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# CHAPTER 1
# Cellular Physiology
Volume and Composition of Body Fluids, 1
Characteristics of Cell Membranes, 4
Transport Across Cell Membranes, 5
Diffusion Potentials and Equilibrium
Potentials, 14
Resting Membrane Potential, 18
Action Potentials, 19
Synaptic and Neuromuscular Transmission, 26
Skeletal Muscle, 34
Smooth Muscle, 40
Summary, 43
Challenge Yourself, 44
Understanding the functions of the organ systems
requires profound knowledge of basic cellular mecha -
nisms. Although each organ system differs in its overall
function, all are undergirded by a common set of physi -
ologic principles.
The following basic principles of physiology are
introduced in this chapter: body fluids, with particular
emphasis on the differences in composition of intracel -
lular fluid and extracellular fluid; creation of these
concentration differences by transport processes in cell
membranes; the origin of the electrical potential differ -
ence across cell membranes, particularly in excitable
cells such as nerve and muscle; generation of action
potentials and their propagation in excitable cells;
transmission of information between cells across syn -
apses and the role of neurotransmitters; and the
mechanisms that couple the action potentials to con -
traction in muscle cells.
These principles of cellular physiology constitute a
set of recurring and interlocking themes. Once these principles are understood, they can
be applied and integrated into the function of each organ system.
# VOLUME AND COMPOSITION OF BODY FLUIDS
Distribution of Water in the Body Fluid Compartments
In the human body, water constitutes a high proportion of body weight. The total
amount of fluid or water is called total body water , which accounts for 50% to 70%
of body weight. For example, a 70-kilogram (kg) man whose total body water is 65%
of his body weight has 45.5 kg or 45.5 liters (L) of water (1 kg water 1 L water). In
general, total body water correlates inversely with body fat. Thus total body water is a
higher percentage of body weight when body fat is low and a lower percentage when
body fat is high. Because females have a higher percentage of adipose tissue than males,
they tend to have less body water. The distribution of water among body fluid compart -
ments is described briefly in this chapter and in greater detail in Chapter 6.
Total body water is distributed between two major body fluid compartments: intracel -
lular fluid (ICF) and extracellular fluid (ECF) (Fig. 1.1). The ICF is contained within the
cells and is two-thirds of total body water; the ECF is outside the cells and is one-third
of total body water. ICF and ECF are separated by the cell membranes.
ECF is further divided into two compartments: plasma and interstitial fluid. Plasma
is the fluid circulating in the blood vessels and is the smaller of the two ECF 2 Physiology
equivalent of chloride (Cl ). Likewise, one mole of
calcium chloride (CaCl 2) in solution dissociates into
two equivalents of calcium (Ca 2+) and two equivalents
of chloride (Cl ); accordingly, a Ca 2+ concentration of
1 mmol/L corresponds to 2 mEq/L.
One osmole is the number of particles into which a
solute dissociates in solution. Osmolarity is the con -
centration of particles in solution expressed as osmoles
per liter. If a solute does not dissociate in solution (e.g.,
glucose), then its osmolarity is equal to its molarity. If
a solute dissociates into more than one particle in
solution (e.g., NaCl), then its osmolarity equals the
molarity multiplied by the number of particles in solu -
tion. For example, a solution containing 1 mmol/L
NaCl is 2 mOsm/L because NaCl dissociates into two
particles.
pH is a logarithmic term that is used to express
hydrogen (H +) concentration. Because the H + concen -
tration of body fluids is very low (e.g., 40 10 9 Eq/L
in arterial blood), it is more conveniently expressed as
a logarithmic term, pH. The negative sign means that
pH decreases as the concentration of H + increases, and
pH increases as the concentration of H + decreases. Thus
pH H= +
log [ ]10
subcompartments. Interstitial fluid is the fluid that
actually bathes the cells and is the larger of the two
subcompartments. Plasma and interstitial fluid are
separated by the capillary wall. Interstitial fluid is an
ultrafiltrate of plasma, formed by filtration processes
across the capillary wall. Because the capillary wall is
virtually impermeable to large molecules such as
plasma proteins, interstitial fluid contains little, if any,
protein.
The method for estimating the volume of the body
fluid compartments is presented in Chapter 6.
Composition of Body Fluid Compartments
The composition of the body fluids is not uniform. ICF
and ECF have vastly different concentrations of various
solutes. There are also certain predictable differences
in solute concentrations between plasma and interstitial
fluid that occur as a result of the exclusion of protein
from interstitial fluid.
Units for Measuring Solute Concentrations
Typically, amounts of solute are expressed in moles,
equivalents, or osmoles. Likewise, concentrations of
solutes are expressed in moles per liter (mol/L),
equivalents per liter (Eq/L), or osmoles per liter
(Osm/L). In biologic solutions, concentrations of
solutes are usually quite low and are expressed in
milli moles per liter (mmol/L), milli equivalents per liter
(mEq/L), or milli osmoles per liter (mOsm/L).
One mole is 6 10 23 molecules of a substance. One
millimole is 1/1000 or 10 3 moles. A glucose concentra -
tion of 1 mmol/L has 1 10 3 moles of glucose in 1 L
of solution.
An equivalent is used to describe the amount of
charged (ionized) solute and is the number of moles
of the solute multiplied by its valence. For example,
one mole of potassium chloride (KCl) in solution dis -
sociates into one equivalent of potassium (K +) and one
> TOTAL BODY WATER
> Intracellular fluid Extracellular fluid Cell membrane Capillary wall Interstitial fluid Plasma
> Fig. 1.1 Body fluid compartments.
SAMPLE PROBLEM. Two men, Subject A and
Subject B, have disorders that cause excessive acid
production in the body. The laboratory reports the
acidity of Subject As blood in terms of [H +] and the
acidity of Subject Bs blood in terms of pH. Subject
A has an arterial [H +] of 65 10 9 Eq/L, and Subject
B has an arterial pH of 7.3. Which subject has the
higher concentration of H + in his blood?
SOLUTION. To compare the acidity of the blood of
each subject, convert the [H +] for Subject A to pH
as follows:
pH HEq/L Eq/L
= = = +
log [ ]log ( )log ( . )
log 10 10 910 8165 10 6 5 10 0010 810 86 5 0 81 10 8 0 6 5 10 0 81 8 0 7 19 . .log .log . . ( . ) .
== = + = =
pH =( . ) .7 19 7 19
Thus Subject A has a blood pH of 7.19 computed
from the [H +], and Subject B has a reported blood
pH of 7.3. Subject A has a lower blood pH, reflecting
a higher [H +] and a more acidic condition.
Electroneutrality of Body Fluid Compartments
Each body fluid compartment must obey the principle
of macroscopic electroneutrality ; that is, each 1Cellular Physiology 3
Creation of Concentration Differences
Across Cell Membranes
The differences in solute concentration across cell
membranes are created and maintained by energy-
consuming transport mechanisms in the cell membranes.
The best known of these transport mechanisms is
the Na +-K + ATPase (Na +-K + pump), which transports
Na + from ICF to ECF and simultaneously transports K +
from ECF to ICF. Both Na + and K + are transported
against their respective electrochemical gradients;
therefore an energy source, adenosine triphosphate
(ATP), is required. The Na +-K + ATPase is responsible
for creating the large concentration gradients for Na +
and K + that exist across cell membranes (i.e., the low
intracellular Na + concentration and the high intracel -
lular K + concentration).
