B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 38
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Whatis the molarity of ethanol in a person at this legal limit?C.How many 12-oz (355-mL) bottles of 5% beer coulda 70-kg person drink and remain under the legal limit? A70-kg person contains about 40 liters of water. Ignore themetabolism of ethanol, and assume that the water contentof the person remains constant.89CHAPTER 2 END-OF-CHAPTER PROBLEMSD.Ethanol is metabolized at a constant rate of about120 mg per hour per kg body weight, regardless of its concentration. If a 70-kg person were at twice the legal limit(160 mg/100 mL), how long would it take for their bloodalcohol level to fall below the legal limit?2–10 A histidine side chain is known to play an important role in the catalytic mechanism of an enzyme; however, it is not clear whether histidine is required in its protonated (charged) or unprotonated (uncharged) state. Toanswer this question you measure enzyme activity over arange of pH, with the results shown in Figure Q2–1.
Whichform of histidine is required for enzyme activity?Figure Q2–1 Enzymeactivity as a functionof pH (Problem 2–10).activity (% of maximum)1000456pH78910Problemsp2.20/2.11/Q2.1Figure Q2–2 Three molecules that illustrate theOPO–OCOseven most common functional groups in biology(Problem 2–11). 1,3-Bisphosphoglycerate andpyruvate are intermediates in glycolysis andcysteine is an amino acid.HO CHCH2CO–OPO–OOO–1,3-bisphosphoglycerate2–13 Polymerization of tubulin subunits into microtubules occurs with an increase in the orderliness of the subunits.
Yet tubulin polymerization occurs with an increasein entropy (decrease in order). How can that be?2–14 A 70-kg adult human (154 lb) could meet his orher entire energy needs for one day by eating 3 moles ofglucose (540 g). (We do not recommend this.) Each molecule of glucose generates 30 molecules of ATP when it isoxidized to CO2. The concentration of ATP is maintained incells at about 2 mM, and a 70-kg adult has about 25 litersof intracellular fluid. Given that the ATP concentrationremains constant in cells, calculate how many times perday, on average, each ATP molecule in the body is hydrolyzed and resynthesized.2–15 Assuming that there are 5 × 1013 cells in the humanbody and that ATP is turning over at a rate of 109 ATPmolecules per minute in each cell, how many watts is thehuman body consuming? (A watt is a joule per second.)Assume that hydrolysis of ATP yields 50 kJ/mole.2–11 The three molecules in Figure Q2–2 contain theseven most common reactive groups in biology.
Most molecules in the cell are built from these functional groups.Indicate and name the functional groups in these molecules.–OBefore you do any calculations, try to guess whether themolecules are moving at a slow crawl (<1 km/hr), an easywalk (5 km/hr), or a record-setting sprint (40 km/hr).CSHCH2OCH3pyruvateCHNH3 +2–16 Does a Snickers™ candy bar (65 g, 1360 kJ) provide enough energy to climb from Zermatt (elevation 1660m) to the top of the Matterhorn (4478 m, Figure Q2–3),or might you need to stop at HÖrnli Hut (3260 m) to eatanother one? Imagine that you and your gear have a massof 75 kg, and that all of your work is done against gravity(that is, you are just climbing straight up).
Remember fromyour introductory physics course thatwork (J) = mass (kg) × g (m/sec2) × height gained (m)where g is acceleration due to gravity (9.8 m/sec2). Onejoule is 1 kg m2/sec2.What assumptions made here will greatly underestimate how much candy you need?OCO–cysteine2–12 “Diffusion” sounds slow—and over everyday distances it is—but on the scale of a cell it is very fast. The average instantaneous velocity of a particle in solution—that is,the velocity between the very frequent collisions—isv = (kT/m)½where k = 1.38 × 10–16 g cm2/K sec2, T = temperature in K(37°C is 310 K), and m = mass in g/molecule.Calculate the instantaneous velocity of a watermolecule (molecular mass = 18 daltons), a glucose molecule (molecular mass = 180 daltons), and a myoglobinmolecule (molecular mass = 15,000 daltons) at 37°C. Justfor fun, convert these numbers into kilometers/hour.Figure Q2–3 TheMatterhorn (Problem2–16).
