Transport of Substances Through Cell Membranes (Guyton & Hall 14th ed) PDF

CHAPTER 4 Transport of Substances Through Cell...

CHAPTER 4 Transport of Substances Through Cell UNIT II Membranes Figure 4-1 lists the approximate concentrations of impor- THE CELL MEMBRANE IS A LIPID tant electrolytes and other substances in the extracellular BILAYER WITH CELL MEMBRANE fluid and intracellular fluid. Note that the extracellular TRANSPORT PROTEINS fluid contains a large amount of sodium but only a small amount of potassium. The opposite is true of the intra- The structure of the membrane covering the outside of cellular fluid. Also, the extracellular fluid contains a large every cell of the body is discussed in Chapter 2 and illus- amount of chloride ions, whereas the intracellular fluid trated in Figure 2-3 and Figure 4-2. This membrane con- contains very little of these ions. However, the concentra- sists almost entirely of a lipid bilayer with large numbers tions of phosphates and proteins in the intracellular fluid of protein molecules in the lipid, many of which penetrate are considerably greater than those in the extracellular all the way through the membrane. fluid. These differences are extremely important to the life The lipid bilayer is not miscible with the extracellular of the cell. The purpose of this chapter is to explain how fluid or the intracellular fluid. Therefore, it constitutes a the differences are brought about by the cell membrane barrier against movement of water molecules and water- transport mechanisms. soluble substances between the extracellular and intracel- lular fluid compartments. However, as shown in Figure 4-2 by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. EXTRACELLULAR INTRACELLULAR The membrane protein molecules interrupt the con- Intra FLUID FLUID tinuity of the lipid bilayer, constituting an alternative - Na+ --------------- 142 mEq/L --------- 10 mEq/L pathway through the cell membrane. Many of these pen- K+ ----------------- 4 mEq/L ------------ 140 mEq/L etrating proteins can function as transport proteins. Some Ca2+ -------------- 2.4 mEq/L ---------- 0.0001 mEq/L Mg2+ -------------- 1.2 mEq/L ---------- 58 mEq/L proteins have watery spaces all the way through the mole- Cl– ---------------- 103 mEq/L --------- 4 mEq/L cule and allow free movement of water, as well as selected HCO3– ------------ 24 mEq/L ----------- 10 mEq/L ions or molecules; these proteins are called channel pro- Phosphates----- 4 mEq/L -------------75 mEq/L SO4= -------------- 1 mEq/L -------------2 mEq/L teins. Other proteins, called carrier proteins, bind with Glucose --------- 90 mg/dl ------------ 0 to 20 mg/dl molecules or ions that are to be transported, and confor- Amino acids ---- 30 mg/dl ------------ 200 mg/dl ? mational changes in the protein molecules then move the substances through the interstices of the protein to the Cholesterol Channel Carrier proteins Phospholipids 0.5 g/dl-------------- 2 to 95 g/dl protein Neutral fat PO2 --------------- 35 mm Hg --------- 20 mm Hg ? PCO2 ------------- 46 mm Hg --------- 50 mm Hg ? pH ----------------- 7.4 ------------------- 7.0 Proteins ---------- 2 g/dl ---------------- 16 g/dl (5 mEq/L) (40 mEq/L) Energy Simple Facilitated diffusion diffusion Figure 4-1. Chemical compositions of extracellular and intracel- lular fluids. The question marks indicate that the precise values Diffusion Active transport for intracellular fluid are unknown. The red line indicates the cell Figure 4-2. Transport pathways through the cell membrane and the membrane. basic mechanisms of transport. 51 UNIT II Membrane Physiology, Nerve, and Muscle Ions diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. DIFFUSION THROUGH THE CELL MEMBRANE Diffusion through the cell membrane is divided into two subtypes, called simple diffusion and facilitated diffusion. Simple diffusion means that kinetic movement of mol- Figure 4-3. Diffusion of a fluid molecule during one thousandth of a second. ecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion other side of the membrane. Channel proteins and carrier is determined by the amount of①substance available, the proteins are usually selective for the types of molecules or ②velocity of kinetic motion, and the⑤number and sizes of ions that are allowed to cross the membrane. openings in the membrane through which the molecules or ions can move. “Diffusion” Versus “Active Transport.” Transport Facilitated diffusion requires interaction of a carrier through the cell membrane, either directly through the li- protein. The carrier protein aids passage of molecules or pid bilayer or through the proteins, occurs via one of two ions through the membrane by binding chemically with basic processes, diffusion or active transport. them and shuttling them through the membrane in this Although many variations of these basic mechanisms form. exist, diffusion means random molecular movement of Simple diffusion can occur through the cell membrane substances molecule by molecule, either through inter- by two pathways: (1) through the interstices of the lipid molecular spaces in the membrane or in combination bilayer if the diffusing substance is lipid-soluble; and with a carrier protein. The energy that causes diffusion is (2) through watery channels that penetrate all the way the energy of the normal kinetic motion of matter. through some of the large transport proteins, as shown to In contrast, active transport means movement of ions the left in Figure 4-2. or other substances across the membrane in combina- tion with a carrier protein in such a way that the carrier Diffusion of Lipid-Soluble Substances Through the protein causes the substance to move against an energy Lipid Bilayer. The lipid solubility of a substance is an gradient, such as from a low-concentration state to a high- important factor for determining how rapidly it diffuses concentration state. This movement requires an additional through the lipid bilayer. For example, the lipid solubili- source of energy besides kinetic energy. A more detailed ties of oxygen, nitrogen, carbon dioxide, and alcohols are explanation of the basic physics and physical chemistry of high, and all these substances can dissolve directly in the these two processes is provided later in this chapter.