2 chemistry of life - weeblymschneidersci.weebly.com/.../30889339/bio_book_chapter_2.pdf ·...

CHAPTER 2 Chemistry of Life 27

Looking AheadQuick ReviewAnswer the following without referring toearlier sections of your book.

1. Identify seven properties of life. (Chapter 1,Section 1)

2. List seven themes of biology. (Chapter 1,Section 1)

3. Distinguish between metabolism andhomeostasis. (Chapter 1, Section 1)

Did you have difficulty? For help, review thesections indicated.

Section 1Nature of Matter

AtomsChemical Bonding

Section 2Water and Solutions

Water in Living ThingsAqueous Solutions

Section 3Chemistry of Cells

Carbon Compounds

Section 4Energy and Chemical Reactions

Energy for Life Processes Enzymes

www.scilinks.orgNational Science Teachers Association sciLINKS Internet resources are located throughout this chapter.

Reading ActivityBefore you read this chapter, write a short list of all the things you know about the chemistry of organisms. Then write a list of the things that you want to know about the chemistry oforganisms. Save your list, and to assess whatyou have learned, see how many questions you can answer after reading this chapter.

What do these horses have in common with the grassunder their feet? They and all other organisms arecomposed of chemical substances that include water,carbohydrates, proteins, and fats.

Chemistryof Life

CHAPTER

2

4B

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Section 1 Nature of Matter

AtomsCooking requires an understanding of how the ingredients in foodsinteract. A cook’s application of chemical principles while preparingrecipes affects the flavor and texture of foods. Just as a cook canbenefit from a knowledge of basic chemistry, you can better under-stand principles of biology if you also understand the fundamentalsof chemistry. Chemistry will help you learn about biology becauseorganisms, including yourself, are chemical machines.

What does all matter have in common? Matter consists of atoms.An is the smallest unit of matter that cannot be broken downby chemical means. Atoms are so small and dynamic that their exactstructure is difficult to determine. Scientists have developed models,such as the one shown in Figure 1, to explain the structure and prop-erties of atoms.

As shown in Figure 1, atoms consist of three kinds of particles:electrons, protons, and neutrons. Protons, shown in red, and neu-trons, shown in blue, make up the nucleus, or core, of an atom.The region around the nucleus that electrons may occupy at anytime is called the electron cloud, shown as a blue haze around thenucleus. Electrons are negatively charged, so the electron cloudhas a negative charge. Protons are positively charged and neutronshave no charge, so the nucleus has a positive charge. Because pro-tons and electrons are oppositely charged, they attract oneanother. Atoms typically have one electron for each proton, so theyhave no electrical charge.

ElementsAn is a pure substance made of only one kindof atom. There are more than 100 known elements,and each is represented by a one-, two-, or three-lettersymbol. For example, the elements hydrogen, oxygen,and carbon are represented by the symbols H, O, andC, respectively. Elements differ in the number ofprotons their atoms contain. Atoms of the simplest ele-ment, hydrogen, each contain one proton and oneelectron. In contrast, oxygen atoms contain eight pro-tons and eight electrons. The number of neutrons inan atom is often but not always equal to the number ofprotons in the atom. Atoms of an element that containdifferent numbers of neutrons are called isotopes. Forexample, three common isotopes of carbon, C, arecarbon-12, carbon-13, and carbon-14. Each containssix protons, however carbon-13 contains seven neu-trons, and carbon-14 contains eight neutrons.

element

atom

Objectives● Differentiate between

atoms and elements.

● Analyze how compounds areformed.

● Distinguish between cova-lent bonds, hydrogen bonds,and ionic bonds.

Key Terms

atomelementcompoundmoleculeion

Proton

Neutron

Electron cloud

28 CHAPTER 2 Chemistry of Life

Figure 1 Atom. Theelectron cloud is the regionof an atom where electronsare most likely to be found.The nucleus of this atomcontains six protons andsix neutrons.


Chemical BondingAtoms can join with other atoms to form stable substances. A forcethat joins atoms is called a chemical bond. A is a sub-stance made of the joined atoms of two or more different elements.For example, when sodium atoms, Na, bond with chlorine atoms, Cl,the compound sodium chloride (table salt) forms. Every compound isrepresented by a chemical formula that identifies the elements in thecompound and their proportions. The formula for sodium chloride,NaCl, shows that there is one sodium atom for every chlorine atom inthe compound.

Covalent BondsCovalent bonds form when two or more atoms share electrons toform a molecule. A , such as the water molecule shown inFigure 2, is a group of atoms held together by covalent bonds. Like therivets and welds that connect steel girders in a skyscraper, covalentbonds join the atoms in molecules. Because the number of protons isequal to the number of electrons in a molecule, the molecule has nonet electrical charge. Other examples of molecules include carbondioxide, CO2, and oxygen gas, O2.