Similarly, the intracellular Ca 2+ concentration is
maintained at a level much lower than the extracellular
Ca 2+ concentration. This concentration difference is
established, in part, by a cell membrane Ca 2+ ATPase
that pumps Ca 2+ against its electrochemical gradient.
Like the Na +-K + ATPase, the Ca 2+ ATPase uses ATP as a
direct energy source.
In addition to the transporters that use ATP directly,
other transporters establish concentration differences
across the cell membrane by utilizing the transmem -
brane Na + concentration gradient (established by the
Na +-K + ATPase) as an energy source. These transporters
create concentration gradients for glucose, amino acids,
Ca 2+, and H + without the direct utilization of ATP.
Clearly, cell membranes have the machinery to
establish large concentration gradients. However, if
cell membranes were freely permeable to all solutes,
these gradients would quickly dissipate. Thus it is
critically important that cell membranes are not freely
permeable to all substances but, rather, have selec -
tive permeabilities that maintain the concentration
gradients established by energy-consuming transport
processes.
Directly or indirectly, the differences in composition
between ICF and ECF underlie every important physi -
ologic function, as the following examples illustrate: (1)
The resting membrane potential of nerve and muscle
critically depends on the difference in concentration of
K+ across the cell membrane; (2) The upstroke of the
action potential of these same excitable cells depends
on the differences in Na + concentration across the cell
membrane; (3) Excitation-contraction coupling in
muscle cells depends on the differences in Ca 2+ concen -
tration across the cell membrane and the membrane of
the sarcoplasmic reticulum (SR); and (4) Absorption of
essential nutrients depends on the transmembrane Na +
concentration gradient (e.g., glucose absorption in the
small intestine or glucose reabsorption in the renal
proximal tubule).
compartment must have the same concentration, in
mEq/L, of positive charges (cations) as of negative
charges (anions) . There can be no more cations than
anions, or vice versa. Even when there is a potential
difference across the cell membrane, charge balance
still is maintained in the bulk (macroscopic) solutions.
(Because potential differences are created by the sepa -
ration of just a few charges adjacent to the membrane,
this small separation of charges is not enough to
measurably change bulk concentrations.)
Composition of Intracellular Fluid and
Extracellular Fluid
The compositions of ICF and ECF are strikingly differ -
ent, as shown in Table 1.1. The major cation in ECF is
sodium (Na +), and the balancing anions are chloride
(Cl ) and bicarbonate (HCO 3). The major cations in
ICF are potassium (K +) and magnesium (Mg 2+), and the
balancing anions are proteins and organic phosphates.
Other notable differences in composition involve Ca 2+
and pH. Typically, ICF has a very low concentration of
ionized Ca 2+ (10 7 mol/L), whereas the Ca 2+ concentra -
tion in ECF is higher by approximately four orders of
magnitude. ICF is more acidic (has a lower pH) than
ECF. Thus substances found in high concentration in
ECF are found in low concentration in ICF, and vice
versa.
Remarkably, given all of the concentration differ -
ences for individual solutes, the total solute concentra -
tion (osmolarity) is the same in ICF and ECF. This
equality is achieved because water flows freely across
cell membranes. Any transient differences in osmolar -
ity that occur between ICF and ECF are quickly dissi -
pated by water movement into or out of cells to
reestablish the equality.
> TABLE 1.1 Approximate Compositions of Extracellular
> and Intracellular Fluids
> Substance and Units
> Extracellular
> Fluid
> Intracellular
> Fluid a
> Na +(mEq/L) 140 14
> K+(mEq/L) 4120
> Ca 2+, ionized (mEq/L) 2.5 b110 4
> Cl (mEq/L) 105 10
> HCO 3(mEq/L) 24 10
> pH c7.4 7.1
> Osmolarity (mOsm/L) 290 290
> aThe major anions of intracellular fluid are proteins and organic
> phosphates.
> bThe corresponding total [Ca 2+] in extracellular fluid is 5 mEq/L
> or 10 mg/dL.
> cpH is log 10 of the [H +]; pH 7.4 corresponds to [H +] of 40
> 10 9Eq/L. 4Physiology
Phospholipid Component of Cell Membranes
Phospholipids consist of a phosphorylated glycerol
backbone (head) and two fatty acid tails (Fig. 1.2).
The glycerol backbone is hydrophilic (water soluble),
and the fatty acid tails are hydrophobic (water insolu -
ble). Thus phospholipid molecules have both hydro -
philic and hydrophobic properties and are called
amphipathic . At an oil-water interface (see Fig. 1.2A),
molecules of phospholipids form a monolayer and
orient themselves so that the glycerol backbone dis -
solves in the water phase and the fatty acid tails dis -
solve in the oil phase. In cell membranes (see Fig.
1.2B), phospholipids orient so that the lipid-soluble
fatty acid tails face each other and the water-soluble
glycerol heads point away from each other, dissolving
in the aqueous solutions of the ICF or ECF. This orienta -
tion creates a lipid bilayer .
Protein Component of Cell Membranes
Proteins in cell membranes may be either integral or
peripheral, depending on whether they span the mem -
brane or whether they are present on only one side.
The distribution of proteins in a phospholipid bilayer
is illustrated in the fluid mosaic model , shown in
Figure 1.3.
Integral membrane proteins are embedded in, and
anchored to, the cell membrane by hydrophobic
interactions . To remove an integral protein from the
cell membrane, its attachments to the lipid bilayer
must be disrupted (e.g., by detergents). Some inte -
gral proteins are transmembrane proteins , meaning
they span the lipid bilayer one or more times; thus
Concentration Differences Between
Plasma and Interstitial Fluids
As previously discussed, ECF consists of two subcom -
partments: interstitial fluid and plasma. The most sig -
nificant difference in composition between these two
compartments is the presence of proteins (e.g., albumin)
in the plasma compartment. Plasma proteins do not
readily cross capillary walls because of their large
molecular size and therefore are excluded from inter -
stitial fluid.
The exclusion of proteins from interstitial fluid has
secondary consequences. The plasma proteins are
negatively charged, and this negative charge causes a
redistribution of small, permeant cations and anions
across the capillary wall, called a Gibbs-Donnan equil -
ibrium . The redistribution can be explained as follows:
The plasma compartment contains the impermeant,
negatively charged proteins. Because of the requirement
for electroneutrality, the plasma compartment must
have a slightly lower concentration of small anions
(e.g., Cl ) and a slightly higher concentration of small
cations (e.g., Na + and K +) than that of interstitial fluid.
The small concentration difference for permeant ions
is expressed in the Gibbs-Donnan ratio , which gives
the plasma concentration relative to the interstitial fluid
concentration for anions and interstitial fluid relative to
plasma for cations. For example, the Cl concentration
in plasma is slightly less than the Cl concentration in
interstitial fluid (due to the effect of the impermeant
plasma proteins); the Gibbs-Donnan ratio for Cl is
0.95, meaning that [Cl ]plasma /[Cl ]interstitial fluid equals 0.95.
For Na +, the Gibbs-Donnan ratio is also 0.95, but Na +,
being positively charged, is oriented the opposite way,
and [Na +]interstitial fluid /[Na +]plasma equals 0.95. Generally,
these minor differences in concentration for small
cations and anions between plasma and interstitial
fluid are ignored.
# CHARACTERISTICS OF CELL
# MEMBRANES
Cell membranes are composed primarily of lipids and
proteins. The lipid component consists of phospholip -
ids, cholesterol, and glycolipids and is responsible for
the high permeability of cell membranes to lipid-soluble
substances such as carbon dioxide, oxygen, fatty acids,
and steroid hormones. The lipid component of cell
membranes is also responsible for the low permeability
of cell membranes to water-soluble substances such as
ions, glucose, and amino acids. The protein component
of the membrane consists of transporters, enzymes,
hormone receptors, cell-surface antigens, and ion and
water channels.
