2 Chemistry of Life

2 Chemistry of Life

2 Chemistry of Life


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2 <strong>Chemistry</strong><strong>of</strong> <strong>Life</strong>CHAPTERKEY CONCEPTS2.1 Atoms, Ions, and MoleculesAll living things are based on atoms and their interactions.2.2 Properties <strong>of</strong> WaterWater’s unique properties allow life to exist on Earth.2.3 Carbon-Based MoleculesCarbon-based molecules are the foundation <strong>of</strong> life.2.4 Chemical Reactions<strong>Life</strong> depends on chemical reactions.2.5 EnzymesEnzymes are catalysts for chemical reactions in living things.BIOLOGYCLASSZONE.COMBIOLOGYView animated chapterconcepts.• Hydrogen Bonding• Energy and ChemicalReactions• Calorimetry• Atoms and BondingKeep current with biology news.• News feeds• Careers• PollsRESOURCE CENTERGet more information on• Elements <strong>of</strong> <strong>Life</strong>• Acids, Bases, and pH34 Unit 1: Introducing Biology

How can this plantdigest a frog?Like other carnivores, the Venus flytrapeats animals to get nutrients that itneeds to make molecules such as proteinsand nucleic acids. Other chemical compoundsmade by the plant’s cells enablethe Venus flytrap to digest the animalsthat it eats. These chemicals are similar tothe chemicals that allow you to digestthe food that you eat.ConnectingCONCEPTSCell Function The Venusflytrap has specialized cellson the surfaces <strong>of</strong> its leaves.Some <strong>of</strong> these cells allow theplant to snap shut on its preywithin 0.5 seconds. Othercells, such as those thatappear purple in this lightmicrograph, secrete digestivechemicals that allow theplant to consume its prey.(LM; magnification 500)Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 35

Connecting CONCEPTSCell Structure and FunctionSeveral different ions are transportedacross cell membranesduring cell processes. You willlearn how this transport occursin Chapters 3 and 4.MAIN IDEAIons form when atoms gain or lose electrons.An ion is an atom that has gained or lost one or more electrons. An ion formsbecause an atom is more stable when its outermost energy level is full; the gainor loss <strong>of</strong> electrons results in a full outermost energy level. An atom becomesan ion when its number <strong>of</strong> electrons changes and it gains an electrical charge.This charge gives ions certain properties. For example, compounds consistingonly <strong>of</strong> ions—ionic compounds—easily dissolve in water.Some ions are positively charged, and other ions are negatively charged.The type <strong>of</strong> ion that forms depends on the number <strong>of</strong> electrons in an atom’souter energy level. An atom with few electrons in its outer energy level tendsto lose those electrons. An atom that loses one or more electrons becomes apositively charged ion because it has more protons than electrons. In contrast,an atom with a nearly full outer energy level tends to gain electrons. An atomthat gains one or more electrons becomes a negatively charged ion because ithas more electrons than protons.Ions play large roles in organisms. For example, hydrogen ions (H + ) areneeded for the production <strong>of</strong> usable chemical energy in cells. Calcium ions(Ca 2+ ) are necessary for every muscle movement in your body. And chlorideions (Cl – ) are important for a certain type <strong>of</strong> chemical signal in the brain.Ions usually form when electrons are transferred from one atom to another.For example, FIGURE 2.3 shows the transfer <strong>of</strong> an electron from a sodiumatom (Na) to a chlorine atom (Cl). When it loses its one outer electron, thesodium atom becomes a positively charged sodium ion (Na + ). Its secondenergy level, which has eight electrons, is now a full outermost energy level.The transferred electron fills chlorine’s outermost energy level, forming anegatively charged chloride ion (Cl – ). Positive ions, such as Na + , are attractedto negative ions, such as Cl – . An ionic bond forms through the electrical forcebetween oppositely charged ions. Salt, or sodium chloride (NaCl), is an ioniccompound <strong>of</strong> Na + and Cl – . Sodium chloride is held together by ionic bonds.Apply What determines whether an atom becomes a positive ion or a negative ion?FIGURE 2.3 IONS AND IONIC BONDS1 The sodium atom (Na) loses its one outer 2 The positive sodium ion (Na + ) and negative chlorideelectron to the chlorine atom (Cl).ion (Cl – ) attract each other and form an ionic bond.Na loses anelectron to Clionic bondgained electronSodium atom (Na)Chlorine atom (Cl) Sodium ion (Na + ) Chloride ion (Cl – )38 Unit 1: Introducing Biology

MAIN IDEAAtoms share pairs <strong>of</strong> electrons in covalent bonds.Not all atoms easily gain or lose electrons. Rather, the atoms <strong>of</strong> many elementsshare pairs <strong>of</strong> electrons. The shared pairs <strong>of</strong> electrons fill the outermost energylevels <strong>of</strong> the bonded atoms. A covalent bond forms when atoms share a pair<strong>of</strong> electrons. Covalent bonds are generally very strong, and depending on howmany electrons an atom has, two atoms may form several covalent bonds toshare several pairs <strong>of</strong> electrons. FIGURE 2.4 illustrates how atoms <strong>of</strong> carbonand oxygen share pairs <strong>of</strong> electrons in covalent bonds. All three atoms in amolecule <strong>of</strong> carbon dioxide (CO 2 ) have full outer energy levels.VOCABULARYThe prefix co- means“together,” and valent comesfrom a Latin word that means“power” or “strength.”FIGURE 2.4 COVALENT BONDSA carbon atom needs four electrons to fill its outer energy level. An oxygen atom needstwo electrons to fill its outer energy level. In carbon dioxide, carbon makes a doublebond, or shares two pairs <strong>of</strong> electrons, with each oxygen atom.covalent bondsOxygen atom (O)Carbon atom (C)Carbon dioxide (CO 2 )Oxygen atom (O)A molecule is two or more atoms held together by covalent bonds. In thecompound carbon dioxide, each oxygen atom shares two pairs <strong>of</strong> electrons(four electrons) with the carbon atom. Some elements occur naturally in theform <strong>of</strong> diatomic, or “two-atom,” molecules. For example, a molecule <strong>of</strong>oxygen (O 2 ) consists <strong>of</strong> two oxygen atoms that share two pairs <strong>of</strong> electrons.Almost all <strong>of</strong> the substances that make up organisms, from lipids to nucleicacids to water, are molecules held together by covalent bonds.Summarize What happens to electrons in outer energy levels when two atomsform a covalent bond?2.1 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What distinguishes oneelement from another?2. Describe the formation <strong>of</strong> anionic compound.3. What is the difference between anionic bond and a covalent bond?CRITICAL THINKING4. Compare and Contrast How does amolecule differ from an atom?5. Apply Explain why a hydrogenatom can become either an ion ora part <strong>of</strong> a molecule.Connecting CONCEPTS6. <strong>Chemistry</strong> A sodium atom hasone outer electron, and acarbon atom has four outerelectrons. How might thisdifference be related to thetypes <strong>of</strong> compounds formed byatoms <strong>of</strong> these two elements?Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 39

2.2Properties <strong>of</strong> WaterKEY CONCEPT Water’s unique properties allow life to exist on Earth.MAIN IDEAS• <strong>Life</strong> depends on hydrogen bondsin water.• Many compounds dissolve in water.• Some compounds form acidsor bases.VOCABULARYhydrogen bond, p. 41cohesion, p. 41adhesion, p. 41solution, p. 42solvent, p. 42solute, p. 42acid, p. 42base, p. 42pH, p. 42Reviewion, moleculeConnect When you are thirsty, you need to drink something that is mostlywater. Why is the water you drink absolutely necessary? Your cells, and the cells<strong>of</strong> every other living thing on Earth, are mostly water. Water gives cells structureand transports materials within organisms. All <strong>of</strong> the processes necessary for lifetake place in that watery environment. Water’s unique properties, which arerelated to the structure <strong>of</strong> the water molecule, are important for living things.MAIN IDEA<strong>Life</strong> depends on hydrogen bonds in water.How do fish survive a cold winter if their pond freezes? Unlike mostsubstances, water expands when it freezes. Water is less dense as a solid (ice)than as a liquid. In a pond, ice floats and covers the water’s surface. The iceacts as an insulator that allows the water underneath to remain a liquid. Ice’slow density is related to the structure <strong>of</strong> the water molecule.Water and Hydrogen BondsWater is a polar molecule. You canthink about polar molecules in thesame way that you can think about amagnet’s poles. That is, polar moleculeshave a region with a slight positivecharge and a region with a slightnegative charge. Polar molecules, suchas the water molecule shown inFIGURE 2.5, form when atoms in a moleculehave unequal pulls on the electronsthey share. In a molecule <strong>of</strong> water,the oxygen nucleus, with its eightprotons, attracts the shared electronsHOHFIGURE 2.5 In water molecules, theoxygen atom has a slightly negativecharge, and the hydrogen atoms haveslightly positive charges.more strongly than do the hydrogen nuclei, with only one proton each. Theoxygen atom gains a small negative charge, and the hydrogen atoms gain smallpositive charges. Other molecules, called nonpolar molecules, do not have thesecharged regions. The atoms in nonpolar molecules share electrons more equally.40 Unit 1: Introducing Biology

