Osmosis Filtration and Convection ter flow or volume flow (v)across a wall of In filtration(B). KG·(△P-△x)=KG·Pm anisms is achieved through osmosis(diffu- Filtration occurs mainly through capilla sion of water)or filtration. They can occur only walls, which allow the passage of small ions if the wall is water-permeable. This allows and molecules(o-O; see below ), but not of smotic and hydrostatic pressure differences plasma proteins (B, molecule X). Their cor the wall to drive the fluids centration difference leads to an oncotic pr through it. sure difference(△a) that opposes△ P. There Osmotic flow equals the hydraulic conduc- fore, filtration can occur only if△P>△π(→B. u tivity(K])times the osmotic pressure differ- p 152, P 210). J=Kf·△T arried along with the water flow of osmo- The osmotic pressure difference (A)can be sis or filtration. The amount of solvent drag for calculated using van,'t Hoffs law, as modified solute X Ux) depends mainly on osmotic flow by Staverman: Uv)and the mean solute activity ax(p. 382) AT=6·R·T·△Com, (112 at the site of penetration, but also on the where o is the reflection coefficient of the par- degree of reflection from th cles(see below ), R is the universal gas con- brane, tant(+P. 20), T is the absolute temperature, coefficient(o) Solvent drag for solute x ux)is Jv(1-a)ax[.s-I trations, Casm-Cosm(A). Since ACosm, the Larger molecules such as proteins are entirely driving force for osn a negative value, Jy reflected, and o-1(B, molecule X). Reflec is also negative(Eq. 1.11). The water therefore tion of smaller molecules is lower, and o< 1 lows against the concentration gradient of the When urea passes through the wall of the concentration, Cosm, attracts the water. When 0.68. The value(l-o) is also called the sieving the concentration of water is considered in os. coeficient(p. 154) nosis, the H20 concentration in A, a, CRo,is rs ter than that in A, b, Cho. CHo-Cho is molecular substances in plasma bind to pro- therefore the driving force for H2o diffusion teins(C). This hinders the free penetration Osmosis also cannot occur unless the reflection coefficient is greater than zero the glomerular filter( p. 154ff ). At a gl nless the wall of partition is merular filtration fraction of 20%, 20% of a less permeable to the solutes than to water. freely filterable substance is filtered out. If, Aquaporins(AQP) are water channels that however, 9/10 of the substance is bound to ermit the passage of water in many cell mem- plasma proteins, only 2% will be filtered during ranes. a chief cell in the renal collecting duct each renal pass ontains a total of ca 107 water channels. com. Convection rising AQP2 (regulated in the luminal me epithelium of the renal collecting dur ae lie ary tract. The solute is then carried along piece of driftwood. The quantity of solute transported over time com)is the product of vater(A, right panel)is controlled by the in- volume flow Jv(in m.s")and the solute con- ertion and removal of AQP2, which is stored in centration C(mol- m3) 「1.15 ceptors, CAMP: -p. 276), water ch are inserted in the luminal membrane within ed air occurs nutes, thereby ng the water ability of the membrane to around 1.5 x 10-17
241 Fundamentals and Cell Physiology Water flow or volume flow (JV) across a wall of partition (membrane or cell layer), in living organisms is achieved through osmosis (diffusion of water) or filtration. They can occur only if the wall is water-permeable. This allows osmotic and hydrostatic pressure differences (∆π and ∆P) across the wall to drive the fluids through it. Osmotic flow equals the hydraulic conductivity (Kf) times the osmotic pressure difference (∆π) ( A): JV Kf ⋅ ∆π [1.11] The osmotic pressure difference (∆π) can be calculated using van’t Hoff’s law, as modified by Staverman: ∆π σ ⋅ R ⋅ T ⋅ ∆Cosm, [1.12] where σ is the reflection coefficient of the particles (see below), R is the universal gas constant ( p. 20), T is the absolute temperature, and ∆Cosm [osm ⋅ kgH2O–1] is the difference between the lower and higher particle concentrations, Ca osm – Cb osm ( A). Since ∆Cosm, the driving force for osmosis, is a negative value, JV is also negative (Eq. 1.11). The water therefore flows against the concentration gradient