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Prentice Hall Biology

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Chapter 2 The Chemistry of Life

The beautiful feathers of this great egret are made up of protein. Proteins are one of the main groups of carbon compounds found inliving things.

Do large and small molecules behave exactly alike?

Procedure

1. Label six test tubes as follows: tofu, soy sauce, butter, soap, starch, and sugar. Place a tiny amount of each sample inthe appropriate test tube.

2. Half-fill each test tube with water. Stopper the test tubes. Shake each test tube for 2 minutes. Record yourobservations of each test tube.

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Think About It

1. Observing Which substances dissolved easily in water?2. Drawing Conclusions Tofu, starch, and butter consist mostly of large molecules (protein, starch, and fat,

respectively). Soy sauce, sugar, and soap contain smaller molecules that are related to the large molecules. Are thelarger molecules more or less soluble than the smaller molecules?

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Key Concepts

What three subatomicparticles make up atoms?

How are all of the isotopesof an element similar?

What are the two maintypes of chemical bonds?

Vocabulary atom nucleus electron element isotope compound chemical bond ionic bond ion covalent bond molecule van der Waals forcesReading Strategy: UsingPrior Knowledge Before youread, write down what youalready know about atoms,elements, and compounds. Asyou read, note the main newconcepts you learn.

21 The Nature of Matter

Life depends on chemistry. When you eat food or inhale oxygen, your bodyuses these materials in chemical reactions that keep you alive. Just asbuildings are made from bricks, steel, glass, and wood, living things are madefrom chemical compounds. If the first task of an architect is to understandbuilding materials, then the first job of a biologist is to understand thechemistry of life.

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21 The Nature of Matter (continued)Atoms

The study of chemistry begins with the basic unit of matter, the atom. The Greek wordatomos, which means unable to be cut, was first used to refer to matter by the Greekphilosopher Democritus nearly 2500 years ago. Democritus asked a simple question: Ifyou take an object like a stick of chalk and break it in half, are both halves still chalk?The answer, of course, is yes. But what happens if you go on? Suppose you break it inhalf again and again and again. Can you continue to divide without limit, or does therecome a point at which you cannot divide the fragment of chalk without changing itinto something else? Democritus thought that there had to be a limit. He called thesmallest fragment the atom, a name scientists still use today.

Atoms are incredibly small. Placed side by side, 100 million atoms would make a rowonly about 1 centimeter longabout the width of your little finger! Despite itsextremely small size, an atom contains subatomic particles that are even smaller.

The figure below shows the subatomic particles in a helium atom. The subatomicparticles that make up atoms are protons, neutrons, and electrons. Protons andneutrons have about the same mass. However, protons are positively charged particles(+) and neutrons carry no charge. Their name is a reminder that they are neutralparticles. Strong forces bind protons and neutrons together to form the nucleus, whichis at the center of the atom.

Helium Atom Helium atoms contain protons, neutrons, and electrons. The positively charged protons anduncharged neutrons are bound together in the dense nucleus, while the negatively charged electrons move in the spacearound the nucleus.

The electron is a negatively charged particle () with 1/1840 the mass of a proton.Electrons are in constant motion in the space surrounding the nucleus. They areattracted to the positively charged nucleus but remain outside the nucleus because ofthe energy of their motion. Because atoms have equal numbers of electrons andprotons, and because these subatomic particles have equal but opposite charges, atomsare neutral.

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21 The Nature of Matter (continued)Elements and Isotopes

A chemical element is a pure substance that consists entirely of one type of atom.More than 100 elements are known, but only about two dozen are commonly found inliving organisms. Elements are represented by a one- or two-letter symbol. C, forexample, stands for carbon, H for hydrogen, and Na for sodium. The number ofprotons in an atom of an element is the element's atomic number. Carbon's atomicnumber is 6, meaning that each atom of carbon has six protons and, consequently, sixelectrons. See Appendix G, The Periodic Table, which shows the elements.

Isotopes Atoms of an element can have different numbers of neutrons. For example,some atoms of carbon have six neutrons, some have seven, and a few have eight.Atoms of the same element that differ in the number of neutrons they contain areknown as isotopes. The sum of the protons and neutrons in the nucleus of an atom iscalled its mass number. Isotopes are identified by their mass numbers. The figure atright shows the subatomic composition of carbon-12, carbon-13, and carbon-14atoms. The weighted average of the masses of an element's isotopes is called itsatomic mass. Weighted means that the abundance of each isotope in nature isconsidered when the average is calculated. Because they have the same numberof electrons, all isotopes of an element have the same chemical properties.

Isotopes of Carbon

Radioactive Isotopes Some isotopes are radioactive, meaning that their nuclei areunstable and break down at a constant rate over time. The radiation these isotopes giveoff can be dangerous, but radioactive isotopes have a number of important scientificand practical uses.

Geologists can determine the ages of rocks and fossils by analyzing the isotopes foundin them. Radiation from certain isotopes can be used to treat cancer and to killbacteria that cause food to spoil. Radioactive isotopes can also be used as labels ortracers to follow the movements of substances within organisms.

