3 Биологические мембраны. Обмен веществом (1160072), страница 9
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Operational Definition of Lipids How is thedefinition of "lipid" different from the definitions ofother types of biomolecules that we have considered, such as amino acids, nucleic acids, and proteins?13. Effect of Polarity on Solubility Rank, in orderof increasing solubility in water, a triacylglycerol,a diacylglycerol, and a monoacylglycerol, all containing only palmitic acid.14. Intracellular Messengers from Phosphatidylinositols When the hormone vasopressin stimulates cleavage of phosphatidylinositol-4,5-bisphosphate by hormone-sensitive phospholipase C, twoproducts are formed. Compare their properties andsolubilities in water, and predict whether eitherwould be expected to diffuse readily through thecytosol.15. Identification of Unknown Lipids JohannThudichum, who practiced medicine in Londonabout 100 years ago, also dabbled in lipid chemistry in his spare time.
He isolated a variety of lipidsfrom neural tissue, and characterized and namedmany of them. His carefully sealed and labeledvials of isolated lipids were rediscovered manyyears later. How would you confirm, using techniques available to you but not to him, that thevials he labeled "sphingomyelin" and "cerebroside"actually contain these compounds?16. Analysis of Choline-Containing PhospholipidsHow would you distinguish sphingomyelin fromphosphatidylcholine by chemical, physical, or enzymatic tests?C H A P T E RBiological Membranesand TransportThe first living cell probably came into being when a membraneformed, separating that cell's precious contents from the rest of theuniverse.
Membranes define the external boundary of cells and regulate the molecular traffic across that boundary; they divide the internal space into discrete compartments to segregate processes and components (Fig. 10-1); they organize complex reaction sequences; andthey are central to both biological energy conservation and cell-to-cellcommunication.
The biological activities of membranes flow from theirremarkable physical properties. Membranes are tough but flexible,self-sealing, and selectively permeable to polar solutes. Their flexibility permits the shape changes that accompany cell growth and movement (such as amoeboid movement). Their ability to seal over temporary breaks in their continuity allows two membranes to fuse, as inexocytosis, or a single membrane-enclosed compartment to undergofission, yielding two sealed compartments, as in endocytosis or celldivision, without creating gross leaks through the cell surface.
Becausemembranes are selectively permeable, they retain certain compoundsand ions within cells and within specific cellular compartments, andexclude others.Membranes are not merely passive barriers. They include an arrayof proteins specialized for promoting or catalyzing a variety of molecular events. Pumps move specific organic solutes and inorganic ionsacross the membrane against a concentration gradient; energy transducers convert one form of energy into another; receptors on theplasma membrane sense extracellular signals, converting them intomolecular changes within the cell.(d)268(e)Figure 10-1 Viewed in cross section, all intracellular membranes share a characteristic trilaminarappearance.
The protozoan Paramecium contains avariety of specialized membrane-bounded organelles. When a thin section of a Paramecium isstained with osmium tetroxide to highlight membranes, each of the membranes appears as a threelayer structure, 5 to 8 nm thick. The trilaminarimages consist of two electron-dense layers on theinner and outer surfaces separated by a less densecentral region. At left are high-magnification viewsof the membranes of (a) a cell body (plasma andalveolar membranes tightly apposed), (b) a cilium,(c) a mitochondrion, (d) a digestive vacuole, (e) theendoplasmic reticulum, and (f) a secretory vesicle.Chapter 10 Biological Membranes and TransportMembranes are composed of just two layers of molecules, and aretherefore very thin; they can be thought of as essentially two-dimensional.
A large number of cellular processes are associated with membranes (such as the synthesis of lipids and certain proteins, and theenergy transductions in mitochondria and chloroplasts). Because intermolecular collisions are far more probable in this two-dimensionalspace than in three-dimensional space, the efficiency of certain enzyme-catalyzed pathways organized within a two-dimensional membrane is vastly increased.In this chapter we first describe the composition of cellular membranes and their chemical architecture—the physical structure thatunderlies their biological functions.
