THE MOLECULES OF LIFE UNIT 1 06 Free Ch06.qxp 10/8/09

1y ago
4.50 MB
26 Pages
Last View : 10m ago
Last Download : 10m ago
Upload by : Amalia Wilborn

06 free ch06.qxp10/8/092:19 PMPage 99THE MOLECULES OF LIFEUNIT16Lipids, Membranes,and the First CellsK E Y CO N C E P TSPhospholipids are amphipathicmolecules—they have a hydrophilic regionand a hydrophobic region. In solution, theyspontaneously form bilayers that areselectively permeable—meaning that onlycertain substances cross them readily.Ions and molecules diffuse spontaneouslyfrom regions of high concentration toregions of low concentration. Watermoves across lipid bilayers from regionsof high concentration to regions of lowconcentration via osmosis—a special caseof diffusion.In cells, membrane proteins are responsiblefor the passage of ions, polar molecules,and large molecules that can’t cross themembrane on their own because they arenot soluble in lipids.These bacterial cells have been stained with a red compound that inserts itself into the plasmamembrane.The plasma membrane defines the cell—the basic unit of life. In single-celled organismslike those shown here, the membrane creates a physical separation between life on the inside andnonlife on the outside.The research discussed in previous chapters suggeststhat biological evolution began with an RNA moleculethat could make a copy of itself. As the offspring of thismolecule multiplied in the prebiotic soup, natural selectionwould have favoured versions of the molecule that were particularly stable and efficient at catalysis. Another great milestonein the history of life occurred when a descendant of this replicator became enclosed within a membrane. This event createdthe first cell and thus the first organism.The cell membrane, or plasma membrane, is a layer of molecules that surrounds the cell, separating it from the externalenvironment and selectively regulating the passage of moleculesand ions into or out of the cell. The evolution of the plasmaKey ConceptImportant InformationPractise Itmembrane was a momentous development because it separated life from nonlife. Before plasma membranes existed, selfreplicating molecules probably clung to clay-sized mineralparticles, building copies of themselves as they randomlyencountered the appropriate nucleotides in the prebiotic soupthat washed over them. But the membrane made an internalenvironment possible—one that could have a chemical composition different from that of the external environment. This wasimportant for two reasons. First, the chemical reactions necessary for life could occur much more efficiently in an enclosedarea, because reactants could collide more frequently. Second,the membrane could serve as a selective barrier. That is, itcould keep compounds out of the cell that might damage the99