␣ lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these sub- DIFFUSION stances through the membrane is directly proportional to All molecules and ions in the body fluids, including water its lipid solubility. Especially large amounts of oxygen can molecules and dissolved substances, are in constant be transported in this way; therefore, oxygen can be de- motion, with each particle moving in its separate way. The livered to the interior of the cell almost as though the cell motion of these particles is what physicists call “heat”— membrane did not exist.␣ the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero tem- Diffusion of Water and Other Lipid-Insoluble Mole- perature. When a moving molecule, A, approaches a sta- cules Through Protein Channels. Even though water is tionary molecule, B, the electrostatic and other nuclear highly insoluble in the membrane lipids, it readily passes forces of molecule A repel molecule B, transferring some through channels in protein molecules that penetrate all of the energy of motion of molecule A to molecule B. the way through the membrane. Many of the body’s cell Consequently, molecule B gains kinetic energy of motion, membranes contain protein “pores” called aquaporins whereas molecule A slows down, losing some of its that selectively permit rapid passage of water through kinetic energy. As shown in Figure 4-3, a single molecule the membrane. The aquaporins are highly specialized, in a solution bounces among the other molecules—first in and there are at least 13 different types in various cells of one direction, then another, then another, and so forth— mammals. randomly bouncing thousands of times each second. This continual movement of molecules among one another in The rapidity with which water molecules can diffuse through most cell membranes is astounding. -> For example, impressive liquids or gases is called diffusion. the total amount of water that diffuses in each direction 52  Chapter 4 Transport of Substances Through Cell Membranes through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood Pore loop cell. Outside Selectivity Other lipid-insoluble molecules can pass through the filter protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as UNIT II they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% ⑧ greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water pen- etration, this amount of urea penetration still allows rapid Potassium transport of urea through the membrane within② minutes.␣ ion DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND “GATING” OF CHANNELS Inside Computerized three-dimensional reconstructions of pro- Pore helix tein pores and channels have demonstrated tubular path- ways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. Figure 4-4. The structure of a potassium channel. The channel is com- Pores are composed of integral cell membrane proteins posed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the that form open tubes through the membrane and are always pore loops, and carbonyl oxygens line the walls of the selectivity filter, open. However, the diameter of a pore and its electrical forming sites for transiently binding dehydrated potassium ions. The in- charges provide selectivity that permits only certain mole- teraction of the potassium ions with carbonyl oxygens causes the potas- cules to pass through. For example, aquaporins permit rapid sium ions to shed their bound water molecules, permitting the dehy- passage of water through cell membranes but exclude other drated potassium ions to pass through the pore. molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single molecular diameters of the ions because potassium ions file. The pore is too narrow to permit passage of any hydrated are slightly larger than sodium ions. Using x-ray crys- ions. As discussed in Chapters 28 and 76, the density of some tallography, potassium channels were found to have a aquaporins (e.g., aquaporin-2) in cell membranes is not static tetrameric structure consisting of four identical protein but is altered in different physiological conditions. subunits surrounding a central pore (Figure 4-4). At the The protein channels are distinguished by two impor- top of the channel pore are pore loops that form a narrow tant characteristics: (1) they are often selectively perme- selectivity filter. Lining the selectivity filter are carbonyl able to certain substances; and (2) many of the channels oxygens. When hydrated potassium ions enter the selec- can be opened or closed by gates that are regulated by tivity filter, they interact with the carbonyl oxygens and electrical signals (voltage-gated channels) or chemicals shed most of their bound water molecules, permitting the that bind to the channel proteins (ligand-gated channels). dehydrated potassium ions to pass through the channel. Thus, ion channels are flexible dynamic structures, and The carbonyl oxygens are too far apart, however, to enable subtle conformational changes influence gating and ion them to interact closely with the smaller sodium ions, selectivity. which are therefore effectively excluded by the selectivity filter from passing through the pore. Selective Permeability of Protein Channels. Many protein Different selectivity filters for the various ion channels channels are highly selective for transport of one or more are believed to determine, in large part, the specificity of specific ions or molecules. This selectivity results from various channels for cations or anions or for particular specific characteristics of the channel, such as its diam- ions, such as sodium (Na+), potassium (K+), and calcium eter, shape, and the nature of the electrical charges and (Ca2+), that gain access to the channels. chemical bonds along its inside surfaces. One of the most important of the protein channels, Potassium channels permit passage of potassium ions the sodium channel, is only 0.3 to 0.5 nanometer in across the cell membrane about 1000 times more read- diameter, but the ability of sodium channels to discrimi- ily than they permit passage of sodium ions. This high nate sodium ions among other competing ions in the degree of selectivity cannot be explained entirely by the surrounding fluids is crucial for proper cellular function. 