of the solute particles. In other words, the higher concentration, Cb osm, attracts the water. When the concentration of water is considered in osmosis, the H2O concentration in A,a, Ca H2O, is greater than that in A,b, Cb H2O. Ca H2O – Cb H2O is therefore the driving force for H2O diffusion ( A). Osmosis also cannot occur unless the reflection coefficient is greater than zero (σ 0), that is, unless the wall of partition is less permeable to the solutes than to water. Aquaporins (AQP) are water channels that permit the passage of water in many cell membranes. A chief cell in the renal collecting duct contains a total of ca. 107 water channels, comprising AQP2 (regulated) in the luminal membrane, and AQP3 and 4 (permanent) in the basolateral membrane. The permeability of the epithelium of the renal collecting duct to water ( A, right panel) is controlled by the insertion and removal of AQP2, which is stored in the membrane of intracellular vesicles. In the presence of the antidiuretic hormone ADH (V2 receptors, cAMP; p. 276), water channels are inserted in the luminal membrane within minutes, thereby increasing the water permeability of the membrane to around 1.5 10– 17 L s– 1 per channel. In filtration ( B), JV Kf ⋅ (∆P –∆π) = Kf ⋅ Peff [1.13] Filtration occurs mainly through capillary walls, which allow the passage of small ions and molecules (σ = 0; see below), but not of plasma proteins ( B, molecule X). Their concentration difference leads to an oncotic pressure difference (∆π) that opposes ∆P. Therefore, filtration can occur only if ∆P ∆π ( B, p. 152, p. 210). Solvent drag occurs when solute particles are carried along with the water flow of osmosis or filtration. The amount of solvent drag for solute X (JX) depends mainly on osmotic flow (JV) and the mean solute activity ax ( p. 382) at the site of penetration, but also on the degree of particle reflection from the membrane, which is described using the reflection coefficient (σ). Solvent drag for solute X (JX) is therefore calculated as Jx JV (1 – σ) ax [mol ⋅ s–1] [1.14] Larger molecules such as proteins are entirely reflected, and σ =1( B, molecule X). Reflection of smaller molecules is lower, and σ 1. When urea passes through the wall of the proximal renal tubule, for example, σ = 0.68. The value (1–σ) is also called the sieving coefficient ( p. 154). Plasma protein binding occurs when smallmolecular substances in plasma bind to proteins ( C). This hinders the free penetration of the substances through the endothelium or the glomerular filter ( p. 154 ff.). At a glomerular filtration fraction of 20%, 20% of a freely filterable substance is filtered out. If, however, 9/10 of the substance is bound to plasma proteins, only 2% will be filtered during each renal pass. Convection functions to transport solutes over long distances—e.g., in the circulation or urinary tract. The solute is then carried along like a piece of driftwood. The quantity of solute transported over time (Jconv) is the product of volume flow JV (in m3 ⋅ s–1) and the solute concentration C (mol ⋅ m–3): Jconv JV ⋅ C [mol ⋅ s–1]. [1.15] The flow of gases in the respiratory tract, the transmission of heat in the blood and the release of heat in the form of warmed air occurs through convection ( p. 224). Osmosis, Filtration and Convection Edema, diabetes mellitus & insipidus, electrolyte disturbance, infusion solutions
Plate 1.12 Osmosis Filtration and Convection A. Osmosis(water diffusion) Water diffusion from a to b Epithelium Water flux I-K A(-Coum-Cosm) ollecting duct B. Filtration Example Glomerular △P>△ Water filtration Water flux=K(△P-△x) Primary C. Plasma protein binding Prevents excretion (e. g, by binding of heme by hemopexin Blood side (e g-, binding of Fe 3* ions by apotransferrin) Provides rapid access ion stores Helps to dissolve lipophilic substances in blood b/Affects certain medications(e.g.many sulfonamides): tion) functions as an allergen (hapten)
251 Fundamentals and Cell Physiology Plate 1.12 Osmosis, Filtration and Convection ( " $ =& #% " ! # & " # % & & & O O& & P)J;Q K & # & P)J;Q# O & O & & & 3 @ % @ % ,& B J% % % A " $ &"& &" % 1 & & & A /C &" % $ % C7C 5 ! %% & & 9 & & (I = % !6 +* , !( &!$
