Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 105
Текст из файла (страница 105)
The higher a substance’spartition coefficient, the more lipid-soluble it is. The firstand rate-limiting step in transport by passive diffusion ismovement of a molecule from the aqueous solution intothe hydrophobic interior of the phospholipid bilayer,which resembles oil in its chemical properties. This is thereason that the more hydrophobic a molecule is, the fasterit diffuses across a pure phospholipid bilayer.
For example, diethylurea, with an ethyl group (CH3CH2O) attached to each nitrogen atom of urea, has a K of 0.01,whereas urea has a K of 0.0002 (see Figure 7-1). Diethylurea, which is 50 times (0.01/0.0002) more hydrophobicthan urea, will diffuse through phospholipid bilayer membranes about 50 times faster than urea. Diethylurea alsoenters cells about 50 times faster than urea. Similarly,fatty acids with longer hydrocarbon chains are more hydrophobic than those with shorter chains and will diffusemore rapidly across a pure phospholipid bilayer at allconcentrations.If a transported substance carries a net charge, its movement is influenced by both its concentration gradient and themembrane potential, the electric potential (voltage) acrossthe membrane.
The combination of these two forces, calledthe electrochemical gradient, determines the energetically favorable direction of transport of a charged molecule acrossa membrane. The electric potential that exists across mostcellular membranes results from a small imbalance in theconcentration of positively and negatively charged ions onthe two sides of the membrane. We discuss how this ionic imbalance, and resulting potential, arise and are maintained inSections 7.2 and 7.3.Impermeable▲ FIGURE 7-1 Relative permeability of a pure phospholipidbilayer to various molecules. A bilayer is permeable to smallhydrophobic molecules and small uncharged polar molecules,slightly permeable to water and urea, and essentiallyimpermeable to ions and to large polar molecules.Membrane Proteins Mediate Transport of MostMolecules and All Ions Across BiomembranesAs is evident from Figure 7-1, very few molecules and noions can cross a pure phospholipid bilayer at appreciablerates by passive diffusion.
Thus transport of most molecules7.1 • Overview of Membrane Transport123ATP-powered pumps(100−103 ions/s)Ion channels(107−108 ions/s)Transporters(102−104 molecules/s)247ExteriorCytosolClosedATP ADP + PiOpen▲ FIGURE 7-2 Overview of membrane transport proteins.UniporterSymporterAntiporterABCGradients are indicated by triangles with the tip pointing towardlower concentration, electrical potential, or both.
1 Pumps utilizethe energy released by ATP hydrolysis to power movement ofspecific ions (red circles) or small molecules against theirelectrochemical gradient. 2 Channels permit movement ofspecific ions (or water) down their electrochemical gradient.Transporters, which fall into three groups, facilitate movementof specific small molecules or ions. Uniporters transport a singletype of molecule down its concentration gradient 3A.Cotransport proteins (symporters, 3B , and antiporters, 3C )catalyze the movement of one molecule against its concentrationgradient (black circles), driven by movement of one or more ionsdown an electrochemical gradient (red circles).
Differences inthe mechanisms of transport by these three major classes ofproteins account for their varying rates of solute movement.into and out of cells requires the assistance of specializedmembrane proteins. Even transport of molecules with a relatively large partition coefficient (e.g., water and urea) is frequently accelerated by specific proteins because theirtransport by passive diffusion usually is not sufficiently rapidto meet cellular needs.All transport proteins are transmembrane proteins containing multiple membrane-spanning segments that generallyare helices. By forming a protein-lined pathway across themembrane, transport proteins are thought to allow movement of hydrophilic substances without their coming intocontact with the hydrophobic interior of the membrane.Here we introduce the various types of transport proteinscovered in this chapter (Figure 7-2).ATP-powered pumps (or simply pumps) are ATPasesthat use the energy of ATP hydrolysis to move ions or smallmolecules across a membrane against a chemical concentration gradient or electric potential or both.