Opposite charges <strong>of</strong> polar molecules can interactto form hydrogen bonds. A hydrogen bond is anattraction between a slightly positive hydrogenatom and a slightly negative atom, <strong>of</strong>ten oxygenor nitrogen. Hydrogen bonding is shownamong water molecules in FIGURE 2.6, but thesebonds are also found in many other molecules.For example, hydrogen bonds are part <strong>of</strong> thestructures <strong>of</strong> proteins and <strong>of</strong> DNA, which is thegenetic material for all organisms.hydrogenbondBIOLOGYSee hydrogenbonding in action atClassZone.com.Properties Related to Hydrogen BondsIndividual hydrogen bonds are about 20 times weaker than typical covalentbonds, but they are relatively strong among water molecules. As a result, alarge amount <strong>of</strong> energy is needed to overcome the attractions among watermolecules. Without hydrogen bonds, water would boil at a much lowertemperature than it does because less energy would be needed to changeliquid water into water vapor. Water is a liquid at the temperatures thatsupport most life on Earth only because <strong>of</strong> hydrogen bonds in water.Hydrogen bonds are responsible for three important properties <strong>of</strong> water.• High specific heat Hydrogen bonds give water an abnormally high specificheat. This means that water resists changes in temperature. Compared tomany other compounds, water must absorb more heat energy to increasein temperature. This property is very important in cells. The processes thatproduce usable chemical energy in cells release a great deal <strong>of</strong> heat. Waterabsorbs the heat, which helps to regulate cell temperatures.• Cohesion The attraction among molecules <strong>of</strong> a substance is cohesion.Cohesion from hydrogen bonds makes water molecules stick to each other.You can see this when water forms beads, such as on a recently washed car.Cohesion also produces surface tension, which makes a kind <strong>of</strong> skin onwater. Surface tension keeps the spider in FIGURE 2.6 from sinking.• Adhesion The attraction among molecules <strong>of</strong> different substances is calledadhesion. In other words, water molecules stick to other things. Adhesionis responsible for the upward curve on the surface <strong>of</strong> the water in FIGURE 2.7because water molecules are attracted to the glass <strong>of</strong> the test tube. Adhesionhelps plants transport water from their roots to their leaves because watermolecules stick to the sides <strong>of</strong> the vessels that carry water.Compare How are hydrogen bonds similar to ionic bonds?FIGURE 2.6 Water’s surface tensioncomes from hydrogen bonds(left) that cause water moleculesto stick together.FIGURE 2.7 The water’s surface(left, dyed red) is curved downbecause water has greater adhesionthan cohesion. The surface <strong>of</strong>the mercury (right) is curved upbecause mercury has greatercohesion than adhesion.MAIN IDEAMany compounds dissolve in water.Molecules and ions cannot take part in chemical processes inside cellsunless they dissolve in water. Important materials such as sugars and oxygencannot be transported from one part <strong>of</strong> an organism to another unlessthey are dissolved in blood, plant sap, or other water-based fluids.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 41

FIGURE 2.8 A mosquitoinjects a solution containinga protein solutethat prevents blood fromclotting. The mosquito solution outsucks in blood, which is asolution containing solutes suchas ions, sugars, and proteins.Many substances dissolve in thewater in your body. When one substancedissolves in another, a solutionforms. A solution is a mixture <strong>of</strong>VISUAL VOCABThe solvent is the substance that ispresent in the greatest amount, and isthe substance that dissolves solutes.substances that is the same through-out—it is a homogeneous mixture. Asolution has two parts. The solvent is solutionthe substance that is present in thegreater amount and that dissolvesanother substance. A solute is aA solute is the substance that dissolves.substance that dissolves in a solvent.The amount <strong>of</strong> solute dissolved in a certain amount <strong>of</strong> solvent is a solution’sconcentration. One spoonful <strong>of</strong> a drink mix in water has little flavor becauseit has a low concentration. But a solution with four spoonfuls in the sameamount <strong>of</strong> water tastes stronger because it has a higher concentration.The liquid part <strong>of</strong> your blood, called plasma, is about 95percent water. Therefore, the solvent in plasma is water and all<strong>of</strong> the substances dissolved in it are solutes. Most <strong>of</strong> thesesolutes, such as sugars and proteins, dissolve in the water <strong>of</strong>blood plasma because they are polar. Polar molecules dissolvein water because the attraction between the water moleculesand the solute molecules is greater than the attraction amongthe molecules <strong>of</strong> the solute. Similarly, ionic compounds, suchas sodium chloride, dissolve in water because the charges <strong>of</strong>the water molecules attract the charges <strong>of</strong> the ions. The watermolecules surround each ion and pull the compound apart.Nonpolar substances, such as fats and oils, rarely dissolvein water. Nonpolar molecules do not have charged regions, sosolution inthey are not attracted to polar molecules. Polar molecules andnonpolar molecules tend to remain separate, which is why wesay, “Oil and water don’t mix.” But nonpolar molecules willdissolve in nonpolar solvents. For example, some vitamins,such as vitamin E, are nonpolar and dissolve in fat in your body.Connect What are the solvent and solutes in a beverage you drink?MAIN IDEASome compounds form acids or bases.Some compounds break up into ions when they dissolve in water. An acid isa compound that releases a proton—a hydrogen ion (H + )—when it dissolvesin water. An acid increases the concentration <strong>of</strong> H + ions in a solution. Bases arecompounds that remove H + ions from a solution. When a base dissolves inwater, the solution has a low H + concentration. A solution’s acidity, or H + ionconcentration, is measured by the pH scale. In FIGURE 2.9 you can see that pH isusually between 0 and 14. A solution with a pH <strong>of</strong> 0 is very acidic, with a highH + concentration. A solution with a pH <strong>of</strong> 14 is very basic, with a low H +concentration. Solutions with a pH <strong>of</strong> 7 are neutral—neither acidic nor basic.42 Unit 1: Introducing Biology

FIGURE 2.9 Understanding pHThe pH <strong>of</strong> a solution depends on the concentration <strong>of</strong> H + ions.stomach acid pH between 1 and 3blood pH 7.4pure water pH 7 bile pH between 8 and 9pH0 1 2 3 4 5 6 7 8 9 10 11 12 13pH14more acidicneutralmore basicThe concentration <strong>of</strong> H + ions varies depending on how acidic or basic a solution is.high H +concentrationlow H +concentrationSummarize Describe the relationship between the H + concentration and the pH value.Most organisms, including humans, need to keep their pH within a verynarrow range around neutral (pH 7.0). However, some organisms need a verydifferent pH range. The azalea plant thrives in acidic (pH 4.5) soil, and amicroorganism called Picrophilus survives best at an extremely acidic pH <strong>of</strong>0.7. For all <strong>of</strong> these different organisms, pH must be tightly controlled.One way pH is regulated in organisms is by substances called buffers. Abuffer is a compound that can bind to an H ion when the H + concentrationincreases, and can release an H ion when the H concentration decreases. Inother words, a buffer “locks up” H ions and helps to maintain homeostasis.For example, the normal pH <strong>of</strong> human blood is between 7.35 and 7.45, so it isslightly basic. Just a small change in pH can disrupt processes in your cells,and a blood pH greater than 7.8 or less than 6.8, for even a short time, isdeadly. Buffers in your blood help prevent any large changes in blood pH.Apply Cells have higher H + concentrations than blood. Which has a higher pH? Why?ConnectingCONCEPTSHuman Biology In the humanbody, both the respiratorysystem and the excretory systemhelp regulate pH. You will learnabout human systems andhomeostasis in Chapter 28.2.2 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. How do polar molecules formhydrogen bonds?2. What determines whether a compoundwill dissolve in water?3. Make a chart that compares acidsand bases.CRITICAL THINKING4. Compare and Contrast How dopolar molecules differ from nonpolarmolecules? How does thisdifference affect their interactions?5. Connect Describe an example <strong>of</strong>cohesion or adhesion that youmight observe during your daily life.Connecting CONCEPTS6. Cellular Respiration Whensugars are broken down toproduce usable energy for cells,a large amount <strong>of</strong> heat isreleased. Explain how the waterinside a cell helps to keep thecell’s temperature constant.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 43