26 Active Transport ve transport occurs in many parts of the (Al A2), 3 Na and 2" are"pumped"out of body when solutes are transported and into the cell, respectively, while 1 ATP heir concentration gradient(uphill transport) molecule is used to phosphorylate the carrie nd / or, in the case of ions, against an electr tential ( p. 22). All in all, active transport changes the conformation of the protein and ccurs against the electrochemical gradient or subsequently alters the affinities of the Na E p 20 ff. ) they are not appropriate for this moves the binding sites to the opposite side of large portion of chemical energy tion restores the pump to its original state wided by foodstuffs is utilized for active (A2e=f). The pumping rate of the Na-K been made readily avail- ATPase increases when the cytosolic Na*cor able in the form of ATP(p. 41). The energy centration rises-due, for instance, to created by atp hydrolysis is used to drive the creased Na* influx, or when the extracellular ansmembrane transport of numerous ions, Krises. Therefore, Na, K-activatable ATPase is etabolites, and waste products. According to the full name of the pump. Na-"K-ATPase is in- the laws of thermodynamics, the energy ex- hibited by ouabain and cardiac glycosid nded in these reactions pr order econdary active transport occurs when vival and normal function of cells and, there. via a carrier protein (e.g, sodium gluco fore, for the whole organism(p 38 ff. ) transporter type 2, SGLT2)is coupled with the primary active transport, the energy pro- passive(downhill) transport of an ion (in this duced by hydrolysis of ATP goes directly into example Na*: -B1). In this case, the electro- ion transport through an ion pump. This type chemical Na* gradient into the cell(created by of ion pump is called an ATPase. They establish Na*-K -ATPase at another site on the cell mem- the electrochemical gradients rather slowly, brane: A)provides the driving force needed e.g, at a rate of around 1 umol.5- of for secondary active uptake of glucose into the membrane surface area in the case of cell Coupling of the transport of compounds ATPase. The gradient can be exploited oss a membrane is called cotransport achieve rapid (passive) ionic currents in the op- which may be in the form of symport or anti- osite direction after the permeability of ort Symport occurs when the compound and hannes has been increased (p. 32 ff. Na driving ion are transported across the mem can, for example, be driven into a nerve cell at a brane in the same direction ( B1-3). Antiport countertransport)occu ansported in opposite directions. Antiport ATPases occur ubiquitously in cell occurs, for example, when an electrochemical branes(Na'-K*-ATPase)and in the endo- Nat gradient drives H"in the opposite direction plasmic reticulum and plasma membrane by secondary active transport(B4). The re- (Ca-ATPase), renal collecting duct and stom- sulting H' gradient can then be exploited for (H-ATPase). They transport Na* K: Ca*and peptides(-B5)or Fe ions(p 90). H, respectively, by primarily active mecha- Electroneutral transport occurs when the nisms. All except H*-ATPase consist of 2 a-sub- net electrical charge remains balanced during P-type ATPases).The ansport,e.g during Na /H* antiport ( B4 a-subunits are phosphorylated and form the and Na'-CI-symport(B2) Charge transport ion transport channel (A1). ccurs in electrogenic(rheogenic) trans ponsible for main- e. g, in Na*-glucosed symport (B1).Na nance of intracellular na’ and k homeostasis amino acid° symport(→B3),2N’-am and, thus, for maintenance of the cell mem- acid- symport, or H -peptide symport(B5). brane potential. During each transport cyo The chemical Na* gradient provides the sole P