Forensic ScientistJob description: work as a forensic scientist for local,state, or federal investigative agencies in order to conductscientific forensic examinations in criminal investigations

Education: a bachelor's degree in sciencebiology, physics,chemistry, metallurgy; some states require several years offorensic laboratory experience

Skills: analytical, logical, computer literate, detail oriented,able to take meticulous notes and to prepare evidence forpresentation in court as well as to testify as an expert witness

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Highlights: have the opportunity to use logic and science tosolve unique or unusual problems in criminal investigationsand to work collaboratively with other scientists

For: Career links

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21 The Nature of Matter (continued)Chemical Compounds

In nature, most elements are found combined with other elements in compounds. Achemical compound is a substance formed by the chemical combination of two ormore elements in definite proportions. Scientists show the composition of compoundsby a kind of shorthand known as a chemical formula. Water, which contains twoatoms of hydrogen for each atom of oxygen, has the chemical formula H2O. Theformula for table salt, NaCl, indicates that the elements from which table salt formssodium and chlorinecombine in a 1 : 1 ratio.

The physical and chemical properties of a compound are usually very different fromthose of the elements from which it is formed. For example, hydrogen and oxygen,which are gases at room temperature, can combine explosively and form liquid water.Sodium is a silver-colored metal that is soft enough to cut with a knife. It reactsexplosively with cold water. Chlorine is very reactive, too. It is a poisonous, greenishgas that was used to kill many soldiers in World War I. Sodium and chlorine combineto form sodium chloride (NaCl), or table salt. Sodium chloride is a white solid thatdissolves easily in water. As you know, sodium chloride is not poisonous. In fact, it isessential for the survival of most living things.

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21 The Nature of Matter (continued)Chemical Bonds

The atoms in compounds are held together by chemical bonds. Much of chemistry isdevoted to understanding how and when chemical bonds form. Bond formationinvolves the electrons that surround each atomic nucleus. The electrons that areavailable to form bonds are called valence electrons. The main types of chemicalbonds are ionic bonds and covalent bonds.

Ionic Bonds An ionic bond is formed when one or more electrons are transferredfrom one atom to another. Recall that atoms are electrically neutral because they haveequal numbers of protons and electrons. An atom that loses electrons has a positivecharge. An atom that gains electrons has a negative charge. These positively andnegatively charged atoms are known as ions.

Energy Levels and IonicBonding

The figure at right shows how ionic bonds form between sodium and chlorine in tablesalt. A sodium atom easily loses its one valence electron and becomes a sodium ion(Na+). A chlorine atom easily gains an electron and becomes a chloride ion (Cl). In asalt crystal, there are trillions of sodium and chloride ions. These oppositely chargedions have a strong attraction. The attraction between oppositely charged ions is anionic bond.

Ionic Bonding

Covalent Bonds Sometimes electrons are shared by atoms instead of beingtransferred. What does it mean to share electrons? It means that the movingelectrons actually travel in the orbitals of both atoms. A covalent bond forms whenelectrons are shared between atoms. When the atoms share two electrons, the bond iscalled a single covalent bond. Sometimes the atoms share four electrons and form adouble bond. In a few cases, atoms can share six electrons and form a triple bond.

Covalent Bonding

The structure that results when atoms are joined together by covalent bonds is called amolecule. The molecule is the smallest unit of most compounds. The diagram, below,of a water molecule shows that each hydrogen atom forms a single covalent bond withthe oxygen atom.

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Covalent Bonding The chemical bond in which electrons are shared between atoms is called acovalent bond. In a water molecule, each hydrogen atom shares two electrons with the oxygen atom.

Van der Waals Forces Because of their structures, atoms of different elements donot all have the same ability to attract electrons. Some atoms have a stronger attractionfor electrons than do other atoms. Therefore, when the atoms in a covalent bond shareelectrons, the sharing is not always equal. Even when the sharing is equal, the rapidmovement of electrons can create regions on a molecule that have a tiny positive ornegative charge.

When molecules are close together, a slight attraction can develop between theoppositely charged regions of nearby molecules. Chemists call such intermolecularforces of attraction van der Waals forces, after the scientist who discovered them.Although van der Waals forces are not as strong as ionic bonds or covalent bonds,they can hold molecules together, especially when the molecules are large.

People who keep geckos as pets have already seen van der Waals forces in action.These remarkable little lizards can climb up vertical surfaces, even smooth glass walls,and then hang on by a single toe despite the pull of gravity. How do they do it? No,they do not have some sort of glue on their feet and they don't have suction cups.

A gecko foot like the one shown at right is covered by as many as half a million tinyhairlike projections. Each projection is further divided into hundreds of tiny, flat-surfaced fibers. This design allows the gecko's foot to come in contact with anextremely large area of the wall at the molecular level. Van der Waals forces formbetween molecules on the surface of the gecko's foot and molecules on the surface ofthe wall. The combined strength of all the van der Waals forces allows the gecko tobalance the pull of gravity. When the gecko needs to move its foot, it peels the foot offat an angle and reattaches it at another location on the wall.