53 UNIT II Membrane Physiology, Nerve, and Muscle Outside Gate Na+ Na+ the bottom panel of Figure 4-5, the potassium gates closed are on the intracellular ends of the potassium chan- Gate open – – nels, and they open when the inside of the cell mem- – – –– – –– – brane becomes positively charged. The opening of – –– – –– –– –– these gates is partly responsible for terminating the – – – – action potential, a process discussed in Chapter 5. – – – – Inside 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical sub- stance (a ligand) with the protein, which causes a conformational or chemical bonding change in the Outside protein molecule that opens or closes the gate. One of the most important instances of chemical gat- ing is the effect of the neurotransmitter acetylcho- line on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the Gate open gate of this channel, providing a negatively charged Gate pore about 0.65 nanometer in diameter that allows Inside closed K+ K+ uncharged molecules or positive ions smaller than Figure 4-5. Transport of sodium and potassium ions through protein this diameter to pass through. This gate is exceed- channels. Also shown are conformational changes in the protein mol- ingly important for the transmission of nerve sig- ecules to open or close the “gates” guarding the channels. nals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle The narrowest part of the sodium channel’s open pore, contraction (see Chapter 7).␣ the selectivity filter, is lined with strongly negatively charged amino acid residues, as shown in the top panel Open-State Versus Closed-State of Gated Channels. of Figure 4-5. These strong negative charges can pull Figure 4-6A shows two recordings of electrical current small dehydrated sodium ions away from their hydrat- flowing through a single sodium channel when there was ing water molecules into these channels, although the an approximately 25-millivolt potential gradient across ions do not need to be fully dehydrated to pass through the membrane. Note that the channel conducts current the channels. Once in the channel, the sodium ions dif- in an all-or-none fashion. That is, the gate of the channel fuse in either direction according to the usual laws of snaps open and then snaps closed, with each open state diffusion. Thus, the sodium channel is highly selective lasting for only a fraction of a millisecond, up to sever- for passage of sodium ions.␣ al milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the Gating of Protein Channels. Gating of protein chan- protein gates. At one voltage potential, the channel may nels provides a means of controlling ion permeability of remain closed all the time or almost all the time, whereas the channels. This mechanism is shown in both panels of at another voltage, it may remain open either all or most Figure 4-5 for selective gating of sodium and potassium of the time. At in-between voltages, as shown in the fig- ions. Some of the gates are thought to be gatelike exten- ure, the gates tend to snap open and closed intermittently, sions of the transport protein molecule, which can close resulting in an average current flow somewhere between the opening of the channel or can be lifted away from the the minimum and maximum.␣ opening by a conformational change in the shape of the protein molecule. Patch Clamp Method for Recording Ion Current Flow The opening and closing of gates are controlled in two Through Single Channels. The patch clamp method for principal ways: recording ion current flow through single protein chan- 1. Voltage gating. In the case of voltage gating, the nels is illustrated in Figure 4-6B. A micropipette with a molecular conformation of the gate or its chemi- tip diameter of only 1 or 2 micrometers is abutted against cal bonds responds to the electrical potential across the outside of a cell membrane. Suction is then applied the cell membrane. For example, in the top panel of inside the pipette to pull the membrane against the tip of Figure 4-5, a strong negative charge on the inside the pipette, which creates a seal where the edges of the of the cell membrane may cause the outside sodium pipette touch the cell membrane. The result is a minute gates to remain tightly closed. Conversely, when the membrane “patch” at the tip of the pipette through which inside of the membrane loses its negative charge, electrical current flow can be recorded. these gates open suddenly and allow sodium to pass Alternatively, as shown at the bottom right in Figure inward through the sodium pores. This process is 4-6B, the small cell membrane patch at the end of the the basic mechanism for eliciting action potentials pipette can be torn away from the cell. The pipette with in nerves that are responsible for nerve signals. In its sealed patch is then inserted into a free solution, which 54  Chapter 4 Transport of Substances Through Cell Membranes Open sodium channel Simple diffusion 3 Vmax Rate of diffusion Picoamperes 0 Facilitated UNIT II diffusion 3 0 0 2 4 6 8 10 Concentration of substance A Milliseconds Figure 4-7. Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated Recorder diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the Vmax. FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS Facilitated diffusion is also called carrier-mediated diffu- sion because a substance transported in this manner dif- To recorder fuses through the membrane with the help of a specific carrier protein. That is, the carrier facilitates diffusion of the substance to the other side. Facilitated diffusion differs from simple diffusion in the following important way. Although the rate of simple dif- fusion through an open channel increases proportionately with the concentration of the diffusing substance, in facili- tated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facil- itated diffusion is demonstrated in Figure 4-7. The figure shows that as the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase Membrane proportionately but, in the case of facilitated diffusion, the “patch” rate of diffusion cannot rise higher than the Vmax level. What is it that limits the rate of facilitated diffusion? A B probable answer is the mechanism illustrated in Figure Figure 4-6. A, Recording of current flow through a single voltage- 4-8. This Figure shows a carrier protein with a pore large gated sodium channel, demonstrating the all or none principle for enough to transport a specific molecule partway through. opening and closing of the channel. B, Patch clamp method for re- cording current flow through a single protein channel. To the left, the It also shows a binding receptor on the inside of the pro- recording is performed from a “patch” of a living cell membrane. To tein carrier. The molecule to be transported enters the the right, the recording is from a membrane patch that has been torn pore and becomes bound. Then, in a fraction of a second, away from the cell. a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side allows the concentrations of ions both inside the micropi- of the membrane. Because the binding force of the recep- pette and in the outside solution to be altered as desired. tor is weak, the thermal motion of the attached molecule Also, the voltage between the two sides of the membrane causes it to break away and be released on the opposite can be set, or “clamped,” to a given voltage. side of the membrane. The rate at which molecules can It has been possible to make such patches small enough be transported by this mechanism can never be greater so that only a single channel protein is found in the mem- than the rate at which the carrier protein molecule can brane patch being studied. By varying the concentrations undergo change back and forth between its two states. of different ions, as well as the voltage across the mem- Note specifically, though, that this mechanism allows the brane, one can determine the transport characteristics of transported molecule to move—that is, diffuse—in either the single channel, along with its gating properties.␣ direction through the membrane. 55 UNIT II Membrane Physiology, Nerve, and Muscle Transported Outside Inside molecule Binding point Co Ci Carrier protein A Membrane and conformational change − − – + − − – + − − − − − − − − − − − − − − − − − − − − −− − − − − − − − − − − − − − − − − − − − − − − − − − Release B of binding Figure 4-8. Postulated mechanism for facilitated diffusion. Among the many substances that cross cell mem- branes by facilitated diffusion are glucose and most of the Piston P1 P2 amino acids. In the case of glucose, at least 14 members of a family of membrane proteins (called GLUT) that trans- port glucose molecules have been discovered in various C tissues. Some of these GLUT proteins transport other Figure 4-9. Effect of concentration difference (A), electrical poten- monosaccharides that have structures similar to that of tial difference affecting negative ions (B), and pressure difference (C) glucose, including galactose and fructose. One of these, to cause diffusion of molecules and ions through a cell membrane. glucose transporter 4 (GLUT4), is activated by insulin, Co, concentration outside the cell; Ci, concentration inside the cell; which can② increase the rate of facilitated diffusion of glu- P1 pressure 1; P2 pressure 2. cose as much as 10- to 20-fold in insulin-sensitive tissues. in which Co is the concentration outside and Ci is the con- This is the principal mechanism whereby insulin controls centration inside the cell.␣ glucose use in the body, as discussed in Chapter 79.␣ Membrane Electrical Potential and Diffusion of Ions—The “Nernst Potential.” If an electrical poten- FACTORS THAT AFFECT NET RATE OF tial is applied across the membrane, as shown in Figure DIFFUSION 4-9B, the electrical charges of the ions cause them to By now, it is evident that many substances can diffuse move through the membrane even though no concen- through the cell membrane. What is usually important tration difference exists to cause movement. Thus, in the is the net rate of diffusion of a substance in the desired left panel of Figure 4-9B, the concentration of negative direction. This net rate is determined by several factors. ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the Net Diffusion Rate Is Proportional to the Concen- membrane, and a negative charge has been applied to the tration Difference Across a Membrane. Figure 4-9A left, creating an electrical gradient across the membrane. shows a cell membrane with a high concentration of a The positive charge attracts the negative ions, whereas the substance on the outside and a low concentration of a negative charge repels them. Therefore, net diffusion oc- substance on the inside. The rate at which the substance curs from left to right. After some time, large quantities of diffuses inward is proportional to the concentration of negative ions have moved to the right, creating the condi- molecules on the outside because this concentration de- tion shown in the right panel of Figure 4-9B, in which a termines how many molecules strike the outside of the concentration difference of the ions has developed in the membrane each second. Conversely, the rate at which direction opposite to the electrical potential difference. molecules diffuse outward is proportional to their con- The concentration difference now tends to move the ions centration inside the membrane. Therefore, the rate of net to the left, whereas the electrical difference tends to move diffusion into the cell is proportional to the concentration them to the right. When the concentration difference on the outside minus the concentration on the inside: rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical dif- Net diffusion ∝ (Co − Ci ) ference that will balance a given concentration difference 56  Chapter 4 Transport of Substances Through Cell Membranes of univalent ions—such as Na+ ions—can be determined Water NaCl solution from the following formula, called the Nernst equation: C1 EMF (in millivolts ) = ±61log C2 in which EMF is the electromotive force (voltage) UNIT II between side 1 and side 2 of the membrane, C1 is the con- centration on side 1, and C2 is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5.