Biology III Cell Structure And Function
Biology III Cell Structure And Function BIOL 23100
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This 19 page Class Notes was uploaded by Gordon Beier Jr. on Saturday September 19, 2015. The Class Notes belongs to BIOL 23100 at Purdue University taught by Peter Hollenbeck in Fall. Since its upload, it has received 64 views. For similar materials see /class/207831/biol-23100-purdue-university in Biological Sciences at Purdue University.
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mcnnms 38 5c 39 03 E as D50 2010 P J Honnnnnok EIOL 231 The Cell Bmlngy nprnhelm Rnadng cnan 20 pp 595793 7007707 m 957100ExamIV39057 1 Epnmun orgnnm unethical A Analogous In memhr us form wrmnuousshzns Lhatar mzhoundaryhz zwzm Lhzumdzof u Enamnnal c a mmzoan organmn and h oumd world ms s analogous to h nuasnaa nannmann ofa cc forming a boundary nmnnn Lhz cytoplam and h txmczllular nnnna z Enamnnal shzms hav man o h boundary pronan that w hav aknady dzsmhrd oronn mmth Th at oonnnuons snlnonvnlynnmnanln asymmnno mnond to nonnal Sigms nandmz nnraonons a Two primary natures of pnhzhal onus that mak um pronan nosnnun gtSPECIA1JZED CHLVCELL JUNCTIONS gts1gtEcmc mm norms yummy LOCATED 1 Cominuouswilh oulsideworld 3 50 anatomy Izsson u A an dch widnn your body aka up 01 and rzlmsa col aka up nume sum as gums and ammo suds Elma was prode sum as ma Eu unllk a singlzrczlkd g ng In a pond a or In our nody Ex gcs matmal Lthz oumdz world n c mmnwm nssunspaon and txmcdlular uid that sun at 395 In mm txahan matmal w lood 39r moo n m txahangcs ma nnal shzmsofcpmuzhalczlls Exampla Lhz lung or 55m in hdncy forum not for gnoosn and ammo ands Hnms Lhz cavzpalrmng man mad nlnom umrmuaus I Kunm a an 0 001 y 7 Hnss 92 2 All of the epithelial cells In your body mm a CONTINUOUS SHEET This 15 and the anus It is less obvmns fox epithelia that form glands and oxgans such as the l I A at Ham and deep into the ePithelial tubes and chamba39ls ofthese mng without when Dill anatomy lntemallzes some ofvhe Dn39slde Wmld by deep and complex lnfoldlngs of cell sheets Heie is my sketch ofthls ldea 5th P e Pancreas K kmer C Specialized vs cnmmnnprnper es l 39 the kldna39y the llvex and pancreas the GI uact lung and 5km a am 2 Thus as we move along this conunuous sheet theie ale zones Dfuansltmn l r k t e to a much somei mmsta39l one in the oial cavlty N m l epithelial cells and so some sunctnlal features are also common to them all 11 Epithelia have mechanical strength r n r m connective tissue molecules oi basementmembxane that the cell rests on Fox some oses the basal and lamal domalns togethei form a slngle basolateial domaln B Junctions and lamen Between adjacent cells the uncuonal comglsx in the lamal domain contains tlghtjnnctmns adha39la39ns Junctmns and desrnnsomes Faithei down mom basal ale the hemiuesmosomes The majorjunelions found in Ihe laleral junelional complex of epilhelial cells Tight junction Adherens Junction Desmnsame 1 Tight junctions g 2023 help make the epithelial sheet semipermeable They d 0 tr are comprise ansrnembrane proteins from adjacent cells that interact strongly to bring the membranes oftwo adjacent cells very close together Even smallmolecules ike ions cannot pass between the tightest ofthese junctions They typically form a continuous belt or gasket around each cell 2 Adherens junctions g 202425 promote strong speci c adhesion betw en neighboring cells and play a prominent role in sign ing within epithelial cells The junctions are comprised of transmembrane proteins of the cadherin family Cadherins from adjacent cells bind to each other viatheir extracellular domains in a manner that requires free Call At their intracellular end cadherins bind adaptor proteins catenins which in turn interact with Factin These junctions like tight junctions encircle the entire cell 3 Desmosomes g 2027 are focal contacts between cells that allow the entire epithelial sheet to have resistance to mechanical disruption Similar to adherens junctions they involve members of the cadherin family of transrnernbrane proteins binding to each other in a Ca dependent manner at their extracellular ends and binding to adaptor 39 1 39 39 ends L p protein of desmosomes bind not to Factin but 39 mameni Because each 39 39 these in turn interact with other desmosomes the