The arrangement of their electrons determines how atoms bondtogether. Electrons are grouped into different levels. The levels clos-est to the nucleus have less energy than the levels farther from thenucleus. Electron levels can hold a limited number of electrons. Theouter electron levels of hydrogen and helium can hold up to twoelectrons. All other atoms, however, have outer electron levels thatcan hold up to eight electrons. An atom becomes stable when itsouter electron level is full. If the outer electron level is not full, anatom will react readily with atoms that can provide electrons to fillits outer level. As Figure 2 shows, water, H2O, forms when an oxygenatom, which has six outer electrons, combines with two hydrogenatoms, which have one outer electron each.

Hydrogen BondsThe electrons in a water molecule are sharedby oxygen and hydrogen atoms. However, theshared electrons are attracted more stronglyby the oxygen nucleus than by the hydrogennuclei. The water molecule therefore has par-tially positive and negative ends, or poles. Asshown in Figure 3, the partially positive endof one water molecule is attracted to the neg-ative end of another water molecule.Molecules with an unequal distribution ofelectrical charge, such as water molecules,are called polar molecules. This attractionbetween two water molecules is an exampleof a hydrogen bond—a weak chemical attrac-tion between polar molecules.

molecule

compound

SECTION 1 Nature of Matter 29

Figure 2 Water molecule.Each water molecule is heldtogether by covalent bondsbetween two hydrogen atomsand one oxygen atom.

Hydrogen bonds

H H

–

+ +

O

Water molecules are attracted to each other byhydrogen bonds.

Figure 3 Hydrogen bonds in water


Differentiate between atoms and elements.

Describe how an atom differs from a molecule.

Distinguish between covalent bonds and ionicbonds.

Identify the type of weak bond that formsbetween water molecules.

Critical Thinking Recognizing DifferencesExplain the difference between polar moleculesand nonpolar molecules. Give an example of apolar molecule.

Sodium chloride is an example of A a compound. C an isotope.B a molecule. D an ion.

TAKS Test PrepTAKS Test Prep

Section 1 Review

Ionic BondsSometimes atoms or molecules gain or lose electrons. An atom ormolecule that has gained or lost one or more electrons is called an

(IE ahn). Ions have an electrical charge because they contain anunequal number of electrons and protons. An atom that has lostelectrons is positively charged, whereas an atom that has gainedelectrons is negatively charged.

Ions of opposite charge may interact to form an ionic bond. Forexample, an atom of sodium is unstable because it has only one elec-tron in its outer level. Sodium readily gives up this electron tobecome a stable, positively charged sodium ion, Na+. An atom ofchlorine is also unstable because it has seven electrons in its outerlevel. Chlorine readily accepts an electron to become a stable, nega-tively charged chloride ion, Cl–. The negative charge of a chlorideion is attracted to the positive charge of a sodium ion. Thus, sodiumatoms and chlorine atoms readily form an ionic bond to becomesodium chloride, as shown in Figure 4.

ion


Real LifeCells contain water and are vulnerable tofreezing. Some species of fish canswim in icy water becausetheir bodies contain chem-icals similar to automotiveantifreeze. Analyzing Information Humans lack thesechemicals. Why then canwe tolerate short-termexposure to subzerotemperatures?

Chloride ion, Cl–

Sodium ion, Na+

Ionic bonds between sodium ions and

chloride ions

Salt crystal

Table salt (NaCl)

Ionic bonds in sodium chloride, NaCl, are formed by the interaction betweensodium ions, Na+, and chloride ions, Cl–.

Figure 4 Ionic bonds in sodium chloride


Water in Living ThingsYou may not realize it, but nearly 70 percent of your body is madeof water. About two-thirds of the molecules in your body are watermolecules. Your body’s cells are filled with water, and water is themedium in which most cellular events take place. Your cells are alsosurrounded by water, and water helps move nutrients and other sub-stances into and out of your cells. What are some of the propertiesof water that make it such an important substance for life?

Storage of Heat Water heats more slowly and retains heat longer than many othersubstances. For example, a pot of boiling water removed from astove takes a long time to cool down to room temperature. Manyorganisms release excess heat through water evaporation. For exam-ple, humans cool themselves by sweating. The water vapor lostthrough the evaporation of sweat carries heat away from the body. Inorganisms, this ability to control temperature enables cells to main-tain a constant internal temperature when the external temperaturechanges drastically. Water thus helps cells maintain homeostasis.

Cohesion and AdhesionThe hydrogen bonds between water molecules cause the cohesion ofliquid water. (koh HEE zhuhn) is an attraction betweensubstances of the same kind. Because of cohesion, water and otherliquids form thin films and drops, such as those shown in Figure 5.Molecules at the surface of water are linked together by hydrogenbonds like a crowd of people linked by holding hands. This attrac-tion between water molecules causes a condition known as surfacetension. Surface tension prevents the surface of water from stretch-ing or breaking easily.