Van der Waals Forces

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21 The Nature of Matter

1. Key Concept Describe the structure of an atom. 2. Key Concept Why do all isotopes of an element have the same chemical properties? In what way do

isotopes of an element differ? 3. Key Concept What is a covalent bond? An ionic bond? 4. What is a compound? How are compounds related to molecules? 5. How do van der Waals forces hold molecules together? 6. Critical Thinking Comparing and Contrasting How are ionic bonds and van der Waals forces similar?

How are they different?

Writing an ArticleWrite an article for your school newspaper on forensic science as a career. Assume that you have alreadyinterviewed a forensic scientist who works for a law enforcement agency. The article should be about 500 wordslong. Hint: Consider the interests of your readers.

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Key Concepts

Why are water moleculespolar?

What are acidicsolutions? What are basicsolutions?

Vocabulary cohesion adhesion mixture solution solute solvent suspension pH scale acid base bufferReading Strategy: UsingVisuals Before you read,preview the figuresHydrogen Bonds and NaClDissolving in Water. As youread, note how these twofigures are related.

22 Properties of Water

After several days in space, one of the first astronauts to travel to the moonlooked back longingly at Earth and marveled at its distant beauty. If there areother beings who have seen Earth, he said, they must surely call it the blueplanet. The astronaut was referring to the blue appearance of the water in theoceans, which cover three fourths of Earth's surface. Water is also the singlemost abundant compound in most living things.

Water is one of the few compounds that is a liquid at the temperatures foundover much of Earth's surface. Unlike most substances, water expands as itfreezes. Thus, ice is less dense than liquid water, which explains why ice floatson the surface of lakes and rivers. If the ice sank to the bottom, the situationwould be disastrous for fish and plant life in regions with cold winters, to saynothing of the sport of ice skating!

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22 Properties of Water (continued)The Water Molecule

Like all molecules, a water molecule (H2O) is neutral. The positive charges on its 10protons balance out the negative charges on its 10 electrons. However, there is more tothe story.

For: Links on properties ofwaterVisit: www.SciLinks.orgWeb Code: cbn-1022

Polarity With 8 protons in its nucleus, an oxygen atom has a much stronger attractionfor electrons than does the hydrogen atom with a single proton in its nucleus. Thus, atany moment, there is a greater probability of finding the shared electrons near theoxygen atom than near the hydrogen atom. Because the water molecule has a bentshape, as shown in the figure below, the oxygen atom is on one end of the moleculeand the hydrogen atoms are on the other. As a result, the oxygen end of the moleculehas a slight negative charge and the hydrogen end of the molecule has a slight positivecharge.

A Water Molecule The unequal sharing of electrons causes a water molecule to be polar. Thehydrogen end of the molecule is slightly positive, and the oxygen end is slightly negative.

A molecule in which the charges are unevenly distributed is called a polar moleculebecause the molecule is like a magnet with poles. A water molecule is polarbecause there is an uneven distribution of electrons between the oxygen andhydrogen atoms. The negative pole is near the oxygen atom and the positive pole isbetween the hydrogen atoms.

Hydrogen Bonds Because of their partial positive and negative charges, polarmolecules such as water can attract each other, as shown in the figure below. Thecharges on a polar molecule are written in parentheses, () or (+), to show that theyare weaker than the charges on ions such as Na+ and Cl. The attraction between thehydrogen atom on one water molecule and the oxygen atom on another water moleculeis an example of a hydrogen bond. Hydrogen bonds are not as strong as covalent orionic bonds, but water's ability to form multiple hydrogen bonds is responsible formany of its special properties.

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Hydrogen Bonds The illustration shows the hydrogen bonds that form between water molecules.

A single water molecule may be involved in as many as four hydrogen bonds at thesame time. The ability of water to form multiple hydrogen bonds is responsible formany of water's properties. Cohesion is an attraction between molecules of the samesubstance. Because of hydrogen bonding, water is extremely cohesive. Water'scohesion causes molecules on the surface of water to be drawn inward, which is whydrops of water form beads on a smooth surface. Cohesion also explains why someinsects and spiders can walk on a pond's surface.

Adhesion is an attraction between molecules of different substances. Have you everbeen told to read the volume in a graduated cylinder at eye level? The surface of thewater in the graduated cylinder dips slightly in the center because the adhesionbetween water molecules and glass molecules is stronger than the cohesion betweenwater molecules. Adhesion between water and glass also causes water to rise in anarrow tube against the force of gravity. This effect is called capillary action.Capillary action is one of the forces that draw water out of the roots of a plant and upinto its stems and leaves. Cohesion holds the column of water together as it rises.

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22 Properties of Water (continued)Solutions and Suspensions

Water is not always pureit is often found as part of a mixture. A mixture is amaterial composed of two or more elements or compounds that are physically mixedtogether but not chemically combined. Salt and pepper stirred together constitute amixture. So do sugar and sand. Earth's atmosphere is a mixture of gases. Living thingsare in part composed of mixtures involving water. Two types of mixtures that can bemade with water are solutions and suspensions.

