Answers to Test & Quantitative and Thought Questions
(2) binding to a membrane protein that normally
stimulates acid secretion but not itself triggering
acid secretion, thereby preventing the body’s natural
messengers from binding (competition); or (3) having
an allosteric effect on the binding sites, which would
increase the afﬁ nity of the sites that normally bind
inhibitor messengers or decrease the afﬁ nity of those
sites that normally bind stimulatory messengers.
The reason for a lack of insulin effect could be either
a decrease in the number of available binding sites
insulin can bind to or a decrease in the afﬁ nity of the
binding sites for insulin so that less insulin is bound.
A third possibility, which does not involve insulin
binding, would be a defect in the way the binding site
triggers a cell response once it has bound insulin.
(a) Acid secretion could be increased to 40 mmol/h
by (1) increasing the concentration of compound X
from 2 pM to 8 pM, thereby increasing the number
of binding sites occupied; or (2) increasing the
afﬁ nity of the binding sites for compound X, thereby
increasing the amount bound without changing the
concentration of compound X. (b) Increasing the
concentration of compound X from 18 to 28 pM will
not increase acid secretion because, at 18 pM, all the
binding sites are occupied (the system is saturated),
and there are no further binding sites available.
The maximum rate at which the end product E can
be formed is 5 molecules per second, the rate of the
slowest (rate-limiting) reaction in the pathway.
Under normal conditions, the concentration of oxygen
at the level of the mitochondria in cells, including
muscle at rest, is sufﬁ cient to saturate the enzyme that
combines oxygen with hydrogen to form water. The
rate-limiting reactions in the electron transport chain
depend on the available concentrations of ADP and P
which are combined to form ATP.
Thus, increasing the oxygen concentration above
normal levels will not increase ATP production. If a
muscle is contracting, it will break down ATP into
ADP and P
, which become the major rate-limiting
substrates for increasing ATP production. With
intense muscle activity, the level of oxygen may fall
below saturating levels, limiting the rate of ATP
production, and intensely active muscles must then
use anaerobic glycolysis to provide additional ATP.
Under these circumstances, increasing the oxygen
concentration in the blood will increase the rate of
ATP production. As discussed in Chapter 12, it is
not the concentration of oxygen in the blood that is
increased during exercise but the rate of blood ﬂ ow
to a muscle, resulting in greater quantities of oxygen
delivery to the tissue.
During starvation, in the absence of ingested glucose,
the body’s stores of glycogen are rapidly depleted.
Glucose, which is the major fuel used by the brain,
must now be synthesized from other types of
molecules. Most of this newly formed glucose comes
from the breakdown of proteins to amino acids and
their conversion to glucose. To a lesser extent, the
glycerol portion of fat is converted to glucose. The fatty
acid portion of fat cannot be converted to glucose.
Ammonia is formed in most cells during the oxidative
deamination of amino acids and then travels to the
liver via the blood. The liver detoxiﬁ es the ammonia
by converting it to the nontoxic compound urea.
Because the liver is the site in which ammonia is
converted to urea, diseases that damage the liver can
lead to an accumulation of ammonia in the blood,
which is especially toxic to nerve cells. Note that it is
not the liver that produces the ammonia.
Channels are proteins that span the membrane and are
opened by ligands, voltage, or mechanical stimuli.
Facilitated diffusion does not require ATP. Recall
that secondary active transport
ATP because ion pumps were needed to establish the
electrochemical gradient for a particular ion (such as
After the initial movement of water out of the cells
due to osmosis, the urea concentration quickly
equilibrates across each cell’s plasma membrane,
removing any osmotic stimulus.
Segregation of function on different surfaces of the
cell, and the ability to secrete chemicals (e.g., from the
pancreas), are two of the most important features of
Diffusion is slowed by the resistance of a membrane.
Because ions are charged, both the chemical and the
electrical gradients determine its rate and direction of
Quantitative and Thought Questions
(a) During diffusion, the net ﬂ ux always occurs from
high to low concentration. Thus, it will be from 2 to
1 in A and from 1 to 2 in B. (b) At equilibrium, the
concentrations of solute in the two compartments
will be equal: 4 mM in case A and 31 mM in case B.
(c) Both will reach diffusion equilibrium at the same
rate because the difference in concentration across the
membrane is the same in each case, 2 mM [(3 – 5) = –2,
and (32 – 30) = 2]. The two one-way ﬂ
uxes will be
much larger in B than in A, but the net ﬂ ux has the
same magnitude in both cases, although it is oriented
in opposite directions.
The ability of one amino acid to decrease the ﬂ
of a second amino acid across a cell membrane is an
example of the competition of two molecules for the
same binding site, as explained in Chapter 3. The
binding site for alanine on the transport protein can
also bind leucine. The higher the concentration of
alanine, the greater the number of binding sites that it
occupies, and the fewer available for binding leucine.
Thus, less leucine will move into the cell.