> Water
A Water Water Oil B
> Fig. 1.2 Orientation of phospholipid molecules at oil and
> water interfaces. Depicted are the orientation of phospholipid
> at an oil-water interface (A) and the orientation of phospholipid
> in a bilayer, as occurs in the cell membrane (B). 1Cellular Physiology
5
hydrogen bonds. One example of a peripheral mem -
brane protein is ankyrin , which anchors the
cytoskeleton of red blood cells to an integral mem -
brane transport protein, the Cl -HCO 3 exchanger
(also called band 3 protein).
# TRANSPORT ACROSS CELL
# MEMBRANES
Several types of mechanisms are responsible for trans -
port of substances across cell membranes (Table 1.2).
Substances may be transported down an electro -
chemical gradient (downhill) or against an electro -
chemical gradient (uphill). Downhill transport occurs
by diffusion, either simple or facilitated, and requires
no input of metabolic energy. Uphill transport occurs
by active transport, which may be primary or second -
ary. Primary and secondary active transport processes
transmembrane proteins are in contact with both
ECF and ICF. Examples of transmembrane integral
proteins are ligand-binding receptors (e.g., for hor -
mones or neurotransmitters), transport proteins
(e.g., Na +-K + ATPase), pores, ion channels, cell
adhesion molecules, and GTP-binding proteins (G
proteins). A second category of integral proteins is
embedded in the lipid bilayer of the membrane but
does not span it. A third category of integral proteins
is associated with membrane proteins but is not
embedded in the lipid bilayer.
Peripheral membrane proteins are not embedded
in the membrane and are not covalently bound to
cell membrane components. They are loosely
attached to either the intracellular or extracellular
side of the cell membrane by electrostatic interac -
tions (e.g., with integral proteins) and can be
removed with mild treatments that disrupt ionic or
> Lipid bilayer Intracellular fluid Peripheral protein Integral protein Gated ion channel Extracellular fluid
> Fig. 1.3 Fluid mosaic model for cell membranes.
> TABLE 1.2 Summary of Membrane Transport
Type of Transport Active or Passive
Carrier-
Mediated
Uses Metabolic
Energy Dependent on Na + Gradient
Simple diffusion Passive; downhill No No No
Facilitated diffusion Passive; downhill Yes No No
Primary active transport Active; uphill Yes Yes; direct No
Cotransport Secondary active a Yes Yes; indirect Yes (solutes move in same direction
as Na + across cell membrane)
Countertransport Secondary active a Yes Yes; indirect Yes (solutes move in opposite
direction as Na + across cell
membrane)
> aNa +is transported downhill, and one or more solutes are transported uphill. 6Physiology
Stereospecificity . The binding sites for solute on the
transport proteins are stereospecific. For example,
the transporter for glucose in the renal proximal
tubule recognizes and transports the natural isomer
> D
-glucose, but it does not recognize or transport the
unnatural isomer L-glucose. In contrast, simple dif -
fusion does not distinguish between the two glucose
isomers because no protein carrier is involved.
Competition . Although the binding sites for trans -
ported solutes are quite specific, they may recognize,
bind, and even transport chemically related solutes.
For example, the transporter for glucose is specific
for D-glucose, but it also recognizes and transports
a closely related sugar, D-galactose. Therefore the
presence of D-galactose inhibits the transport of
> D
-glucose by occupying some of the binding sites
and making them unavailable for glucose.
Simple Diffusion
Diffusion of Nonelectrolytes
Simple diffusion occurs as a result of the random
thermal motion of molecules, as shown in Figure 1.5.
Two solutions, A and B, are separated by a membrane
that is permeable to the solute. The solute concentra -
tion in A is initially twice that of B. The solute molecules
are in constant motion, with equal probability that a
given molecule will cross the membrane to the other
solution. However, because there are twice as many
solute molecules in Solution A as in Solution B, there
will be greater movement of molecules from A to B
than from B to A. In other words, there will be net
diffusion of the solute from A to B, which will continue
until the solute concentrations of the two solutions
become equal (although the random movement of
molecules will go on forever).
are distinguished by their energy source. Primary active
transport requires a direct input of metabolic energy;
secondary active transport utilizes an indirect input of
metabolic energy.
Further distinctions among transport mechanisms
are based on whether the process involves a protein
carrier. Simple diffusion is the only form of transport
that is not carrier mediated. Facilitated diffusion,
primary active transport, and secondary active trans -
port all involve integral membrane proteins and are
called carrier-mediated transport . All forms of carrier-
mediated transport share the following three features:
saturation, stereospecificity, and competition.
Saturation . Saturability is based on the concept that
carrier proteins have a limited number of binding
sites for the solute. Figure 1.4 shows the relationship
between the rate of carrier-mediated transport and
solute concentration. At low solute concentrations,
many binding sites are available and the rate of
transport increases steeply as the concentration
increases. However, at high solute concentrations,
the available binding sites become scarce and the
rate of transport levels off. Finally, when all of the
binding sites are occupied, saturation is achieved at
a point called the transport maximum , or Tm. The
kinetics of carrier-mediated transport are similar to
Michaelis-Menten enzyme kineticsboth involve
proteins with a limited number of binding sites. (The
Tm is analogous to the V max of enzyme kinetics.)
Tm-limited glucose transport in the proximal tubule
of the kidney is an example of saturable transport.
> Concentration Transport rate Simple diffusion Carrier-mediated transport Tm
> Fig. 1.4 Kinetics of carrier-mediated transport.
> Tm, Trans -
> port maximum.
> Membrane
A B
> Fig. 1.5 Simple diffusion. The two solutions, Aand B, are
> separated by a membrane, which is permeable to the solute
> (circles). Solution A initially contains a higher concentration of the
> solute than does Solution B. 1Cellular Physiology 7
> THICKNESS OF THE MEMBRANE ( X)
The thicker the cell membrane, the greater the distance
the solute must diffuse and the lower the rate of
diffusion.
> SURFACE AREA (A)
The greater the surface area of membrane available, the
higher the rate of diffusion. For example, lipid-soluble
gases such as oxygen and carbon dioxide have particu -
larly high rates of diffusion across cell membranes.
These high rates can be attributed to the large surface
area for diffusion provided by the lipid component of
the membrane.
To simplify the description of diffusion, several of
the previously cited characteristics can be combined
into a single term called permeability (P) . Permeability
includes the partition coefficient, the diffusion coeffi -
cient, and the membrane thickness. Thus
P KD x
=
By combining several variables into permeability, the
rate of net diffusion is simplified to the following
expression:
J PA C CA B= ( )
where
J Net rate of diffusion mmol/s P Permeability (cm/s A Surfa
=
==
( ))cce area for diffusion cm C Concentration in Solution A ( A( )2
= mmmol/L) C Concentration in Solution B mmol/L B = ( )
Net diffusion of the solute is called flux , or flow (J) ,
and depends on the following variables: size of the
concentration gradient, partition coefficient, diffusion
coefficient, thickness of the membrane, and surface
area available for diffusion.
> CONCENTRATION GRADIENT (C ACB)
The concentration gradient across the membrane is the
driving force for net diffusion. The larger the difference
in solute concentration between Solution A and Solu -
tion B, the greater the driving force and the greater the
net diffusion. It also follows that, if the concentrations
in the two solutions are equal, there is no driving force
and no net diffusion.