Water molecules are also attracted to manyother similarly polar substances. (adHEE zhuhn) is an attraction between differentsubstances. Because of adhesion, some sub-stances get wet. Adhesion powers a process,called capillary action, in which water mol-ecules move upward through a narrow tube,such as the stem of a plant. The attraction ofwater to the walls of the tube sucks the waterup more strongly than gravity pulls it down.Water moves upward through a plant fromroots to leaves through a combination of cap-illary action, cohesion, and other factors.

Adhesion

Cohesion

Water and Solutions Section 2

Objectives● Analyze the properties of

water.

● Describe how waterdissolves substances.

● Distinguish between acidsand bases.

Key Terms

cohesionadhesionsolutionacidbase

Figure 5 Cohesion.Because of cohesion, waterforms drops like those on thisplant.

SECTION 2 Water and Solutions 31Copyright © by Holt, Rinehart and Winston. All rights reserved.

www.scilinks.orgTopic: Properties of WaterKeyword: HX4149


When sodium chloride, NaCl, is dissolved in water, sodium ions, Na+, andchloride ions, Cl–, become surrounded by water molecules, H2O.

Figure 6 Water dissolves ionic compounds

Chloride ion, Cl–

Sodium ion, Na+Water molecules, H2O

NaCl

Sodiumchloride,

NaCl

Aqueous SolutionsMany substances dissolve in water. For example, when you add saltto water, the resulting mixture is a saltwater solution. A isa mixture in which one or more substances are evenly distributed inanother substance. Many important substances in the body havebeen dissolved in blood or other aqueous fluids. Because these sub-stances can dissolve in water, they can more easily move within andbetween cells. For example, sugar could not be delivered to yourcells if it were not dissolved in water.

PolarityThe polarity of water enables many substances to dissolve in water.Ionic compounds and polar molecules dissolve best in water. Whenionic compounds are dissolved in water, the ions become sur-rounded by polar water molecules. As Figure 6 shows, ions areattracted to the ends of water molecules with the opposite charge.The resulting solution is a mixture of water molecules and ions. Asimilar attraction results when polar molecules are dissolved inwater. In both cases, the ions or molecules become evenly distrib-uted in the water.

Nonpolar molecules do not dissolve well in water. When nonpolarsubstances, such as oil, are placed in water, the water molecules aremore attracted to each other than to the nonpolar molecules. As aresult, the nonpolar molecules are shoved together. This explainswhy oil forms clumps or beads in water. The inability of nonpolarmolecules to dissolve in polar molecules is important to organisms.For example, the shape and function of cell membranes depend onthe interaction of polar water with nonpolar membrane molecules.

solution


Acids and BasesWhile the bonds in water molecules are strong, at any given time atiny fraction of those bonds might break, forming a hydrogen ion,H+, and a hydroxide ion, OH–:

H2O → H+ + OH–

As a result, pure water always has a low concentration of hydro-gen ions and hydroxide ions, which are present in equal numbers.Compounds that form hydrogen ions when dissolved in water arecalled . When an acid is added to water, the concentration ofhydrogen ions in the solution is increased above that of pure water.

In contrast, compounds that reduce the concentration of hydro-gen ions in a solution are called . Many bases form hydroxideions when dissolved in water. Such bases lower the concentration ofhydrogen ions because hydroxide ions react with hydrogen ions toform water molecules.

The pH scale shown in Figure 7 measures the concentration ofhydrogen ions in solutions. All solutions have a pH value between 0and 14. Pure water has a pH value of 7. Acidic solutions have pH val-ues below 7, and basic solutions have pH values above 7. Eachwhole number represents a factor of 10 on the scale. A solution witha pH value of 5, for example, has 10 times as many hydrogen ions asone with a pH value of 6.

bases

acids

SECTION 2 Water and Solutions 33

Reading EffectivelyAs you read, you mayencounter the terms alkalineor alkalinity. Basic solu-tions—whose pH is above7—are often called alkalinesolutions. Solutions with pH values below 7 areusually referred to as acidic solutions.

Figure 7 The pH scale.The pH scale measures theconcentration of hydrogen ionsin a solution.

Neutral More basicMore acidic

0pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Lemon Milk AntacidVinegarHand soap

Householdammonia

Section 2 Review

Distinguish between adhesion and cohesion.

Identify a substance that would not dissolve wellin water. Explain why.

Differentiate between acids and bases.

Critical Thinking Inferring RelationshipsWhen salt is added to water, the freezing point ofthe water decreases. Explain why this occurs.

The pH of solution A is 2. ThepH of solution B is 4. How does the concentrationof hydrogen ions in A ([H+]A) compare with theconcentration of hydrogen ions in B ([H+]B)?A [H+]A � 2 � [H+]B. C [H+]A � 100 � [H+]B. B [H+]B � 2 � [H+]A. D [H+]B � 100 � [H+]A.