(Courtesy ofZermatt Tourism.)2–17 In the absence of oxygen, cells consume glucoseat a high, steady rate. When oxygen is added, glucose consumption drops precipitously and is then maintained atthe lower rate. Why is glucose consumed at a high rate inProblems p2.31/2.18/Q2.4the absenceof oxygen and at a low rate in its presence?PANEL 2–1: Chemical Bonds and Groups Commonly Encountered in Biological Molecules90CARBON SKELETONSCarbon has a unique role in the cell because of itsability to form strong covalent bonds with othercarbon atoms. Thus carbon atoms can join to form:branched treesringschainsCCCCCCCCCCCCCHYDROCARBONSA covalent bond forms when two atoms come very closetogether and share one or more of their electrons. In a singlebond, one electron from each of the two atoms is shared; ina double bond, a total of four electrons are shared.Each atom forms a fixed number of covalent bonds in adefined spatial arrangement.
For example, carbon forms foursingle bonds arranged tetrahedrally, whereas nitrogen formsthree single bonds and oxygen forms two single bonds arrangedas shown below.NODouble bonds exist and have a different spatial arrangement.CCCalso written asCOVALENT BONDSCCCCalso written asalso written asCCCCCCNOCarbon and hydrogen combinetogether to make stablecompounds (or chemical groups)called hydrocarbons.
These arenonpolar, do not formhydrogen bonds, and aregenerally insoluble in water.Atoms joined by twoor more covalent bondscannot rotate freelyaround the bond axis.This restriction is amajor influence on thethree-dimensional shapeof many macromolecules.HHCHHHCHHmethanemethyl groupH2CCH2ALTERNATING DOUBLE BONDSThe carbon chain can include doublebonds. If these are on alternate carbonatoms, the bonding electrons movewithin the molecule, stabilizing thestructure by a phenomenon calledresonance.CCCCCCCCCAlternating double bonds in a ringcan generate a very stable structure.HHCCCCCCCCH2HHHH2CHCCH2H2CCthe truth is somewhere betweenthese two structuresCCH2H2CHCH2CHHHbenzeneHCH2H2CHCH2H3Coften written aspart of the hydrocarbon “tail”of a fatty acid molecule91C–O CHEMICAL GROUPSC–N CHEMICAL GROUPSMany biological compounds contain a carbonbonded to an oxygen.
For example,Amines and amides are two important examples ofcompounds containing a carbon linked to a nitrogen.Amines in water combine with an H+ ion to becomepositively charged.HalcoholCThe –OH is called ahydroxyl group.OHaldehydeCOH+NHHOCOOCCOcarboxylic acidOHacidamineCHOCCOHNH2H2OOalcoholacidCOesterSULFHYDRYL GROUPTheCamideHNitrogen also occurs in several ring compounds, includingimportant constituents of nucleic acids: purines and pyrimidines.OCH2OCNEsters are formed by a condensation reactionbetween an acid and an alcohol.OCCH2NOHThe –COOH is called acarboxyl group.
In waterthis loses an H_+ ion tobecome –COO .CCH+NAmides are formed by combining an acid and anamine. Unlike amines, amides are uncharged in water.An example is the peptide bond that joins amino acidsin a protein.— is called aThe C—Ocarbonyl group.CestersCHCketoneHHHNCCCNHcytosine (a pyrimidine)HHSH is called a sulfhydryl group. In the amino acid cysteine, the sulfhydryl groupmay exist in the reduced form,CSHor more rarely in an oxidized, cross-bridging form,CSSCPHOSPHATESInorganic phosphate is a stable ion formed fromphosphoric acid, H3PO4. It is also written as Pi .Phosphate esters can form between a phosphate and a free hydroxyl group.Phosphate groups are often attached to proteins in this way.OHOO_OPCOHHO_O_OP_OOCO_OPalsowritten asH2OC_OOPThe combination of a phosphate and a carboxyl group, or two or more phosphate groups, gives an acid anhydride.Because compounds of this kind are easily hydrolysed in the cell, they are sometimes said to contain a “high-energy” bond.HOCH2OOOOHO_OPOC_OOOH2OOPOH_OHO_OPPOO_H2O_OO_O_OPOH 2Ohigh-energy acyl phosphatebond (carboxylic–phosphoricacid anhydride) found insome metabolitesOOOPO__phosphoanhydride—a highenergy bond found inmolecules such as ATPalso written asOCOPalso written asOPPPANEL 2–2: Water and Its Influence on the Behavior of Biological Molecules92WATERWATER STRUCTURETwo atoms, connected by a covalent bond, may exert different attractions forthe electrons of the bond.