Special Solutions:Suspensions and Colloids

Solutions If a crystal of table salt is placed in a glass of warm water, sodium andchloride ions on the surface of the crystal are attracted to the polar water molecules.Ions break away from the crystal and are surrounded by water molecules, as illustratedin the figure below. The ions gradually become dispersed in the water, forming a typeof mixture called a solution. All the components of a solution are evenly distributedthroughout the solution. In a saltwater solution, table salt is the solutethe substancethat is dissolved. Water is the solventthe substance in which the solute dissolves.Water's polarity gives it the ability to dissolve both ionic compounds and other polarmolecules, such as sugar. Without exaggeration, water is the greatest solvent on Earth.

NaCl Dissolving in Water When an ionic compound such as sodium chloride is placed in water, watermolecules surround and separate the positive and negative ions.

Suspensions Some materials do not dissolve when placed in water but separate intopieces so small that they do not settle out. The movement of water molecules keeps thesmall particles suspended. Such mixtures of water and nondissolved material areknown as suspensions. Some of the most important biological fluids are bothsolutions and suspensions. The blood that circulates through your body is mostlywater, which contains many dissolved compounds. However, blood also contains cellsand other undissolved particles that remain in suspension as the blood moves throughthe body.

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22 Properties of Water (continued)Acids, Bases, and pH

A water molecule can react to form ions. This reaction can be summarized by achemical equation in which double arrows are used to show that the reaction can occurin either direction.

How often does this happen? In pure water, about 1 water molecule in 550 millionreacts and forms ions. Because the number of positive hydrogen ions produced isequal to the number of negative hydroxide ions produced, water is neutral.

The pH scale Chemists devised a measurement system called the pH scale toindicate the concentration of H+ ions in solution. As the figure at right shows, the pHscale ranges from 0 to 14. At a pH of 7, the concentration of H+ ions and OH ions isequal. Pure water has a pH of 7. Solutions with a pH below 7 are called acidicbecause they have more H+ ions than OH ions. The lower the pH, the greater theacidity. Solutions with a pH above 7 are called basic because they have more OH

ions than H+ ions. The higher the pH, the more basic the solution. Each step on the pHscale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10times as many H+ ions as a liter of a solution with a pH of 5.

The pH Scale

Acids Where do all those extra H+ ions in a low-pH solution come from? They comefrom acids. An acid is any compound that forms H+ ions in solution. Acidicsolutions contain higher concentrations of H+ ions than pure water and have pHvalues below 7. Strong acids tend to have pH values that range from 1 to 3. Thehydrochloric acid produced by the stomach to help digest food is a strong acid.

Bases A base is a compound that produces hydroxide ions (OH ions) in solution. Basic, or alkaline, solutions contain lower concentrations of H+ ions than

pure water and have pH values above 7. Strong bases, such as lye, tend to have pHvalues ranging from 11 to 14.

Buffers The pH of the fluids within most cells in the human body must generally bekept between 6.5 and 7.5. If the pH is lower or higher, it will affect the chemicalreactions that take place within the cells. Thus, controlling pH is important formaintaining homeostasis. One of the ways that the body controls pH is throughdissolved compounds called buffers. Buffers are weak acids or bases that can reactwith strong acids or bases to prevent sharp, sudden changes in pH.

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Are foods acidic or basic?

Materials pH paper, samples of food, paper towel, scalpel, dropper pipette

Procedure

1. Predicting Predict whether most foods are acidic or basic.

2. If using a pH probe, see your teacher for instructions.3. Tear off a small piece of pH paper for each sample you will test. Place these pieces on a paper towel.4. Construct a data table in which you will record the name and pH of each food sample.5. Use a scalpel to cut a piece off each solid. CAUTION: Be careful not to cut yourself. Do not eat the

food. Touch the cut surface of each sample to a square of pH paper. Use a dropper pipette to place a dropof any liquid sample on a square of pH paper. Record the pH of each sample in your data table.

Analyze and Conclude 1. Analyzing Data Were most of the samples acidic or basic?2. Evaluating and Revising Was your prediction correct?

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22 Properties of Water

1. Key Concept Use the structure of a water molecule to explain why it is polar. 2. Key Concept Compare acidic and basic solutions in terms of their H+ ion and OH ion concentrations.

3. What is the difference between a solution and a suspension? 4. What does pH measure? 5. Critical Thinking Predicting The strong acid hydrogen fluoride (HF) can be dissolved in pure water. Will

the pH of the solution be greater or less than 7?

Creating a Concept MapDraw a concept map on the properties of water. Include the following terms in your concept map: hydrogenbonds, polarity, cohesion, adhesion, capillary action, and solvent.

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Key Concept

What are the functions ofeach group of organiccompounds?

Vocabulary monomer polymer carbohydrate monosaccharide polysaccharide lipid nucleic acid nucleotide ribonucleic acid (RNA) deoxyribonucleic acid (DNA) protein amino acidReading Strategy:Summarizing As you read,find the key ideas. Write downa few key words from eachmain idea. Then, use the keywords in your summary.Reread your summary, keepingonly the most important ideas.

23 Carbon Compounds

Until the early 1800s, many chemists thought that compounds created byorganismsorganic compoundswere distinctly different from compoundsin nonliving things. In 1828, a German chemist was able to synthesize theorganic compound urea from a mineral called ammonium cyanate. Chemistssoon realized that the principles governing the chemistry of nonliving thingscould be applied to living things. Scientists still use the term organicchemistry, but now it describes something a little different. Today, organicchemistry is the study of all compounds that contain bonds between carbonatoms.

