate into the citric acid cycle.
It is one of the many examples of how metabolic pathways
are carefully coordinated to work together to maintain
appropriate concentrations of all metabolites required by
the cell (see Figure A13–6).
ADP
+ P
ATP
CO 2
pyruvate
carboxylase
oxaloacetate
pyruvate
acetyl CoA
CITRIC
ACID
CYCLE
citrate
O O
C
H C OH
O
As
O OH
O –
O – C
H C OH + AsO
3– 4 +
CH 2 O P
CH 2 O P
H 2 O
Figure A13–2
H +
Figure A13–6
ANSWER 13–7 The carbon atoms in sugar molecules are
already partially oxidized. In contrast, only the very first
carbon atom in the acyl chains of fatty acids is oxidized.
Thus, two carbon atoms from glucose are lost as CO 2 during
the conversion of pyruvate to acetyl CoA (see Figure 13−3),
leaving only four carbons to enter the citric acid cycle,
ECB5 eA13.02/A13.02
ECB5 eA13.06/A13.06