Glucose(monosaccharide)

A potato contains many granules that are filled with starch.

Starch contains hundreds oflinked glucose molecules.

Starch (polysaccharide)

Section 3 Chemistry of Cells


Carbon Compounds Most matter in your body that is not water is made of organiccompounds. Organic compounds contain carbon atoms that arecovalently bonded to other elements—typically hydrogen, oxygen,and other carbon atoms. Four principal classes of organic com-pounds are found in living things: carbohydrates, lipids, proteins, andnucleic acids. Without these compounds, cells could not function.

Carbohydratesare organic compounds made of carbon, hydrogen,

and oxygen atoms in the proportion of 1:2:1. Carbohydrates are akey source of energy, and they are found in most foods—especiallyfruits, vegetables, and grains. The building blocks of carbohydratesare single sugars, called (mahn oh SAK uhreyedz), such as glucose, C6H12O6, and fructose. Simple sugars suchas glucose are a major source of energy in cells. Disaccharides aredouble sugars formed when two monosaccharides are joined. Forexample, sucrose, or common table sugar, consists of both glucoseand fructose. Polysaccharides such as starch, shown in Figure 8, arechains of three or more monosaccharides. A polysaccharide is anexample of a macromolecule, a large molecule made of manysmaller molecules.

In organisms, some polysaccharides function as storehouses of theenergy contained in sugars. Two polysaccharides that store energy inthis way are starch, which is made by plants, and glycogen, which ismade by animals. Both starch and glycogen are made of hundreds oflinked glucose molecules. Cellulose is a polysaccharide that providesstructural support for plants. Humans cannot digest cellulose. Thus,you cannot digest wood, which is mostly cellulose.

monosaccharides

Carbohydrates

Objectives● Summarize the characteris-

tics of organic compounds.

● Compare the structures andfunction of different types ofbiomolecules.

● Describe the components ofDNA and RNA.

● State the main role of ATP in cells.

Key Terms

carbohydratemonosaccharidelipidproteinamino acidnucleic acidnucleotideDNARNAATP

Figure 8 Structure ofpolysaccharides. Starch is along chain of many linkedglucose molecules.

C O

C C

CCH

OH H

H OH

CH 2OH

H

OH

OH

H

9A

6A

9A


Fatty acids can be saturated or unsaturated.

Figure 9 Structure of fats

Lipids(LIHP ihdz) are nonpolar molecules that are not soluble in

water. They include fats, phospholipids, steroids, and waxes. Lipidsare an important part of the structure and functioning of cell mem-branes. Phospholipids make up the lipid bilayer of cell membranes.Steroids include cholesterol, which is found in animal cell mem-branes. Other lipids include some light-absorbing compounds calledpigments, such as the plant pigment chlorophyll.

Fats are lipids that store energy. As Figure 9 shows, a typical fatcontains three fatty acids bonded to a glycerol molecule backbone.Glycerol is a three-carbon organic molecule. A fatty acid is a longchain of carbon atoms, shown in green, with hydrogen atoms bondedto them. Most carbon atoms in a fatty acid are bonded to either oneor two hydrogen atoms, shown in blue. Because bonds between car-bon and hydrogen are rich in energy, fats can store a lot of energy.

In a saturated fatty acid, all of the carbon atoms in the chain arebonded to two hydrogen atoms (except the carbon atom on the end,which is bonded to three hydrogen atoms). Most animal fats—suchas those in butter, lard, and grease from cooked meats—containprimarily saturated fatty acids. Saturated fatty acids are straightmolecules and are generally solid at room temperature.

In an unsaturated fatty acid, some of the carbon atoms are linkedby a “double” covalent bond, each with only one hydrogen atom,producing kinks in the molecule, as shown in Figure 9. Most plantoils, such as olive oil, and some fish oils contain mainly unsaturatedfatty acids and are generally liquid at room temperature.Hydrogenated vegetable oils contain naturally unsaturated fattyacids that have been saturated artificially by the addition ofhydrogen atoms. Thus, hydrogenated vegetable oils, such as those inmargarine and vegetable shortening, are generally solid at roomtemperature.

Lipids

SECTION 3 Chemistry of Cells 35

Saturated fats, such asbutter, are solid at roomtemperature.

Saturated fatty acid

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C H

H

O

CHO

Unsaturated fatty acid

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

C

H

H

C

H

C

H

C

H

H

C

H

C

H

H

C

H

H

C H

H

C

H

H

C

O

CHO

Unsaturated fats, such asolive oil, are liquid at roomtemperature.