261 Fundamentals and Cell Physiology Active transport occurs in many parts of the body when solutes are transported against their concentration gradient (uphill transport) and/or, in the case of ions, against an electrical potential ( p. 22). All in all, active transport occurs against the electrochemical gradient or potential of the solute. Since passive transport mechanisms represent “downhill” transport ( p. 20 ff.), they are not appropriate for this task. Active transport requires the expenditure of energy. A large portion of chemical energy provided by foodstuffs is utilized for active transport once it has been made readily available in the form of ATP ( p. 41). The energy created by ATP hydrolysis is used to drive the transmembrane transport of numerous ions, metabolites, and waste products. According to the laws of thermodynamics, the energy expended in these reactions produces order in cells and organelles—a prerequisite for survival and normal function of cells and, therefore, for the whole organism ( p. 38 ff.). In primary active transport, the energy produced by hydrolysis of ATP goes directly into ion transport through an ion pump. This type of ion pump is called an ATPase. They establish the electrochemical gradients rather slowly, e.g., at a rate of around 1 µmol ⋅ s–1 ⋅ m–2 of membrane surface area in the case of Na+ -K+ - ATPase. The gradient can be exploited to achieve rapid (passive) ionic currents in the opposite direction after the permeability of ion channels has been increased ( p. 32 ff.). Na+ can, for example, be driven into a nerve cell at a rate of up to 1000 µmol ⋅ s–1 ⋅ m–2 during an action potential. ATPases occur ubiquitously in cell membranes (Na+ -K+ -ATPase) and in the endoplasmic reticulum and plasma membrane (Ca2+-ATPase), renal collecting duct and stomach glands (H+ ,K+ -ATPase), and in lysosomes (H+ -ATPase). They transport Na+ , K+ , Ca2+ and H+ , respectively, by primarily active mechanisms. All except H+ -ATPase consist of 2 α-subunits and 2 -subunits (P-type ATPases). The α-subunits are phosphorylated and form the ion transport channel ( A1). Na+-K+-ATPase is responsible for maintenance of intracellular Na+ and K+ homeostasis and, thus, for maintenance of the cell membrane potential. During each transport cycle ( A1, A2), 3 Na+ and 2 K+ are “pumped” out of and into the cell, respectively, while 1 ATP molecule is used to phosphorylate the carrier protein ( A2b). Phosphorylation first changes the conformation of the protein and subsequently alters the affinities of the Na+ and K+ binding sites. The conformational change is the actual ion transport step since it moves the binding sites to the opposite side of the membrane ( A2b ⇒ d). Dephosphorylation restores the pump to its original state ( A2e ⇒ f). The pumping rate of the Na+ -K+ - ATPase increases when the cytosolic Na+ concentration rises—due, for instance, to increased Na+ influx, or when the extracellular K+ rises. Therefore, Na+ ,K+ -activatable ATPase is the full name of the pump. Na-+ K+ -ATPase is inhibited by ouabain and cardiac glycosides. Secondary active transport occurs when uphill transport of a compound (e.g., glucose) via a carrier protein (e.g., sodium glucose transporter type 2, SGLT2) is coupled with the passive (downhill) transport of an ion (in this example Na+ ; B1). In this case, the electrochemical Na+ gradient into the cell (created by Na+ -K+ -ATPase at another site on the cell membrane; A) provides the driving force needed for secondary active uptake of glucose into the cell. Coupling of the transport of compounds across a membrane is called cotransport, which may be in the form of symport or antiport. Symport occurs when the compound and driving ion are transported across the membrane in the same direction ( B1–3). Antiport (countertransport) occurs when they are transported in opposite directions. Antiport occurs, for example, when an electrochemical Na+ gradient drives H+ in the opposite direction by secondary active transport ( B4). The resulting H+ gradient can then be exploited for tertiary active symport of molecules such as peptides ( B5) or Fe ions (p. 90). Electroneutral transport occurs when the net electrical charge remains balanced during transport, e.g., during Na+ /H+ antiport ( B4) and Na+ -Cl– symport (B2). Charge transport occurs in electrogenic (rheogenic) transport, e.g., in Na+ -glucose0 symport (B1), Na+ - amino acid0 symport (B3), 2 Na+ -amino acid– symport, or H+ -peptide0 symport (B5). The chemical Na+ gradient provides the sole Active Transport Neural and muscular excitability disorders, anoxia and consequences, cardiac glycosides
P 27 A. Na-K-ATPase 3 Na INa o KJo C‖α ADP Ki Phosphorylation Discharge Conformation e Conformational e Dephosphorylation Partly after P. Auger)