desmosomeF network creates a multicellular continuum of resistance to shearin 4 Hernidesmosomes g 2048 are sites in the basal domain where transmembrane proteins called integrins interact NOT with adjacent cells but with the protein bers of L 39FCMAL quot quot quot 391 5 mm Jul in a manner similar to the desmosomes This anchors the epithelial sheet to the underlying basal lamina basement membrane Protein linking Signals transmitted to cyloplasm 39 o catenin p120 catenln More detailed schematic structure of an adherens junction including signaling from catenins 39 Desmo Lipid bilayer Cytoplasmic Desmoplakin plaque More detailed schematic structure of a desmosome Plasma membrane h i intermediate Ilamen s Plaque containing 7 protein plectln Basement membrane Anchoring filament Laminirr5 i in qulagen l ers Type Vll Collagen fibrils Schematic structure of a hemidesmosome C Junctions and signaling Cell cell junctions are NOT just sites where cells adhere to each other or just sites that separate apical from basolateral membrane domains It has become clear that they are also very important sites of cell signaling This is true for all four types of junction that we are considering I Signaling proteins located on the cytoplasmic side of junctions convey information to the cell about its interaction with adjacent cells or extracellular matrix and its behavior and identity as part of an epithelial sheet This information is conveyed in ways that you already understand through conformational changes binding of additional signaling proteins cascades of G protein activity phosphorylation etc the machinery of the cell including its gene expression and cell cycle control 4 2 Cell signaling arising at nonjunction surface receptors also conveys information FROM the cell TO the junctions regulating their structure in concert with other aspects of the cell s condition and environment III Secretion by epithelia exocytosis A Sorting vesicles to a domain 1 Most epithelia need to secrete material by exocytosis to both the apical and basolateral domains 2 Example The extracellular matrix molecules of the basement membrane are secreted only at the basal surface They assemble outside the cell into a brous matrix 3 Example A different set of molecules is secreted at the apical surface These differ according to what epithelium you re examining but they can include mucus digestive enzymes amp acid in the GI tract mucus and surfactant in the lung etc 4 Since different molecules all synthesized and inserted into the ER end up undergoing exocytosis in completely different places the vesicles that contain them must be segregated or sorted at the level of the Golgi apparatus and transported to different parts of the plasma membrane B Constitutive vs stimulated secretion 1 Many secreting epithelia have a pathway of organelle traffic by which they constantly bud off vesicles from the Golgi and transport them to the surface where they ise and release their contents 2 Some secretion occurs by the accumulation of vesicles near the cell surface until a specific signal is received by the cell This triggers a signaling pathway that results in the lSiOl l of the secretory vesicles with the plasma membrane and the release of their contents The secretion of digestive enzymes in the GI tract is an example IV Glucose transport by epithelia g 1218 p 399 A Membrane pumps amp transporters recall lectures 14 amp 15 The glucose is lower in the lumen of the GI tract apical side of epithelium outside world AND in the extracellular uid beneath the epithelial sheet basal side inside world than it is inside the cell How does the cell move glucose from the gut across the epithelium and eventually into the bloodstream Three membrane transporters are necessary 1 Na Glucose smporter this transmembrane protein complex couples the energetically favorable inward movement of Na to the energetically unfavorable import of glucose into the cell stoichiometry 2Nal glucose lecture 15 2 Glucose transporter this transmembrane complex allows glucose to cross the membrane moving down its concentration gradient Note that the transporter simply provides an avenue for an energetically favorable movement to occur 3 NaK ATPase or pump lecture 15 this transmembrane complex couples the hydrolysis of ATP to the energetically unfavorable movement