Solution Manual for Human Physiology, 16th Edition

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Solution Manual for Human Physiology, 16th Edition delivers clear and concise summaries of essential textbook content.

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1
Fox: Human Physiology, 16th Edition
Chapter 1 Answers to In-chapter Questions
Test Your Understanding
13. Epithelial membranes can be simple (one cell deep) or stratified (many cells deep).
Simple membranes provide a barrier that allows selective transport from the external
environment to the tissues that underlie the epithelial membrane. Stratified
membranes provide more protection but not transport. Stratified squamous keratinized
epithelium of the skin provides the greatest degree of protection.
14. Bone, blood, and the dermis are all connective tissues because they have abundant
extracellular material. However, they differ on what material comprises the extracellular
material and regarding their cells. The extra cellular material of blood is the fluid and
plasma. The dermis contains connective tissue fibers. Bone also has connective tissue
fibers, but the extracellular matrix is calcified.
15. A single negative feedback loop helps to maintain homeostasis by counteracting
changes in one direction. For example, insulin helps to prevent blood glucose from
getting too high. A different hormone counteracts too great a fall in blood glucose.
When these two negative feedback mechanisms act antagonistically to each other, they
afford a finer degree of control in maintaining homeostasis.
16. Insulin is secreted by the pancreatic islets in response to a rise in blood glucose above its
set point. The insulin then acts to lower the blood glucose by promoting the movement
of glucose from the blood into tissue cells. As a result, the blood glucose is lowered
back to the normal range. The lowering of blood glucose causes the pancreatic islets to
reduce their secretion of insulin.
17. In phase I clinical trials, the drug is tested on healthy human volunteers. In phase II
clinical trials, the drug is tested on the target population (those for whom the drug is
intended). Phase III clinical trials involve a much larger and more diverse test
population. Phase IV tests other potential uses of the drug. However, before a drug ever
reaches clinical trials, its development and early testing require animal research.
18. Claude Barnard proposed that the conditions within the body maintain constancy
despite challenges to that constancy. This idea led to the concept of homeostasis and
the concept that physiological mechanisms exist to maintain that internal constancy.

2
Test Your Analytical Ability
19. Positive feedback mechanisms accentuate changes. Thus, if the body temperature were
to rise, positive feedback mechanisms would make it become even hotter. Conversely, if
body temperature were to fall, positive feedback mechanisms would make it become
even colder. A fall in blood glucose would become an even greater fall in response to
positive feedback. Homeostasis could not be maintained and life could not exist under
these conditions.
20. Negative feedback mechanisms may have been acting earlier, but they’re effects don’t
become evident until 40 minutes after injection, when blood glucose starts to rise. The
initial blood glucose concentration appears to be restored 120 minutes after the
injection. Quantitative measurements are needed to determine the effect of insulin and
the effectiveness of antagonistic mechanisms in this instance, and in all cases where
physiological mechanisms are being evaluated.
21. There must be communication between the intracellular fluid and the extracellular fluid
in order for a cell to obtain nutrients and eliminate wastes. There must also be
communication between the interstitial fluid and the blood plasma in order for these
molecules to be obtained and distributed to the cells and for homeostasis to be
maintained.
22. If the blood pressure has fallen so low that the person has collapsed, one would expect,
on the basis of homeostasis and negative feedback, that mechanisms would be set in
motion to help raise the blood pressure. Logically, a more rapid and powerful heartbeat
would have that effect and also cause a more rapid pulse.
23. Adult stem cells are found in the red bone marrow, where they give rise to the blood
cells. Adult stem cells are also found in skeletal muscles and in the adult brain, as well as
in hair follicles and other locations. Unlike embryonic stem cells, adult stem cells cannot
form all of the body tissues and a complete organism.
Test Your Quantitative Ability
24. The set point, as calculated as the average value, is 37.15 degrees.
25. The range of values is 1.10 degrees.
26. The sensitivity is 0.55 degrees.