2.3Carbon-Based MoleculesKEY CONCEPT Carbon-based molecules are the foundation <strong>of</strong> life.MAIN IDEAS• Carbon atoms have uniquebonding properties.• Four main types <strong>of</strong> carbon-basedmolecules are found in living things.VOCABULARYmonomer, p. 45polymer, p. 45carbohydrate, p. 45lipid, p. 46fatty acid, p. 46protein, p. 47amino acid, p. 47nucleic acid, p. 48Reviewatom, molecule, covalent bondConnect Car manufacturers <strong>of</strong>ten build several types <strong>of</strong> cars from the sameinternal frame. The size and style <strong>of</strong> the cars might differ on the outside, but theyhave the same structure underneath. Carbon-based molecules are similar, butthey are much more varied. There are millions <strong>of</strong> different carbon-based molecules,but they form around only a few simple frames composed <strong>of</strong> carbon atoms.MAIN IDEACarbon atoms have unique bonding properties.Carbon is <strong>of</strong>ten called the building block <strong>of</strong> life because carbon atoms are thebasis <strong>of</strong> most molecules that make up living things. These molecules form thestructure <strong>of</strong> living things and carry out most <strong>of</strong> the processes that keep organismsalive. Carbon is so important because its atomic structure gives it bondingproperties that are unique among elements. Each carbon atom has fourunpaired electrons in its outer energy level. Therefore, carbon atoms can formcovalent bonds with up to four other atoms, including other carbon atoms.As FIGURE 2.10 shows, carbon-based molecules have three fundamentalstructures—straight chains, branched chains, and rings. All three types <strong>of</strong>molecules are the result <strong>of</strong> carbon’s ability to form four covalent bonds.Carbon chains can bond with carbon rings to form very large, very complexmolecules. These large molecules can be made <strong>of</strong> many small molecules thatare bonded together. In a sense, the way these molecules form is similar to theway in which individual links <strong>of</strong> metal come together to make a bicycle chain.FIGURE 2.10 CARBON CHAINS AND RINGSStraight chainBranched chainRingOHH H H H HC C C C CH H H HCH 3CHCHCCHA simplified structure can also be shown as:CH 3 CH 2 CH 2 CH CH 2PenteneCH 3CH CH 2 CH 3CH 2HexaneCHCCOHVanillinOCH 344 Unit 1: Introducing Biology

In many carbon-based molecules,small molecules are subunits <strong>of</strong> anentire molecule, like links in a chain.Each subunit in the complete moleculeis called a monomer. When monomersare linked, they form molecules calledpolymers. A polymer is a large molecule,or macromolecule, made <strong>of</strong>many monomers bonded together. All<strong>of</strong> the monomers in a polymer may bethe same, as they are in starches, or theymay be different, as they are in proteins.VISUAL VOCABEach smaller molecule is a subunitcalled a monomer.mono- = onepoly- = manymonomerpolymerA polymer is a molecule that containsmany monomers bonded together.Synthesize Write your own analogy for the formation <strong>of</strong> a polymer from monomers.MAIN IDEAFour main types <strong>of</strong> carbon-based molecules arefound in living things.All organisms are made <strong>of</strong> four types <strong>of</strong> carbon-based molecules: carbohydrates,lipids, proteins, and nucleic acids. These molecules have different structures andfunctions, but all are formed around carbon chains and rings.TAKING NOTESUse a content frame to help youunderstand monomers and polymersin carbon-based molecules.Monomer Polymer Example FunctionCarbohydratesFruits and grains are in different food groups, but they both contain largeamounts <strong>of</strong> carbohydrates. Carbohydrates are molecules composed <strong>of</strong> carbon,hydrogen, and oxygen, and they include sugars and starches. Carbohydratescan be broken down to provide a source <strong>of</strong> usable chemical energy for cells.Carbohydrates are also a major part <strong>of</strong> plant cell structure.The most basic carbohydrates are simple sugars, or monosaccharides(MAHN-uh-SAK-uh-RYDZ). Many simplesugars have either five or six carbon atoms.Fruits contain a six-carbon sugar calledCfructose. Glucose, one <strong>of</strong> the sugars made by Hplant cells during photosynthesis, is anotherCsix-carbon sugar. Simple sugars can beHObonded to make larger carbohydrates. Forexample, two sugars bonded together makeHthe disaccharide you know as table sugar,shown in FIGURE 2.11. Many glucose moleculescan be linked to make polysaccharides (PAHLee-SAK-uh-RYDZ),which are polymers <strong>of</strong> monosaccharides.CH 2 OHHOHCOHCOHCHOHGlucose (C 6 H 12 O 6 ) can be ring shaped andis <strong>of</strong>ten shown as a simplified hexagon.Starches, glycogen, and cellulose are polysaccharides. Starches and glycogenare similar, but they differ from cellulose because their glucose monomers arebonded together differently. Most starches are branched chains <strong>of</strong> glucosemolecules. Starches are made and stored by plants, and they can be brokendown as a source <strong>of</strong> energy by plant and animal cells. Glycogen, which is madeand stored in animals, is more highly branched than plant starches.FIGURE 2.11 Household sugar(sucrose) is a disaccharide, or twosugarmolecule, <strong>of</strong> glucose (inset)and fructose.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 45

FIGURE 2.12 CARBOHYDRATE STRUCTUREPolymer (starch)Starch is a polymer <strong>of</strong> glucosemonomers that <strong>of</strong>ten has abranched structure.monomerPolymer (cellulose)Cellulose is a polymer <strong>of</strong>glucose monomers that has astraight, rigid structure.Connecting CONCEPTSCell Structure A cell wall made<strong>of</strong> cellulose surrounds themembrane <strong>of</strong> plant cells. Youwill learn more about cell wallsin Chapter 3.FIGURE 2.13 Fatty acids can beeither saturated or unsaturated.Saturated fatty acidOHOOHOCCCH 2Unsaturated fatty acidCH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 2CH 3CHCH 2 CCHCH 2CHCCellulose is somewhat different from starch and glycogen. Its straight, rigidstructure, shown in FIGURE 2.12, makes the cellulose molecule a major buildingblock in plant cell structure. Cellulose makes up the cell wall that is the toughouter covering <strong>of</strong> plant cells. You have eaten cellulose in the stringy fibers <strong>of</strong>vegetables such as celery, so you know that it is tough to chew and break up.LipidsLipids are nonpolar molecules that include fats, oils, and cholesterol. Likecarbohydrates, most lipids contain chains <strong>of</strong> carbon atoms bonded to oxygenand hydrogen atoms. Some lipids are broken down as a source <strong>of</strong> usableenergy for cells. Other lipids are parts <strong>of</strong> a cell’s structure.Fats and oils are two familiar types <strong>of</strong> lipids. They store large amounts <strong>of</strong>chemical energy in organisms. Animal fats are found in foods such as meatand butter. You know plant fats as oils, such as olive oil and peanut oil. Thestructures <strong>of</strong> fats and oils are similar. They both consist <strong>of</strong> a molecule calledglycerol (GLIHS-uh-RAWL) bonded to molecules called fatty acids. Fatty acidsare chains <strong>of</strong> carbon atoms bonded to hydrogen atoms. Two different types <strong>of</strong>fatty acids are shown in FIGURE 2.13.Many lipids, both fats and oils, contain three fatty acids bonded to glycerol.They are called triglycerides. Most animal fats are saturated fats, which meansthey have the maximum number <strong>of</strong> hydrogen atoms possible. That is, everyplace that a hydrogen atom can bond to a carbon atom is filled with a hydrogenatom, and all carbon–carbon bonds are single bonds. You can think <strong>of</strong> thefatty acid as being “saturated” with hydrogen atoms. In contrast, fatty acids inoils have fewer hydrogen atoms because there is at least one double bondbetween carbon atoms. These lipids are called unsaturated fats because thefatty acids are not saturatedCH CH 2 CH 2 CH 2CH 2 CH 2 CH 2 CH 3Saturated fats containfatty acids in which allcarbon–carbon bondsare single bonds.Unsaturated fats havefatty acids with atleast one carbon–carbon double bond.with hydrogen atoms. Fats andoils are very similar, but whyare animal fats solid and plantoils liquid? The double bondsin unsaturated fats make kinksin the fatty acids. As a result,the molecules cannot packtogether tightly enough t<strong>of</strong>orm a solid.46 Unit 1: Introducing Biology

All cell membranes are made mostly <strong>of</strong> another type <strong>of</strong> lipid, called aphospholipid (FAHS-foh-LIHP-ihd). A phospholipid consists <strong>of</strong> glycerol, tw<strong>of</strong>atty acids, and a phosphate group (PO 4– ) that is part <strong>of</strong> the polar “head” <strong>of</strong> themolecule. The fatty acids are the nonpolar “tails” <strong>of</strong> a phospholipid. Comparethe structure <strong>of</strong> a phospholipid to the structure <strong>of</strong> a triglyceride in FIGURE 2.14.FIGURE 2.14 LIPID STRUCTUREPhospholipidPO 4–headtailsA phospholipidhas nonpolar fattyacid “tails” and apolar “head” thatcontains a phosphategroup.TriglycerideA triglyceride hasthree fatty acidsand a molecule<strong>of</strong> glycerol, butno phosphategroup.Cholesterol (kuh-LEHS-tuh-RAWL) is a lipid that has a ring structure. Youmay hear about dangers <strong>of</strong> eating foods that contain a lot <strong>of</strong> cholesterol, such aseggs, but your body needs a certain amount <strong>of</strong> it to function. For example,cholesterol is a part <strong>of</strong> cell membranes, and your body uses it to make chemicalscalled steroid hormones. Cholesterol-based steroids have many functions.Some regulate your body’s response to stress. Others, such as testosterone andestrogen, control sexual development and the reproductive system.ProteinsProteins are the most varied <strong>of</strong> the carbon-based molecules in organisms. Inmovement, eyesight, or digestion, proteins are at work. A protein is a polymermade <strong>of</strong> monomers called amino acids. Amino acids are molecules that containcarbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Organisms use 20different amino acids to build proteins. Your body can make 12 <strong>of</strong> the aminoacids. The others come from foods you eat, such as meat, beans, and nuts.Look at FIGURE 2.15 to see the amino acid serine. All amino acids have similarstructures. As FIGURE 2.16 shows, each amino acid monomer has a carbonatom that is bonded to four other parts. Three <strong>of</strong> these parts are the same inevery amino acid: a hydrogen atom, an amino group (NH 2 ), and a carboxylgroup (COOH). Amino acids differ only in their side group, or the R-group.Amino acids form covalent bonds, called peptide bonds, with each other.The bonds form between the amino group <strong>of</strong> one amino acid and the carboxylgroup <strong>of</strong> another amino acid. Through peptide bonds, amino acids arelinked into chains called polypeptides. A protein is one or more polypeptides.FIGURE 2.15 Serine is one <strong>of</strong> 20amino acids that make up proteinsin organisms.OHHHNCH 2CHCOOHFIGURE 2.16 AMINO ACID AND PROTEIN STRUCTUREAll amino acids have a carbon atombonded to a hydrogen atom, anamino group (NH 2 ), and a carboxylgroup (COOH). Different amino acidshave different side groups (R).HHNRCHCOOHMonomer (amino acid)OCNHHpeptide bondsPeptide bonds form between the aminogroup <strong>of</strong> one amino acid and the carboxylgroup <strong>of</strong> another amino acid.RCOCNHPolymer (protein)peptide bondsA polypeptide is a chain <strong>of</strong> precisely orderedamino acids linked by peptide bonds. A proteinis made <strong>of</strong> one or more polypeptides.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 47