of Na out of the cell and K in with a ratio of 3Na2K It establishes the Na gradient that drives the Naglucose symporter B Membrane domains 1 The presence ofthese 3 transporters will not accomplish glucose transport across the epithelium from the gut to the bloodstream UNLESS they are each in the 2 The Naglucose syrnporter must be limited to the apical membrane or it would not recover glucose from the digested food inthe gut 3 The glucose transporter must be limited to the basolateral domain or it would allow glucose to pass out of the cell back into the gut instead of into the extracellular uid and bloodstream 4 The NaK ATPase is limited to the basolateral domainbecause the ionic composition of the extracellular intracellular uid is closely regulated withinthe body ike in the lumen ofthe gut Na gtucuse colansuovkr LECTURE 4 30 August 2010 P J Hollenbeck BIOL 231 STRUCTURES OF BIOLOGICAL MACROMOLECULES Read Chap 2 pp 5059 Panels 23 thru 26 DVD 2123 Problems 1113 Q2 19 ltDownload from the web syllabus the set of extra gures that accompany this lecturegt I Questions and classes A What we want to know about the macromolecules that make up the cell 1 There are 4 major classes of macromolecules in the cell that we will consider repeatedly through out this course They have the common chemical composition of C H O N P S They are synthesized from smaller building block molecules which allows efficient recycling and turnover of the macromolecules in the cell But they are otherwise very different 2 Take the opportunity now to refamiliarize yourself with each of these 4 groups of macromolecules As you study them ask yourself What are the properties of each Which of the seven functional groups do they contain What building blocks comprise them and how are they constructed Do they form larger polymers or other associations Where are they found in the cell and what are their functions B 4 classes of macromolecules l m 7 we will consider steroids and phospholipids which between them constitute the bulk of the plasma and organelle membranes 2 Carbohydrate 7 monosaccharides eg glucose ribose etc are the building blocks for oligosaccharides e g the carbohydrate chains attached to membrane proteins and polysaccharides e g cellulose glycogen starch 3 Nucleic acids 7 nucleotides are the building blocks for DNA and RNA as well as energy substrates and phosphate sources for a multitude of cellular reactions 4 Proteins 7 amino acids are the building blocks of polypeptides protein molecules 11 Structure and function A Lipids l Lipids in the cell come in 2 general avors 7 those with acyl chains DVD 22 and those with carbon ring backbones 2 Those with ring backbones are steroids such as cholesterol see fig below which shares a 4ring structure with other steroids but has substituent groups that are unique The ring structure is rigid and planar The 7OH group confers some polar character 3 Lipids with acyl chains include triglycerides and phospholipids see extra figures These have a glycerol backbone connected by ester linkages to fatty acid chains 2 each for phospolipids 3 for triacyglycerols The long acyl chains can have entirely single saturated C C bonds or a mixture of single and double unsaturated CC bonds The double bonds create kinks that affect the packing of lipids in membranes see fig next page a subject we will revisit Triacylglycerols are storage forms of 1 lipid they are the triglycerides that your doctor measures in your blood lipid screening 4 A phospholipid has a polar head group such as choline or seIine connected by a phosphodiester linkage to the third position of the glycerol backbone Different phospholipids are distinguished by their polar head groups Tnslearate Linseed ull Two different triacylglycerol molecules The one at left tristearate has all 3 fatty acid chains fully saturated with single CC bonds the linseed oil at right has 12 unsaturated CC bonds in each chain Cholesterol Phosphate Polar Glycerol Fatty head group backbone acid chains Cholesterol top is an abundant steroid lipid found in most cellular membranes It is a rigid planar structure with a single OH group conferring some polar character Phosphatidyl choline above is a common phospholipid with an amine containing polar head group 2 ltLipids do not make large structures by polymerization of subunits