06 free ch06.qxp10010/8/092:19 PMPage 100Unit 1 The Molecules of Lifereplicator, but it might allow the entry of compounds requiredby the replicator. The membrane not only created the cell butalso made it into an efficient and dynamic reaction vessel.The goal of this chapter is to investigate how membranesbehave, with an emphasis on how they differentiate the internalenvironment from the external environment. Let’s begin byexamining the structure and properties of the most abundantmolecules in plasma membranes: the “oily” or “fatty” compounds called lipids. Then we can delve into analyzing the waylipids behave when they form membranes. Which ions andmolecules can pass through a membrane that consists of lipids?Which cannot, and why? The chapter ends by exploring howproteins that become incorporated into a lipid membrane cancontrol the flow of materials across the membrane.(a) In solution, lipids form water-filled vesicles.50 nm(b) Red blood cells resemble vesicles.6.1 LipidsMost biochemists are convinced that the building blocks ofmembranes, called lipids, existed in the prebiotic soup. Thisconclusion is based on the observation that several types oflipids have been produced in experiments designed to mimicthe chemical and energetic conditions that prevailed early inEarth’s history. For example, the spark-discharge experimentsreviewed in Chapter 3 succeeded in producing at least two typesof lipids.An observation made by A. D. Bangham illustrates why thisresult is interesting. In the late 1950s, Bangham performedexperiments to determine how lipids behave when they areimmersed in water. But until the electron microscope wasinvented, he had no idea what these lipid–water mixtureslooked like. Once transmission electron microscopes becameavailable, Bangham was able to produce high-magnification,high-resolution images of his lipid–water mixtures. (Transmission electron microscopy is introduced in BioSkills 8.) Theimages that resulted, called micrographs, were astonishing.As Figure 6.1a shows, the lipids had spontaneously formedenclosed compartments filled with water. Bangham called thesemembrane-bound structures vesicles and noted that theyresembled cells (Figure 6.1b). Bangham had not done anythingspecial to the lipid–water mixtures; he had merely shaken themby hand.The experiment raises a series of questions: How couldthese structures have formed? Is it possible that vesicles likethese existed in the prebiotic soup? If so, could they havesurrounded a self-replicating molecule and become the firstplasma membrane? Let’s begin answering these questions byinvestigating what lipids are and how they behave.What Is a Lipid?Earlier chapters analyzed the structures of the organic molecules called amino acids, nucleotides, and monosaccharides50 μmFIGURE 6.1 Lipids Can Form Cell-like Vesicles When in Water.(a) Transmission electron micrograph showing a cross section throughthe tiny, bag-like compartments that formed when a researcher shook amixture of lipids and water. (b) Scanning electron micrograph showingred blood cells from humans. Note the scale bars.and explored how these monomers polymerize to form macromolecules. Here let’s focus on another major type of mid-sizedmolecule found in living organisms: lipids.Lipid is a catch-all term for carbon-containing compoundsthat are found in organisms and are largely nonpolar andhydrophobic—meaning that they do not dissolve readily inwater. (Recall from Chapter 2 that water is a polar solvent.)Lipids do dissolve, however, in liquids consisting of nonpolarorganic compounds.To understand why lipids do not dissolve in water, examine the five-carbon compound called isoprene illustrated inFigure 6.2a; notice that it consists of a group of carbon atomsbonded to hydrogen atoms. Molecules that contain only carbonand hydrogen, such as isoprene or octane (see Chapter 2) areknown as hydrocarbons. Hydrocarbons are nonpolar, becauseelectrons are shared equally in carbon–hydrogen bonds. Thisproperty makes hydrocarbons hydrophobic. Thus, the reasonlipids do not dissolve in water is that they have a significanthydrocarbon component. Figure 6.2b is a type of compoundcalled a fatty acid, which consists of a hydrocarbon chainbonded to a carboxyl (COOH) functional group. Isoprene

06 free ch06.qxp10/8/092:19 PMPage 101Chapter 6 Lipids, Membranes, and the First Cells(a) IsopreneOCH2CCarboxylgroupA Look at Three Types of Lipids Found in CellsH2CCH3Unlike amino acids, nucleotides, and carbohydrates, lipids aredefined by a physical property—their solubility—instead oftheir chemical structure. As a result, the structure of lipids varieswidely. To drive this point home, consider the structures of themost important types of lipids found in cells: fats, steroids, andphospholipids.CH2CH 2CCHand fatty acids are key building blocks of the lipids found inorganisms.(b) Fatty acidHOCH2CH2H2CCH2H2CCH2H2CHydrocarbonchain CH2CH2H2CCH2H3CFIGURE 6.2 Hydrocarbon Groups Make Lipids Hydrophobic.(a) Isoprenes are hydrocarbons. Isoprene subunits can be linked end toend to form long hydrocarbon chains. (b) Fatty acids typically contain atotal of 14–20 carbon atoms, most found in their long hydrocarbon tails.EXERCISE Circle the hydrophobic portion of a fatty acid.GlycerolHHHHCCCOHOHOHH2OHOOCHFats are composed of three fatty acids that are linked to athree-carbon molecule called glycerol. Because of this struc-ture, fats are also called triacylglycerols or triglycerides.As Figure 6.3a shows, fats form when a dehydration reaction occurs between a hydroxyl group of glycerol and thecarboxyl group of a fatty acid. The glycerol and fatty-acidmolecules become joined by an ester linkage, which isanalogous to the peptide bonds, phosphodiester bonds, andglycosidic linkages in proteins, nucleic acids, and carbohydrates, respectively. Fats are not polymers, however, andfatty acids are not monomers. As Figure 6.3b shows, fattyacids are not linked together to form a macromolecule in theway that amino acids, nucleotides, and monosaccharidesH2C(a) Fats form via dehydration reactions.101(b) Fats consist of glycerol linked by ester linkages to three fatty acids.HHHHCCCOOOCO CO CHOEsterlinkagesDehydrationreactionFatty acidFIGURE 6.3 Fats Are One Type of Lipid Found in Cells. (a) When glycerol and a fatty acid react, a water molecule leaves.(b) The covalent bond that results from this reaction is termed an ester linkage.The fat shown here as a structural formulaand a space-filling model is tristearin, the most common type of fat in beef.