Real LifeFat-free potato chipsfried in artificial fatscontain fewer caloriesthan those fried innatural fats. Unfortunately, someartificial fats may reducevitamin absorption andcause indigestion in somepeople.Finding Information Research the benefits andpotential shortcomings ofartificial fats. 3B



ProteinsA (PROH teen) is a large molecule formed bylinked smaller molecules called amino acids.

are the building blocks of proteins. Twenty dif-ferent amino acids are found in proteins. Some aminoacids are polar, and others are nonpolar. Some aminoacids are electrically charged, and others are notcharged. As Figure 10 shows, proteins tend to fold intocompact shapes determined by how the protein’samino acids interact with water and one another.

Some proteins are enzymes and promote chemicalreactions. Other proteins have important structuralfunctions. For example, the protein collagen (KAHL uhjuhn) is found in skin, ligaments, tendons, and bones.Your hair and muscles contain structural proteins andso do the fibers of a blood clot. Other proteins calledantibodies help your body defend against infection.Specialized proteins in muscles enable your musclesto contract. In your blood, a protein called hemoglobincarries oxygen from your lungs to body tissues.

acidsAmino

protein

Globular protein

Linked aminoacids

Proteins are chains of amino acids folded intocompact shapes.

Figure 10 Structure of proteins

Foods as Fuel

Most foods contain a mixtureof carbohydrates, proteins,

and fats. The body can use thesemolecules to build new tissues,but it uses them mostly as anenergy source. Your body’s cellsharvest the energy in food mol-ecules for metabolism. Theenergy value of food molecules ismeasured in kilocalories (kcal).

The minimal rate of energy useper hour (h), called the basalmetabolic rate, is about 70 kcal/hfor men and 60 kcal/h for women.Typically, walking uses about 200kcal/h and jogging uses about600 kcal/h. If more kilocaloriesare consumed than are used, thebody will store the excess kilo-calories as fat, regardless ofwhether the consumed kilocalo-ries are contained in carbohy-drates, proteins, or fats.

CarbohydratesMost carbohydrates in foodscome from plant products, suchas fruits, grains, and vegetables.Other sources are milk, whichcontains the sugar lactose, andvarious meats, which containsome glycogen. Candy and softdrinks also contain sugars. About4 kcal of energy is supplied by 1gram (g) of carbohydrates.

ProteinsPrimary sources of dietary pro-tein include legumes, eggs, milk,fish, poultry, and meat. As withcarbohydrates, proteins supplyabout 4 kcal/g. Dietary protein isan important source of aminoacids. Proteins also provide rawmaterials for other compounds,such as nucleic acids.

FatsFats are found mainly in veg-etable oils, such as olive oil; dairyproducts, such as milk and but-ter; and meat, such as beef andpork. Fats contain more energyper gram than do carbohydratesand proteins; fats supply about9.5 kcal/g of energy.

www.scilinks.orgTopic: Foods as FuelKeyword: HX4086


SECTION 3 Chemistry of Cells 37

P

P PP

P

P

P

P

P

P

P

PP

P

P

P

P

PP

P

P

P

Phosphategroup

P

Base

Sugar

Nucleotide

DNA is made of two strands of multiple, linked nucleotides.

Figure 11 Structure of nucleic acids

Nucleic Acids All of your cells contain nucleic acids. A is a long chainof smaller molecules called nucleotides. A has threeparts: a sugar, a base, and a phosphate group, which contains phos-phorus and oxygen atoms. There are two types of nucleic acids—DNA and RNA—and each type contains four kinds of nucleotides.

, or deoxyribonucleic acid, consists of two strands ofnucleotides that spiral around each other, as shown in Figure 11.Chromosomes contain long strands of DNA, which stores heredi-tary information.

, or ribonucleic acid, consists of a single strand of nucleo-tides. RNA plays many key roles in the manufacture of proteins.RNA can also act as an enzyme, promoting the chemical reactionsthat link amino acids to form proteins.

ATPAnother important biological molecule is ATP. , or adenosine (uhDEHN uh seen) triphosphate, is a single nucleotide with two extraenergy-storing phosphate groups. When food molecules are brokendown inside cells, some of the energy in the molecules is storedtemporarily in ATP. Cells need a steady supply of ATP to function.

ATP

RNA

DNA

nucleotidenucleic acid

Section 3 Review

Identify what all organic compounds have incommon, and list the four principal classes oforganic compounds.

Compare the structures of saturated andunsaturated lipids. 9A

Describe the three parts of a nucleotide andhow they are attached to one another. 6A

Critical Thinking Inferring RelationshipsCompare the role of ATP in cells with the roles of RNA. 9A

Molecule X contains a sugarand a phosphate group. What is molecule X? 9A

A a carbohydrate C a fatty acidB a nucleotide D an amino acid




Section 4 Energy and ChemicalReactions

Energy for Life ProcessesYou are surrounded by energy. Energy is in food, in the motion of aspeeding car, in the sound of a guitar, and in the warmth of a blazingfire. is the ability to move or change matter. Energy exists inmany forms—including light, heat, chemical energy, mechanicalenergy, and electrical energy—and it can be converted from one formto another. If you kick a ball, for example, the energy of your kickmakes the ball move. If you cook an egg in a hot skillet, heat causesthe egg to change color and solidify, as shown in Figure 12. Theenergy transferred to the egg by heat rearranges the atoms and mol-ecules in the egg. The bioluminescent click beetle, also shown inFigure 12, uses energy to produce light.