> PARTITION COEFFICIENT (K)
The partition coefficient, by definition, describes the
solubility of a solute in oil relative to its solubility in
water. The greater the relative solubility in oil, the
higher the partition coefficient and the more easily the
solute can dissolve in the cell membranes lipid bilayer.
Nonpolar solutes tend to be soluble in oil and have
high values for partition coefficient, whereas polar
solutes tend to be insoluble in oil and have low values
for partition coefficient. The partition coefficient can be
measured by adding the solute to a mixture of olive oil
and water and then measuring its concentration in the
oil phase relative to its concentration in the water
phase. Thus
K Concentration in olive oil Concentration in water
=
> DIFFUSION COEFFICIENT (D)
The diffusion coefficient depends on such characteris -
tics as size of the solute molecule and the viscosity of
the medium. It is defined by the Stokes-Einstein equa -
tion (see later). The diffusion coefficient correlates
inversely with the molecular radius of the solute and
the viscosity of the medium. Thus small solutes in
nonviscous solutions have the largest diffusion coeffi -
cients and diffuse most readily; large solutes in viscous
solutions have the smallest diffusion coefficients and
diffuse least readily. Thus
D KT
r
= 6
where
D Diffusion coefficient K Boltzmann constant T Absolute temp
=
== eerature K
r Molecular radius Viscosity of the medium ( )
==
SAMPLE PROBLEM. Solution A and Solution B are
separated by a membrane whose permeability to
urea is 2 10 5 cm/s and whose surface area is
1 cm 2. The concentration of urea in A is 10 mg/mL,
and the concentration of urea in B is 1 mg/mL. The
partition coefficient for urea is 10 3, as measured in
an oil-water mixture. What are the initial rate and
direction of net diffusion of urea?
SOLUTION. Note that the partition coefficient is
extraneous information because the value for per -
meability, which already includes the partition
coefficient, is given. Net flux can be calculated by
substituting the following values in the equation for
net diffusion: Assume that 1 mL of water = 1 cm 3.
Thus
J PA C CA B= ( )8 Physiology
(In contrast, simple diffusion will proceed as long as
there is a concentration gradient for the solute.)
An excellent example of facilitated diffusion is the
transport of D-glucose into skeletal muscle and adipose
cells by the GLUT4 transporter. Glucose transport
can proceed as long as the blood concentration of
glucose is higher than the intracellular concentration of
glucose and as long as the carriers are not saturated.
Other monosaccharides such as D-galactose, 3-O-methyl
glucose, and phlorizin competitively inhibit the trans -
port of glucose because they bind to transport sites on
the carrier. The competitive solute may itself be trans -
ported (e.g., D-galactose), or it may simply occupy the
binding sites and prevent the attachment of glucose
(e.g., phlorizin). As noted previously, the nonphysio -
logic stereoisomer, L-glucose, is not recognized by the
carrier for facilitated diffusion and therefore is not
bound or transported.
Primary Active Transport
In active transport, one or more solutes are moved
against an electrochemical potential gradient (uphill).
In other words, solute is moved from an area of low
concentration (or low electrochemical potential) to an
area of high concentration (or high electrochemical
potential). Because movement of a solute uphill is
work, metabolic energy in the form of ATP must be
provided. In the process, ATP is hydrolyzed to adenos -
ine diphosphate (ADP) and inorganic phosphate (P i),
releasing energy from the terminal high-energy phos -
phate bond of ATP. When the terminal phosphate is
released, it is transferred to the transport protein, initi -
ating a cycle of phosphorylation and dephosphoryla -
tion. When the ATP energy source is directly coupled
to the transport process, it is called primary active
transport. Three examples of primary active transport
in physiologic systems are the Na +-K + ATPase present
in all cell membranes, the Ca 2+ ATPase present in SR
and endoplasmic reticulum, and the H+-K + ATPase
present in gastric parietal cells and renal -intercalated
cells.
> Na +-K +ATPase (Na +-K +Pump)
Na +-K + ATPase is present in the membranes of all cells.
It pumps Na + from ICF to ECF and K + from ECF to ICF
(Fig. 1.6). Each ion moves against its respective elec -
trochemical gradient. The stoichiometry can vary but,
typically, for every three Na + ions pumped out of the
cell, two K + ions are pumped into the cell. This stoichi -
ometry of three Na + ions per two K + ions means that,
for each cycle of the Na +-K + ATPase, more positive
charge is pumped out of the cell than is pumped into
the cell. Thus the transport process is termed electro -
genic because it creates a charge separation and a
potential difference. The Na +-K + ATPase is responsible
> Diffusion of Electrolytes
Thus far, the discussion concerning diffusion has
assumed that the solute is a nonelectrolyte (i.e., it is
uncharged). However, if the diffusing solute is an ion
or an electrolyte , there are two additional consequences
of the presence of charge on the solute.
First, if there is a potential difference across the
membrane, that potential difference will alter the net
rate of diffusion of a charged solute. (A potential dif -
ference does not alter the rate of diffusion of a nonelec -
trolyte.) For example, the diffusion of K + ions will be
slowed if K + is diffusing into an area of positive charge,
and it will be accelerated if K + is diffusing into an area
of negative charge. This effect of potential difference
can either add to or negate the effects of differences in
concentrations, depending on the orientation of the
potential difference and the charge on the diffusing ion.
If the concentration gradient and the charge effect are
oriented in the same direction across the membrane,
they will combine; if they are oriented in opposite
directions, they may cancel each other out.
Second, when a charged solute diffuses down a
concentration gradient, that diffusion can itself gener -
ate a potential difference across a membrane called a
diffusion potential . The concept of diffusion potential
will be discussed more fully in a following section.
Facilitated Diffusion
Like simple diffusion, facilitated diffusion occurs down
an electrochemical potential gradient; thus it requires
no input of metabolic energy. Unlike simple diffusion,
however, facilitated diffusion uses a membrane carrier
and exhibits all the characteristics of carrier-mediated
transport: saturation, stereospecificity, and competi -
tion. At low solute concentration, facilitated diffusion
typically proceeds faster than simple diffusion (i.e., is
facilitated) because of the function of the carrier.
However, at higher concentrations, the carriers will
become saturated and facilitated diffusion will level off.
> where
> Jcm/s cm mg/mL mg/mL Jcm/s cm mg/
> ==
> 210 110 1210 110 5252()
> (ccm mg/cm mg/s 33
> 411 8 10
> =
> ).
> The magnitude of net flux has been calculated as
> 1.8 10 4mg/s. The direction of net flux can be
> determined intuitively because net flux will occur
> from the area of high concentration (Solution A) to
> the area of low concentration (Solution B). Net dif -
> fusion will continue until the urea concentrations of
> the two solutions become equal, at which point the
> driving force will be zero. 1Cellular Physiology
9
glycosides inhibit the Na +-K + ATPase by binding to the
E2~P form near the K +-binding site on the extracellular
side, thereby preventing the conversion of E 2~P back
to E1. By disrupting the cycle of phosphorylation-
dephosphorylation, these drugs disrupt the entire
enzyme cycle and its transport functions.