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23 Carbon Compounds (continued)The Chemistry of Carbon

Is carbon so interesting that a whole branch of chemistry should be set aside just tostudy carbon compounds? It is indeed, for two reasons. First, carbon atoms have fourvalence electrons. Each electron can join with an electron from another atom to form astrong covalent bond. Carbon can bond with many elements, including hydrogen,oxygen, phosphorus, sulfur, and nitrogen.

For: Articles on organicchemistry

Even more important, a carbon atom can bond to other carbon atoms, which givescarbon the ability to form chains that are almost unlimited in length. These carbon-carbon bonds can be single, double, or triple covalent bonds. Chains of carbon atomscan even close upon themselves to form rings, as shown at right. Carbon has theability to form millions of different large and complex structures. No other elementeven comes close to matching carbon's versatility.

Carbon Compounds

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23 Carbon Compounds (continued)Macromolecules

Many of the molecules in living cells are so large that they are known asmacromolecules, which means giant molecules. Macromolecules are made fromthousands or even hundreds of thousands of smaller molecules.

Macromolecules are formed by a process known as polymerization (pah-lih-mur-ih-ZAY-shun), in which large compounds are built by joining smaller ones together. Thesmaller units, or monomers, join together to form polymers. The monomers in apolymer may be identical, like the links on a metal watch band; or the monomers maybe different, like the beads in a multicolored necklace. The figure below illustrates theformation of a polymer from more than one type of monomer.

Polymerization When small molecules called monomers join together, they form polymers, or large molecules.

It would be difficult to study the millions of organic compounds if they were notclassified into groups. Four groups of organic compounds found in livingthings are carbohydrates, lipids, nucleic acids, and proteins. Sometimes theseorganic compounds are referred to as biomolecules. As you read about thesemolecules, compare their structures and functions.

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23 Carbon Compounds (continued)Carbohydrates

Carbohydrates are compounds made up of carbon, hydrogen, and oxygen atoms,usually in a ratio of 1 : 2 : 1. Living things use carbohydrates as their mainsource of energy. Plants and some animals also use carbohydrates for structuralpurposes. The breakdown of sugars, such as glucose, supplies immediate energy forall cell activities. Living things store extra sugar as complex carbohydrates known asstarches. As shown below, the monomers in starch polymers are sugar molecules.

Carbohydrates Starches and sugars are examples of carbohydrates that are used by living things as asource of energy. Starches form when sugars join together in a long chain.

Single sugar molecules are also called monosaccharides (mahn-oh-SAK-uh-rydz).Besides glucose, monosaccharides include galactose, which is a component of milk,and fructose, which is found in many fruits.

The large macromolecules formed from monosaccharides are known aspolysaccharides. Many animals store excess sugar in a polysaccharide calledglycogen, or animal starch. When the level of glucose in your blood runs low,glycogen is released from your liver. The glycogen stored in your muscles supplies theenergy for muscle contraction and, thus, for movement.

Plants use a slightly different polysaccharide, called plant starch, to store excess sugar.Plants also make another important polysaccharide called cellulose. Tough, flexiblecellulose fibers give plants much of their strength and rigidity. Cellulose is the majorcomponent of both wood and paper, so you are actually looking at cellulose when youare reading a textbook.

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23 Carbon Compounds (continued)Lipids

Lipids are a large and varied group of biological molecules that are generally notsoluble in water. Lipids are made mostly from carbon and hydrogen atoms. Thecommon categories of lipids are fats, oils, and waxes. Lipids can be used to storeenergy. Some lipids are important parts of biological membranes and waterproofcoverings. Steroids are lipids as well. Many steroids serve as chemical messengers.

Many lipids are formed when a glycerol molecule combines with compounds calledfatty acids, as shown in the figure at right. If each carbon atom in a lipid's fatty acidchains is joined to another carbon atom by a single bond, the lipid is said to besaturated. The term saturated is used because the fatty acids contain the maximumpossible number of hydrogen atoms.

Lipid Structure

If there is at least one carbon-carbon double bond in a fatty acid, the fatty acid is saidto be unsaturated. Lipids whose fatty acids contain more than one double bond are saidto be polyunsaturated. If the terms saturated and polyunsaturated seem familiar, youhave probably seen them on food package labels. Lipids such as olive oil, whichcontains unsaturated fatty acids, tend to be liquid at room temperature. Cooking oils,such as corn oil, sesame oil, canola oil, and peanut oil, contain polyunsaturated lipids.

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23 Carbon Compounds (continued)Nucleic Acids

Nucleic acids are macromolecules containing hydrogen, oxygen, nitrogen, carbon, andphosphorus. Nucleic acids are polymers assembled from individual monomers knownas nucleotides. Nucleotides consist of three parts: a 5-carbon sugar, a phosphategroup, and a nitrogenous base, as shown in the figure below. Individual nucleotidescan be joined by covalent bonds to form a polynucleotide, or nucleic acid.

Nucleotide Structure Nucleic acids store and transmit genetic information. The monomers that makeup a nucleic acid are nucleotides. Each nucleotide has a 5-carbon sugar, a phosphate group, and a nitrogenous base.