06 free ch06.qxp2:19 PMPage 102Unit 1 The Molecules of Life(a) A aticHOdroiSteCH3gsNonpolar(hydrophobic)ri nCH3HC CH3Isoprene chain10210/8/09H2CCH2H2CHCCH3H3C(b) A phospholipidH3CCH3N CH3CH2H2COPolar head(hydrophilic)HOPHHOCCCOOHO–CholineHOC OH2CCH2CH2H2CH 2CCH2CH2H 2CH2CCH2CH2H2CH 2CCHCH2H2CCHCH2 H C2H2CCH2CH2 H C2H2CCH2CH2 H C2H3CCH2H 2CCH3PhosphateGlycerolCFatty acidNonpolar tail(hydrophobic)Fatty acidH2CFIGURE 6.4 Amphipathic Lipids Contain Hydrophilic and Hydrophobic Elements. (a) All steroids have a distinctivefour-ring structure. (b) All phospholipids consist of a glycerol that is linked to a phosphate group and to either two chainsof isoprene or two fatty acids.QUESTION What makes cholesterol—the steroid shown in part (a)—different from other steroids?QUESTION If these molecules were in solution, where would water molecules interact with them?are.After studying the structure in Figure 6.3b, youshould be able to explain why fats store a great deal ofchemical energy, and why they are hydrophobic. Steroids are a family of lipids distinguished by the four-ringstructure shown in solid orange in Figure 6.4a. The varioussteroids differ from one another by the functional groups or

06 free ch06.qxp10/8/092:19 PMPage 103Chapter 6 Lipids, Membranes, and the First Cellsside groups attached to those rings. The molecule picturedin Figure 6.4a is cholesterol, which is distinguished by ahydrocarbon “tail” formed of isoprene subunits. Cholesterolis an important component of plasma membranes in manyorganisms. In mammals, it is also used as the starting pointfor the synthesis of several of the signalling molecules calledhormones. Estrogen, progesterone, and testosterone areexamples of hormones derived from cholesterol. Thesemolecules are responsible for regulating sexual developmentand activity in humans. Phospholipids consist of a glycerol that is linked to a phos-phate group (PO422) and to either two chains of isoprene ortwo fatty acids. In some cases, the phosphate group isbonded to another small organic molecule, such as thecholine shown on the phospholipid in Figure 6.4b. Phospholipids with isoprene tails are found in the domain Archaeaintroduced in Chapter 1; phospholipids composed of fattyacids are found in the domains Bacteria and Eukarya. In allthree domains of life, phospholipids are critically importantcomponents of the plasma membrane.To summarize, the lipids found in organisms have a widearray of structures and functions. In addition to storing chemical energy and serving as signals between cells, lipids act as pigments that capture or respond to sunlight, form waterproofcoatings on leaves and skin, and act as vitamins used in anarray of cellular processes. The most important lipid function,however, is their role in the plasma membrane. Let’s take acloser look at the specific types of lipids found in membranes.The Structures of Membrane LipidsNot all lipids can form the artificial membranes that Banghamand his colleagues observed. In fact, just two types of lipids areusually found in plasma membranes.Membrane-forminglipids have a polar, hydrophilic region in addition to the nonpolar, hydrophobic region found in all lipids. To better understand this structure, take another look at the phospholipidillustrated in Figure 6.4b. Notice that the molecule has a“head” region containing highly polar covalent bonds as wellas positive and negative charges. The charges and polar bondsin the head region interact with water molecules when a phospholipid is placed in solution. In contrast, the long isoprene orfatty-acid tails of a phospholipid are nonpolar. Water molecules cannot form hydrogen bonds with the hydrocarbon tail,so they do not interact with this part of the molecule.Compounds that contain both hydrophilic and hydrophobicelements are amphipathic (“dual-sympathy”). Phospholipidsare amphipathic. As Figure 6.4a shows, cholesterol is alsoamphipathic. It has both hydrophilic and hydrophobic regions.The amphipathic nature of phospholipids is far and awaytheir most important feature biologically. It is responsible fortheir presence in plasma membranes.103Check Your UnderstandingIf you understand that Fats, steroids, and phospholipids differ in structure andfunction: Fats store chemical energy; amphipathic steroidsare important components of cell membranes;phospholipids are amphipathic and are usually the mostabundant component of cell membranes.You should be able to 1) Draw a generalized version of a fat, a steroid, and aphospholipid.2) Use these diagrams to explain why cholesterol andphospholipids are amphipathic.3) Explain how the structure of a fat correlates with itsfunction in the cell.6.2 Phospholipid BilayersPhospholipids do not dissolve when they are placed in water.Water molecules interact with the hydrophilic heads of thephospholipids, but not with their hydrophobic tails. Instead ofdissolving in water, then, phospholipids may form one of twotypes of structures: micelles or lipid bilayers.Micelles (Figure 6.5a) are tiny droplets created when thehydrophilic heads of phospholipids face the water and thehydrophobic tails are forced together, away from the water.Lipids with compact tails tend to form micelles. Because theirdouble-chain tails are often too bulky to fit in the interior of amicelle, most phospholipids tend to form bilayers. Phospholipidbilayers, or simply, lipid bilayers, are created when two sheetsof phospholipid molecules align. As Figure 6.5b shows, thehydrophilic heads in each layer face a surrounding solutionwhile the hydrophobic tails face one another inside the bilayer.In this way, the hydrophilic heads interact with water while thehydrophobic tails interact with each other. Micelles tend toform from phospholipids with relatively short tails; bilayerstend to form from phospholipids with longer tails.Once you understand the structure of micelles and phospholipid bilayers, the most important point to recognize aboutthem is that they form spontaneously. No input of energy isrequired. This concept can be difficult to grasp, because theormation of these structures clearly decreases entropy. Micellesand lipid bilayers are much more highly organized than phospholipids floating free in the solution. The key is to recognize thatmicelles and lipid bilayers are much more stable energeticallythan are independent molecules in solution. Stated anotherway, micelles and lipid bilayers have much lower potentialenergy than do independent phospholipids in solution. Independent phospholipids are unstable in water because theirhydrophobic tails disrupt hydrogen bonds that otherwise