Ca 2+ ATPase (Ca 2+ Pump)
Most cell (plasma) membranes contain a Ca 2+ ATPase,
or plasma-membrane Ca 2+ ATPase (PMCA) , whose
function is to extrude Ca 2+ from the cell against an
electrochemical gradient; one Ca 2+ ion is extruded for
each ATP hydrolyzed. PMCA is responsible, in part, for
maintaining the very low intracellular Ca 2+ concentra -
tion. In addition, the sarcoplasmic reticulum (SR) of
muscle cells and the endoplasmic reticulum of other
cells contain variants of Ca 2+ ATPase that pump two
Ca 2+ ions (for each ATP hydrolyzed) from ICF into the
interior of the SR or endoplasmic reticulum (i.e., Ca 2+
sequestration). These variants are called SR and endo -
plasmic reticulum Ca 2+ ATPase (SERCA) . Ca 2+ ATPase
functions similarly to Na +-K + ATPase, with E 1 and E 2
states that have, respectively, high and low affinities
for Ca 2+. For PMCA, the E 1 state binds Ca 2+ on the
intracellular side, a conformational change to the E 2
state occurs, and the E 2 state releases Ca 2+ to ECF. For
SERCA, the E 1 state binds Ca 2+ on the intracellular side
and the E 2 state releases Ca 2+ to the lumen of the SR or
endoplasmic reticulum.
H+-K + ATPase (H +-K + Pump)
H+-K + ATPase is found in the parietal cells of the gastric
mucosa and in the -intercalated cells of the renal
collecting duct. In the stomach, it pumps H + from the
ICF of the parietal cells into the lumen of the stomach,
where it acidifies the gastric contents. Omeprazole ,
an inhibitor of gastric H +-K + ATPase, can be used thera -
peutically to reduce the secretion of H + in the treatment
of some types of peptic ulcer disease.
for maintaining concentration gradients for both Na +
and K + across cell membranes, keeping the intracellular
Na + concentration low and the intracellular K + concen -
tration high.
The Na +-K + ATPase consists of and subunits. The
subunit contains the ATPase activity, as well as
the binding sites for the transported ions, Na + and K +.
The Na +-K + ATPase switches between two major con -
formational states, E 1 and E 2. In the E1 state , the binding
sites for Na + and K + face the ICF and the enzyme has
a high affinity for Na +. In the E2 state , the binding sites
for Na + and K + face the ECF and the enzyme has a high
affinity for K +. The enzymes ion-transporting function
(i.e., pumping Na + out of the cell and K + into the cell)
is based on cycling between the E 1 and E 2 states and is
powered by ATP hydrolysis.
The transport cycle is illustrated in Figure 1.6. The
cycle begins with the enzyme in the E 1 state, bound to
ATP. In the E 1 state, the ion-binding sites face the ICF,
and the enzyme has a high affinity for Na +; three Na +
ions bind, ATP is hydrolyzed, and the terminal phos -
phate of ATP is transferred to the enzyme, producing a
high-energy state, E 1~P. Now, a major conformational
change occurs, and the enzyme switches from E 1~P to
E2~P. In the E 2 state, the ion-binding sites face the ECF,
the affinity for Na + is low, and the affinity for K + is high.
The three Na + ions are released from the enzyme to
ECF, two K + ions are bound, and inorganic phosphate
is released from E 2. The enzyme now binds intracellular
ATP, and another major conformational change occurs
that returns the enzyme to the E 1 state; the two K +
ions are released to ICF, and the enzyme is ready for
another cycle.
Cardiac glycosides (e.g., ouabain and digitalis ) are
a class of drugs that inhibits Na +-K + ATPase. Treat -
ment with this class of drugs causes certain predict -
able changes in intracellular ionic concentration: The
intracellular Na + concentration will increase, and the
intracellular K + concentration will decrease. Cardiac
> ATP ATP
> Extracellular fluid Intracellular fluid Extracellular fluid Intracellular fluid Cardiac glycosides 3Na +Cardiac glycosides ADP + P iATP Na +K+2K +3Na +2K +E1E1
~ P E2~ PE2
> Fig. 1.6 Na +-K +pump of cell membranes.
> ADP, Adenosine diphosphate;
> ATP, adenosine tri -
> phosphate;
> E, Na +-K +ATPase;
> E~P, phosphorylated Na +-K +ATPase;
> Pi, inorganic phosphate. 10 Physiology
two specific recognition sites, one for Na + ions and the
other for glucose. When both Na + and glucose are
present in the lumen of the small intestine, they bind
to the transporter. In this configuration, the cotransport
protein rotates and releases both Na + and glucose to
the interior of the cell. (Subsequently, both solutes
are transported out of the cell across the basolateral
membraneNa + by the Na +-K + ATPase and glucose by
facilitated diffusion.) If either Na + or glucose is missing
from the intestinal lumen, the cotransporter cannot
rotate. Thus both solutes are required, and neither can
be transported in the absence of the other (Box 1.1).
Finally, the role of the intestinal Na +-glucose cotrans -
port process can be understood in the context of overall
intestinal absorption of carbohydrates. Dietary carbo -
hydrates are digested by gastrointestinal enzymes to an
absorbable form, the monosaccharides. One of these
monosaccharides is glucose, which is absorbed across
the intestinal epithelial cells by a combination of Na +-
glucose cotransport in the luminal membrane and
facilitated diffusion of glucose in the basolateral mem -
brane. Na +-glucose cotransport is the active step, allow -
ing glucose to be absorbed into the blood against an
electrochemical gradient.
Countertransport
Countertransport (antiport or exchange) is a form of
secondary active transport in which solutes move in
opposite directions across the cell membrane. Na + moves
into the cell on the carrier down its electrochemical
gradient; the solutes that are countertransported or
exchanged for Na + move out of the cell. Countertrans -
port is illustrated by Ca 2+-Na + exchange (Fig. 1.8) and
by Na +-H + exchange. As with cotransport, each process
Secondary Active Transport
Secondary active transport processes are those in which
the transport of two or more solutes is coupled. One
of the solutes, usually Na +, moves down its electro -
chemical gradient (downhill), and the other solute
moves against its electrochemical gradient (uphill). The
downhill movement of Na + provides energy for the
uphill movement of the other solute. Thus metabolic
energy, as ATP, is not used directly, but it is supplied
indirectly in the Na + concentration gradient across the
cell membrane. (The Na +-K + ATPase, utilizing ATP,
creates and maintains this Na + gradient.) The name
secondary active transport therefore refers to the indi -
rect utilization of ATP as an energy source.
Inhibition of the Na +-K + ATPase (e.g., by treatment
with ouabain) diminishes the transport of Na + from ICF
to ECF, causing the intracellular Na + concentration to
increase and thereby decreasing the size of the trans -
membrane Na + gradient. Thus indirectly, all secondary
active transport processes are diminished by inhibitors
of the Na +-K + ATPase because their energy source, the
Na + gradient, is diminished.
There are two types of secondary active transport,
distinguishable by the direction of movement of the
uphill solute. If the uphill solute moves in the same
direction as Na +, it is called cotransport , or symport .
If the uphill solute moves in the opposite direction
of Na +, it is called countertransport, antiport , or
exchange .
Cotransport
Cotransport (symport) is a form of secondary active
transport in which all solutes are transported in the
same direction across the cell membrane. Na + moves
into the cell on the carrier down its electrochemical
gradient; the solutes, cotransported with Na +, also
move into the cell. Cotransport is involved in several
critical physiologic processes, particularly in the
absorbing epithelia of the small intestine and the
renal tubule. For example, Na +-glucose cotransport
(SGLT) and Na +-amino acid cotransport are present
in the luminal membranes of the epithelial cells of
both small intestine and renal proximal tubule. Another
example of cotransport involving the renal tubule is
Na +-K +-2Cl cotransport , which is present in the luminal
membrane of epithelial cells of the thick ascending
limb. In each example, the Na + gradient established by
the Na +-K + ATPase is used to transport solutes such as
glucose, amino acids, K +, or Cl against electrochemical
gradients.