FIGURE 2.17 Hemoglobin in redblood cells transports oxygen. Thestructure <strong>of</strong> hemoglobin dependson hydrogen bonds between specificamino acids. Just one aminoacid change causes red blood cellsto have the curved shape characteristic<strong>of</strong> sickle cell anemia.(colored SEM; magnification 3500)For more information oncarbon-based molecules,visit scilinks.org.Keycode: MLB002Proteins differ in the number and order <strong>of</strong> aminoacids. The specific sequence <strong>of</strong> amino acids determinesa protein’s structure and function. Two types <strong>of</strong>interactions between the side groups <strong>of</strong> some aminoacids are especially important in protein structure.First, some side groups contain sulfur atoms. Thehydrogen bond sulfur atoms can form covalent bonds that force theprotein to bend into a certain shape.Second, hydrogen bonds can form between the side groups <strong>of</strong> some aminoacids. These hydrogen bonds cause the protein to fold into a specific shape.For example, FIGURE 2.17 shows the structure <strong>of</strong> one <strong>of</strong> the four polypeptidesthat makes up hemoglobin, the protein in your red blood cells that transportsoxygen. Each <strong>of</strong> the four polypeptides contains an iron atom that bonds to anoxygen molecule. The four polypeptides are folded in a way that puts the fouroxygen-carrying sites together in a pocketlike structure inside the molecule.If a protein has incorrect amino acids, the structure may change in a way thatprevents the protein from working properly. Just one wrong amino acid <strong>of</strong> the574 amino acids in hemoglobin causes the disorder sickle cell anemia.Nucleic AcidsDetailed instructions to build proteins are stored in extremely long carbonbasedmolecules called nucleic acids. Nucleic acids are polymers that are madeup <strong>of</strong> monomers called nucleotides. A nucleotide is composed <strong>of</strong> a sugar, aphosphate group, and a nitrogen-containing molecule called a base. There aretwo general types <strong>of</strong> nucleic acids: DNA and RNA.Nucleic acids differ from the other types <strong>of</strong> carbon-based molecules.Carbohydrates, lipids, and proteins have a large number <strong>of</strong> structures andfunctions. Nucleic acids have just one function. They work together to makeproteins. DNA stores the information for putting amino acids together tomake proteins, and RNA helps to build proteins. DNA is the basis <strong>of</strong> genes andheredity, but cannot do anything by itself. Instead, the structure <strong>of</strong> DNA—theorder <strong>of</strong> nucleotides—provides the code for the proper assembly <strong>of</strong> proteins.You will learn more about nucleic acids and how they build proteins in Unit 3.Apply What is the relationship between proteins and nucleic acids?2.3 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What is the relationship between apolymer and a monomer?2. Explain how both nucleic acids andproteins are polymers. Be sure todescribe the monomers that makeup the polymers.CRITICAL THINKING3. Compare and Contrast How arecarbohydrates and lipids similar?How are they different?4. Infer Explain how the bondingproperties <strong>of</strong> carbon atoms result inthe large variety <strong>of</strong> carbon-basedmolecules in living things.Connecting CONCEPTS5. Biochemistry Why mightfatty acids, amino acids, andnucleic acids increase thehydrogen ion (H ) concentration<strong>of</strong> a solution? Explain youranswer.48 Unit 1: Introducing Biology

DATA ANALYSISIndependent andDependent VariablesIDENTIFYING VARIABLESDATA ANALYSISClassZone.comIn an experiment, a scientist determines theeffect one variable has on another. A scientistchanges, or manipulates, the independentvariable and measures or observes thedependent variables. Therefore, data from anexperiment are measurements <strong>of</strong> dependentvariables. Changes in dependent variables“depend upon” the independent variable.EXAMPLEA scientist studied the effect <strong>of</strong> jogging on thenumber <strong>of</strong> Calories used. (The Calories in foodare kilocalories, or 1000 calories.) People joggedfor three different lengths <strong>of</strong> time—10 minutes,20 minutes, and 30 minutes. The number <strong>of</strong>Calories used was measured, recorded, andplotted on a graph like the one shown on theright. What are the independent and dependentvariables?• The independent variable is the length <strong>of</strong>time spent jogging (10 minutes, 20 minutes,or 30 minutes).• The dependent variable is the number <strong>of</strong>Calories used while jogging—the number <strong>of</strong>Calories “depends on” time.dependentvariableindependentvariableGRAPH 1. JOGGING AND CALORIES IDENTIFY VARIABLESA company that makes nutritional products is developing a new type <strong>of</strong>protein drink for athletes. A scientist at the company is studying the pH atwhich a digestive enzyme best breaks down the different proteins in thedrink. The scientist uses the following experimental procedure:• Five test tubes each contain 2 mL <strong>of</strong> the protein drink.• Five different solutions contain the digestive enzyme, but each solutionhas a different pH—1.5, 2.0, 2.5, 3.0, and 3.5. One enzyme solution is addedto each test tube <strong>of</strong> protein drink.• Protein levels are measured in each <strong>of</strong> the five test tubes.1. Identify What are the independent and dependent variables in theexperiment? Explain your answers.2. Apply Time is <strong>of</strong>ten used as a dependent variable in experiments. Describehow time could be used as a dependent variable in this experiment.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 49

2.4Chemical ReactionsKEY CONCEPT <strong>Life</strong> depends on chemical reactions.MAIN IDEAS• Bonds break and form duringchemical reactions.• Chemical reactions release orabsorb energy.VOCABULARYchemical reaction, p. 50reactant, p. 50product, p. 50bond energy, p. 51equilibrium, p. 51activation energy, p. 53exothermic, p. 53endothermic, p. 53Reviewatom, moleculeConnect When you hear the term chemical reaction, what comes to mind?Maybe you think <strong>of</strong> liquids bubbling in beakers. You probably do not think <strong>of</strong>the air in your breath, but most <strong>of</strong> the carbon dioxide and water vapor that youbreathe out are made by chemical reactions in your cells.MAIN IDEABonds break and form during chemical reactions.Plant cells make cellulose by linking simple sugars together. Plant and animalcells break down sugars to get usable energy. And all cells build protein moleculesby bonding amino acids together. These are just a few <strong>of</strong> the chemicalreactions in living things. Chemical reactions change substances into differentsubstances by breaking and forming chemical bonds.Reactants, Products, and Bond EnergyYour cells need the oxygen molecules that you breathe in. Oxygen (O 2 ) plays apart in a series <strong>of</strong> chemical reactions that provides usable energy for your cells.These reactions, which are described in detail in Chapter 4, break down thesimple sugar glucose (C 6 H 12 O 6 ). The process uses oxygen and glucose andresults in carbon dioxide (CO 2 ), water (H 2 O), and usable energy. Oxygen andglucose are the reactants. Reactants are the substances changed during achemical reaction. Carbon dioxide and water are the products. Products arethe substances made by a chemical reaction. Chemical equations are used toshow what happens during a reaction. The overall equation for the processthat changes oxygen and glucose into carbon dioxide and water is6O 2 + C 6 H 12 O 66CO 2 + 6H 2OReactants Direction ProductsFIGURE 2.18 The breakdown<strong>of</strong> glucose provides chemicalenergy for all activities,including speed skating.The reactants are on the left side <strong>of</strong> the equation, and the products are onthe right side. The arrow shows the direction <strong>of</strong> the reaction. This process,which is called cellular respiration, makes the carbon dioxide and water vaporthat you breathe out. But for carbon dioxide and water to be made, bondsmust be broken in the reactants, and bonds must form in the products. Whatcauses bonds in oxygen and glucose molecules to break? And what happenswhen new bonds form in carbon dioxide and water?50 Unit 1: Introducing Biology