06 free ch06.qxp10410/8/092:19 PMPage 104Unit 1 The Molecules of Life(a) Lipid micelles(b) Lipid bilayersWaterNo waterWaterHydrophilic heads interact with waterHydrocarbon surroundedby water moleculesHydrophobic tails interact with each otherFIGURE 6.6 Hydrocarbons Disrupt Hydrogen Bonds between WaterMolecules. Hydrocarbons are unstable in water because they disrupthydrogen bonding between water molecules.EXERCISE Label the area where no hydrogen bonding is occurringbetween water molecules.QUESTION Hydrogen bonds pull water molecules closer together.Which way are the water molecules in this figure being pulled, relative tothe hydrocarbon?Hydrophilic heads interact with waterFIGURE 6.5 Phospholipids Form Bilayers in Solution. In (a) a micelleor (b) a lipid bilayer, the hydrophilic heads of lipids face out, towardwater; the hydrophobic tails face in, away from water. Plasma membranesconsist in part of lipid bilayers.would form between water molecules ( Figure 6.6; see alsoFigure 2.13b). As a result, amphipathic molecules are muchmore stable in aqueous solution when their hydrophobic tailsavoid water and instead participate in the hydrophobic (vander Waals) interactions introduced in Chapter 3. In this case,the loss of potential energy outweighs the decrease in entropy.Overall, the free energy of the system decreases. Lipid bilayerformation is exergonic and spontaneous.If you understand this reasoning, you should be able toadd water molecules that are hydrogen-bonded to eachhydrophilic head in Figure 6.5, and explain the logic behindyour drawing.Artificial Membranes as an Experimental SystemWhen lipid bilayers are agitated by shaking, the layers break andre-form as small, spherical structures. This is what happened inBangham’s experiment. The resulting vesicles had water on theinside as well as the outside because the hydrophilic heads ofthe lipids faced outward on each side of the bilayer.Researchers have produced these types of vesicles by usingdozens of different types of phospholipids. Artificial membrane-bound vesicles like these are called liposomes. The ability tocreate them supports an important conclusion: If phospholipidmolecules accumulated during chemical evolution early in Earth’shistory, they almost certainly formed water-filled vesicles.To better understand the properties of vesicles and plasmamembranes, researchers began creating and experimentingwith liposomes and other types of artificial bilayers. Some ofthe first questions they posed concerned the permeability oflipid bilayers. The permeability of a structure is its tendency toallow a given substance to pass across it. Once a membraneforms a water-filled vesicle, can other molecules or ions pass inor out? If so, is this permeability selective in any way? Thepermeability of membranes is a critical issue, because if certainmolecules or ions pass through a lipid bilayer more readilythan others, the internal environment of a vesicle can becomedifferent from the outside. This difference between exterior andinterior environments is a key characteristic of cells.Figure 6.7 shows the two types of artificial membranes thatare used to study the permeability of lipid bilayers. Figure 6.7ashows liposomes, roughly spherical vesicles. Figure 6.7b illustrates planar bilayers, which are lipid bilayers constructedacross a hole in a glass or plastic wall separating two aqueous(watery) solutions.Using liposomes and planar bilayers, researchers can studywhat happens when a known ion or molecule is added to oneside of a lipid bilayer (Figure 6.7c). Does the ion or moleculecross the membrane and show up on the other side? If so, how