Nucleic acids store and transmit hereditary, or genetic, information. There aretwo kinds of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid(DNA). As their names indicate, RNA contains the sugar ribose and DNA contains thesugar deoxyribose.

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23 Carbon Compounds (continued)Proteins

Proteins are macromolecules that contain nitrogen as well as carbon, hydrogen, andoxygen. Proteins are polymers of molecules called amino acids. Amino acids arecompounds with an amino group (NH2) on one end and a carboxyl group (COOH)on the other end.

The figure below shows one reason why proteins are among the most diversemacromolecules. More than 20 different amino acids are found in nature. All aminoacids are identical in the regions where they may be joined together by covalentbonds. This uniformity allows any amino acid to be joined to any other amino acidby bonding an amino group to a carboxyl group.

Amino Acids Amino acids are the monomers of proteins. All amino acids have an amino group at one end and acarboxyl group at the other end. What distinguishes one amino acid from another is the R-group section of the molecule.

The portion of each amino acid that is different is a side chain called an R-group.Some R-groups are acidic and some are basic. Some are polar and some are nonpolar.Some contain carbon rings. The instructions for arranging amino acids into manydifferent proteins are stored in DNA. Each protein has a specific role. Someproteins control the rate of reactions and regulate cell processes. Some are usedto form bones and muscles. Others transport substances into or out of cells orhelp to fight disease.

Proteins can have up to four levels of organization. The first level is the sequence ofamino acids in a protein chain. Second, the amino acids within a chain can be twistedor folded. Third, the chain itself is folded. If a protein has more than one chain, eachchain has a specific arrangement in space as shown by the red and blue structures inthe figure at right. Van der Waals forces and hydrogen bonds help maintain a protein'sshape. In the next section, you will learn why a protein's shape is so important.

Protein Structure

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23 Carbon Compounds

1. Key Concept Name four groups of organic compounds found in living things. 2. Key Concept Describe at least one function of each group of organic compounds. 3. What properties of carbon explain carbon's ability to form many different macromolecules?4. Critical Thinking Applying Concepts Explain why proteins are considered polymers but lipids are not.5. Critical Thinking Comparing and Contrasting Compare the structures and functions of the

biomolecules lipids and starches.

Levels of OrganizationUse what you learned about levels of organization in Section 1-3 to discuss the levels of organization inmacromolecules. Begin your discussion with the smallest structure.

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Key Concepts

What happens to chemicalbonds during chemicalreactions?

How do energy changesaffect whether a chemicalreaction will occur?

Why are enzymesimportant to living things?

Vocabulary chemical reaction reactant product activation energy catalyst enzyme substrateReading Strategy:Building Vocabulary Afteryou read, write a phrase orsentence in your own words todefine or describe eachvocabulary term.

24 Chemical Reactions and Enzymes

Living things, as you have seen, are made up of chemical compoundssomesimple and some complex. But chemistry isn't just what life is made ofchemistry is also what life does. Everything that happens in an organismitsgrowth, its interaction with the environment, its reproduction, and even itsmovementis based on chemical reactions.

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24 Chemical Reactions and Enzymes (continued)Chemical Reactions

A chemical reaction is a process that changes, or transforms, one set of chemicalsinto another set. An important scientific principle is that mass and energy areconserved during chemical transformations. This is also true for chemical reactionsthat occur in living organisms. Some chemical reactions occur slowly, such as thecombination of iron and oxygen to form an iron oxide called rust. Other reactionsoccur quickly. The elements or compounds that enter into a chemical reaction areknown as reactants. The elements or compounds produced by a chemical reaction areknown as products. Chemical reactions always involve changes in thechemical bonds that join atoms in compounds.

One example of an important chemical reaction that occurs in your body involvescarbon dioxide. Your cells constantly produce carbon dioxide as a normal part of theiractivity. This carbon dioxide is carried to your lungs through the bloodstream, andthen is eliminated as you exhale. However, carbon dioxide is not very soluble in water.The bloodstream could not possibly dissolve enough carbon dioxide to carry it awayfrom your tissues were it not for a chemical reaction. As it enters the blood, carbondioxide reacts with water to produce a highly soluble compound called carbonic acid,H2CO3.

The reaction shown above enables the bloodstream to carry carbon dioxide to thelungs. In the lungs, the reaction is reversed.

This reverse reaction produces carbon dioxide gas, which is released as you exhale.

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24 Chemical Reactions and Enzymes (continued)Energy in Reactions

Energy is released or absorbed whenever chemical bonds form or are broken. Becausechemical reactions involve breaking and forming bonds, they involve changes inenergy.

Energy Changes Some chemical reactions release energy, and other reactionsabsorb energy. Energy changes are one of the most important factors in determiningwhether a chemical reaction will occur. Chemical reactions that release energyoften occur spontaneously. Chemical reactions that absorb energy will not occurwithout a source of energy. An example of an energy-releasing reaction is hydrogengas burning, or reacting, with oxygen to produce water vapor.

The energy is released in the form of heat, and sometimeswhen hydrogen gasexplodeslight and sound.