Figure 1.7 illustrates the principles of cotransport
using the example of Na +-glucose cotransport ( SGLT1 ,
or Na +-glucose transport protein 1) in intestinal epithe -
lial cells. The cotransporter is present in the luminal
membrane of these cells and can be visualized as having
> ATP ATP
> Intestinal epithelial cell Lumen Blood Na +SGLT1 Glucose Glucose Basolateral membrane Luminal or apical membrane 2K +3Na +
> Fig. 1.7 Na +-glucose cotransport in an intestinal epithelial
> cell.
> ATP, Adenosine triphosphate;
> SGLT1, Na +-glucose transport
> protein 1. 1Cellular Physiology 11
uses the Na + gradient established by the Na +-K + ATPase
as an energy source; Na + moves downhill and Ca 2+ or
H+ moves uphill.
Ca 2+-Na + exchange is one of the transport mecha -
nisms, along with the Ca 2+ ATPase, that helps maintain
the intracellular Ca 2+ concentration at very low levels
(10 7 molar). To accomplish Ca 2+-Na + exchange, active
transport must be involved because Ca 2+ moves out of
the cell against its electrochemical gradient. Figure 1.8
illustrates the concept of Ca 2+-Na + exchange in a muscle
cell membrane. The exchange protein has recognition
sites for both Ca 2+ and Na +. The protein must bind Ca 2+
on the intracellular side of the membrane and, simul -
taneously, bind Na + on the extracellular side. In this
configuration, the exchange protein rotates and delivers
Ca 2+ to the exterior of the cell and Na + to the interior
of the cell.
The stoichiometry of Ca 2+-Na + exchange varies
between different cell types and may even vary for a
single cell type under different conditions. Usually,
however, three Na + ions enter the cell for each Ca 2+ ion
extruded from the cell. With this stoichiometry of three
Na + ions per one Ca 2+ ion, three positive charges move
into the cell in exchange for two positive charges
leaving the cell, making the Ca 2+-Na + exchanger
electrogenic .
Osmosis
Osmosis is the flow of water across a semipermeable
membrane because of differences in solute concentra -
tion. Concentration differences of impermeant solutes
establish osmotic pressure differences, and this osmotic
pressure difference causes water to flow by osmosis.
Osmosis of water is not diffusion of water: Osmosis
occurs because of a pressure difference, whereas diffu -
sion occurs because of a concentration (or activity)
difference of water.
> BOX 1.1 Clinical Physiology: Glucosuria Due to
> Diabetes Mellitus
DESCRIPTION OF CASE. At his annual physical
examination, a 14-year-old boy reports symptoms of
frequent urination and severe thirst. A dipstick test
of his urine shows elevated levels of glucose. The
physician orders a glucose tolerance test, which
indicates that the boy has type I diabetes mellitus.
He is treated with insulin by injection, and his
dipstick test is subsequently normal.
EXPLANATION OF CASE. Although type I diabetes
mellitus is a complex disease, this discussion is
limited to the symptom of frequent urination and
the finding of glucosuria (glucose in the urine).
Glucose is normally handled by the kidney in the
following manner: Glucose in the blood is filtered
across the glomerular capillaries. The epithelial
cells, which line the renal proximal tubule, then
reabsorb all of the filtered glucose so that no glucose
is excreted in the urine. Thus a normal dipstick test
would show no glucose in the urine. If the epithelial
cells in the proximal tubule do not reabsorb all of
the filtered glucose back into the blood, the glucose
that escapes reabsorption is excreted. The cellular
mechanism for this glucose reabsorption is the Na +-
glucose cotransporter in the luminal membrane of
the proximal tubule cells. Because this is a carrier-
mediated transporter, there is a finite number of
binding sites for glucose. Once these binding sites
are fully occupied, saturation of transport occurs
(transport maximum).
In this patient with type I diabetes mellitus, the
hormone insulin is not produced in sufficient
amounts by the pancreatic cells. Insulin is required
for normal uptake of glucose into liver, muscle, and
other cells. Without insulin, the blood glucose
concentration increases because glucose is not taken
up by the cells. When the blood glucose concentra -
tion increases to high levels, more glucose is filtered
by the renal glomeruli and the amount of glucose
filtered exceeds the capacity of the Na +-glucose
cotransporter. The glucose that cannot be reabsorbed
because of saturation of this transporter is then
spilled in the urine.
TREATMENT. Treatment of the patient with
type I diabetes mellitus consists of administering
exogenous insulin by injection. Whether secreted
normally from the pancreatic cells or adminis -
tered by injection, insulin lowers the blood glucose
concentration by promoting glucose uptake into
cells. When this patient received insulin, his blood
glucose concentration was reduced; thus the amount
of glucose filtered was reduced, and the Na +-glucose
cotransporters were no longer saturated. All of the
filtered glucose could be reabsorbed, and there -
fore no glucose was excreted, or spilled, in the
urine.
> ATP ATP
> 2K +3Na +Muscle cell 3Na +Ca 2+
> Fig. 1.8 Ca 2+-Na +countertransport (exchange) in a muscle
> cell.
> ATP, Adenosine triphosphate. 12 Physiology
> Osmotic Pressure
Osmosis is the flow of water across a semipermeable
membrane due to a difference in solute concentration.
The difference in solute concentration creates an
osmotic pressure difference across the membrane and
that pressure difference is the driving force for osmotic
water flow.
Figure 1.9 illustrates the concept of osmosis. Two
aqueous solutions, open to the atmosphere, are shown
in Figure 1.9A. The membrane separating the solutions
is permeable to water but is impermeable to the solute.
Initially, solute is present only in Solution 1. The solute
in Solution 1 produces an osmotic pressure and causes,
by the interaction of solute with pores in the membrane,
a reduction in hydrostatic pressure of Solution 1. The
resulting hydrostatic pressure difference across the
membrane then causes water to flow from Solution 2
into Solution 1. With time, water flow causes the
volume of Solution 1 to increase and the volume of
Solution 2 to decrease.
Figure 1.9B shows a similar pair of solutions;
however, the preparation has been modified so that
water flow into Solution 1 is prevented by applying
pressure to a piston. The pressure required to stop the
flow of water is the osmotic pressure of Solution 1.
The osmotic pressure ( ) of Solution 1 depends on
two factors: the concentration of osmotically active
particles and whether the solute remains in Solution 1
(i.e., whether the solute can cross the membrane or
not). Osmotic pressure is calculated by the vant Hoff
equation (as follows), which converts the concentra -
tion of particles to a pressure, taking into account
whether the solute is retained in the original solution.