06 free ch06.qxp10/8/092:19 PMPage 105Chapter 6 Lipids, Membranes, and the First Cells(a) Liposomes: Artificial membrane-bound vesiclesWaterWater50 nm(b) Planar bilayers: Artificial membranes105factor changes from one experimental treatment to the next.Control, in turn, is why experiments are such an effectivemeans of exploring scientific questions. You might recall fromChapter 1 that good experimental design allows researchers toalter one factor at a time and determine what effect, if any,each has on the process being studied.Equally important for experimental purposes, liposomesand planar bilayers provide a clear way to determine whether agiven change in conditions has an effect. By sampling thesolutions on both sides of the membrane before and after thetreatment and then analyzing the concentration of ions andmolecules in the samples, researchers have an effective way todetermine whether the treatment had any consequences.Using such systems, what have biologists learned aboutmembrane permeability?Selective Permeability of Lipid BilayersWaterWaterLipidbilayer(c) Artificial-membrane experimentsHow rapidly can differentsolutes cross the membrane(if at all) when .Solute(ion ormolecule)?1. Different types ofphospholipids are used tomake the membrane?2. Proteins or othermolecules are added tothe membrane?FIGURE 6.7 Liposomes and Planar Bilayers Are ImportantExperimental Systems. (a) Electron micrograph of liposomes in crosssection (left) and a cross-sectional diagram of the lipid bilayer in aliposome. (b) The construction of planar bilayers across a hole in a glasswall separating two water-filled compartments (left), and a close-upsketch of the bilayer. (c) A wide variety of experiments are possible withliposomes and planar bilayers; a few are suggested here.rapidly does the movement take place? What happens when adifferent type of phospholipid is used to make the artificialmembrane? Does the membrane’s permeability change whenproteins or other types of molecules become part of it?Biologists describe such an experimental system as elegantand powerful because it gives them precise control over whichWhen researchers put molecules or ions on one side of a liposomeor planar bilayer and measure the rate at which the moleculesarrive on the other side, a clear pattern emerges: Lipid bilayersare highly selective. Selective permeability means that somesubstances cross a membrane more easily than other substancescan. Small, nonpolar molecules move across bilayers quickly.In contrast, large molecules and charged substances cross themembrane slowly, if at all. According to the data in Figure 6.8,small, nonpolar molecules such as oxygen (O2) move acrossselectively permeable membranes more than a billion times fasterthan do chloride ions (Cl2). Very small and uncharged moleculessuch as water (H 2 O) can also cross membranes relativelyrapidly, even if they are polar. Small, polar molecules such asglycerol and urea have intermediate permeability.The leading hypothesis to explain this pattern is that chargedcompounds and large, polar molecules can’t pass through thenonpolar, hydrophobic tails of a lipid bilayer. Because of theirelectrical charge, ions are more stable in solution where theyform hydrogen bonds with water than they are in the interiorof membranes, which is electrically neutral.If you understand this hypothesis, you should be able to predict whetheramino acids and nucleotides will cross a membrane readily.To test the hypothesis, researchers have manipulated the sizeand structure of the tails in liposomes or planar bilayers.Does the Type of Lipid in a Membrane AffectIts Permeability?Theoretically, two aspects of a hydrocarbon chain could affectthe way the chain behaves in a lipid bilayer: (1) the numberof double bonds it contains and (2) its length. Recall fromChapter 2 that when carbon atoms form a double bond, theattached atoms are found in a plane instead of a (threedimensional) tetrahedron. The carbon atoms involved are