The reverse reaction, in which water is changed into hydrogen and oxygen gas,absorbs so much energy that it generally doesn't occur by itself. In fact, the onlypractical way to reverse the reaction is to pass an electrical current through water todecompose water into hydrogen gas and oxygen gas. Thus, in one direction thereaction produces energy, and in the other direction the reaction requires energy.

In order to stay alive, organisms need to carry out reactions that require energy.Because matter and energy are conserved in chemical reactions, every organism musthave a source of energy to carry out chemical reactions. Plants get that energy bytrapping and storing the energy from sunlight in energy-rich compounds. Animals gettheir energy when they consume plants or other animals. Humans release the energyneeded to grow tall, to breathe, to think, and even to dream through the chemicalreactions that occur when humans metabolize, or break down, digested food.

Activation Energy Even chemical reactions that release energy do not always occurspontaneously. That's a good thing because if they did, the pages in textbooks mightburst into flames. The cellulose in paper burns in the presence of oxygen and releasesheat and light. However, the cellulose will burn only if you light it with a match,which supplies enough energy to get the reaction started. Chemists call the energy thatis needed to get a reaction started the activation energy. As the figure below shows,activation energy is a factor in whether the overall chemical reaction releases energyor absorbs energy.

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Activation Energy Chemical reactions that release energy often occur spontaneously. Chemicalreactions that absorb energy will occur only with a source of energy. The peak of each graph represents the energyneeded for the reaction to go forward. The difference between this required energy and the energy of the reactants is theactivation energy.

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24 Chemical Reactions and Enzymes (continued)Enzymes

Some chemical reactions that make life possible are too slow or have activationenergies that are too high to make them practical for living tissue. These chemicalreactions are made possible by a process that would make any chemist proudcellsmake catalysts. A catalyst is a substance that speeds up the rate of a chemicalreaction. Catalysts work by lowering a reaction's activation energy. For: Links on enzymesVisit: www.SciLinks.org

Web Code: cbn-1024

Enzymes are proteins that act as biological catalysts. Enzymes speed upchemical reactions that take place in cells. Like other catalysts, enzymes act bylowering the activation energies, as illustrated by the graph below. Lowering theactivation energy has a dramatic effect on how quickly the reaction is completed. Howbig an effect does it have? Consider the reaction in which carbon dioxide combineswith water to produce carbonic acid.

Effect of Enzymes Enzymes speed up chemical reactions that take place in cells. Notice how theaddition of an enzyme lowers the activation energy in this reaction. This action speeds up the reaction.

Left to itself, this reaction is so slow that carbon dioxide might build up in the bodyfaster than the bloodstream could remove it. Your bloodstream contains an enzymecalled carbonic anhydrase that speeds up the reaction by a factor of 10 million. Withcarbonic anhydrase on the job, the reaction takes place immediately and carbondioxide is removed from the blood quickly.

Enzymes are very specific, generally catalyzing only one chemical reaction. For this

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reason, part of an enzyme's name is usually derived from the reaction it catalyzes.Carbonic anhydrase gets its name because it catalyzes the reaction that removes waterfrom carbonic acid.

How Does pH Affect an Enzyme?Catalase is an enzyme that helps decompose the toxichydrogen peroxide that is produced during normal cellactivities. The products of this reaction are water andoxygen gas. The pressure of the oxygen gas in a closedcontainer increases as oxygen is produced. Any increase inthe rate of the reaction will cause an increase in thepressure of the oxygen.

The purple line on the graph represents the normal rate ofthe reaction in a water solution of hydrogen peroxide andcatalase. The red line represents the rate of reaction whenan acid is added to the solution. The blue line representsthe rate of reaction when a base is added to the solution.

1. Applying Concepts What variable is plotted onthe x-axis? What variable is plotted on the y-axis?

2. Interpreting Graphics How did the rate ofreaction change over time in the control reaction?

3. Inferring Suggest an explanation for the change in the control reaction at about 40 seconds.4. Drawing Conclusions What effect do acids and bases have on the enzyme catalase?5. Drawing Conclusions Would it be valid to conclude that if a base were added, the rate of the reaction

would slow down? Explain.6. Going Further Predict what would happen if vinegar were added to a water solution of hydrogen

peroxide and catalase.

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24 Chemical Reactions and Enzymes (continued)Enzyme Action

How do enzymes do their jobs? For a chemical reaction to take place, the reactantsmust collide with enough energy so that existing bonds will be broken and new bondswill be formed. If the reactants do not have enough energy, they will be unchangedafter the collision.

The Enzyme-Substrate Complex Enzymes provide a site where reactants can bebrought together to react. Such a site reduces the energy needed for reaction. Thereactants of enzyme-catalyzed reactions are known as substrates.

Enzymatic Reactions

The figure at right provides an example of an enzyme-catalyzed reaction. The enzymeis hexokinase. The substrates are glucose and ATP. During the reaction, a phosphategroup is transferred from ATP to the glucose molecule. Recall that each protein has aspecific, complex shape. The substrates bind to a site on the enzyme called the activesite. The active site and the substrates have complementary shapes. The fit is soprecise that the active site and substrates are often compared to a lock and key.