Each phospholipid for example is H synthesized from glycerol fatty acids and polar head group building blocks However a cell membrane contains a large number of phospholipids that have hydrophobic interactions with each other NOT covalent connections to each other But the other 3 H 39m major kinds of macromolecules in cells 7 carbohydrates nucleic acids and proteins 7 are large Polymerization of subunits to form a large macromolecule covalent Polymers Of repeating subunits These this is how polysaccharaides nucleic acids and proteins form polymerizations occur with the loss of water and require energy as we will seegt B Carbohydrates CHZOn simple sugars and polysaccharides 1 Simple sugars such as hexoses like glucose DVD 21 can assume a linear or ring structure They serve as energy sources metabolic intermediates and subunits for the construction of larger polymers called polysaccharides D Fructose DGIUCOSE DGIUCOSE qDGiucose mimiucose H H iRing FBmIathl Haworth projection chair iumii i I 6 H c 0H g cHZOH SCHQOH H H C OH H SCOH H H5 0 H WHO I I I i 4 H 1 H0 H Ho crH H0 3H 4c 3H H I OH H H 5 H H0 7 OH HO i wagoni HCOH H 3 H H00 l l ggc H OH H C OH H I OH l 1 l H OH H C OH H C OH I H H 2 Building a polysaccharide Polymers of simple sugars are linked by covalent bonds called glycosidic bonds formed between the rst carbon C1 of one sugar and an 43H group on another carbon usually C4 Glycosidic bonds form between sugars in one of two arrangements referred to as or and 5 depending on the position of the 70H groups relative to the ring The 1 versus 5 linkages create differences in the structure of the polysaccharide chain that in turn underlie differences in function Cellulose with 5 linkages forms straight chains that can associate sidebyside to form tough structural cables Starch and glycogen with or linkages are used for energy storage Starch polysaccharides form large helices while glycogen polysaccharides are branched see lecture 4 extra gures C Nucleic acids ltThe basic building block of DNA and RNA is the nucleotide which is a base covalently linked to a ribose sugar which is in turn covalently linked to phosphates To build a nucleotide l A G T C and U Start with the nitrogenous bases the double ringed purines adenine and guanine and the single ringed pyrimidines cytosine and thymine and uracil in RNA These differ from each other in the functional groups attached to them These groups are responsible for the H bonds that produce base pairing 3 Phosphate 2 A pentose sugar ribose Bases are covalently linked to ribose for RNA or deoxyribose for DNA One of the nitrogens in the base is linked to the 1 carbon in the ribose note how the carbons are numbered gure at left DVD 23 3 Phosphate groups A phosphoester bond links phosphate to the 5 carbon of the ribose see gure below The form used in synthesis of DNA or RNA is the triphosphate Sugar The nucleotide monophosphate dAMP 4 Building a nucleic acid polymer strand Starting with nucleotide triphosphates the enzymes that synthesize RNA or DNA use the energy of the bond between the rst and second phosphate to link two subunits together At each step a nucleotide triphosphate is linked through the phosphate group to the free 43H group attached to the 3 carbon of the growing nucleic acid polymer see lecture 4 extra gures The polymer is always synthesized in this direction with new subunits being added to the free 3 carbon at the end NOT by addition to the free phosphate group attached to the 5 carbon of a sugar at the other end We will return to this 5 to3 growth when we consider DNA replication and transcription later in the course 5 DNA of course has a higher order structure A new strand is always made as a copy of an eXisting strand and two strands form a double heliX stabilized by hydrogen bonds between bases see lecture 4 extra gures and some van der Waals interactions Ionic bonds are also important for some proteinDNA interactions why We will return to this later as well D Proteins or polypeptides l The building blocks for proteins are amino acids These are amphoteric molecules 7 they can act as both acids and bases There are 20 different amino acids commonly found in proteins but they all share a common core structure THE AMINO ACID The general formula ofan amino acid is avcarbon morn H ca rlmxyl group amino group