06 free ch06.qxp10610/8/092:19 PMPage 106Unit 1 The Molecules of Life(a) Permeability scale (cm/s)(b) Size and charge affect the rate of diffusion across a membrane.Phospholipid bilayer100High permeabilitySmall, nonpolar moleculesO2, CO2, N2Small, uncharged polar moleculesH2O, urea,glycerolLarge, uncharged polar moleculesGlucose, sucroseO2 ,CO 2–210H2O10–4Glycerol, urea10–6Glucose10–810–10Cl –K Na –1210Low permeabilityIonsCl – , K , Na FIGURE 6.8 Selective Permeability of Lipid Bilayers. (a) The numbers represent “permeability coefficients,” or therate (cm/s) at which an ion or molecule crosses a lipid bilayer. (b) The relative permeabilities of various molecules and ions,based on data like those presented in part (a).QUESTION About how fast does water cross the lipid bilayer?also locked into place. They cannot rotate freely, as they do incarbon–carbon single bonds. As a result, a double bond betweencarbon atoms produces a “kink” in an otherwise straighthydrocarbon chain (Figure 6.9).CH2Double bondscause kinks Unsaturatedfatty acidSaturatedfatty acidFIGURE 6.9 Unsaturated Hydrocarbons Contain Carbon–CarbonDouble Bonds. A double bond in a hydrocarbon chain produces a“kink.”The icon on the right indicates that one of the hydrocarbon tailsin a phospholipid is unsaturated and therefore kinked.EXERCISE Draw the structural formula and a schematic diagram foran unsaturated fatty acid containing two double bonds.When a double bond exists between two carbon atoms ina hydrocarbon chain, the chain is said to be unsaturated.Conversely, hydrocarbon chains without double bonds are saidto be saturated. This choice of terms is logical, because if ahydrocarbon chain does not contain a double bond, it is saturated with the maximum number of hydrogen atoms that canattach to the carbon skeleton. If it is unsaturated, then fewerthan the maximum number of hydrogen atoms are attached.Because they contain more C–H bonds, which have much morefree energy than C@C bonds, saturated fats have much morechemical energy than unsaturated fats do. People who aredieting are often encouraged to eat fewer saturated fats. Foodsthat contain lipids with many double bonds are said to bepolyunsaturated and are advertised as healthier than foodswith more-saturated fats.Why do double bonds affect the permeability of membranes?When hydrophobic tails are packed into a lipid bilayer, thekinks created by double bonds produce spaces among the tightlypacked tails. These spaces reduce the strength of hydrophobicinteractions among the tails. Because the interior of themembrane is “glued together” less tightly, the structure shouldbecome more fluid and more permeable (Figure 6.10).Hydrophobic interactions also become stronger as saturatedhydrocarbon tails increase in length. Membranes dominated byphospholipids with long, saturated hydrocarbon tails should bestiffer and less permeable because the interactions among thetails are stronger.