An Enzyme-CatalyzedReaction

The figure below shows a substrate fitting into an active site on an enzyme. Theenzyme and substrate are bound together by intermolecular forces and form anenzyme-substrate complex. They remain bound together until the reaction is done.Once the reaction is over, the products of the reaction are released and the enzyme isfree to start the process again.

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Enzyme-Substrate Complex This space-filling model shows how a substrate binds to an active site on anenzyme.

Regulation of Enzyme Activity Because they are catalysts for reactions, enzymescan be affected by any variable that influences a chemical reaction. Enzymes,including those that help digest food, work best at certain pH values. Many enzymesare affected by changes in temperature. Not surprisingly, those enzymes produced byhuman cells generally work best at temperatures close to 37C, the normal temperatureof the human body.

Cells can regulate the activities of enzymes in a variety of ways. Most cells containproteins that help to turn key enzymes on or off at critical stages in the life of thecell. Enzymes play essential roles in regulating chemical pathways, making materialsthat cells need, releasing energy, and transferring information.

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24 Chemical Reactions and Enzymes

1. Key Concept What happens to chemical bonds during chemical reactions?2. Key Concept Describe the role of energy in chemical reactions. 3. Key Concept What are enzymes, and how are they important to living things? 4. Describe how enzymes work, including the role of the enzyme-substrate complex. 5. Critical Thinking Applying Concepts A change in pH can change the shape of a protein. How might a

change in pH affect the function of an enzyme such as hexokinase? (Hint: Think about the analogy of thelock and key.)

ModelingMake a model that demonstrates how an active site and a substrate are like a lock and a key. Give a brief talk inwhich you refer to your model as you explain how enzymes work.

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Chapter 2 The Chemistry of Life

Click on a Key Concept to link to the page where the concept is explained.

21 The Nature of Matter Key Concepts

The subatomic particles that make up atoms areprotons, neutrons, and electrons.Because they have the same number of electrons,all isotopes of an element have the same chemicalproperties.The main types of chemical bonds are covalentbonds and ionic bonds.

Vocabulary

atom nucleus electron element isotope compound chemical bond ionic bond ion covalent bond molecule van der Waals forces

22 Properties of Water Key Concepts

A water molecule is polar because there is anuneven distribution of electrons between the oxygenand hydrogen atoms.Acidic solutions contain higher concentrations of H+ions than pure water and have pH values below 7.Basic, or alkaline, solutions contain lowerconcentrations of H+ ions than pure water and havepH values above 7.

Vocabulary

cohesion adhesion mixture solution solute solvent suspension pH scale acid base buffer

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Thinking Visually How Enzymes Work: Cycle Diagram

23 Carbon Compounds Key Concepts

Four groups of organic compounds found in livingthings are carbohydrates, lipids, nucleic acids, andproteins.Living things use carbohydrates as their mainsource of energy. Plants and some animals alsouse carbohydrates for structural purposes.Lipids can be used to store energy. Some lipids areimportant parts of biological membranes andwaterproof coverings.Nucleic acids store and transmit hereditary, orgenetic, information.Some proteins control the rate of reactions andregulate cell processes. Some proteins build tissuessuch as bone and muscle. Others transportmaterials or help to fight disease.

Vocabulary

monomer polymer carbohydrate monosaccharide polysaccharide lipid nucleic acid nucleotide ribonucleic acid (RNA) deoxyribonucleic acid (DNA) protein amino acid

24 Chemical Reactions and Enzymes Key Concepts

Chemical reactions always involve changes in thechemical bonds that join atoms in compounds.Chemical reactions that release energy often occurspontaneously. Chemical reactions that absorbenergy will not occur without a source of energy.Enzymes speed up chemical reactions that takeplace in cells.

Vocabulary

chemical reaction reactant product activation energy catalyst enzyme substrate

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Van der Waals Forces Van der Waals forces help geckos to grip smooth, vertical surfaces. Applying Concepts Whichproduct(s) might be developed based on van der Waals forces? Explain.


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Isotopes of Carbon Because they have the same number of electrons, these isotopes of carbon have the samechemical properties. The difference among the isotopes is the number of neutrons in their nuclei.


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Ionic Bonding The chemical bond in which electrons are transferred from one atom to another is called an ionic bond.The compound sodium chloride forms when sodium loses its valence electron to chlorine.




The pH Scale The concentration of H+ ions determines whether solutions are acidic or basic. The most acidic material onthis pH scale is stomach acid. The most basic material on this scale is oven cleaner.




Carbon Compounds Carbon can form single, double, or triple bonds with other carbon atoms. Each line between atoms in amolecular drawing represents one covalent bond. Observing How many covalent bonds are there between the carbon atoms inacetylene?




Lipid Structure Lipids are used to store energy. Lipid molecules are made up of fatty acids and glycerol. Liquid lipids, suchas olive oil, contain mainly unsaturated fatty acids.




Protein Structure Proteins help to carry out chemical reactions, transport small molecules in and out of cells, and fightdiseases. Proteins are made up of chains of amino acids folded into complex structures.




An Enzyme-Catalyzed Reaction The enzyme hexokinase converts the substrates glucose and ATP into glucose-6-phosphateand ADP. Predicting What happens to the hexokinase after the products are released?

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