HZN C COOH R sidechain g roup R is commonly one of 20 different side chains At pH 7 both the amino and carboxyl groups are ionized 2 The R groups of the amino acids distinguish them They can be acidic basic uncharged polar or nonpolar hydrophobic as we will discuss in detail soon 3 To build a protein amino acids are connected in an unbranched chain via the amide linkages that we have already discussed Because they lose their free amino and carboxyl groups in the polymerization we refer to the subunits contained in a polypeptide as amino acid residues pvf f39 398 i m 4 Direction proteins are always synthesized by l the addition a new amino acid to the free carboxyl H group at the end of the growing chain never to the Phe A H amino group at the other end This notion of an i am ino or N terminus to the protein and a carboxyl or Cterminus will be a major topic soon when we study the structure of proteins and the mechanism of protein synthesis st H c 7mme l NH G H39 z l I l if 5 The backbone of a protein is the continuous 06 7 0 chain of covalentlybonded atoms from N to 1 KPH gt I carbon to C to the N of the next residue to 1 Lys H I lrlil lrl39ll 1H 47 carbon and so on As we will see next time this OC i i l I backbone can take on very different higher order CV EMJWS U structures depending on the R groups of the amino pulvpepude cham acid residues ltthe upshot these are important molecules to understand in any area of biological sciences Don t memorize a lot of structures 7 instead analyze the differences among these macromolecules their properties their building blocks the nature of their polymers their location and function in the cellgt ghmmalln Cell wall In nucleus KEY Protein Carbohydrate Llpid I DNA I RNA Carlmllyllmle Starch grain in chloroplast Plasma membrane 1 Plolem RNA Ribosome Micmtubules Milnchondrlon 5 from sm aller Type of PLIPIDS CARBOHYDRATES NUCLEIC ACIDS PROTEINS macromolecule polypeptides building block phospholipids sugars nucleotides amino acids any smaller glycerol bases SUbunitS fatty acids ribose or phosphate deoxyribose poar head group 39phosphate What bonds ester phosphoester make bldg bl CK phosphodiester NC building blocks into polymer subunits What s the polysaccharides nucleic acids DNA proteins P ymer7 linear or branched RNA unbranChed unbranched What bonds link glycosidic Cl and 3 phosphodiester peptide an amide bond Some functions ofmacro molecule in the cell form membranes energy storage structure energy storage information storage cataytic rRNA structu re cataysis reguation One other interesting difference between the 3 polymers Carbohydrates form different structures glycogen starch cellulose by connecting essentially IDENTICAL building blocks glucose molecules together in di erent ways or B or branched But the differences in structure and function among proteins and nucleic acids are achieved by connecting DIFFERENT building bloclcs in the SAME wayi LECTURE 3 27 August 2010 P J Hollenbeck BIOL 231 BIOLOGICAL CHEMISTRY Read Chap 2 pp 3950 5963 Panels 21 pp 645 22 pp 667 Panel 27 pp 767 DVD 24 Problem assignment 8 10 ltLast time we dealt with the boundaries of the cell and organelles Now we re ready to dive into the cell 7 but rst we need to understand its chemical building blocksgt 1 H20 protons and carbon A H20 the chemistry of cells is as much about WATER as it is about CARBON 1 Water is a POLAR molecule although it s not charged The two covalent bonds are both polarized such that the O atom is negative relative to the H atoms This asymmetric distribution of charge allows all 3 atoms to participate in interactions with other water molecules so that 2 Water is a very COHESIVE liquid At each moment each molecule in liquid water is interacting on average with 34 other molecules This only rises to 4 when water freezes 3 We call the part of the cell dominated by water a HYDROPHILIC environment It is energetically favorable for molecules that are completely ionized or have asymmetricallydistributed charge to reside there Examples are sodium ions or glucose The other part of the cell mainly membranes is a HYDROPHOBIC environment It excludes water and other molecules with asymmetricallydistributed charge and contains molecules like lipids that have extended regions of uncharged hydrocarbons An example is stearic acid a fatty acid with