1
Fox: Human Physiology, 16th Edition
Chapter 2 Answers to In-chapter Questions
Test Your Understanding
16. Nonpolar covalent bonds are formed when the valence electrons are shared equally.
Polar covalent bonds are formed when the valence electrons are shared unequally.
Ionic bonds are formed by negative and positive ions that are attracted to each other,
not by the sharing of electrons.
17. An acid is a molecule that can donate protons (H+) to solution. A base is a molecule that
can directly or indirectly remove protons from the solution.
18. Starch in a potato can be hydrolyzed in the digestive system into its glucose subunits,
which can be absorbed into the blood. The liver can remove glucose from the blood and
through dehydration synthesis reaction synthesize glycogen (animal starch). Then the
blood glucose fall, hydrolysis of liver glycogen releases free glucose for the blood.
19. All fats are lipids because they are nonpolar and not water-soluble. Not all lipids are fats,
because fats are triglycerides and there are other categories of lipids, such as steroids
and phospholipids.
20. Both fats and oils are triglycerides. Fats are solid at room temperature, whereas oils are
liquid. Generally, fats contain more saturated fatty acids then oils. There is evidence
that a diet too high in saturated fatty acids as part of saturated fats can contribute to
atherosclerosis, and thus cardiovascular disease. This may be because saturated fatty
acids help raise the LDL-cholesterol and lower the HDL-cholesterol.
21. DNA serves as a template because of complementary base pairing. An adenine must
pair with thymine, and a guanine must pair with a cytosine. One strand of a parent DNA
is duplicated in this way, so that the new DNA molecule contains one strand that is
“conserved” from the parent DNA and one strand that was newly formed from it.
Test Your Analytical Ability
22. The primary structure refers to the sequence of amino acids in the protein. The
secondary structure refers to its helical structure, and the tertiary structure refers to its
3D structure. How the polypeptide chain forms its higher order structure is determined
by the particular amino acid functional groups and their positions in the polypeptide
chain.

2
23. In order to extract the hormone it must be soluble in the fluid used. If the hormone is
not soluble in water, it may be some type of lipid. This idea is supported by the
observation that the benzene (a nonpolar solvent) extract did have a hormonal effect.
24. It is chemically correct in the sense that the product is free of cholesterol. It is
misleading in the sense that it could be free of cholesterol yet high in saturated fat and
therefore unhealthy despite the lack of cholesterol.
25. Hydrogenation is a method of making butter substitutes that increase fatty acid
saturation and leads to the production of trans fats. Both saturated fats and trans fats
appear to contribute to high blood cholesterol, which is a risk factor in the development
of atherosclerosis.
26. When you cook meat, the heat can cause weak hydrogen bonds and others to break and
lead to a change in the consistency of the meat compared to its raw state. However,
cooking does not break the much stronger covalent peptide bonds, so it doesn’t lead to
a soup of amino acids.
Test Your Quantitative Ability
27. H2O has a molecular weight of 18. C6H12O6 has a molecular weight of 180.
28. Fructose must have the same molecular weight as glucose: 180.
29. Sucrose has a molecular weight of 342.
30. Fructose and glucose each have a molecular weight of 180, so the two together would
be doubled to 360. However, they come together by dehydration synthesis, so water
with a molecular weight of 18 is removed. Therefore, 180 + 180 − 18 = 342.