␣ Effect of a Pressure Difference Across the Membrane. At times, a considerable pressure difference develops be- tween the two sides of a diffusible membrane. This pres- Osmosis sure difference occurs, for example, at the blood capillary Figure 4-10. Osmosis at a cell membrane when a sodium chloride membranes in all tissues of the body. The pressure in solution is placed on one side of the membrane and water is placed on the other side. many capillaries is about 20 mm Hg greater inside than outside. Pressure actually means the sum of all the forces of the other side. Water molecules pass through the cell mem- different molecules striking a unit surface area at a given brane with ease, whereas sodium and chloride ions pass instant. Therefore, having a higher pressure on one side of a through only with difficulty. Therefore, sodium chloride membrane than on the other side means that the sum of all solution is actually a mixture of permeant water mole- the forces of the molecules striking the channels on that side cules and nonpermeant sodium and chloride ions, and of the membrane is greater than on the other side. In most the membrane is said to be selectively permeable to water cases, this situation is caused by greater numbers of mole- but much less so to sodium and chloride ions. Yet, the cules striking the membrane per second on one side than on presence of the sodium and chloride has displaced some the other side. The result is that increased amounts of energy of the water molecules on the side of the membrane are available to cause a net movement of molecules from where these ions are present and, therefore, has reduced the high-pressure side toward the low-pressure side. This the concentration of water molecules to less than that of effect is demonstrated in Figure 4-9C, which shows a pis- pure water. As a result, in the example shown in Figure ton developing high pressure on one side of a pore, thereby 4-10, more water molecules strike the channels on the causing more molecules to strike the pore on this side and, left side, where there is pure water, than on the right side, therefore, more molecules to diffuse to the other side.␣ where the water concentration has been reduced. Thus, net movement of water occurs from left to right—that is, osmosis occurs from the pure water into the sodium OSMOSIS ACROSS SELECTIVELY chloride solution. PERMEABLE MEMBRANES—“NET DIFFUSION” OF WATER Osmotic Pressure By far, the most abundant substance that diffuses through If in Figure 4-10 pressure were applied to the sodium the cell membrane is water. Enough water ordinarily dif- chloride solution, osmosis of water into this solution fuses in each direction through the red blood cell mem- would be slowed, stopped, or even reversed. The amount brane per second to equal about 100 times the volume of of pressure required to stop osmosis is called the osmotic the cell itself. Yet, the amount that normally diffuses in pressure of the sodium chloride solution. the two directions is balanced so precisely that zero net The principle of a pressure difference opposing osmo- movement of water occurs. Therefore, the volume of the sis is demonstrated in Figure 4-11, which shows a selec- cell remains constant. However, under certain conditions, tively permeable membrane separating two columns of a concentration difference for water can develop across fluid, one containing pure water and the other contain- a membrane. When this concentration difference for ing a solution of water and any solute that will not pen- water develops, net movement of water does occur across etrate the membrane. Osmosis of water from chamber B the cell membrane, causing the cell to swell or shrink, into chamber A causes the levels of the fluid columns to depending on the direction of the water movement. This become farther and farther apart, until eventually a pres- process of net movement of water caused by a concentra- sure difference develops between the two sides of the tion difference of water is called osmosis. membrane that is great enough to oppose the osmotic To illustrate osmosis, let us assume the conditions effect. The pressure difference across the membrane at shown in Figure 4-10, with pure water on one side of the this point is equal to the osmotic pressure of the solution cell membrane and a solution of sodium chloride on the that contains the nondiffusible solute. 57 UNIT II Membrane Physiology, Nerve, and Muscle Thus, a solution that has 1 osmole of solute dissolved Chamber A Chamber B in each kilogram of water is said to have an osmolality of 1 osmole per kilogram, and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the cm H2O extracellular and intracellular fluids is about 300 millios- moles per kilogram of water.␣ Relationship of Osmolality to Osmotic Pressure. At normal body temperature, 37°C (98.6°F), a concentration Semipermeable of 1 osmole per liter will cause 19,300 mm Hg osmotic membrane pressure in the solution. Likewise, 1 milliosmole per lit- er concentration is equivalent to 19.3 mm Hg osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, Figure 4-11. Demonstration of osmotic pressure caused by osmosis are highly attracted to one another; consequently, they at a semipermeable membrane. cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on aver- Importance of Number of Osmotic Particles (Molar age, the actual osmotic pressure of the body fluids is about Concentration) in Determining Osmotic Pressure. 0.93 times the calculated value.