a C18 carbon chain ltRemember the structure of molecules detemiines which environment 7 hydrophilic or hydrophobic 7 they will be found in Hydrophilic locations include the cell s cytoplasm lumens of organelles and the extracellular space It is no surprise that molecules with both polar and hydrophobic domains such as phospholipids are found at and form the hydrophobic boundaries between watery locationsgt B PH 1 Water dissociates and we can think of the products as IF and OH In pure water Hl OH 10 7M so the dissociation constant KW Hl OH 10 1 Auseful measure of the Hl that you know very well is pH 2 recall that pH 10g1H 7logH We need to be acutely aware of pH in biological chemistry because the most important ionizations of biological macromolecules involve losing or gaining Hl C CC bonds single and double 1 Carbon can make 4 equivalent single bonds A C C single bond allows free rotation of the connected C atoms and their constituents 2 A CC double bond does NOT allow rotation so the groups attached to the 2 C atoms must lie in the same plane Look at the example gs 2 19 20 ofa lipid carbon chain with either a double bond unsaturated or with only single bonds saturated II Important functional groups refer to Panel 21 pp 6465 A Structure and ionization l lVTETHYL Not ionized under normal conditions 2 CARBONYL Not ionized under normal conditions 3 HYDROXYL under some conditions can be ionized can lose a Hl as pH increases 4 SULFHYDRYL under some conditions can be ionized can lose a Hl as pH increases 5 PHOSPHATE under some conditions can be ionized can lose 2 H as pH increases 6 CARBOXYL under some conditions can be ionized can lose a HT as pH increases 7 AlVHNO under some conditions can be ionized can gain a H as pH decreases B REAL biological acids and bases 1 The most important acids and bases that we ll deal with in the cell are Carboxyl groups they re acidic in that they can lose a proton and become negatively charged Note the difference between carboxyl and carbonyl groups Primary amines they re bases in that they can accept an extra proton and become positively charged 2 The pH at which these ionizations occur is called the pK see below and next page If we want to understand how functional groups behave we need to know whether they are charged or not at a particular pH If we know the pK of a functional group and whether it gains or loses charge when it gains a H we can determine this easily ff lilH 012 1 3 739 F zla 1 1 E Hcll H2quot I Mquot C HN CH 9 TH liiH 9H2 9H2 939 H 150 quot 5 5 39 p 7 00 1311 EH 5H2 5 39 Htli rIIH HN CH c coon fooquot 3 CH 9H2 5 1 2 E 5 9H 39 1 aspartic glutamic histidine lysine arginine acid acid pK47 pK47 pK65 pK102 pK12 For the amino acid side chains that can be ionized the charge changes with pH Realize that this is just another chemical equilibrium they gain or lose a proton and the pH at which this occurs depends upon their affinity for protons which is a fundamental property of each side chain Think ofthese molecules as either binding a proton from the solution becoming protonated or losing a proton to the solution becoming de protonatedquot By definition when a side chain is below its pK it will be protonated above its pK it will be deprotonated There s only one twist to remember when an acidic residue at left above is below its pK and thus protonated it is uncharged and as pH rises above its pK and it loses the proton it becomes negatively charged 71 A basic residue at right above goes from being positively charged 1 when protonated to being uncharged when it loses a proton C Attaching functional groups to carbon backbones gures below 1 ester linkage alcohol plus acid we ll see this in phospholipids for example 2 phosphoester linkage alcohol plus phosphate eg phosphorylation of serine threonine and tyrosine amino acid side chains ltalso look at phosphodiester bond pp 58 and 75gt 3 phosphoanhydride linkage phosphate plus phosphate e g in nucleotides 4 amide linkage amine plus acid eg the peptide bond in proteins carboxylic acid alcohol 0 II I c 0H Ho c I szO H I c o o I ester phosphate alcohol 0 II I P OH HO O imp 0 phosphoester Phosphate Phosphate O 0 II II P OH H0 P I I o 0 in o 0 ll O IP 0 O phosphoanhydride Carboxylic acid Amine O g 0H Hgv lt opr H l l H Amide peptide bond Note that each of these bonds is formed with