1
Fox: Human Physiology, 16th Edition
Chapter 3 Answers to In-chapter Questions
Test Your Understanding
17. The plasma membrane can invaginate to form vesicles in endocytosis and can perform
the opposite function in exocytosis. In certain cells, pseudopods can be extended to
allow the cell to move by amoeboid motion. Within the membrane are proteins that
perform a variety of functions that dynamically affect cellular activities.
18. A nucleosome is a particle composed of histone proteins around which two turns of
DNA are wound. Histone proteins are believed to suppress DNA expression in areas
where the DNA is highly compacted to form heterochromatin. This function is affected
by acetylation/deacetylation of the histone proteins.
19. The genetic code is the sequence of bases in DNA that specify the production of RNA.
Messenger RNA in turn codes for proteins, which have many functions, including
enzyme activity. As a result of the production of specific proteins and their actions, the
genetic code thereby influences the structure and function of the body.
20. The DNA code determines the structure of its complementary mRNA, and in that way
carries information regarding the structure of proteins. However, in order for the
genetic code as translated into an mRNA code to become realized as a specific sequence
of amino acids, the actions of transfer RNA molecules are needed. In this way, tRNA
translates the sequence of codons into a sequence of amino acids.
21. Proteins that are produced for the cell’s own use are mostly released from the rough
endoplasmic reticulum into the cytoplasm. However, some are packaged into vesicles
by the Golgi apparatus and sent to the plasma membrane where they function. All the
proteins for secretion are internalized into the ER and then the Golgi apparatus, then
sent for exocytosis within vesicles that bud from the Golgi apparatus.
22. The genome refers to all of the genes within an organism, whereas the proteome refers
to all of the proteins in an organism. The proteome is coded for by the genome.
23. The Golgi complex communicates with the ER, perhaps through the release of vesicles
or perhaps more directly. In this way, proteins produced by the ER are packaged within
the Golgi complex into vesicles. The vesicles released by the Golgi complex migrate to
the plasma membrane, where they fuse with it. This can release secreted products and
it can add new material to the plasma membrane.

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2
24. The two centrioles of a centrosome are required for the production of the microtubules
of the spindle fibers, which attach to the chromosomes during cell division. In a
nondividing cell, the centrioles of a centrosome organize microtubules to help produce
nonmotile primary cilia. In the apical membrane of some epithelial cells, the
centrosomes form motile cilia.
25. A nondividing cell is in the phase of mitosis called interphase, which is divided into three
phases. In the G1 phase, the DNA is actively directing the affairs of the cell. If a cell is
going to divide, it replicates its DNA during the S phase. During the G2 phase, the DNA
condenses into shorter, thicker structures, called chromosomes. Cell division occurs as
a result of the stages of the mitotic phase. The progression through these steps is
regulated in part by proteins called cyclins.
26. Oncogenes are those that promote cancer, and tumor suppressor genes help protect
against cancer. Oncogenes may be genes that normally control cell division and
apoptosis, so that the oncogenes promote the division of cancer cells and prevent their
self-destruction. Tumor suppressor genes, such as p53, have the opposite effect.
27. Apoptosis is a form of programmed cell death that has characteristic features that
distinguish it from necrosis. Apoptosis occurs as part of normal, physiological processes,
including the normal turnover of epithelial cells and blood cells and for tissue
remodeling during embryonic development.
28. Epigenetic inheritance refers to the silencing of specific genes in the gametes that
formed an embryo, so that the expression of those genes is suppressed from one
generation to another. This is a newly discovered form of inheritance that could explain
how metabolic effects of starvation in the parents could affect the offspring, for
example.
Test Your Analytical Ability
29. The histone proteins can repress gene expression, in part by affecting how the DNA is
wound in the nucleosomes. The proteins influence the degree of compaction of the
nucleosomes. The more compact the chromatin, the less access RNA polymerase has to
the sites in the DNA where it must bind to begin transcription. Also, there are
transcription factors, which are proteins that are needed for the expression of specific
genes. Hormones can influence the activity of transcription factors, and thereby affect
genetic expression.
30. The tumor suppressor gene p53 can help guard against cancer by either stopping cell
division at a particular checkpoint or by promoting the apoptosis of a mutated
cancerous cell. One possible way that cancer could be treated would be the