We then turn to membrane transport, the protein-mediated transmembrane passage of solutes. In laterchapters we will discuss the role of membranes in energy transduction,lipid synthesis, signal transduction, and protein synthesis.The Molecular Constituents of MembranesOne approach to u n d e r s t a n d i n g m e m b r a n e function is to study membrane composition—to determine, for example, which components a r ecommonly present in m e m b r a n e s a n d which are u n i q u e to m e m b r a n e swith specific functions. Knowledge of composition is also invaluable instudies of m e m b r a n e structure, as any viable model for m e m b r a n estructure m u s t conform to and explain t h e known composition.
Beforedescribing m e m b r a n e structure and function, we therefore considerthe molecular components of m e m b r a n e s .Proteins and polar lipids account for almost all of t h e m a s s of biological membranes; t h e small a m o u n t of carbohydrate p r e s e n t is generally part of glycoproteins or glycolipids. The relative proportions ofprotein and lipid differ in different m e m b r a n e s (Table 10-1), reflectingthe diversity of biological roles. The myelin sheath, which serves as apassive electrical insulator wrapped around certain neurons, consistsprimarily of lipids, but t h e m e m b r a n e s of bacteria, mitochondria, andchloroplasts, in which m a n y enzyme-catalyzed metabolic processestake place, contain more protein t h a n lipid.Table 10—1 Major components of plasma membranesof different species*Protein Phospholipid(%)(%)Mouse liverCorn leafYeast4547522726756754025Otherlipids—GalactolipidsTriacylglycerolsSteryl estersSterol(%) Sterol type2574CholesterolSitosterolErgosterol40StigmasterolParamecium(ciliate protist)E.
coli———* Values are given as weight percentages.Each Membrane Has a Characteristic Lipid CompositionFor studies of membrane composition, it is essential first to isolate themembrane of interest. When eukaryotic cells are subjected to mechani-269Part II Structure and Catalysis270Table 10-2 Lipid composition of organelle membranes of a rat liver cell*Plasma membraneGolgi complexSmooth endoplasmic reticulumRough endoplasmic reticulumNuclear membraneLysosomal membraneMitochondrial membraneInnerOuterChoiPCPEPSPIPGCLSM308106184050551115211694031055203142513046787700000000200514101233243545452423261323184351'••: Values are given as weight percentages. Choi designates cholesterol; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine; PI, phosphatidylinositol; PG.
phosphatidylglycerol; CL, cardiolipin; SM, sphingomyelin.200,000-116,00097,00066,000-43,000-31,000-cal shear, their plasma membranes are torn and fragmented, releasingcytosolic components and membrane-bounded organelles: mitochondria, chloroplasts, lysosomes, nuclei, and others. The plasma membrane fragments and intact organelles can be isolated by centrifugaltechniques described in Chapter 2 (see Fig. 2-24).Chemical analysis of membranes isolated from various sourcesreveals certain common properties. Membrane lipid composition ischaracteristic for each kingdom, each species, each tissue, and eachorganelle within a given cell type (Table 10-2).
Cells clearly havemechanisms to control the kinds and amounts of membrane lipids synthesized and to target specific lipids to particular organelles. Thesedistinct combinations doubtless confer advantages on cells and organisms during evolution, but in most cases the functional significance ofthese characteristic lipid compositions remains to be discovered.2100014.000-Figure 10-2 Membranes with specialized functionsdiffer in protein composition, as revealed by electrophoretic separation on a polyacrylamide gel in thepresence of the detergent SDS (p. 141).
The purplemembrane of Halobacterium and the rod-cell outersegment membrane are very rich in bacteriorhodopsin and rhodopsin, respectively. The myelin sheathalso contains relatively few kinds of proteins. Theother membranes shown have more complex functions, reflected in a wider variety of membrane proteins.Membranes with Different FunctionsHave Different ProteinsThe protein composition of membranes from different sources (Fig.10-2) varies even more widely than their lipid composition, reflectingfunctional specialization. The outer segment of the rod cells of the vertebrate retina is highly specialized for the reception of light; more than90% of its membrane protein is the light-absorbing protein rhodopsin(see Fig.
9-18). The less-specialized plasma membrane of the erythrocyte has about 20 prominent proteins as well as dozens of minor ones;many of these serve as transporters, each responsible for moving aspecific solute across the membrane. The inner (plasma) membrane ofE. coli contains hundreds of different proteins, various transporters, aswell as many enzymes involved in energy-conserving metabolism, lipidsynthesis, protein export, and cell division. The outer membrane ofE. coli has a different function (protection) and a different set of proteins.
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