␣ The osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the The Term Osmolarity. Osmolarity is the osmolar con- number of particles per unit volume of fluid, not by the centration expressed as osmoles per liter of solution rather mass of the particles. The reason for this is that each par- than osmoles per kilogram of water. Although, strictly ticle in a solution, regardless of its mass, exerts, on aver- speaking, it is osmoles per kilogram of water (osmolality) age, the same amount of pressure against the membrane. that determines osmotic pressure, the quantitative differ- That is, large particles, which have greater mass (m) than ences between osmolarity and osmolality are less than 1% small particles, move at a slower velocity (v). The small for dilute solutions such as those in the body. Because it is particles move at higher velocities in such a way that their far more practical to measure osmolarity than osmolality, average kinetic energies (k), as determined by the follow- measuring osmolarity is the usual practice in physiologi- ing equation, cal studies.␣ mv 2 k= 2 ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES are the same for each small particle as for each large parti- cle. Consequently, the factor that determines the osmotic At times, a large concentration of a substance is required pressure of a solution is the concentration of the solution in the intracellular fluid, even though the extracellular in terms of the number of particles (which is the same as fluid contains only a small concentration. This situation its molar concentration if it is a nondissociated molecule), is true, for example, for potassium ions. Conversely, it is not in terms of mass of the solute.␣ important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the Osmolality—The Osmole. To express the concentration extracellular fluid are high. This situation is especially true of a solution in terms of numbers of particles, a unit called for sodium ions. Neither of these two effects could occur by the osmole is used in place of grams. simple diffusion because simple diffusion eventually equil- One osmole is 1 gram molecular weight of osmotically ibrates concentrations on the two sides of the membrane. active solute. Thus, 180 grams of glucose, which is 1 gram Instead, some energy source must cause excess movement molecular weight of glucose, is equal to 1 osmole of glucose of potassium ions to the inside of cells and excess move- because glucose does not dissociate into ions. If a solute dis- ment of sodium ions to the outside of cells. When a cell sociates into two ions, 1 gram molecular weight of the solute membrane moves molecules or ions uphill against a con- will become 2 osmoles because the number of osmotically centration gradient (or uphill against an electrical or pres- active particles is now twice as great as for the nondissociated sure gradient), the process is called active transport. solute. Therefore, when fully dissociated, 1 gram molecular Some examples of substances that are actively trans- weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. ported through at least some cell membranes include 58  Chapter 4 Transport of Substances Through Cell Membranes sodium, potassium, calcium, iron, hydrogen, chloride, 3Na+ Outside iodide, and urate ions, several different sugars, and most of the amino acids. 2K+ Primary Active Transport and Secondary Active Transport. Active transport is divided into two types ac- UNIT II cording to the source of the energy used to facilitate the transport, primary active transport and secondary active transport. In primary active transport, the energy is de- rived directly from the breakdown of adenosine triphos- ATPase phate (ATP) or some other high-energy phosphate com- pound. In secondary active transport, the energy is derived 3Na+ ADP ATP secondarily from energy that has been stored in the form + Inside 2K+ Pi of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell mem- Figure 4-12. Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phos- brane, created originally by primary active transport. In phate ion. both cases, transport depends on carrier proteins that pen- etrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein 2. It has two binding sites for potassium ions on the functions differently from the carrier in facilitated diffusion outside. because it is capable of imparting energy to the transported 3. The inside portion of this protein near the sodium substance to move it against the electrochemical gradient. binding sites has adenosine triphosphatase (AT- The following sections provide some examples of primary Pase) activity. active transport and secondary active transport, with more When two potassium ions bind on the outside of detailed explanations of their principles of function.␣ the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes RN PRIMARY ACTIVE TRANSPORT activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenos- Sodium-Potassium Pump Transports ine diphosphate (ADP) and liberating a high-energy Sodium Ions Out of Cells and Potassium phosphate bond of energy. This liberated energy is Ions into Cells believed to cause a chemical and conformational Among the substances that are transported by primary change in the protein carrier molecule, extruding active transport are sodium, potassium, calcium, hydro- three sodium ions to the outside and two potassium gen, chloride, and a few other ions. The active transport ions to the inside. mechanism that has been studied in greatest detail is the As with other enzymes, the Na+-K+ ATPase pump can sodium-potassium (Na+-K+) pump, a transporter that run in reverse. If the electrochemical gradients for Na+ pumps sodium ions ⑧ outward through the cell membrane and K+ are experimentally increased to the degree that the of all cells and, at the same time, pumps potassium ions energy stored in their gradients is greater than the chemi- from the outside to the o inside. This pump is responsible cal energy of ATP hydrolysis, these ions will move down for maintaining the sodium and potassium concentra- their concentration gradients, and the Na+-K+ pump will tion differences across the cell membrane, as well as for synthesize ATP from ADP and phosphate. The phosphor- establishing a negative electrical voltage inside the cells. ylated