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3
development of a drug that restored p53 activity in a person with a mutated p53 gene,
which occurs in most cancers.
31. Destroying all of the lysosomes in all of the cells in a person’s body would have many
negative consequences. Lysosomes are needed for programmed cell death, so all of the
physiological functions dependent on apoptosis would cease. Also, in order for cells to
turn over worn out organelles, they need lysosomes to digest them, so they and other
products within the cell would accumulate. In addition, the ability of macrophages and
other phagocytic cells to function would cease.
32. Since actinomycin D did not stop the hormone’s action, genetic transcription (RNA
synthesis) was not required for the action of the hormone. Thus, the action was not
dependent of the synthesis of new proteins, such as enzymes. However, the hormone’s
action was blocked by puromycin, so that means that the mRNA coding for that protein
was already present in the cell. Puromycin blocked the translation of that mRNA so that
the protein needed for that hormone’s action could not be produced.
33. It was once thought that each protein is coded by a different gene, but this cannot be
true if the proteome is larger than the genome. Once way that this could happen is if
the exons produced by a gene can combine in different ways to form different proteins,
or if the intervening sequences (introns) for one protein can function as exons in the
production of a different protein.
34. Short interfering RNA (siRNA) is a short double stranded RNA that can, after
modification within a RISC particle, bind by complementary base pairing to a specific
mRNA and cause its destruction. Micro RNA (miRNA) is a short single stranded RNA that
can enter a RISC particle and bind to part of a number of particular mRNA molecules.
This can cause their destruction, but more often just suppresses their expression to a
degree.
35. Ubiquitin is a 76 amino acid peptide that is added to certain regulatory proteins, or
defective proteins, in order to tag them for destruction. Such tagged proteins can then
be digested within proteasomes, which are complexes of protease enzymes.
Test Your Quantitative Ability
36. Since 3 bases comprise one codon for each amino acid, and there are 600 amino acids in
the protein, 1,800 bases are needed (not counting the start and stop sequences and
regulatory sequences).
37. The gene must have 3 exons in order for it to have 2 introns.

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1
Fox: Human Physiology, 16th Edition
Chapter 4 Answers to In-chapter Questions
Test Your Understanding
11. An enzyme is a protein with a specific secondary and tertiary structure that gives it a
unique shape and position of active groups to bind in a specific way to its substrates.
The tertiary structure is held in place by weak bonds, including hydrogen bonds.
Anything that alters this bonding can alter the enzyme’s shape and reduce its action.
This 3D structure can be altered and the enzyme function reduced by changes in pH and
by increases in temperature.
12. An increase in the concentrations of the enzyme’s substrates will drive the reaction to
the right (increase its rate), and a decrease will drive the reaction to the left (decreasing
the reaction rate). Conversely, an increased concentration of products will drive the
reaction to the left, decreasing the reaction rate. Consequently, the reaction rate will be
high when the concentration of substrates is high and the concentration of products is
low.
13. As the reaction products accumulate, they can exert an inhibitory effect on the enzyme,
slowing the reaction rate. This is a form of negative feedback, because a rise in products
inhibits their own formation, preventing excessive accumulation of the products.
14. According to the first law, energy can be transformed but cannot be created or
destroyed. So, some of the chemical energy released by the metabolism of molecules,
such as glucose, can be transformed into the chemical bond energy that binds
phosphate to ADP to form ATP. According to second law, no energy transformation is
100% efficient, and so some energy is released as heat. Also, according to second law,
energy must be needed to produce a more ordered, lower entropy state of ATP from
the less ordered, higher entropy state of ADP plus phosphate.
15. Shuttling of hydrogen atoms involves transfer of electrons, so the gaining of hydrogen is
accompanied by the gaining of an electron and is a reduction, while the loss of hydrogen
is the loss of an electron and an oxidation reaction. NAD and FAD can gain hydrogens
and electrons and move from one location (inside the matrix of the mitochondrion) to
another (the cristae) to transfer hydrogens and couple reduction and oxidation
reactions.