Energy can be stored or released by chemical reactions. A chem-ical reaction is a process during which chemical bonds betweenatoms are broken and new ones are formed, producing one or moredifferent substances. At any moment, thousands of chemical reac-tions are occurring in every cell of your body. The starting materialsfor chemical reactions are called reactants. The newly formed sub-stances are called products. Chemical reactions are summarized bychemical equations, which are written in the following form:

Reactants → Products

The arrow is read as “changes to” or “forms.” For example, dissolv-ing sodium chloride in water causes the following reaction:

NaCl → Na+ + Cl–

Energy

Objectives● Evaluate the importance

of energy to living things.

● Relate energy and chemicalreactions.

● Describe the role ofenzymes in chemicalreactions.

● Identify the effect of enzymeson food molecules.

Key Terms

energyactivation energyenzymesubstrate active site

An egg becomes solid when it is heated. A chemical reaction causes the bioluminescent click beetle, Pyrophorusnoctilucus, to give off light energy.

Figure 12 Evidence of chemical reactions

9C

9C

12A

4B


Energy in Chemical Reactions In chemical reactions, energy is absorbed or released when chemi-cal bonds are broken and new ones are formed. The graphs shownin Figure 13 compare a chemical reaction that releases energy witha chemical reaction that absorbs energy. The freezing and melting ofwater are good examples of how energy is released or absorbed dur-ing chemical reactions. When water freezes, the process that leadsto the formation of ice crystals causes heat energy to be released.When you fill an ice-cube tray with water and place it in the freezerto make ice, heat is released from the water as the water freezes.When you remove ice cubes from the freezer, the ice begins to melt.When ice melts, it absorbs heat from the environment. When youhold a piece of ice, your hand gets cold and heat is transferred fromyour hand to the ice as the ice begins to melt.

Metabolism (muh TAB uh lihz uhm) is the term used to describeall of the chemical reactions that occur within an organism. Yourcells get most of the energy needed for metabolism from the food youeat. As food is digested, chemical reactions convert the chemicalenergy in food molecules to forms of energy that can be used by cells.

Activation EnergyThe heat from a flame transfers enough energy to ignite the logs in acampfire. The spark from a spark plug causes the gasoline in an auto-mobile engine to ignite. In both cases, energy is needed to start achemical reaction. The energy needed to start a chemical reaction iscalled . To better understand activation energy,think of rolling a boulder down a hill. To get the boulder rolling down-hill, you must first push it. Activation energy is simply a chemical“push” that starts a chemical reaction. Even in a chemical reactionthat releases energy, activation energy must be supplied before thereaction can occur.

activation energy

SECTION 4 Energy and Chemical Reactions 39

Reactants

Products

Energyreleased

Energ

y

Reaction progress

Reactants

Products

Energyabsorbed

Energ

yReaction progress

Energy-Releasing Reaction Energy-Absorbing Reaction

Chemical reactions absorb or release energy.

Figure 13 Energy and chemical reactions

www.scilinks.orgTopic: Chemical ReactionsKeyword: HX4040


EnzymesLike engines, cells consume fuel because they need energy to func-tion. Just as an engine requires a spark of energy to begin burninggasoline, most biochemical reactions—chemical reactions thatoccur in cells—require activation energy to begin. The chemicalreactions in cells occur quickly and at relatively low temperaturesbecause of the action of many enzymes. are substancesthat increase the speed of chemical reactions. Most enzymes areproteins. Enzymes are catalysts (KAT uh lists), which are substancesthat reduce the activation energy of a chemical reaction. As shownin Figure 14, an enzyme increases the speed of a chemical reactionby reducing the activation energy of the reaction.

Enzymes help organisms maintain homeostasis. Withoutenzymes, chemical reactions would not occur quickly enough tosustain life. For example, consider a reaction that takes place inyour blood. Blood carries carbon dioxide, CO2, (a waste productmade by cells) to your lungs, where it is eliminated as you breatheout. In the lungs, carbon dioxide reacts with water, H2O, to formcarbonic acid, H2CO3, as shown by the following equation:

carbonic anhydraseCO2 � H2O → H2CO3←

The reverse reaction occurs in your lungs, converting carbonic acidback to carbon dioxide and water. Most enzyme-assisted reactionsare reversible, meaning they can proceed in the opposite direction.