QUICK LABMODELINGChemical BondingYou use energy to put things together, but chemical bonding is different.Energy is added to break bonds, and energy is released whenbonds form.MATERIALS2 flat magnetsPROBLEM How is chemical bonding similar to the interactionbetween two magnets?PROCEDURE1. Bring the magnets close to each other until they snap together.2. Pull the magnets away from each other.ANALYZE AND CONCLUDE1. Infer How is bond formation represented by the snapping sound?2. Apply How is bond energy related to your separation <strong>of</strong> the magnets?First, energy is added to break bonds in molecules <strong>of</strong> oxygen and glucose.Bond energy is the amount <strong>of</strong> energy that will break a bond between twoatoms. Bonds between different types <strong>of</strong> atoms have different bond energies.A certain amount <strong>of</strong> energy is needed to break bonds in an oxygen molecule.A different amount <strong>of</strong> energy is needed to break bonds in a glucose molecule.Energy is released when bonds form, such as when molecules <strong>of</strong> water andcarbon dioxide are made. When a bond forms, the amount <strong>of</strong> energy releasedis equal to the amount <strong>of</strong> energy that breaks the same bond. For example,energy is released when hydrogen and oxygen atoms bond to form awater molecule. The same amount <strong>of</strong> energy is needed to break apart awater molecule.Chemical EquilibriumSome reactions go from reactants to products until the reactants are used up.However, many reactions in living things are reversible. They move in bothdirections at the same time. These reactions tend to go in one direction or theother depending on the concentrations <strong>of</strong> the reactants and products. One suchreaction lets blood, shown in FIGURE 2.19, carry carbon dioxide. Carbon dioxidereacts with water in blood to form a compound called carbonic acid (H 2 CO 3 ).Your body needs this reaction to get rid <strong>of</strong> carbon dioxide waste from your cells.CO 2 + H 2 O H 2 CO 3The arrows in the equation above show that the reaction goes in bothdirections. When the carbon dioxide concentration is high, as it is around yourcells, the reaction moves toward the right and carbonic acid forms. In yourlungs, the carbon dioxide concentration is low. The reaction goes in the otherdirection, and carbonic acid breaks down.When a reaction takes place at an equal rate in both directions, the reactantand product concentrations stay the same. This state is called equilibrium.Equilibrium (EE-kwuh-LIHB-ree-uhm) is reached when both the reactants andproducts are made at the same rate.Apply Explain why concentration is important in a chemical reaction.FIGURE 2.19 Blood cells andplasma transport materialsthroughout the body. Carbonicacid dissolves in the blood so thatcarbon dioxide can be transportedto the lungs. (compositecolored SEM; magnification 1000)Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 51

FIGURE 2.20 Energy and Chemical ReactionsEnergy is required to break bonds in reactants, and energy is releasedwhen bonds form in products. Overall, a chemical reaction either absorbsor releases energy.BIOLOGYWatch exothermicand endothermicreactions atClassZone.com.ACTIVATION ENERGYWhen enough activation energy is added to the reactants,bonds in the reactants break and the reaction begins.activation energyenergy inEnergyreactantsReaction progressEXOTHERMIC REACTION Energy ReleasedENDOTHERMIC REACTION Energy Absorbedtotal energyaddedtotal energyreleasedtotal energyaddedtotal energyreleasedEnergyreactantsdifference in energy(released)productsEnergyreactantsdifference in energy(absorbed)productsReaction progressReaction progressThe products in an exothermic reaction have a lower bondenergy than the reactants, and the difference in bond energyis released to the surroundings.The products in an endothermic reaction have a higher bondenergy than the reactants, and the difference in bond energyis absorbed from the surroundings.CRITICALVIEWINGIs the amount <strong>of</strong> activation energy related to whether a reactionis exothermic or endothermic? Why or why not?52 Unit 1: Introducing Biology

MAIN IDEAChemical reactions release or absorb energy.All chemical reactions involve changes in energy. Energy that is added to thereactants breaks their chemical bonds. When new bonds form in the products,energy is released. This means that energy is both absorbed and releasedduring a chemical reaction. Some chemical reactions release more energy thanthey absorb. Other chemical reactions absorb more energy than they release.Whether a reaction releases or absorbs energy depends on bond energy.Some energy must be absorbed by the reactants in any chemical reaction.Activation energy is the amount <strong>of</strong> energy that needs to be absorbed for achemical reaction to start. Activation energy is like the energy you would needto push a rock up a hill. Once the rock is at the top <strong>of</strong> the hill, it rolls downthe other side by itself. A graph <strong>of</strong> the activation energy that is added tostart a chemical reaction is shown at the top <strong>of</strong> FIGURE 2.20.An exothermic chemical reaction releases more energy than it absorbs.If the products have a lower bond energy than the reactants, the reaction isexothermic. The excess energy—the difference in bond energy between thereactants and products—is <strong>of</strong>ten given <strong>of</strong>f as heat or light. Some animals,such as squids and fireflies, give <strong>of</strong>f light that comes from exothermicreactions, as shown in FIGURE 2.21. Cellular respiration, the process that usesglucose and oxygen to provide usable energy for cells, is also exothermic.Cellular respiration releases not only usable energy for your cells but alsoheat that keeps your body warm.An endothermic chemical reaction absorbs more energy than it releases.If products have a higher bond energy than reactants, the reaction is endothermic.Energy must be absorbed to make up the difference. One <strong>of</strong> themost important processes for life on Earth, photosynthesis, is endothermic.During photosynthesis, plants absorb energy from sunlight and use thatenergy to make simple sugars and complex carbohydrates.Analyze How is activation energy related to bond energy?VOCABULARYThe prefix exo- means “out,”and the prefix endo- means“in.” Energy “moves out <strong>of</strong>”an exothermic reaction, andenergy “moves into” anendothermic reaction.FIGURE 2.21 The glow <strong>of</strong>the bugeye squid comes froman exothermic reaction thatreleases light.2.4 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. Hydrogen peroxide (H 2 O 2 )breaks down into water (H 2 O)and oxygen (O 2 ). Explain whythis is a chemical reaction.What are the reactants andthe products in the reaction?2. How do endothermic andexothermic reactions differ?CRITICAL THINKING3. Infer The process below is exothermic.What must be true about the bondenergies <strong>of</strong> the reactants and theproducts? Explain.6O 2 + C 6 H 12 O 66CO 2 + 6H 2O4. Evaluate Why might it not always bepossible to determine the reactants andthe products in a reaction? Explain youranswer in terms <strong>of</strong> chemical equilibrium.Connecting CONCEPTS5. Biochemistry A chemicalreaction can start whenenough activation energy isadded to the reactants. Do youthink the activation energy forchemical reactions in livingthings is high or low? Explainyour answer.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 53

2.5 EnzymesKEY CONCEPT Enzymes are catalysts for chemical reactions in living things.MAIN IDEAS• A catalyst lowers activation energy.• Enzymes allow chemical reactions to occurunder tightly controlled conditions.VOCABULARYcatalyst, p. 54enzyme, p. 55substrate, p. 56Reviewchemical reaction,activation energy, protein,hydrogen bondConnect Just how can a Venus flytrap digest a frog? It happens through theaction <strong>of</strong> proteins called enzymes. Enzymes help to start and run chemicalreactions in living things. For example, enzymes are needed to break down foodinto smaller molecules that cells can use. Without enzymes, a Venus flytrapcouldn’t break down its food, and neither could you.MAIN IDEAA catalyst lowers activation energy.Remember what you learned about activation energy in Section 2.4. Activationenergy for a chemical reaction is like the energy that is needed to push arock up a hill. When enough energy is added to get the rock to the top <strong>of</strong> ahill, the rock can roll down the other side by itself. Activation energy gives asimilar push to a chemical reaction. Once a chemical reaction starts, it cancontinue by itself, and it will go at a certain rate.Often, the activation energy for a chemical reaction comes from anincrease in temperature. But even after a chemical reaction starts, it mayhappen very slowly. The reactants may not interact enough, or they may not beat a high enough concentration, to quickly form the products <strong>of</strong> the reaction.However, both the activation energy and rate <strong>of</strong> a chemical reaction can bechanged by a chemical catalyst, as shown in FIGURE 2.22. A catalyst (KAT-l-ihst)is a substance that decreases the activation energy needed to start a chemicalreaction and, as a result, also increases the rate <strong>of</strong> the chemical reaction.FIGURE 2.22 CATALYSTS AND ACTIVATION ENERGYUnder normal conditions, acertain amount <strong>of</strong> activationenergy is needed to start achemical reaction. A catalystdecreases the activationenergy needed.activation energy(uncatalyzed)productsNormal reactionCatalyzed reactionEnergyreactantsactivation energy(catalyzed)Reaction progress54 Unit 1: Introducing Biology