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2
16. An inborn error of metabolism often involves single gene mutations that result in single
defective enzymes. Such is the case with albinism and PKU. Both conditions are
inherited defects in a single enzyme involved in the metabolism of phenylalanine. PKU is
caused by a defect in the enzyme that converts phenylalanine to tyrosine. Albinism is
caused by a defect in a later enzyme in this pathway, which converts DOPA to melanin.
17. Niacin and riboflavin are needed to produce NAD and FAD, respectively. We need NAD
and FAD to shuttle hydrogens and electrons from the metabolic pathways that
breakdown glucose and other molecules to the location in mitochondria where most of
the ATP is produced. We don’t directly get energy from NAD and FAD, but we need
these molecules in order to derive energy (ATP) from food molecules.
Test Your Analytical Ability
18. Metabolic pathways are frequently branched, with one branch promoted by one
enzyme and a different branch promoted by a different enzyme. The enzymes behave
as switches on these branched pathways, determining which direction the path will
proceed according to the relative amounts of the two branch point enzymes.
19. The LDH enzyme interconverts pyruvic and lactic acids, according to the relative
concentrations of each. It is normally active within the cell cytoplasm but can enter the
blood when cells die. Once it is in the blood, it is inactive because conditions in the
blood do not favor either reaction.
20. First, the plant generally has a pH greater than that of the stomach, so the enzymes
would be unlikely to be very active in the stomach and would likely denature at the
great acidity of gastric juice. Since the denatured enzyme is a protein, it would likely be
digested by pepsin in the stomach and other enzymes in the small intestine.
21. According to the second law of thermodynamics, energy is lost as heat in every energy
transformation, including the energy transformations that occur when animals eat
plants or other animals. So, it requires most total biomass of plants to support a smaller
biomass of herbivores, and a greater biomass of herbivores to support a smaller
biomass of carnivores.
22. A person who hyperventilates exhales more carbon dioxide than is exhaled in normal
breathing. This lowers the blood CO2, which lowers the amount of carbonic acid formed
in the blood. The lower than normal formation of carbonic acid lowers the amount of
H+, thereby raising the blood pH.

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3
Test Your Quantitative Ability
23. The pH optimum of pepsin is close to 2. The pH optimum of salivary amylase is close to
7. The pH optimum of trypsin is close to 9.
24. Half-maximum activity of salivary amylase appears to occur at a pH of about 6 and a pH
of about 8.
25. Half-maximum activity of pepsin appears to occur at pH a little over 1, and a pH a little
less than 4.
26. Pepsin and salivary amylase appear to have overlapping activities at a pH of about 5.
27. Since salivary amylase has a pH optimum near 7, and almost no activity at acidic pH
values, once the swallowed saliva is mixed with the very acidic gastric juice the activity
of salivary amylase should be greatly decreased.

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1
Fox: Human Physiology, 16th Edition
Chapter 5 Answers to In-chapter Questions
Test Your Understanding
14. The lactic acid pathway has the advantage that it is shorter than the aerobic pathway
and can quickly generate ATP in the absence of oxygen. However, it only produces a net
gain of 2 ATP, which is much less than the ATP produced by aerobic respiration of
glucose and other molecules. It also produces an acid, lactic acid, which can accumulate
and lower the pH.
15. Pyruvate is formed as the end product of the glycolytic pathway. In this process, NAD is
reduced to NADH+ + H+. In order to regenerate NAD to continue this process, the two
hydrogens are donated to pyruvate to produce lactate. During aerobic respiration, the
oxidation of NADH+ + H+ to NAD is accomplished by the electron transport chain in the
mitochondria.
16. Cyanide inhibits the final step in aerobic respiration, the passage of the electrons to
oxygen. When this step is blocked, the components of the electron transport chain stay
in the reduced state because they cannot pass the electrons to the next step. As a
result, the electron transport chain cannot accept electrons from NADH. When there is
insufficient NAD+ available for the citric acid cycle, it stops. Therefore, the production of
ATP must stop, and the cell dies because of lack of sufficient energy.
17. Glucose can be converted by glycolysis into 3-phospgoglyeraldehyde, which can be
converted into glycerol. Also, glucose can be converted into acetyl CoA, which can be
used to produce fatty acids. The glycerol and fatty acids can then condense to form
triglycerides. End-product inhibition by ATP inhibits the steps of aerobic respiration, so
the intermediate molecules can be funneled into the production of fat.
18. The metabolic pathway by which fat is broken down for energy is called beta-oxidation.
In this process, each fatty acid chain is successively broken down into two-carbon
molecules that form acetyl CoA. At each step, an ATP and reduced NADH and FADH2 are
generated, which can donate electrons to the electron transport chain to generate ATP
by oxidative phosphorylation. In this way, the breakdown of fatty acids yields many
more ATP than the breakdown of glucose.
19. Through the process of oxidative deamination, amino acids can be converted into
pyruvate, acetyl CoA, and a few different intermediates of the citric acid cycle. Once in
the aerobic respiratory pathway, these molecules can be metabolized for energy the
same they are when they are derived from glucose. However, the release of the amine