In such cases the bond is polar, with one endslightly negatively charged (δ–) and the other slightly positively charged (δ+).Molecules of water join together transientlyin a hydrogen-bonded lattice. Even at 37oC,15% of the water molecules are joined tofour others in a short-lived assembly knownas a “flickering cluster.”Hδ+electropositiveregionOδH–electronegativeregionδ+δ–Although a water molecule has an overall neutral charge (having the samenumber of electrons and protons), the electrons are asymmetrically distributed,which makes the molecule polar. The oxygen nucleus draws electrons awayfrom the hydrogen nuclei, leaving these nuclei with a small net positive charge.The excess of electron density on the oxygen atom creates weakly negativeregions at the other two corners of an imaginary tetrahedron.The cohesive nature of water isresponsible for many of its unusualproperties, such as high surface tension,specific heat, and heat of vaporization.HYDROGEN BONDSBecause they are polarized, twoadjacent H2O molecules can forma linkage known as a hydrogenbond.
Hydrogen bonds haveonly about 1/20 the strengthof a covalent bond.Hydrogen bonds are strongest whenthe three atoms lie in a straight line.HH2δOHδbond lengthsδ+δ++δHhydrogen bond0.17 nmH+OO2δ+hydrogen bondHHO0.10 nmcovalent bond+HYDROPHILIC MOLECULESHYDROPHOBIC MOLECULESSubstances that dissolve readily in water are termed hydrophilic. They arecomposed of ions or polar molecules that attract water molecules throughelectrical charge effects. Water molecules surround each ion or polar moleculeon the surface of a solid substance and carry it into solution.HHHOH HHOHOHOO–O HHHH δ+ H+δδ–Hδ_HN HO++Hδ ClNa –HO δ–δ OHO CHO H δ+δ+ Oδ–HHHOHON HHHHOHOHHHHOIonic substances such as sodium chlorideHHdissolve because water molecules arePolar substances such as ureaattracted to the positive (Na+) or negative_dissolve because their molecules(Cl ) charge of each ion.form hydrogen bonds with thesurrounding water molecules.Molecules that contain a preponderanceof nonpolar bonds are usually insoluble inwater and are termed hydrophobic.
This istrue, especially, of hydrocarbons, whichcontain many C–H bonds. Water moleculesare not attracted to such molecules and sohave little tendency to surround them andcarry them into solution.HH OH HHHCCHHHOHHHHCHOOHHOHOHOHH93WATER AS A SOLVENTMany substances, such as household sugar, dissolve in water. That is, theirmolecules separate from each other, each becoming surrounded by water molecules.When a substance dissolves in aliquid, the mixture is termed a solution.The dissolved substance (in this casesugar) is the solute, and the liquid thatdoes the dissolving (in this case water)is the solvent. Water is an excellentsolvent for many substances becauseof its polar bonds.sugardissolveswatermoleculesugar crystalsugar moleculeACIDSHYDROGEN ION EXCHANGESubstances that release hydrogen ions into solutionare called acids.Positively charged hydrogen ions (H ) can spontaneouslymove from one water molecule to another, thereby creatingtwo ionic species.+HClH+Cl–hydrochloric acid(strong acid)hydrogen ionchloride ionHOMany of the acids important in the cell are only partiallydissociated, and they are therefore weak acids—for example,the carboxyl group (–COOH), which dissociates to give ahydrogen ion in solution.OH+HHOHO–Ohydronium ionhydroxyl ion(water acting as (water acting asa weak base)a weak acid)H2OCOHH+O HHoften written as:OCHH+OH–hydrogenionhydroxylionNote that this is a reversible reaction.Since the process is rapidly reversible, hydrogen ions arecontinually shuttling between water molecules.
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