This process, referred to as active transport, is an example of a coupledchemical reaction (Chapter 2). In this case, transport of ionsor small molecules “uphill” against an electrochemical gradient, which requires energy, is coupled to the hydrolysis ofATP, which releases energy. The overall reaction—ATPhydrolysis and the “uphill” movement of ions or small molecules—is energetically favorable.Channel proteins transport water or specific types of ionsand hydrophilic small molecules down their concentration orelectric potential gradients.
Such protein-assisted transportsometimes is referred to as facilitated diffusion. Channel proteins form a hydrophilic passageway across the membranethrough which multiple water molecules or ions move simultaneously, single file at a very rapid rate. Some ion chan-nels are open much of the time; these are referred to as nongated channels. Most ion channels, however, open only in response to specific chemical or electrical signals; these arereferred to as gated channels.Transporters (also called carriers) move a wide varietyof ions and molecules across cell membranes. Three types oftransporters have been identified. Uniporters transport a single type of molecule down its concentration gradient via facilitated diffusion. Glucose and amino acids cross the plasmamembrane into most mammalian cells with the aid of uniporters.
In contrast, antiporters and symporters couple themovement of one type of ion or molecule against its concentration gradient with the movement of one or more different ions down its concentration gradient. These proteinsoften are called cotransporters, referring to their ability totransport two different solutes simultaneously.Like ATP pumps, cotransporters mediate coupled reactions in which an energetically unfavorable reaction(i.e., uphill movement of molecules) is coupled to an energetically favorable reaction. Note, however, that the natureof the energy-supplying reaction driving active transportby these two classes of proteins differs.
ATP pumps use energy from hydrolysis of ATP, whereas cotransporters usethe energy stored in an electrochemical gradient. This latter process sometimes is referred to as secondary activetransport.Table 7-1 summarizes the four mechanisms by whichsmall molecules and ions are transported across cellularmembranes. In this chapter, we focus on the properties andoperation of the membrane proteins that mediate the threeprotein-dependent transport mechanisms. Conformationalchanges are essential to the function of all transport proteins.248CHAPTER 7 • Transport of Ions and Small Molecules Across Cell MembranesTABLE 7-1Mechanisms for Transporting Ions and Small Molecules Across Cell MembranesTransport MechanismPropertyPassive DiffusionFacilitated DiffusionActive TransportCotransport*Requires specificproteinSolute transportedagainst its gradientCoupled to ATPhydrolysisDriven by movement ofa cotransported iondown its gradientExamples of moleculestransportedO2, CO2, steroidhormones, many drugsGlucose and aminoacids (uniporters); ionsand water (channels)Ions, small hydrophilicmolecules, lipids (ATPpowered pumps)Glucose and aminoacids (symporters);various ions andsucrose (antiporters)*Also called secondary active transport.ATP-powered pumps and transporters undergo a cycle ofconformational change exposing a binding site (or sites) toone side of the membrane in one conformation and to theother side in a second conformation.
Because each such cycleresults in movement of only one (or a few) substrate molecules, these proteins are characterized by relatively slow ratesof transport ranging from 100 to 104 ions or molecules persecond (see Figure 7-2). Ion channels shuttle between aclosed state and an open state, but many ions can passthrough an open channel without any further conformational change.
For this reason, channels are characterized byvery fast rates of transport, up to 108 ions per second.Several Features Distinguish UniportTransport from Passive DiffusionThe protein-mediated movement of glucose and other smallhydrophilic molecules across a membrane, known as uniporttransport, exhibits the following distinguishing properties:1. The rate of facilitated diffusion by uniporters is far higherthan passive diffusion through a pure phospholipid bilayer.2. Because the transported molecules never enter the hydrophobic core of the phospholipid bilayer, the partitioncoefficient K is irrelevant.3.
Transport occurs via a limited number of uniportermolecules, rather than throughout the phospholipid bilayer.Consequently, there is a maximum transport rate Vmax thatis achieved when the concentration gradient across themembrane is very large and each uniporter is working atits maximal rate.4. Transport is specific.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