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2
groups provides substrates for urea synthesis. So, a starving person who is breaking
down protein for energy will be producing more urea.
20. Only the liver can secrete glucose into the blood because only the liver has the enzyme
glucose-6-phosphatase to remove the phosphate group to produce free glucose for
secretion. Other organs lack this enzyme, and organic molecules with phosphate groups
cannot pass through the plasma membrane.
21. The production of lactic acid from glucose is termed fermentation by analogy with the
way yeast cells convert glucose to ethanol. In both the production of lactic acid and of
ethanol, the last electron acceptor is an organic molecule rather than oxygen. In the
case of lactic acid fermentation, the final electron acceptor is pyruvic acid.
22. Brown adipose cells have uncoupling protein, located in the inner mitochondrial
membrane, which allows H+ to leak back into the matrix and thus not be used for
oxidative phosphorylation. As a result, the efficiency of oxidative phosphorylation is
reduced (reducing the amount of ATP production) and metabolic heat production
increases. This suggests that in white adipose cells, which lack uncoupling protein, the
greater amount of ATP makes metabolism slower by end-product inhibition.
23. Three molecules that can be used for gluconeogenesis are lactate, amino acids, and
glycerol. Blood lactate levels rise during exercise, and lactate is converted into glucose
following exercise (Cori cycle). Amino acids can likewise be converted into glucose when
needed, as during starvation. Glycerol is a 3-carbon long molecule that can be converted
into pyruvate when triglycerides are hydrolyzed.
Test Your Analytical Ability
24. Fat has the highest amount of energy (calories) per weight, so eliminating fat could help
with reducing her caloric intake and weight loss. However, calories can be obtained
from carbohydrates and proteins too, and if the caloric loss from the elimination of fat is
made up for by eating proteins and carbohydrates, weight will not be lost. (In fact, a
person could get hungrier on such a diet and gain weight.) Also, we need to eat some
fats and oils to get essential fatty acids and the fat-soluble vitamins.
25. If the drug caused the loss of the H+ gradient between the intermembrane space and
the matrix, there will be less H+ passing through cytochrome oxidase to generate ATP.
The cellular ATP would decrease, and with less end-product inhibition, the rate of
aerobic metabolism would increase, generating heat to raise body temperature. The
increased metabolism of adipose cells would cause weight loss.

