H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 103
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Similarly, organelleswithin the cell often have a different internal environmentfrom that of the surrounding cytosol, and organelle membranes contain specific transport proteins that maintainthis difference.We begin our discussion by reviewing some general principles of transport across membranes and distinguishingthree major classes of transport proteins. In subsequent sections, we describe the structure and operation of specific examples of each class and show how members of families ofhomologous transport proteins have different properties thatenable different cell types to function appropriately.
We alsoexplain how specific combinations of transport proteins indifferent subcellular membranes enable cells to carry out essential physiological processes, including the maintenanceof cytosolic pH, the accumulation of sucrose and salts inplant cell vacuoles, and the directed flow of water in bothplants and animals. Epithelial cells, such as those lining thesmall intestine, transport ions, sugars and other small molecules, and water from one side to the other.
We shall see how,in order to do this, their plasma membranes are organizedinto at least two discrete regions, each with its own set oftransport proteins. The last two sections of the chapter focuson the panoply of transport proteins that allow nerve cells togenerate and conduct the type of electric signal called anaction potential along their entire length and to transmitthese signals to other cells, inducing a change in the electricalproperties of the receiving cells.OUTLINE7.1 Overview of Membrane Transport7.2 ATP-Powered Pumps and the Intracellular IonicEnvironment7.3 Nongated Ion Channels and the RestingMembrane Potential7.4 Cotransport by Symporters and Antiporters7.5 Movement of Water7.6 Transepithelial Transport7.7 Voltage-Gated Ion Channels and thePropagation of Action Potentials in Nerve Cells7.8 Neurotransmitters and Receptor and TransportProteins in Signal Transmission at Synapses245246CHAPTER 7 • Transport of Ions and Small Molecules Across Cell Membranes7.1Overview of Membrane TransportThe phospholipid bilayer, the basic structural unit of biomembranes, is essentially impermeable to most watersoluble molecules, ions, and water itself.
After describing thefactors that influence the permeability of lipid membranes,we briefly compare the three major classes of membraneproteins that increase the permeability of biomembranes. Wethen examine operation of the simplest type of transportprotein to illustrate basic features of protein-mediated transport. Finally, two common experimental systems used instudying the functional properties of transport proteins aredescribed.Few Molecules Cross Membranesby Passive DiffusionGases, such as O2 and CO2, and small, uncharged polar molecules, such as urea and ethanol, can readily move by passive(simple) diffusion across an artificial membrane composed ofpure phospholipid or of phospholipid and cholesterol (Figure7-1). Such molecules also can diffuse across cellular membranes without the aid of transport proteins. No metabolicenergy is expended because movement is from a high to alow concentration of the molecule, down its chemical con-GasesCO2, N2, O2SmallunchargedpolarmoleculesEthanolONH2LargeunchargedpolarmoleculesC NH2UreaPermeablePermeableH2 OWaterSlightlypermeableGlucose, fructoseImpermeableIonsK +, Mg2 +, Ca2 +, Cl −,HCO3−, HPO42 −ImpermeableChargedpolarmoleculesAmino acids, ATP,glucose 6-phosphate,proteins, nucleic acidscentration gradient.
As noted in Chapter 2, such transportreactions are spontaneous because they have a positive Svalue (increase in entropy) and thus a negative G (decreasein free energy).The relative diffusion rate of any substance across a purephospholipid bilayer is proportional to its concentration gradient across the layer and to its hydrophobicity and size;charged molecules are also affected by any electric potentialacross the membrane (see below). When a phospholipid bilayer separates two aqueous compartments, membrane permeability can be easily determined by adding a small amountof radioactive material to one compartment and measuringits rate of appearance in the other compartment.
The greaterthe concentration gradient of the substance, the faster its rateof diffusion across a bilayer.The hydrophobicity of a substance is measured by itspartition coefficient K, the equilibrium constant for itspartition between oil and water. 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.
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