Compare the activation energies and the reaction rates in the graph inFIGURE 2.22. Under normal conditions, the reaction requires a certain amount<strong>of</strong> activation energy, and it occurs at a certain rate. When a catalyst is present,less energy is needed and the products form faster. Although catalysts takepart in chemical reactions, catalysts are not considered to be either reactantsor products because catalysts are not changed or used up during a reaction.Summarize Describe two functions <strong>of</strong> catalysts in chemical reactions.MAIN IDEAEnzymes allow chemical reactions to occur undertightly controlled conditions.Chemical reactions in organisms have to take place at an organism’s bodytemperature. Often, reactants are found in low concentrations. Because thereactions must take place very quickly, they usually need a catalyst. Enzymesare catalysts for chemical reactions in living things. Enzymes, like othercatalysts, lower the activation energy and increase the rate <strong>of</strong> chemical reactions.In reactions that are reversible, such as the carbon dioxide and carbonicacid reaction described in Section 2.4, enzymes do not affect chemical equilibrium.This means that enzymes do not change the direction <strong>of</strong> a reaction—they just change the amount <strong>of</strong> time needed for equilibrium to be reached.Enzymes are involved in almost every process in organisms. From breakingdown food to building proteins, enzymes are needed. For example, amylase isan enzyme in saliva that breaks down starch into simpler sugars. This reactionoccurs up to a million times faster with amylase than without it. Enzymes arealso an important part <strong>of</strong> your immune system, as shown in FIGURE 2.23.Almost all enzymes are proteins.These enzymes, like other proteins, arelong chains <strong>of</strong> amino acids. Each enzymealso depends on its structure to functionproperly. Conditions such as temperatureand pH can affect the shape and function,or activity, <strong>of</strong> an enzyme. Enzymeswork best in a small temperature rangearound the organism’s normal bodytemperature. At only slightly highertemperatures, the hydrogen bonds in anenzyme may begin to break apart. Theenzyme’s structure changes and it losesits ability to function. This is one reasonwhy a very high fever is so dangerous toa person. A change in pH can also affectthe hydrogen bonds in enzymes. Manyenzymes in humans work best at thenearly neutral pH that is maintainedwithin cells <strong>of</strong> the human body.FIGURE 2.23 The inset micrograph (top) shows a white blood cell engulfingan invading pathogen. The larger micrograph shows a pathogen after it hasbeen captured. Once inside a white blood cell, enzymes are used to destroythe pathogen. (inset image: colored SEM; magnification about 3000; large image:colored TEM; magnification 11,000)Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 55

Connecting CONCEPTSBiochemistry The order <strong>of</strong>amino acids determines thestructure and function <strong>of</strong> anenzyme. An enzyme’s structure<strong>of</strong>ten depends on hydrogenbonds between amino acids.Enzyme structure is important because each enzyme’s shape allows onlycertain reactants to bind to the enzyme. The specific reactants that an enzymeacts on are called substrates. For example, amylase only breaks down starch.Therefore, starch is the substrate for amylase. Substrates temporarily bind toenzymes at specific places called active sites. Like a key fits into a lock, substratesexactly fit the active sites <strong>of</strong> enzymes. This is why if an enzyme’s structurechanges, it may not work at all. This idea <strong>of</strong> enzyme function, which iscalled the lock-and-key model, is shown below.1Substrates bind to anenzyme at certain placescalled active sites.2 3The enzyme bringssubstrates together andweakens their bonds.The catalyzed reaction formsa product that is releasedfrom the enzyme.The lock-and-key model helps explain how enzymes work. First, enzymesbring substrate molecules close together. Because <strong>of</strong> the low concentrations <strong>of</strong>reactants in cells, many reactions would be unlikely to take place withoutenzymes bringing substrates together. Second, enzymes decrease activationenergy. When substrates bind to the enzyme at the enzyme’s active site, thebonds inside these molecules become strained. If bonds are strained, orstretched slightly out <strong>of</strong> their normal positions, they become weaker. Lessactivation energy is needed for these slightly weakened bonds to be broken.The lock-and-key model is a good starting point for understandingenzyme function. However, scientists have recently found that the structures<strong>of</strong> enzymes are not fixed in place. Instead, enzymes actually bend slightlywhen they are bound to their substrates. In terms <strong>of</strong> a lock and key, it is as ifthe lock bends around the key to make the key fit better. The bending <strong>of</strong> theenzyme is one way in which bonds in the substrates are weakened.Apply How does the structure <strong>of</strong> an enzyme affect its function?2.5 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. How does a catalyst affect theactivation energy <strong>of</strong> a chemicalreaction?2. Describe how the interactionbetween an enzyme and itssubstrates changes a chemicalreaction.CRITICAL THINKING3. Infer Some organisms live in veryhot or very acidic environments.Would their enzymes function in aperson’s cells? Why or why not?4. Predict Suppose that the aminoacids that make up an enzyme’sactive site are changed. How mightthis change affect the enzyme?Connecting CONCEPTS5. Homeostasis Organisms needto maintain homeostasis, orstable internal conditions. Whyis homeostasis important forthe function <strong>of</strong> enzymes?56 Unit 1: Introducing Biology

CHAPTER 2MATERIALS• 5 test tubes• test tube rack• marker• 7 10-mL graduatedcylinders• 4 mL each <strong>of</strong> solutions<strong>of</strong> pH 3, 5, 7, 9, and 11• 2 mL 60% catalasesolution• 1 mL 3% hydrogenperoxide solution• metric rulerPROCESS SKILLS• Identifying Variables• Observing• Measuring• Collecting Data• Interpreting DataINVESTIGATIONEnzymatic ActivityEnzymes are necessary for many processes, including digestion and fighting disease.Conditions such as temperature and pH must be tightly controlled so that enzymescan function properly. In this lab, you will study an enzyme called catalase thathelps break down hydrogen peroxide into water and oxygen.PROBLEM How does pH affect enzymatic activity?PROCEDURE1. Label 5 test tubes pH 3, pH 5, pH 7, pH 9, pH 11. Place them in the test tube rack.2. Add 4 mL <strong>of</strong> the appropriate pH solution to each test tube. Be sure to use adifferent graduated cylinder for each <strong>of</strong> the solutions.3. Add 2 mL <strong>of</strong> the catalase enzyme solution to each <strong>of</strong> the test tubes. Gently swirlthe test tubes to mix the solutions. Allow the test tubes to sit for 5 minutes.4. Design a data table that has rows labeled with the independent variable andcolumns labeled with the dependent variable. Read step 6 to determine whetherthe foam height is the independent variable or the dependent variable.5. Add 1 mL <strong>of</strong> the hydrogen peroxide solution to each test tube. Allow 5 minutesfor the solutions to react. Foam should appear on the solution tops.Caution: Avoid skin contact with hydrogen peroxide.6. Measure the distance in millimeters from the bottom <strong>of</strong> each test tube to thetop <strong>of</strong> the foam in the test tube. Record your measurements in your data table.ANALYZE AND CONCLUDE1. Identify Variables What are the independent and dependent variables? What isthe operational definition <strong>of</strong> the dependent variable?2. Analyze Choose a type <strong>of</strong> graph to appropriately display your data. Constructyour graph.3. Analyze How is the enzymatic activity <strong>of</strong> catalase related to pH? What does thistell you about the pH <strong>of</strong> your cells?4. Infer The activity <strong>of</strong> an enzyme depends upon its structure. What do your resultssuggest about the effect <strong>of</strong> pH on the structure <strong>of</strong> catalase? Explain.5. Experimental Design Why is it important that each test tube have the sameamount <strong>of</strong> each solution? What other sources <strong>of</strong> experimental error mayhave existed?6. Apply Gelatin recipes thatinclude fruit <strong>of</strong>ten say not to usepineapple. Gelatin is mostlyprotein. Pineapple contains anenzyme that is <strong>of</strong>ten used inmeat tenderizers. What effectmight pineapple have on gelatin?Why?Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 57

CHAPTER 2OPTIONS FOR INQUIRYUse these inquiry-based labs and online activities to deepen yourunderstanding <strong>of</strong> the importance <strong>of</strong> chemistry in biology.INVESTIGATIONTesting pHUniversal indicator paper changes color depending onthe pH <strong>of</strong> the solution being tested. Many substancesaround your home are acids and have a low pH. Othersare bases and have a high pH. In this lab you will use pHindicator paper to investigate the pH <strong>of</strong> severalcommon substances.SKILLS Observing, Analyzing, InferringPROBLEM How acidic or basic are householdsubstances?MATERIALS• 6 test tubes• test tube rack• 6 10-mL graduated cylinders• lemon juice• vinegar• mouthwash• window cleaner• 2 unknown solutions• pH indicator paperPROCEDURE1. Use a different graduated cylinder to pour5 mL <strong>of</strong> each <strong>of</strong> the six solutions into each <strong>of</strong>six test tubes.2. Test the pH <strong>of</strong> each known solutionwith a different strip <strong>of</strong> pH indicatorpaper. Use the color scale on theindicator paper package to estimatethe pH <strong>of</strong> each solution. Record thepH <strong>of</strong> the known solutions.3. Construct a pH scale ranging from 0(very acidic) to 14 (very basic), with 7(neutral) in the center. Label your pHscale with the known solutions.4. Test the pH <strong>of</strong> both unknown solutionswith different strips <strong>of</strong> pH indicatorpaper, and record the pH <strong>of</strong> each.5. Add labels for the unknown solutions to the pH scale that you made in step 3.ANALYZE AND CONCLUDE1. Identify Find out from your teacher what the unknown solutions are. Is the pH<strong>of</strong> any <strong>of</strong> the solutions different than you might have expected? Why or why not?2. Apply What is the pH range <strong>of</strong> the solutions you tested? Are any <strong>of</strong> them very acidicor basic? What does this indicate about many common substances found in the home?3. Analyze Describe the hydrogen ion concentrations in each <strong>of</strong> the six solutions.EXTEND YOUR INVESTIGATIONUse red cabbage juice as a pH indicator to test the six solutions that you testedearlier. Is the red cabbage juice as accurate as the pH indicator paper? Explain.58 Unit 1: Introducing Biology