06 free ch06.qxp10/8/092:19 PMPage 107Chapter 6 Lipids, Membranes, and the First CellsLipid bilayer withno unsaturatedfatty acidsLower permeabilityLipid bilayer withmany unsaturatedfatty acidsHigher permeabilityFIGURE 6.10 Fatty-Acid Structure Changes the Permeability ofMembranes. Lipid bilayers containing many unsaturated fatty acidshave more gaps and should be more permeable than are bilayers withfew unsaturated fatty acids.A biologist would predict, then, that bilayers made of lipidswith long, straight, saturated fatty-acid tails should be muchless permeable than membranes made of lipids with short,kinked, unsaturated fatty-acid tails. Experiments on liposomeshave shown exactly this pattern. Phospholipids with long,saturated tails form membranes that are much less permeablethan membranes consisting of phospholipids with shorter,unsaturated tails.The central point here is that the degree of hydrophobicinteractions dictates the behaviour of these molecules. Thisis another example in which the structure of a molecule—specifically, the number of double bonds in the hydrocarbonchain and its overall length—correlates with its properties andfunction.These data are also consistent with the basic observation thathighly saturated fats are solid at room temperature (Figure 6.11a).(a) Saturated lipidsOHOLipids that have extremely long hydrocarbon tails, as waxesdo, form stiff solids at room temperature due to the extensivehydrophobic interactions that occur (Figure 6.11b). Birds, seaotters, and many other organisms synthesize waxes and spreadthem on their exterior surface as a waterproofing; plant cellssecrete a waxy layer that covers the surface of leaves and stemsand keeps water from evaporating. In contrast, highly unsaturated fats are liquid at room temperature (Figure 6.11c). Liquidtriacylglycerides are called oils.Besides exploring the role of hydrocarbon chain length anddegree of saturation on membrane permeability, biologists haveinvestigated the effect of adding cholesterol molecules. Becausethe steroid rings in cholesterol are bulky, adding cholesterol toa membrane should increase the density of the hydrophobicsection. As predicted, researchers found that adding cholesterolmolecules to liposomes dramatically reduced the permeabilityof the liposomes. The data behind this claim are presented inFigure 6.12. The graph in this figure makes another importantpoint, however: Temperature has a strong influence on thebehaviour of lipid bilayers.Why Does Temperature Affect the Fluidity andPermeability of Membranes?At about 25 C—or “room temperature”—the phospholipidsfound in plasma membranes are liquid, and bilayers have theconsistency of olive oil. This fluidity, as well as the membrane’spermeability, decreases as temperature decreases. As temperatures drop, individual molecules in the bilayer move moreslowly. As a result, the hydrophobic tails in the interior of membranes pack together more tightly. At very low temperatures,(b) Saturated lipids with longhydrocarbon tailsButter(c) Unsaturated lipidsBeeswaxOOCOHOCCFIGURE 6.11 The Fluidity of Lipids Depends on the Characteristics of Their Hydrocarbon Chains. The fluidity of alipid depends on the length and saturation of its hydrocarbon chain. (a) Butter consists primarily of saturated lipids.(b) Waxes are lipids with extremely long hydrocarbon chains. (c) Oils are dominated by “polyunsaturates”—lipids withhydrocarbon chains that contain multiple double bonds.QUESTION Why are waxes so effecti

the tiny,bag-like compartments that formed when a researcher shook a mixture of lipids and water.(b) Scanning electron micrograph showing red blood cells from humans.Note the scale bars. 06_free_ch06.qxp 10/8/09 2:19 PM Page 100

Related Documents:

May 02, 2018 · D. Program Evaluation ͟The organization has provided a description of the framework for how each program will be evaluated. The framework should include all the elements below: ͟The evaluation methods are cost-effective for the organization ͟Quantitative and qualitative data is being collected (at Basics tier, data collection must have begun)

Silat is a combative art of self-defense and survival rooted from Matay archipelago. It was traced at thé early of Langkasuka Kingdom (2nd century CE) till thé reign of Melaka (Malaysia) Sultanate era (13th century). Silat has now evolved to become part of social culture and tradition with thé appearance of a fine physical and spiritual .

On an exceptional basis, Member States may request UNESCO to provide thé candidates with access to thé platform so they can complète thé form by themselves. Thèse requests must be addressed to esd rize unesco. or by 15 A ril 2021 UNESCO will provide thé nomineewith accessto thé platform via their émail address.

̶The leading indicator of employee engagement is based on the quality of the relationship between employee and supervisor Empower your managers! ̶Help them understand the impact on the organization ̶Share important changes, plan options, tasks, and deadlines ̶Provide key messages and talking points ̶Prepare them to answer employee questions

Dr. Sunita Bharatwal** Dr. Pawan Garga*** Abstract Customer satisfaction is derived from thè functionalities and values, a product or Service can provide. The current study aims to segregate thè dimensions of ordine Service quality and gather insights on its impact on web shopping. The trends of purchases have

Chính Văn.- Còn đức Thế tôn thì tuệ giác cực kỳ trong sạch 8: hiện hành bất nhị 9, đạt đến vô tướng 10, đứng vào chỗ đứng của các đức Thế tôn 11, thể hiện tính bình đẳng của các Ngài, đến chỗ không còn chướng ngại 12, giáo pháp không thể khuynh đảo, tâm thức không bị cản trở, cái được

CARBON-BASED MOLECULES Reinforcement KEY CONCEPT Carbon-based molecules are the foundation of life. Carbon atoms are the basis of most molecules that make up living things. Many carbon-based molecules are large molecules called polymers that are made of many smaller, repeating molecules called monomers. There are four main types of

The molecules in water have a small positive and a small negative charge, a bit like the poles on a magnet. We call such molecules with partial charges polar molecules. The molecules in oil have almost no positive or negative charge. We call these molecules non-polar molecules. Different kinds of molecules do not mix well with each other. Polar