Thus
= g C R T
where
=
=
Osmotic pressure atm or mm Hg g Number of particles per ( )
mmole in solution (Osm/mol C Concentration (mmol/L Reflect ))== iion coefficient varies from to R Gas constant L atm ( )
( . 0 1
0 082 = //mol KT Absolute temperature K
=
)( )
The reflection coefficient ( ) is a dimensionless
number ranging between 0 and 1 that describes the
> Osmolarity
The osmolarity of a solution is its concentration of
osmotically active particles, expressed as osmoles per
liter or milliosmoles per liter. To calculate osmolarity,
it is necessary to know the concentration of solute and
whether the solute dissociates in solution. For example,
glucose does not dissociate in solution; theoretically,
NaCl dissociates into two particles and CaCl 2 dissoci -
ates into three particles. The symbol g gives the
number of particles in solution and also takes into
account whether there is complete or only partial dis -
sociation. Thus if NaCl is completely dissociated into
two particles, g equals 2.0; if NaCl dissociates only
partially, then g falls between 1.0 and 2.0. Osmolarity
is calculated as follows:
Osmolarity g C =
where
Osmolarity Concentration of particles (mOsm/L
g Number of p
=
=
)
aarticles per mole in solution (Osm/mol C Concentration (mmol )
= //L)
If two solutions have the same calculated osmolarity,
they are called isosmotic . If two solutions have differ -
ent calculated osmolarities, the solution with the higher
osmolarity is called hyperosmotic and the solution
with the lower osmolarity is called hyposmotic .
> Osmolality
Osmolality is similar to osmolarity, except that it is the
concentration of osmotically active particles, expressed
as osmoles (or milliosmoles) per kilogram of water.
Because 1 kg of water is approximately equivalent to
1 L of water, osmola rity and osmola lity will have
essentially the same numerical value.
> The two solutions do not have the same calcu -
> lated osmolarity; therefore they are not isosmotic.
> Solution A has a higher osmolarity than Solution B
> and is hyperosmotic; Solution B is hyposmotic.
> SAMPLE PROBLEM. Solution A is 2 mmol/L urea,
> and Solution B is 1 mmol/L NaCl. Assume that g NaCl
> =1.85. Are the two solutions isosmotic?
> SOLUTION. Calculate the osmolarities of both solu -
> tions to compare them. Solution A contains urea,
> which does not dissociate in solution. Solution B
> contains NaCl, which dissociates partially in solu -
> tion but not completely (i.e., g <2.0). Thus
> Osmolarity Osm/mol mmol/L mOsm/L Osmolarity Osm/mo AB
> =
> ==
> 12
> 21 85 .llmmol/L mOsm/L
> =
> 11 85 .1Cellular Physiology
13
> Semipermeable membrane
APiston applies pressure to stop water flow 1 2 1 2
> Time
1 2 1 2
> atm Time
B
> Fig. 1.9 Osmosis across a semipermeable membrane. A, Solute
> (circles) is present on one
> side of a semipermeable membrane; with time, the osmotic pressure created by the solute causes
> water to flow from Solution 2 to Solution 1. The resulting volume changes are shown. B, The
> solutions are closed to the atmosphere, and a piston is applied to stop the flow of water
> into Solution 1. The pressure needed to stop the flow of water is the effective osmotic pressure
> of Solution 1.
> atm, Atmosphere.
ease with which a solute crosses a membrane. Reflec -
tion coefficients can be described for the following
three conditions (Fig. 1.10):
= 1.0 (see Fig. 1.10A). If the membrane is imper -
meable to the solute, is 1.0, and the solute will be
retained in the original solution and exert its full
osmotic effect. In this case, the effective osmotic
pressure will be maximal and will cause maximal
water flow. For example, serum albumin and intra -
cellular proteins are solutes where = 1.
= 0 (see Fig. 1.10C). If the membrane is freely
permeable to the solute, is 0, and the solute will
diffuse across the membrane down its concentration
gradient until the solute concentrations of the two
solutions are equal. In other words, the solute
behaves as if it were water. In this case, there will
be no effective osmotic pressure difference across
the membrane and therefore no driving force for
osmotic water flow. Refer again to the vant Hoff
equation and notice that, when = 0, the calculated
effective osmotic pressure becomes zero. Urea is an
example of a solute where = 0 (or nearly 0).
= a value between 0 and 1 (see Fig. 1.10B). Most
solutes are neither impermeant ( = 1) nor freely
permeant ( = 0) across membranes, but the reflec -
tion coefficient falls somewhere between 0 and 1. In
such cases, the effective osmotic pressure lies
between its maximal possible value (when the solute
is completely impermeable) and zero (when the
solute is freely permeable). Refer once again to the
vant Hoff equation and notice that, when is
between 0 and 1, the calculated effective osmotic
pressure will be less than its maximal possible value
but greater than zero.
When two solutions separated by a semipermeable
membrane have the same effective osmotic pressure, 14 Physiology
they are isotonic ; that is, no water will flow between
them because there is no effective osmotic pressure
difference across the membrane. When two solutions
have different effective osmotic pressures, the solution
with the lower effective osmotic pressure is hypotonic
and the solution with the higher effective osmotic pres -
sure is hypertonic . Water will flow from the hypotonic
solution into the hypertonic solution ( Box 1.2 ).
A
> = 1 Membrane
B
> = between 0 and 1
C
> = 0
> Fig. 1.10 Reflection coefficient ( ).
SAMPLE PROBLEM. A solution of 1 mol/L NaCl is
separated from a solution of 2 mol/L urea by a
semipermeable membrane. Assume that NaCl is
completely dissociated, that NaCl = 0.3, and urea =
0.05. Are the two solutions isosmotic and/or isotonic?
Is there net water flow, and what is its direction?
SOLUTION
Step 1. To determine whether the solutions are
isosmotic, simply calculate the osmolarity of each
solution (g C) and compare the two values. It was
stated that NaCl is completely dissociated (i.e., sepa -
rated into two particles); thus for NaCl, g = 2.0. Urea
does not dissociate in solution; thus for urea,
g = 1.0.
NaCl Osmolarity g C mol/L Osm/L :.
=
= =
2 0 12
Urea Osmolarity g C mol/L Osm/L :.
=
= =
1 0 22
Each solution has an osmolarity of 2 Osm/L
they are indeed isosmotic.
Step 2. To determine whether the solutions are
isotonic, the effective osmotic pressure of each solu -
tion must be determined. Assume that at 37C
(310 K), RT = 25.45 L-atm/mol. Thus
NaCl g C RT mol/L RTRT atm :...
=
=
==
2 1 0 3 0 6 15 3
Urea g C RT
mol/L RTRT atm :
...
=
=
==
1 2 0 05 0 1 2 5
Although the two solutions have the same calcu -
lated osmolarities and are isosmotic (Step 1), they
have different effective osmotic pressures and they
are not isotonic (Step 2). This difference occurs
because the reflection coefficient for NaCl is much
higher than the reflection coefficient for urea and,
thus NaCl creates the greater effective osmotic pres -
sure. Water will flow from the urea solution into the
NaCl solution, from the hypotonic solution to the
hypertonic solution.
# DIFFUSION POTENTIALS AND
# EQUILIBRIUM POTENTIALS
Ion Channels
Ion channels are integral, membrane-spanning proteins
that, when open, permit the passage of certain ions.
Thus ion channels are selective and allow ions with
specific characteristics to move through them. This
selectivity is based on both the size of the channel and
the charges lining it. For example, channels lined with
negative charges typically permit the passage of cations
but exclude anions; channels lined with positive charges
permit the passage of anions but exclude cations. Chan -
nels also discriminate on the basis of size. For example,
a cation-selective channel lined with negative charges
might permit the passage of Na + but exclude K +; another 1Cellular Physiology 15
The gates on ion channels are controlled by three
types of sensors . One type of gate has sensors that
respond to changes in membrane potential (i.e.,
voltage-gated channels); a second type of gate responds
to changes in signaling molecules (i.e., second
messengergated channels); and a third type of gate
responds to changes in ligands such as hormones or
neurotransmitters (i.e., ligand-gated channels).