INVESTIGATIONEnzymesAs in all organisms, enzymes are critical for chemicalreactions in humans. One reaction producesmelanin, which gives skin a dark color. Melanin helpsprotect skin cells from ultraviolet radiation—thegreater the exposure to ultraviolet radiation, thegreater the production <strong>of</strong> melanin. Melanin is madefrom the amino acid tyrosine by an enzyme calledtyrosinase. Without tyrosinase, the reaction cannottake place. This causes a condition called albinism.SKILL CommunicatingPROBLEM What happens if an enzyme ismissing or defective?RESEARCHResearch the effect <strong>of</strong> one <strong>of</strong> the following enzymedeficiencies:• phenylketonuria• galactosemia• lactose intoleranceFor the enzyme deficiency that you have selected:1. Identify the enzyme involved and its function.2. Examine how a person’s health is affected by thisenzyme deficiency.3. Find out how the deficiency is diagnosed.4. Describe the effects <strong>of</strong> not treating the deficiency.BIOLOGYCLASSZONE.COMVIRTUAL LABCalorimetryHow much energy is in the food you eat? Inthis interactive lab, you will burn differentfood items and measure the number <strong>of</strong>Calories each releases.ANIMATED BIOLOGYAtoms and BondingBuild atoms from a bank <strong>of</strong> protons, neutrons,and electrons. Then explore how two or moreatoms interact to form compounds.People who are lactoseintolerant <strong>of</strong>ten replaceregular milk with soymilk.WEBQUESTPrions are misfolded proteins that cause madcow disease and, in humans, new variantCreutzfeldt-Jakob disease. In this WebQuest,you will learn about prions and how theyinfect people and other animals. Determineif the drastic steps taken to prevent thespread <strong>of</strong> prions actually keep people safe.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 59

CHAPTER2@ CLASSZONE.COMKEY CONCEPTS Vocabulary Games Concept Maps Animated Biology Online Quiz2.1 Atoms, Ions, and MoleculesAll living things are based on atoms and theirinteractions. All matter is composed <strong>of</strong> atomsthat interact. Atoms can become ions by gainingor losing electrons. Compounds and moleculesform when atoms form bonds.Carbon dioxide2.2 Properties <strong>of</strong> WaterWater’s unique properties allow life to exist onEarth. Water is a polar molecule. The slightlycharged regions <strong>of</strong> water molecules formhydrogen bonds that give water properties suchas cohesion and adhesion. Many substancesdissolve in water to form solutions.Polar water moleculeHydrogen bonding2.3 Carbon-Based MoleculesCarbon-based molecules are the foundation <strong>of</strong>life. The four main types <strong>of</strong> carbon-based moleculesin living things are carbohydrates, lipids,proteins, and nucleic acids. The molecules havedifferent functions that are based on their differentstructures.2.4 Chemical Reactions<strong>Life</strong> depends on chemical reactions. A chemicalreaction changes reactants into products. Bondsbreak in the reactants, and new bonds form inthe products. Chemical reactions either releaseenergy (exothermic reactions) or absorb energy(endothermic reactions).2.5 EnzymesEnzymes are catalysts for chemical reactions inliving things. Enzymes increase the rate <strong>of</strong> reactionsand decrease the activation energy forreactions. Each enzyme catalyzes a specific reaction,and a change in an enzyme’s structurechanges its function.Synthesize Your NotesSupporting Main Ideas Use your notes to make a detailedversion <strong>of</strong> the graphic organizer shown below. Makeorganizers for each key concept in the chapter. Be sureto include important details and to mark importantvocabulary terms.Atoms and their interactionsIons—formed when atoms gain or lose electronsCompounds—Concept Map Use conceptmaps like the one on the rightto visualize the relationshipsamong chapter concepts.elementsareatoms <strong>of</strong>different typescombine to formMolecules—60 Unit 1: Introducing Biology

Chapter AssessmentChapter Vocabulary2.1 atom, p. 36element, p. 36compound, p. 37ion, p. 38ionic bond, p. 38covalent bond, p. 39molecule, p. 392.2 hydrogen bond, p. 41cohesion, p. 41adhesion, p. 41solution, p. 42solvent, p. 42solute, p. 42acid, p. 42base, p. 42pH, p. 422.3 monomer, p. 45polymer, p. 45carbohydrate, p. 45lipid, p. 46fatty acid, p. 46protein, p. 47amino acid, p. 47nucleic acid, p. 482.4 chemical reaction, p. 50reactant, p. 50product, p. 50bond energy, p. 51equilibrium, p. 51activation energy, p. 53exothermic, p. 53endothermic, p. 532.5 catalyst, p. 54enzyme, p. 55substrate, p. 56Reviewing VocabularyVocabulary ConnectionsThe vocabulary terms in this chapter are related toeach other in various ways. For each group <strong>of</strong> wordsbelow, write a sentence or two to clearly explain howthe terms are connected. For example, for the termscovalent bond and molecule, you could write “Amolecule is made <strong>of</strong> atoms connected by covalentbonds.”1. atom, ion2. hydrogen bond, cohesion3. solution, solvent4. acid, base, pH5. exothermic, endothermic6. catalyst, enzymeWord Origins7. The word atom comes from the Greek word atomos,which means “indivisible.” Describe the relationshipbetween the Greek term and your understanding <strong>of</strong>atoms.8. The p in pH stands for the German word Potenz, whichmeans “power” or “potential.” The H representshydrogen ions (H + ). How are these related to thedefinition <strong>of</strong> pH?9. The prefix mono- means “one” and the prefix polymeans“many.” Some lipids are monounsaturated andothers are polyunsaturated. Explain the differencebetween the fatty acids in these different types <strong>of</strong> lipids.Reviewing MAIN IDEAS10. Explain how the combination <strong>of</strong> electrons, protons, andneutrons results in the neutral charge <strong>of</strong> an atom.11. Potassium ions (K ) have a positive charge. What happensto a potassium atom’s electrons when it becomes an ion?12. Some types <strong>of</strong> atoms form more than one covalentbond with another atom. What determines how manycovalent bonds two atoms can make? Explain.13. How is hydrogen bonding among water moleculesrelated to the structure <strong>of</strong> the water molecule?14. Explain the difference between solvents and solutes.15. Describe the relationship between hydrogen ions (H )and pH. How is pH related to a solution’s acidity?16. Carbon forms a very large number <strong>of</strong> compounds. Whatcharacteristic <strong>of</strong> carbon atoms allows the formation <strong>of</strong>all <strong>of</strong> these compounds? Explain.17. Identify and explain examples <strong>of</strong> monomers and polymersin carbohydrates, proteins, and nucleic acids.18. Explain the relationship between a protein’s structureand its ability to function.19. What are the components <strong>of</strong> a chemical reaction?20. Explain the difference between exothermic andendothermic reactions.21. Describe the effect <strong>of</strong> a catalyst on activation energyand reaction rate.22. What is the role <strong>of</strong> enzymes in organisms?Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 61

Critical Thinking23. Compare and Contrast How are phospholipidssimilar to lipids such as triglycerides? How are theydifferent?24. Compare and Contrast Briefly describe the similaritiesand differences between hydrogen bonds and ionicbonds. Which type <strong>of</strong> bond do you think is stronger?Why?25. Infer Suppose that you have a cold. What characteristicsmust cold medicine have that allow it to be transportedthroughout your body? Explain.26. Predict Homeostasis involves the maintenance <strong>of</strong>constant conditions in an organism. What might happento a protein if homeostasis is disrupted? Why?27. Compare and Contrast Describe the structures andfunctions <strong>of</strong> starch and cellulose. How are the moleculessimilar? How are they different?28. Apply Suppose you had a friend who wanted toentirely avoid eating fats. What functions <strong>of</strong> lipids couldyou describe to convince that person <strong>of</strong> the importance<strong>of</strong> fats to his or her health?29. Infer The human body can reuse some <strong>of</strong> the enzymesfound in raw fruits and vegetables. Why is this not thecase for cooked fruits and vegetables?Interpreting VisualsThe diagram below shows the lock-and-key model <strong>of</strong>enzyme function. Use it to answer the next threequestions.30. Summarize Briefly explain what is happening at eachstep <strong>of</strong> the process. Be sure to identify each <strong>of</strong> the substances(A–D) shown in each step <strong>of</strong> the process.31. Apply How does Substance A affect the amount <strong>of</strong>activation energy needed by the process? Explain.Analyzing DataUse the graph below to answer the next three questions.ENERGY IN A CHEMICAL REACTION33. Apply Suppose the graph was constructed from datacollected during an experiment. What were theindependent and dependent variables in theexperiment? Explain.34. Analyze How much activation energy is needed to startthe chemical reaction represented by each line on thegraph? How much energy is released from each reaction?35. Apply Explain whether each <strong>of</strong> the chemical reactionsshown on the graph is endothermic or exothermic.Connecting CONCEPTS36. Write About Chemical Equilibrium Carbon dioxidereacts with water in blood plasma to form carbonic acid.The equation for this reaction is shown on page 51.Suppose that you are a molecule <strong>of</strong> carbon dioxide.Describe the chemical reactions that take place whenyou enter the blood and when you leave the blood.Explain what determines how these reactions occur. Besure to include all terms from the chapter that arerelated to the chemical reaction.37. Apply The Venus flytrap shown in the photograph onpage 35 uses enzymes to digest its prey. Describe howpH, solutions, and chemical reactions all play importantroles inside the trap <strong>of</strong> this carnivorous plant.32. Synthesize Describe the importance <strong>of</strong> buffers insolutions in allowing the process shown above totake place.62 Unit 1: Introducing Biology