271 Fundamentals and Cell Physiology Plate 1.13 Active Transport I ./ 4 #;4 1$ ( M+ C N M;C M+ N C N M;C N $ $ 1$ " # ;4 $ $ / 4 - &&! / 4 2 ;4 $ & % / 4 2 ;4 & @- @ " <R " ;C " + C !/ 4 %;4 % 1$
28 Active Transport(continued) p driving force for electr rt molecules such as pi (e g, Na/H"antiport), whereas the negative es, and embrane potential ( p 32ff. provides occurs by ditional driving force for rheogenic cotran- ort into the cell. When secondary active port( p. 12 ff. ) ansport(e.g of glucose)is coupled with the Endocytosis (p 13)can be broken down flux of not one but two Na ions(e.g, SGLT1 into different types, including pinocytosis, porter). the driving force is doubled. The ceptor-mediated endocytosis, and phagocyte- aid of ATPases is necessary. however, if the re- nocytosis is characterized by the cor w quired"uphill" concentration ratio is several tinuous unspecific uptake of extracellular fluid ofH"ions across the luminal membrane of tively small vesicles Receptor-mediated en- arietal cells in the stomach. ATPase-mediated docytosis (C)involves the selective uptake sport can also be electrogenic or elec- of specific macromolecules with the aid of re troneutral, e.g Na'-K-ATPase(3 Na*2"; c. ceptors. This usually begins at small depres- p. 46)or H-K*-ATPase(1H1K') respectively. sions (pits)on the plasma membrane surface. Characteristics of active transport: ince the insides of the pits are often densely o It can be saturated, i.e., it has a limited maxi- coated with the protein clathrin, they ar alled clathrin-coated pits. The receptors in- It is more or less specific, ie, a carrier volved are integral cell membrane proteins olecule will transport only certain chemi- such as those for low-density lipoprotein(LPL; cally similar substances which inhibit the e.g. in hepatocytes)or intrinsic factor-bound transport of each other (competitive inhibi. cobalamin(e.g, in ileal epithelial cells). Thou- sands of the same receptor type or of different Variable quantities of the similar subst: receptors can converge at coated pits (C), re transported at a given concentration, i.e yielding a tremendou each has a different affinity (-1/Kw, see below) of ligand uptake. The endocytosed vesicles are to the transport system. initially coated with clathrin, which is later re- Active transport is inhibited when the leased. The vesicles then transform into early energy supply to the cell is disrupted. endosomes. and most of the associated rece All of these characteristics except the last tors circulate back to the cell membrane carriers, that is, to uniporter- and p. 13). The endocytose mediated(facilitated )diffusion(p. 22). exocytosed on the opposite side of the cell The transport rate of saturable transport (transcytosis, see below ), or is digested by lyso. rding to Mi- somes (C and p 13). Phagocytosis involves naelis-Menten kinetics Isat=max Imol- m"$"1. 11.16 microorganisms or cell debris, by phagocytes where Cis the concentration of the substrate in Small digestion products, such as amino acids, uestion, Jmax is its maximum transport rate, sugars and nucleotides, are transported out of and KM(Michaelis constant)is the substrate the lysosomes into the cytosol, where they can concentration that produces one-half Jmax be used for cellular metabolism or secreted (→p.389m into the extracellular fluid. when Cytosis is a completely different type of ac- mones such as insulin(p. 284)bind to re- ane-bound vesicles with a diameter of receptor complexes can also enter the coated D-400 nm. Vesides are either pinched off pits and are endocytosed (intemalized)and rom the plasma membrane(exocytosis)or in- digested by lysosomes. This reduces the den- rporated into it by invagination(endocyto-. sity of receptors available for hormone bind- sis)in conjunction with the expenditure of ing. In other words, an increased hormone ATP In cytosis, the uptake and release of mac. supply down-regulates the receptor density