Voltage-gated channels have gates that are con -
trolled by changes in membrane potential. For
example, the activation gate on the nerve Na +
channel is opened by depolarization of the nerve cell
membrane; opening of this channel is responsible
for the upstroke of the action potential. Interestingly,
another gate on the Na + channel, an inactivation
gate , is closed by depolarization. Because the activa -
tion gate responds more rapidly to depolarization
than the inactivation gate, the Na + channel first
opens and then closes. This difference in response
times of the two gates accounts for the shape and
time course of the action potential.
Second messengergated channels have gates that
are controlled by changes in levels of intracellular
signaling molecules such as cyclic adenosine mono -
phosphate (cAMP) or inositol 1,4,5-triphosphate
(IP 3). Thus the sensors for these gates are on the
intracellular side of the ion channel. For example,
the gates on Na + channels in cardiac sinoatrial node
are opened by increased intracellular cAMP.
Ligand-gated channels have gates that are controlled
by hormones and neurotransmitters. The sensors for
these gates are located on the extracellular side of
the ion channel. For example, the nicotinic receptor
on the motor end plate is actually an ion channel
that opens when acetylcholine (ACh) binds to it;
when open, it is permeable to Na + and K + ions.
Diffusion Potentials
A diffusion potential is the potential difference gener -
ated across a membrane when a charged solute (an
ion) diffuses down its concentration gradient. Therefore
a diffusion potential is caused by diffusion of ions. It
follows, then, that a diffusion potential can be gener -
ated only if the membrane is permeable to that ion.
Furthermore, if the membrane is not permeable to the
ion, no diffusion potential will be generated no matter
how large a concentration gradient is present.
The magnitude of a diffusion potential, measured
in millivolts (mV), depends on the size of the concen -
tration gradient, where the concentration gradient is
the driving force. The sign of the diffusion potential
depends on the charge of the diffusing ion. Finally,
as noted, diffusion potentials are created by the
cation-selective channel (e.g., nicotinic receptor on the
motor end plate) might have less selectivity and permit
the passage of several different small cations.
Ion channels are controlled by gates , and, depend -
ing on the position of the gates, the channels may be
open or closed. When a channel is open, the ions for
which it is selective can flow through it by passive
diffusion, down the existing electrochemical gradient.
In the open state, there is a continuous path between
ECF and ICF, through which ions can flow. When the
channel is closed, the ions cannot flow through it, no
matter what the size of the electrochemical gradient.
The conductance of a channel depends on the probabil -
ity that it is open. The higher the probability that the
channel is open, the higher is its conductance or
permeability.
> BOX 1.2 Clinical Physiology: Hyposmolarity With
> Brain Swelling
DESCRIPTION OF CASE. A 72-year-old man was
diagnosed recently with oat cell carcinoma of the
lung. He tried to stay busy with consulting work,
but the disease sapped his energy. One evening, his
wife noticed that he seemed confused and lethargic,
and suddenly he suffered a grand mal seizure. In the
emergency department, his plasma Na + concentra -
tion was 113 mEq/L (normal, 140 mEq/L) and his
plasma osmolarity was 230 mOsm/L (normal,
290 mOsm/L). He was treated immediately with an
infusion of hypertonic NaCl and was released from
the hospital a few days later, with strict instructions
to limit his water intake.
EXPLANATION OF CASE. The mans oat cell carci -
noma autonomously secretes antidiuretic hormone
(ADH), which causes syndrome of inappropriate
antidiuretic hormone (SIADH). In SIADH, the high
circulating levels of ADH cause excessive water
reabsorption by the principal cells of the late distal
tubule and collecting ducts. The excess water that
is reabsorbed and retained in the body dilutes the
Na + concentration and osmolarity of the ECF. The
decreased osmolarity means there is also decreased
effective osmotic pressure of ECF and, briefly,
osmotic pressure of ECF is less than osmotic pres -
sure of ICF. The effective osmotic pressure difference
across cell membranes causes osmotic water flow
from ECF to ICF, which results in cell swelling.
Because the brain is contained in a fixed structure
(the skull), swelling of brain cells can cause seizure.
TREATMENT. Treatment of the patient with hyper -
tonic NaCl infusion was designed to quickly raise
his ECF osmolarity and osmotic pressure, which
would eliminate the effective osmotic pressure dif -
ference across the brain cell membranes and stop
osmotic water flow and brain cell swelling. 16 Physiology
The positivity in Solution 2 opposes further diffusion
of Na +, and eventually it is large enough to prevent
further net diffusion. The potential difference that
exactly balances the tendency of Na + to diffuse down
its concentration gradient is the Na + equilibrium
potential . When the chemical and electrical driving
forces on Na + are equal and opposite, Na + is said to be
at electrochemical equilibrium . This diffusion of a few
Na + ions, sufficient to create the diffusion potential,
does not produce any change in Na + concentration in
the bulk solutions.
Example of Cl Equilibrium Potential
Figure 1.12 shows the same pair of solutions as in
Figure 1.11; however, in Figure 1.12, the theoretical
membrane is permeable to Cl rather than to Na +. Cl
will diffuse from Solution 1 to Solution 2 down its
concentration gradient, but Na + will not accompany it.
A diffusion potential will be established, and Solution
2 will become negative relative to Solution 1. The
potential difference that exactly balances the tendency
of Cl to diffuse down its concentration gradient is the
Cl equilibrium potential . When the chemical and
electrical driving forces on Cl are equal and opposite,
then Cl is at electrochemical equilibrium . Again,
diffusion of these few Cl ions will not change the Cl
concentration in the bulk solutions.
Nernst Equation
The Nernst equation is used to calculate the equilibrium
potential for an ion at a given concentration difference
across a membrane, assuming that the membrane is
permeable to that ion. By definition, the equilibrium
potential is calculated for one ion at a time. Thus
E RT
zF C
Cxie
= 2 3 10 . log [ ][ ]
movement of only a few ions, and they do not cause
changes in the concentration of ions in bulk solution.
Equilibrium Potentials
The concept of equilibrium potential is simply an
extension of the concept of diffusion potential. If there
is a concentration difference for an ion across a mem -
brane and the membrane is permeable to that ion, a
potential difference (the diffusion potential) is created.
Eventually, net diffusion of the ion slows and then
stops because of that potential difference. In other
words, if a cation diffuses down its concentration gradi -
ent, it carries a positive charge across the membrane,
which will retard and eventually stop further diffusion
of the cation. If an anion diffuses down its concentra -
tion gradient, it carries a negative charge, which will
retard and then stop further diffusion of the anion. The
equilibrium potential is the diffusion potential that
exactly balances or opposes the tendency for diffusion
down the concentration difference. At electrochemical
equilibrium , the chemical and electrical driving forces
acting on an ion are equal and opposite, and no further
net diffusion occurs.
The following examples of a diffusing cation and a
diffusing anion illustrate the concepts of equilibrium
potential and electrochemical equilibrium.
Example of Na + Equilibrium Potential
Figure 1.11 shows two solutions separated by a theoreti -
cal membrane that is permeable to Na + but not to Cl .
The NaCl concentration is higher in Solution 1 than in
Solution 2. The permeant ion, Na +, will diffuse down
its concentration gradient from Solution 1 to Solution
2, but the impermeant ion, Cl , will not accompany it.
As a result of the net movement of positive charge to
Solution 2, an Na + diffusion potential develops and
Solution 2 becomes positive with respect to Solution 1.
++++Na +Cl Na +Cl 1 2 1 2
> Na +-selective membrane
Na +Cl Na +Cl Time