For more test practice,go to ClassZone.com.1.Glucosidase activity(mol/L/h)Glucosidase Activity at Various Temperatures20015010050030 40 50 60 70 80 90Temperature (°C)This graph shows the activity <strong>of</strong> an enzymecalled glucosidase. What can you conclude fromthese data?A Glucosidase breaks down glucose substrates.B Glucosidase functions best around 70°C.C Glucosidase does not function below 70°C.D Glucosidase is not affected by temperature.2. Proteins are long molecules that are built fromvarious combinations <strong>of</strong>A carbohydrates.B nucleic acids.C lipids.D amino acids.4. An animal’s stomach contains enzymes thatbreak down food into smaller molecules thatthe animal’s cells can use. Enzymes performthis function byA participating in chemical reactions.B increasing the temperature.C changing the ionic concentration.D lowering the pH.5. The primary function <strong>of</strong> DNA is toA provide energy for a cell.B transmit messages between cells.C translate the codes in RNA.D store information for building proteins.6. Which phrase below best describes apolymer?A individual parts <strong>of</strong> a large moleculeB information coded in RNAC molecule made <strong>of</strong> many small moleculesD individual amino acidsTHINK THROUGH THE QUESTIONMany terms in biology are based on roots,prefixes, or suffixes. The prefix poly- means“many.” Consider how this meaning gives a clueto the answer.3.?The diagram shows how an enzyme (black)binds to a substrate (white) during achemical reaction. When this reaction iscomplete, theA enzyme’s shape will be different.B surrounding temperature will have increased.C hydrogen ion concentration will havedecreased.D substrate will be a different molecule.Chapter 2: <strong>Chemistry</strong> <strong>of</strong> <strong>Life</strong> 63

UNIT 1: INTRODUCING BIOLOGYBIOZINEatINTERNET MAGAZINECLASSZONE.COMGo online for thelatest biology newsand updates on allBioZine articles.Expanding the TextbookNews FeedsScience DailyCNNBBCScientists can change anorganism’s genes. Should they?CareersBio BytesOpinion PollStrange BiologyWhen Knowledgeand Ethics CollideOur ability to change living things growsas we learn more about life. But sometimesbiotechnology makes us question whetherwe should change organisms just becausewe can. Maybe the technology is dangerousor maybe it challenges our values.Consider the greenish pig above. A genefrom a fluorescent jellyfish was added toits genome by genetic engineering. Geneticengineering holds great promise for medicine.But how and when should we alter anorganism’s genes?64 Unit 1: Introducing Biology

What is Bioethics?A short answer is that bioethics is the study <strong>of</strong> themoral questions that are raised as a result <strong>of</strong> biologyresearch and its applications. But what do questions<strong>of</strong> ethics have to do with biology? It might seembetter to leave questions about values in a philosophyor social studies class. However, today’s cutting-edgeresearch <strong>of</strong>ten prompts discussions about some <strong>of</strong>our most basic values. In the end, you might find thatbiology class is the best place to consider any number<strong>of</strong> ethical questions.Ethical questions require all <strong>of</strong> us to make decisionsabout “the right thing to do.” Often, the rightthing to do is very clear. The decision benefitsourselves, our families, and our society, and it followsthe accepted values <strong>of</strong> society. However, many times adecision about an ethical issue is not so obvious. It isin these cases when strong feelings on different sides<strong>of</strong> an ethical question can produce conflicts—inourselves, our families, and our society. Can werely upon biology, or any other scientific field, forour decisions?For better or forworse, science can onlyprovide us with information.The knowledge that comes fromscientific research is very useful, and<strong>of</strong>ten necessary, in helping people arrive atdecisions, but science only provides part <strong>of</strong> theanswer to ethical questions. All <strong>of</strong> the advancesin science have given us the ability to do manywondrous things. But bioethics asks us to questionwhether we should actually do all <strong>of</strong> those things.We can add new genes to an organism’s DNA. Wecan clone animals. We can extend human life expectancies.We can test people for genetic diseases. Butshould we, as a society, do all <strong>of</strong> these things? Andwho should decide whether we use all <strong>of</strong> our technologicaladvances? Should these decisions be left toresearchers? to universities? to corporations? Shouldthe government make laws to cover bioethical issues?In the end, any decision based on a bioethical questionwill likely come down to a combination <strong>of</strong>scientific knowledge, personal values, and law.TECHNOLOGYGenetic TestingGenetic testing is used in many ways. We can identify disease-causing genes,determine the guilt or innocence <strong>of</strong> crime suspects, and reunite families thathave been separated. But should genetic testing be used by employers tomake decisions about employees?Suppose that a company secretly obtained and tested DNA samples fromsome <strong>of</strong> its employees. Because <strong>of</strong> rising medical insurance claims, the companywanted to know if the employees had a gene that increased their riskfor developing a certain medical condition. Does this seem like a plot for atelevision show? It isn’t. In 2002, a company had to pay more than $2 millionin damages for testing the DNA <strong>of</strong> employees without their knowledge.Consider another case. In 2005 a basketball player named Eddy Currymissed the end <strong>of</strong> the season due to a potential heart problem. His teamwanted to use a genetic test to find out if he had a life-threateningcondition. Curry refused because the test results could have ended hiscareer. The team refused to let him play. Both the team and Curry madechoices. Who do you think was right? at CLASSZONE.COMBioZine 65

UNIT 1 BIOZINEBioethics and SocietyMany bioethics issues place an individual’s right toprivacy against the right <strong>of</strong> a company to conductbusiness, against the need <strong>of</strong> a community to haveaccess to health information, or against the need <strong>of</strong>scientists to share research. In March <strong>of</strong> 2000, forexample, Iceland’s government sold the genetic andmedical records <strong>of</strong> its 275,000 citizens to a Swiss drugmanufacturer for $200 million. The money helpedIceland’s economy, and any medications or tests forgenetic diseases that result from the medical recordswill be provided for free to all Icelanders.CAREERSGeneticist in ActionDR. CHARMAINE ROYALTITLE Pr<strong>of</strong>essor, Pediatrics,Howard UniversityEDUCATION Ph. D., HumanGenetics, Howard UniversityMany bioethicists focus on the ethical implications<strong>of</strong> technology. Dr. Charmaine Royal, however, isconcerned with the ethics <strong>of</strong> experimental design andthe applications and implications <strong>of</strong> biological research.Dr. Royal, who is a geneticist at the Human GenomeCenter <strong>of</strong> Howard University, points out that somescientists in the past tried to use genetic research tojustify treating non-Caucasians as inferior. She alsonotes that although there is no biological basis for anymeaningful differences among races, many African-Americans are still suspicious <strong>of</strong> genetic research. Many,for example, have been discriminated against when aninsurance company or a prospective employer findsout they have sickle cell anemia, which is a relativelycommon genetic disorder in African-Americans.Dr. Royal, who is Jamaican, wants to ensure thatAfrican-Americans are included and treated fairly inresearch studies, and that they receive the benefits <strong>of</strong>genetic screening and genetic counseling. In 1998Dr. Royal helped start the African-American HereditaryProstate Cancer Study, the first large-scale genetic study<strong>of</strong> African-Americans to be designed and carried out byan almost entirely African-American research team. at CLASSZONE.COMHowever, the government’s actions could also beconsidered to be very troubling. Even though thecitizens were given the option to not be included inthe database, were the Icelanders’ rights to privacyover their genetic records violated? Does anyoneother than the individual have the right to be givenaccess to this very personal information?As biotechnology continuesto advance, you will facenew bioethics questionsthroughout your lifetime.Will you be ready?Health insurance applicants are screened forpreexisting conditions, such as HIV. If a condition isfound, companies might <strong>of</strong>fer insurance to a personat a higher cost. Should insurance companies beallowed to do genetic screening to detect whetherpeople have genes that might increase their risk <strong>of</strong>developing cancer or alcoholism? If not, shouldhealthy people have to pay more to make up forhigher costs the company has to pay for people whorefuse to be screened? From the company’s point <strong>of</strong>view, its responsibility is to make a pr<strong>of</strong>it for itsshareholders. With genetic testing, the company canprotect itself from potentially large costs. As you cansee, a company’s policy based on its ethics may differfrom what others see as ethical.Science alone cannot answer bioethics questions.When these questions arise, we all need to weigh theissues for ourselves. As biotechnology continues toadvance, you will face new bioethics questionsthroughout your lifetime. Will you be ready?Questions to Consider• Should scientists do all <strong>of</strong> the things that technologyhas made it possible for them to do?• Who should decide how biotechnology is used?• Should scientific knowledge and personal beliefsplay equal or unequal roles in decisions aboutbiotechnology? at CLASSZONE.COM66 Unit 1: Introducing Biology

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