form of the Na+-K+ pump, therefore, can either Indeed, Chapter 5 shows that this pump is also the basis donate its phosphate to ADP to produce ATP or use the of nerve function, transmitting nerve signals throughout energy to change its conformation and pump Na+ out of the nervous system. the cell and K+ into the cell. The relative concentrations of Figure 4-12 shows the basic physical components of ATP, ADP, and phosphate, as well as the electrochemical the Na+-K+ pump. The carrier protein is a complex of gradients for Na+ and K+, determine the direction of the two separate globular proteins—a larger one called the α enzyme reaction. For some cells, such as electrically active subunit, with a molecular weight of about 100,000, and a nerve cells, 60% to 70% of the cell’s energy requirement smaller one called the β subunit, with a molecular weight may be o devoted to pumping Na+ out of the cell and K+ of about 55,000. Although the function of the smaller pro- into the cell. tein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has The Na+-K+ Pump Is Important for Controlling Cell three specific features that are important for the function- Volume. One of the most important functions of the ing of the pump: Na+-K+ pump is to control the cell volume. Without func- 1. It has three binding sites for sodium ions on the portion tion of this pump, most cells of the body would swell until of the protein that protrudes to the inside of the cell. they burst. 59 UNIT II Membrane Physiology, Nerve, and Muscle The mechanism for controlling the volume is as fol- lows. Inside the cell are large numbers of proteins and Primary Active Transport of Hydrogen other organic molecules that cannot escape from the cell. Ions Most of these proteins and other organic molecules are Primary active transport of hydrogen ions is especially negatively charged and, therefore, attract large numbers important at two places in the body: (1) in the gastric of potassium, sodium, and other positive ions. All these glands of the stomach; and (2) in the late distal tubules molecules and ions then cause osmosis of water to the and cortical collecting ducts of the kidneys. interior of the cell. Unless this process is checked, the cell In the gastric glands, the deep-lying parietal cells have will swell indefinitely until it bursts. The normal mecha- the most potent primary active mechanism for transport- nism for preventing this outcome is the Na+-K+ pump. ing hydrogen ions of any part of the body. This mechanism Note again that this mechanism pumps three Na+ ions to is the basis for secreting hydrochloric acid in stomach the outside of the cell for every two K+ ions pumped to digestive secretions. At the secretory ends of the gastric the interior. Also, the membrane is far less permeable to gland parietal cells, the hydrogen ion concentration is sodium ions than to potassium ions and, once the sodium increased as much as a million-fold and then is released ions are on the outside, they have a strong tendency to into the stomach, along with chloride ions, to form hydro- stay there. This process thus represents a net loss of ions chloric acid. out the cell, which also initiates osmosis of water out of In the renal tubules, special intercalated cells found in the cell. the late distal tubules and cortical collecting ducts also If a cell begins to swell for any reason, the Na+-K+ transport hydrogen ions by primary active transport. In pump is automatically activated, moving still more ions this case, large amounts of hydrogen ions are secreted to the exterior and carrying water with them. Therefore, from the blood into the renal tubular fluid for the purpose the Na+-K+ pump performs a continual surveillance role of eliminating excess hydrogen ions from the body fluids. in maintaining normal cell volume.␣ The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Electrogenic Nature of the Na+-K+ Pump. The fact Yet, as discussed in Chapter 31, most of these hydrogen that the Na+-K+ pump moves three Na+ ions to the exte- ions combine with tubular fluid buffers before they are rior for every two K+ ions that are moved to the interior eliminated in the urine␣ means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each Energetics of Primary Active Transport cycle of the pump. This action creates positivity outside The amount of energy required to transport a substance the cell but results in a deficit of positive ions inside the actively through a membrane is determined by how much cell; that is, it causes negativity on the inside. Therefore, the substance is concentrated during transport. Com- the Na+-K+ pump is said to be electrogenic because it cre- pared with the energy required to concentrate a sub- ates an electrical potential across the cell membrane. As stance 10-fold, concentrating it 100-fold requires twice discussed in Chapter 5, this electrical potential is a basic as much energy, and concentrating it 1000-fold requires requirement in nerve and muscle fibers for transmitting three times as much energy. In other words, the energy nerve and muscle signals.␣ required is proportional to the logarithm of the degree that the substance is concentrated, as expressed by the Primary Active Transport of Calcium Ions following formula: Another important primary active transport mecha- C1 nism is the calcium pump. Calcium ions are normally Energy (in calories per osmole) = 1400 log C2 maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, Thus, in terms of calories, the amount of energy required at a concentration about 10,000 times less than that to concentrate 1 osmole of a substance 10-fold is about in the extracellular fluid. This level of maintenance 1400 calories, whereas to concentrate it 100-fold, 2800 is achieved mainly by two primary active transport calories are required. One can see that the energy expen- calcium pumps. One, which is in the cell membrane, diture for concentrating substances in cells or for remov- pumps calcium to the ⑧ outside of the cell. The other ing substances from cells against a concentration gradient pumps calcium ions into one or more of the intracel- can be tremendous. Some cells, such as those lining the lular vesicular organelles of the cell, such as the sarco- renal tubules and many glandular cells, expend as much plasmic reticulum of muscle cells and the mitochondria as 90% of their energy for this purpose alone.