Without an enzyme, the reaction that produces carbonic acid isvery slow; only about 2000 molecules of carbonic acid are producedin an hour. This rate is not fast enough for your blood to carry awaythe carbon dioxide released by millions of cells. Fortunately, yourblood contains the enzyme carbonic anhydrase (an HIED rays). Inthe presence of carbonic anhydrase, carbon dioxide and water reactto form about 600,000 molecules of carbonic acid per second! Theenzyme increases the reaction rate about one million times,enabling your body to eliminate carbon dioxide efficiently.

Enzymes


Reactants

Products

Activation energy without an enzyme

Activation energywith an enzyme

Energ

yre

lease

dE

nerg

yabso

rbed

Reaction progress

Activation Energy With and Without an EnzymeFigure 14 Enzymes loweractivation energy. Enzymesdecrease the amount of energyneeded to start a chemicalreaction. Enzymes do notchange the amount of energycontained in either the reac-tants or the products.

Reading EffectivelyAs you read, notice that thenames of most enzymes,such as amylase and cata-lase, end with -ase. This willhelp you identify otherenzymes you will encounterin this book.


BIOgraphic

A substrate attaches to an enzyme's active site.

Enzymes assist biochemical reactions by bringing key molecules together.

Enzyme Action

1 The enzyme reduces the activa-tion energy of the reaction.

2 The enzyme is not changed by the reaction.

3

Activesite

Substrate

Enzyme

Products

Figure 15

Enzyme SpecificityA substance on which an enzyme acts during a chemical reaction iscalled a (SUHB strayt). Enzymes act only on specific sub-strates. For example, the enzyme amylase (AM uh lays) assists in thebreakdown of starch to glucose in the following chemical reaction.In this reaction, starch is amylase’s substrate.

amylasestarch → glucose←

The enzyme catalase (KAT uh lays) assists in the breakdown ofhydrogen peroxide, H2O2, a toxin formed in cells. In this case,hydrogen peroxide is broken down to water, H2O, and oxygen gas,O2. In this reaction, hydrogen peroxide is catalase’s substrate.

catalase2H2O2

→ 2H2O � O2←

An enzyme’s shape determines its activity. Typically, an enzyme isa large protein with one or more deep folds on its surface. Thesefolds form pockets called . As shown in Figure 15, anenzyme’s substrate fits into the active site. An enzyme acts only on aspecific substrate because only that substrate fits into its active site.

Step When an enzyme first attaches to a substrate during achemical reaction, the enzyme’s shape changes slightly sothat the substrate fits more tightly in the enzyme’s active site.

Step At an active site, an enzyme and a substrate interact in away that reduces the activation energy of the reaction, mak-ing the substrate more likely to react.

Step The reaction is complete when products have formed. Theenzyme is now free to catalyze further reactions.

active sites

substrate

SECTION 4 Energy and Chemical Reactions 41

Real LifeSome laundry detergentscontain enzymes.Enzymes break downcarbohydrates or proteinsin common stains such asfood and blood, makingthe stains easier toremove. The enzymes can tolerate the hightemperatures and alkalineconditions required forcleaning fabrics.Finding InformationFind out the source forenzymes used in laundrydetergents. 9C


www.scilinks.orgTopic: Enzyme ActivityKeyword: HX4071


Analysis

1. Name the enzyme thatworks best in highly acidicenvironments.

2. Name the enzyme thatworks best in less-acidicenvironments.

3. Critical ThinkingAnalyzing Data Identify thepH value at which trypsinworks best.

4. Critical ThinkingAnalyzing Data Identifythe pH value at which pepsinworks best.

5. Critical ThinkingInferring RelationshipsWhat does the graph indicateabout the relative acidity of thestomach and small intestine?

Analyzing the Effect of pH on Enzyme ActivityBackground

The graph at right shows the relationship betweenpH and the activity of two digestive enzymes,pepsin and trypsin. Pepsin works in the stomach,while trypsin works in the small intestine. Use thegraph to answer the following questions.

010001011001110101000100100010011100100100010000010100100111010101001000101010010010

1 2

PepsinTrypsin

3 4 5 6 7 8 9

Rate

of

reacti

on

pH

Enzymes and pH

List three ways that organisms use energy. 12A

Summarize how energy is made available bychemical reactions. 4B

Explain how enzymes increase the speed ofchemical reactions. 9C

Describe how the enzyme amylase affects starch.

Critical Thinking Predicting Outcomes Whateffect might a molecule that interferes with theaction of carbonic anhydrase have on your body?