281 Fundamentals and Cell Physiology driving force for electroneutral transport (e.g., Na+ /H+ antiport), whereas the negative membrane potential ( p. 32 ff.) provides an additional driving force for rheogenic cotransport into the cell. When secondary active transport (e.g., of glucose) is coupled with the influx of not one but two Na+ ions (e.g., SGLT1 symporter), the driving force is doubled. The aid of ATPases is necessary, however, if the required “uphill” concentration ratio is several decimal powers large, e.g., 106 in the extreme case of H+ ions across the luminal membrane of parietal cells in the stomach. ATPase-mediated transport can also be electrogenic or electroneutral, e.g., Na+ -K+ -ATPase (3 Na+ /2 K+ ; cf. p. 46) or H+ -K+ -ATPase (1 H+ /1 K+ ), respectively. Characteristics of active transport: ◆ It can be saturated, i.e., it has a limited maximum capacity (Jmax). ◆ It is more or less specific, i.e., a carrier molecule will transport only certain chemically similar substances which inhibit the transport of each other (competitive inhibition). ◆ Variable quantities of the similar substances are transported at a given concentration, i.e., each has a different affinity (~1/KM, see below) to the transport system. ◆ Active transport is inhibited when the energy supply to the cell is disrupted. All of these characteristics except the last apply to passive carriers, that is, to uniportermediated (facilitated) diffusion ( p. 22). The transport rate of saturable transport (Jsat) is usually calculated according to Michaelis–Menten kinetics: Jsat Jmax ⋅ C KM + C [mol ⋅ m–2 ⋅ s–1], [1.16] where C is the concentration of the substrate in question, Jmax is its maximum transport rate, and KM (Michaelis constant) is the substrate concentration that produces one-half Jmax ( p. 389 ff). Cytosis is a completely different type of active transport involving the formation of membrane-bound vesicles with a diameter of 50–400 nm. Vesicles are either pinched off from the plasma membrane (exocytosis) or incorporated into it by invagination (endocytosis) in conjunction with the expenditure of ATP. In cytosis, the uptake and release of macromolecules such as proteins, lipoproteins, polynucleotides, and polysaccharides into and out of a cell occurs by specific mechanisms similar to those involved in intracellular transport ( p. 12 ff.). Endocytosis ( p. 13) can be broken down into different types, including pinocytosis, receptor-mediated endocytosis, and phagocytosis. Pinocytosis is characterized by the continuous unspecific uptake of extracellular fluid and molecules dissolved in it through relatively small vesicles. Receptor-mediated endocytosis ( C) involves the selective uptake of specific macromolecules with the aid of receptors. This usually begins at small depressions (pits) on the plasma membrane surface. Since the insides of the pits are often densely coated with the protein clathrin, they are called clathrin-coated pits. The receptors involved are integral cell membrane proteins such as those for low-density lipoprotein (LPL; e.g., in hepatocytes) or intrinsic factor-bound cobalamin (e.g., in ileal epithelial cells). Thousands of the same receptor type or of different receptors can converge at coated pits ( C), yielding a tremendous increase in the efficacy of ligand uptake. The endocytosed vesicles are initially coated with clathrin, which is later released. The vesicles then transform into early endosomes, and most of the associated receptors circulate back to the cell membrane (C and p. 13). The endocytosed ligand is either exocytosed on the opposite side of the cell (transcytosis, see below), or is digested by lysosomes ( C and p. 13). Phagocytosis involves the endocytosis of particulate matter, such as microorganisms or cell debris, by phagocytes ( p. 94 ff.) in conjunction with lysosomes. Small digestion products, such as amino acids, sugars and nucleotides, are transported out of the lysosomes into the cytosol, where they can be used for cellular metabolism or secreted into the extracellular fluid. When certain hormones such as insulin ( p. 284) bind to receptors on the surface of target cells, hormonereceptor complexes can also enter the coated pits and are endocytosed (internalized) and digested by lysosomes. This reduces the density of receptors available for hormone binding. In other words, an increased hormone supply down-regulates the receptor density. Active Transport (continued) Interaction of medications, malabsorption, glucosuria, electrolyte therapy