␣ in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme SECONDARY ACTIVE TRANSPORT— ATPase, with the same capability to cleave ATP as the CO-TRANSPORT AND COUNTER-TRANSPORT ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for When sodium ions are transported out of cells by pri- calcium instead of for sodium.␣ mary active transport, a large concentration gradient of 60  Chapter 4 Transport of Substances Through Cell Membranes sodium ions across the cell membrane usually develops, Na+ Glucose with a high concentration outside the cell and a low con- centration inside. This gradient represents a storehouse Na-binding Glucose-binding of energy, because the excess sodium o outside the cell site site membrane is always attempting to diffuse to the⑧ interior. Under appropriate conditions, this diffusion energy of UNIT II sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called co- so transport transport, is one form of secondary active transport. For sodium to pull another substance along with it, ④ >both gring a coupling mechanism is required; this is achieved by mode means of still another carrier protein in the cell mem- Na+ Glucose brane. The carrier in this case serves as an attachment Figure 4-13 Postulated mechanism for sodium co-transport of glucose. point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the Na+ Na+ other substance to be transported together to the interior Outside of the cell. In counter-transport, sodium ions again attempt to dif- fuse to the interior of the cell because of their large con- counter centration gradient. However, this time, the substance to transport be transported is on the② inside of the cell and is trans- Inside ported to the outside. Therefore, the sodium ion binds to Ca2+ H+ >opposite the carrier protein, where it projects to the exterior sur- Figure 4-14. Sodium counter-transport of calcium and hydrogen ions. direction face of the membrane, and the substance to be counter- transported binds to the interior projection of the carrier Sodium co-transport of glucose and amino acids protein. Once both have become bound, a conformational occurs especially through the epithelial cells of the intes- change occurs, and energy released by the action of the tinal tract and the renal tubules of the kidneys to promote sodium ion moving to the interior causes the other sub- absorption of these substances into the blood. This pro- stance to move to the exterior. cess will be discussed in later chapters. Other important co-transport mechanisms in at least Co-Transport of Glucose and Amino Acids some cells include co-transport of potassium, chloride, Along with Sodium Ions bicarbonate, phosphate, iodine, iron, and urate ions.␣ Glucose and many amino acids are transported into most cells against large concentration gradients; the Sodium Counter-Transport of Calcium and mechanism of this action is entirely by co- transport, as Hydrogen Ions shown in Figure 4-13. Note that the transport carrier Two especially important counter-transporters (i.e., protein has two binding sites on its exterior side, one for transport in a direction opposite to the primary ion) are sodium and one for glucose. Also, the concentration of sodium-calcium counter-transport and sodium-hydrogen sodium ions is high on the outside and low on the inside, counter-transport (Figure 4-14). which provides energy for the transport. A special prop- Sodium-calcium counter-transport occurs through erty of the transport protein is that a conformational all or almost all cell membranes, with sodium ions mov- change to allow sodium movement to the interior will ing to the interior and calcium ions to the exterior; both not occur until a glucose molecule also attaches. When are bound to the same transport protein in a counter- they both become attached, the conformational change transport mode. This mechanism is in addition to the pri- takes place, and the sodium and glucose are transported mary active transport of calcium that occurs in some cells. to the inside of the cell at the same time. Hence, this Sodium-hydrogen counter-transport occurs in several is a sodium-glucose co-transporter. Sodium-glucose co- tissues. An especially important example is in the proxi- transporters are especially important for transporting mal tubules of the kidneys, where sodium ions move from glucose across renal and intestinal epithelial cells, as dis- the lumen of the tubule to the interior of the tubular cell cussed in Chapters 28 and 66. and hydrogen ions are counter-transported into the tubule Sodium co-transport of amino acids occurs in the same lumen. As a mechanism for concentrating hydrogen ions, manner as for glucose, except that it uses a different set counter-transport is not nearly as powerful as the primary of transport proteins. At least five amino acid transport active transport of hydrogen ions that occurs in the more dis- proteins have been identified, each of which is responsible tal renal tubules, but it can transport extremely large numbers for transporting one subset of amino acids with specific of hydrogen ions, thus making it a key to hydrogen ion control molecular characteristics. in the body fluids, as discussed in detail in Chapter 31.␣ 61 UNIT II Membrane Physiology, Nerve, and Muscle Brush Basement blood from the intestine. These mechanisms are also how border membrane the same substances are reabsorbed from the glomerular filtrate by the renal tubules. Numerous examples of the different types of transport Na+ Na+ discussed in this chapter are provided throughout this Active Connective tissue transport Osmosis text. Na+ Lumen and H2O Active Osmosis Bibliography transport Active Agre P, Kozono D: Aquaporin water channels: molecular mechanisms Na+ Na+ transport for human diseases. 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