Carboxypeptidase is an enzymethat catalyzes reactions in the small intestine. Theproducts of these reactions are amino acids. Whatare the substrates of carboxypeptidase? 9C

A proteins C lipidsB carbohydrates D nucleic acids


Section 4 Review

Factors in Enzyme ActivityAny factor that changes the shape of an enzyme can affect theenzyme’s activity. For example, enzymes operate most efficientlywithin a certain range of temperatures. Temperatures outside thisrange can either break or strengthen some of the enzyme’s bonds,changing its shape. Moreover, each enzyme operates best within acertain range of pH values. A pH value outside this range can causebonds in an enzyme to break, reducing the enzyme’s effectiveness.

The enzymes that are active at any one time in a cell determine whathappens in that cell. Your body’s cells contain many different enzymes,and each enzyme catalyzes a different chemical reaction. Differentkinds of cells contain different collections of enzymes. For example, asyou read this page, the chemical reactions occurring in nerve cells inyour eye are different from the chemical reactions occurring in yourred blood cells.

9C

9D

2C 9C


CHAPTER 2 Highlights 43

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONEKey Terms

Section 1atom (28)element (28)compound (29)molecule (29)ion (30)

Section 4energy (38)activation energy (39)enzyme (40)substrate (41)active site (41)

Section 2cohesion (31)adhesion (31)solution (32)acid (33)base (33)

Section 3carbohydrate (34)monosaccharide (34)lipid (35)protein (36)amino acid (36)nucleic acid (37)nucleotide (37)DNA (37)RNA (37)ATP (37)

Nature of Matter● All matter is made of atoms. Atoms consist of electrons,

protons, and neutrons.● Molecules are groups of atoms linked by covalent bonds.● Hydrogen bonding occurs between polar molecules.● An ion is a charged atom or molecule. Ions of opposite

charge may form an ionic bond.

Water and Solutions● Water, which is essential for life, stores heat efficiently

and binds to itself and other substances.● Water dissolves polar molecules and ionic compounds.● Acids increase the hydrogen ion concentration of

a solution. ● Bases decrease the hydrogen ion concentration of

a solution. ● The pH scale measures the strength of acids and bases.

Chemistry of Cells● Organic compounds are found in living things.● Carbohydrates, such as glucose, are a source of energy

and are used as structural materials in organisms.● Lipids are nonpolar molecules that store energy and are

an important part of cell membranes.● Proteins are chains of amino acids. The sequence of

amino acids determines a protein’s shape and specific function.

● Nucleic acids store and transmit hereditary information. ● ATP is the main energy currency of cells.

Energy and Chemical Reactions● Chemical reactions absorb or release energy.● Starting a chemical reaction requires activation energy.● Enzymes speed up chemical reactions by decreasing

the activation energy of the reactions. ● Enzymes bind only certain substrates. ● Factors such as temperature and pH affect

enzyme activity.

4

3

2

1


Using Key Terms1. Atoms share electrons in a(n) _____ bond.

a. covalent c. ionicb. hydrogen d. nuclear

2. A weak attraction between polar moleculesis called a(n) _____ bond.a. compound c. ionicb. covalent d. hydrogen

3. A substance that forms hydrogen ionswhen dissolved in water is called a(n)a. atom. c. base.b. acid. d. carbohydrate.

4. The energy needed to start a chemicalreaction is called _____ energy.a. cohesive c. activationb. adhesive d. ionic

5. Write a sentence that shows your under-standing of each of the following terms:carbohydrate, lipid, nucleic acid, and protein.

Understanding Key Ideas6. Most atoms contain one or more

a. neutrons. c. protons.b. electrons. d. All of the above

7. Water dissolves ionic compounds becausewater moleculesa. are nonpolar.b. have a pH value of 14 or greater.c. have partially charged ends.d. do not contain atoms.

8. In cells, ATP temporarily storesa. amino acids. c. energy.b. DNA. d. lipids.

9. Energy needed for metabolism does notcome froma. food.b. lipids.c. carbohydrates.d. water.

10. Most enzymes area. lipids.b. carbohydrates.c. proteins.d. nucleic acids.

11. Explain the relationship between an enzymeand the activation energy of the reaction inwhich the enzyme participates.

12. Look at the water strider in the photographbelow. Using what you have learned aboutthe properties of water, explain how theinsect can stand on the water’s surface.

13. Ruby-throated humming-birds migrate 2,000 km every fall. Beforemigrating, they eat nectar and convertmuch of the sugar in the nectar to fat. Whyis it advantageous for these birds to storeenergy as fat rather than as glycogen?

14. Describe how molecules such as carbohy-drates and lipids are important in home-ostasis. (Hint: See Chapter 1, Section 1.)

15. Concept Mapping Make a conceptmap that illustrates the structure of matter.Include the following key terms in yourmap: atom, element, compound, molecule,and ion.

PerformanceZONE

44 CHAPTER 2 Review

CHAPTER REVIEW

9A

9A 9C

9A

9A 12A

9A

2C 3E

9A


2 chemistry of life - weeblymschneidersci.weebly.com/.../30889339/bio_book_chapter_2.pdf ·...

Documents