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26. First, the NADH generated in the cytoplasm by glycolysis has to be shuttled into the
mitochondrion to donate electrons to the electron transport chain. There are two
possible shuttles, one in which 1 cytoplasmic NADH becomes 1 mitochondrial NADH,
and one in which 1 cytoplasmic NADH becomes 1 mitochondrial FADH2. These yield
different numbers of ATP. Second, the ATP generated by oxidative phosphorylation in
the mitochondria must be transported into the cytoplasm, and that transport requires
the use of ATP. These scenarios vary somewhat in different organs, so when any
number is given for the number of ATP produced by oxidative phosphorylation it must
be considered approximate.
27. People who are starving have thin arms and legs because they are breaking down their
muscle protein for energy. The amino acids travel in the blood to the liver, where they
are used for gluconeogenesis. The liver is the only organ that can secrete glucose
derived from stored glycogen or from gluconeogenesis into the blood. These actions
allow the liver to maintain adequate blood glucose during starvation, which is required
for the brain’s metabolism and to keep the person alive.
28. Chicken proteins never enter your body, because they are hydrolyzed in the GI tract to
amino acids. The amino acids from the chicken can then be used for energy or to
synthesize your proteins according to your own genetic code. Similarly, the starch in the
bread never enters your body, only the constituent glucose molecules. These can be
used within your cells for energy or for the synthesis of your own starch (glycogen).
Because chicken proteins and bread starch do not enter your body, it is not strictly true
that you are what you eat.
Test Your Quantitative Ability
29. A twenty-carbon long fatty acid can be broken up into 10 acetyl CoA molecules, since
each is 2 carbons long.
30. Since 1 ATP is broken down every time beta-oxidation of a fatty acid releases an acetyl
CoA, 10 ATP must be broken down in the complete oxidation of a twenty-carbon long
fatty acid.
31. None. All of the ATP is produced by donation of electrons from NADH and FADH2 to the
electron transport chain and oxidative phosphorylation.
32. There will be 10 acetyl CoA generated from a twenty-carbon long fatty acid, and 1 NADH
and 1 FADH2 each time, for a total of 20.
33. 14 ATP is made per each beta-oxidation, as shown in the figure. So with 10 such
processes, the total would be 140 ATP.

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1
Fox: Human Physiology, 16th Edition
Chapter 6 Answers to In-chapter Questions
Test Your Understanding
16. In order for osmosis to occur, two solutions of unequal concentration must be
separated by a semipermeable membrane that is permeable to water but not to one of
the solutes. When that occurs, the random motion of water molecules will cause the net
movement of water from the side of greater to the side of lower water concentration.
17. Simple diffusion occurs in a nonspecific fashion across the plasma membrane.
Facilitated diffusion requires protein carriers in the plasma membrane, and displays the
properties of specificity, competition, and saturation that are characteristic of carrier-
mediated transport.
18. The equilibrium potential for K+ is −90 mV, and that for Na+ is +60 mV. The resting
membrane potential of neurons is about −70 mV. This value is closer to the equilibrium
potential for K+ than that for Na+ because the resting membrane is far more permeable
to K+ than to Na+.
19. The Na+/K+ pumps transport 3 Na+ out of the cell for every 2 K+ they transport into the
cell, thereby adding to the relative negativity of the inside of the resting cell. If gated
Na+ channels were to open, Na+ would diffuse down its electrochemical gradient into
the cell, making it less negative (and even positive) on the inside. If the gated K+
channels opened, K+ would diffuse out of the cell and make the inside of the cell more
negatively charged.
20. Glucose and water molecules cannot penetrate the plasma membrane without the aid
of specific proteins. Carrier proteins for the facilitated diffusion of glucose can be
inserted into the plasma membrane when intracellular vesicles containing these carriers
fuse with the plasma membrane. They can be removed from the plasma membrane
when the membrane invaginates to form a vesicle with the carriers in the membrane.
Water channels in vesicle membranes can similarly be added or removed to increase or
decrease water permeability.
21. The rate of net diffusion across a plasma membrane is influenced by the temperature
(which is relatively constant), the concentration difference across the membrane, the
diffusion distance, and the surface area of membrane through which the diffusing
molecules must pass. Epithelial membranes specialized for diffusion are simple rather
than stratified and generally squamous to minimize the diffusion distance. In some
locations, the cells also have microvilli to increase the surface area for transport.

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