the loss of an H20 The PEPTIDE BOND is an amide linkage see panel 25 p 72 Note that the peptide bond although usually drawn as a single bond has double bond character that is to say rotation around the bond does NOT occur 111 Nonrcovalent bonds see panel 22 27 table 21 p 47 andDVD 24 mm W 7 39 39 bonds is only 1 because they are lower in the presence ofwater ltNote thatforatypical 39 c 39 l b e a 39 39 377 kJmole and the distance between the carbon atoms is 15A angstroms or 015 nmgt A Ionic or electrostatic bonds IONtE BONDS N AQUEOUS SOLUTIONS 1 I W I r tutu simian w W 2 Example a positively charged quotWW amine group and a negatively charged carboxyl group The ionic bond depends on the charge which in turn depends on pH What happens to the ionic bond when pH changes 3 The presence ofwater weakens o H umm ionic bonds since the interaction of quot ate with 39 molecules or soluble ions mask the charge 4 The force ofthe attraction is roportional to the product ofthe h 6 distance between them A typical Ionic bonus alevcry Imvunani u tame lo r between two oppositely charged m13 1 3 2 1quot 39 quot 5 groups would be around 2 ltA typical energy for a single ionic bond in an aqueous environment is approx 12 kImolegt B Hydrogen bonds see panels 2 2 amp 27 Hydrogen bond 1 A hydrogen can be partially shared between two aquot 5 I in 539 electronegative atoms 0 or N in biological systems W eg 1n basepairing in DNA alpha helix in proteins 2 Examples the O of a carbonyl group can share the Hofane hydroxyl group middle example at right 3 The orientation of the atoms is important in H bonds the straighter the path from one electronegative atom through the H to the other electronegative atom the stronger the potential bond A typical Hbond distance is 27 31 A ltI39he energy of single hydrogen bonds varies from 1230 39 ent but is much lower 7 around 4 kJmole 7 in the presence ofHZOgt kJmole in a nonaqueous env1ronm arby hydroxyl group with the O of that 5 4 The energy required to break H bonds is not much greater than the thermal energy present in aqueous solutions within cells thermal energy of molecules at room temp a25 kJmole However the sum of many hydrogen bonds between macromolecules can be large and is an r tL to 1 i essential F C Hydrophobic interactions be sure to study panel 27 pp 7879 1 Nonpolar molecules in an aqueous environment interact with each other more readily than they interact with water In the cell the lipid bilayer is the best example 2 Think of this as a way to minimize the energy state of the system by avoiding imposing order on the Hbonded network of water molecules If many hydrophobic molecules were dispersed in water they would each prevent a large shell of watermolecules from interacting with each other The interactions of water molecules are energetically favorable By interacting with each other and excluding water hydrophobic molecules minimi the number of water molecules that they prevent from interacting ofbinlngical HYDROPHOBIC FORCES r H gtr I f V 77 1 Il quot 39 H r 6 Water forces hydrophobic groups together in order to minimize their disruptive effects on the Hbonded waler network Hydrophobic groups held together in t is way are said to have hy rophobrc bondsquot although the attraction is actually caused by repulsion from water 6 ltA typical u 1 Energy Anructlan 2 E D van der szl s thrzctinns see panel 277 Separallan between cenlevs o alums Al 2 6 lam 11111 1 when atoms approach each other they lnduce transient complementary L L L r r 1 k are 3AA a art it is a nonspecific effectrit doesn tmaLlerwhaLLhe identity othe atoms is which makes it very different from hydrogen bonds for instance 2 As we saw for hydrogen bonds the energy required to disrupt van der typical cell Butwhen large molecules have complementary shapes many of th are brought into position to be attracted to each other by yan der Waals forces and the sum ofthe forces is then s1guf1cantltsee below This relatinnship between shapes uf macmmulecules and their ability tn inmrzct viz large numbers 11139 weak bands hulds fur Lhe DLher nnnrcnvzlent bands mini alnlalhmd length is 34 angstroms gt Waals1910 Nobel Prize Winner ln Physics at http nobelprlze orgnobelszesphyslcslaure ates1910waalsrblo html d2 Wazls tritemelmn 7 omlmal van
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