36 Section Summaries/Chapter 2

Name______________________________ Class __________________ Date ______________

Chapter 2 The Chemistry of Life

The basic unit of matter is the atom. Thesubatomic particles that make up atoms areprotons, neutrons, and electrons. Protonsand neutrons have about the same mass.Protons are positively charged particles (+),and neutrons carry no charge. Protons andneutrons together form the nucleus, at thecenter of the atom. The electron is a nega-tively charged particle (_). Atoms haveequal numbers of electrons and protons,and therefore atoms do not have a charge.

A chemical element is a pure substancethat consists entirely of one type of atom.The number of protons in an atom of an ele-ment is the elements atomic number.Atoms of an element can have differentnumbers of neutrons. Atoms of the sameelement that differ in the number of neu-trons they contain are known as isotopes.Because all the isotopes of an element havethe same number of electrons, they all havethe same chemical properties.

A chemical compound is a substanceformed by the chemical combination of twoor more elements in definite proportions.Atoms in compounds are held together bychemical bonds. An ionic bond is formedwhen one or more electrons are transferredfrom one atom to another. A covalent bondforms when electrons are shared betweenatoms. The structure that results whenatoms are joined together by covalent bondsis called a molecule. Unequal sharing ofelectrons creates regions of positive andnegative charges in molecules. Slight attrac-tion can develop between the oppositelycharged regions of nearby molecules. Suchintermolecular forces of attraction are calledvan der Waals forces.

22 Properties of WaterThe water molecule (H2O) is neutral. Yet, theoxygen end of the molecule has a slight posi-tive charge, and the hydrogen end has aslight negative charge.

A molecule in which the charges areunevenly distributed is called a polarmolecule. Polar molecules can attract eachother. The attraction between the hydrogenatom on one water molecule and the oxygenatom on another water molecule is called ahydrogen bond. Cohesion is an attractionbetween molecules of the same substance.Adhesion is an attraction between moleculesof different substances.

A mixture is a material composed of twoor more elements or compounds that arephysically mixed togetherthe substancesare not chemically combined. A solution is amixture in which all the components areevenly distributed throughout the mixture.In a solution, the substance that is dissolvedis called the solute. The substance in whichthe solute dissolves is called the solvent.Water is the greatest solvent on Earth.

A water molecule can react to form ions.A water molecule (H2O) can form a hydro-gen ion (H+) and a hydroxide ion (OH_). ThepH scale indicates the concentration of H+

ions in a solution. Pure water has a pH of 7.An acid is any compound that forms H+

ions in solution. Acidic solutions containhigher concentrations of H+ ions than purewater. A base is a compound that producesOH_ ions in solution. Basic, or alkaline,solutions contain lower concentrations ofH+ ions than pure water and have pH val-ues above 7.

23 Carbon CompoundsOrganic chemistry is the study of all com-pounds that contain bonds between carbonatoms. Carbon compounds are also calledorganic compounds. Many of the moleculesin living things are so large that they areknown as macromolecules. Macromoleculesare formed in a process called polymerization.Smaller units, called monomers, join togetherto form macromolecules, or polymers.

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Summary

21 The Nature of Matter

Section Summaries/Chapter 2 37

Name______________________________ Class __________________ Date ______________

Four groups of organic compoundsfound in living things are carbohydrates,lipids, nucleic acids, and proteins. Carbo-hydrates are compounds made up of car-bon, hydrogen, and oxygen atoms. Livingthings use carbohydrates as their mainsource of energy. Plants and some animalsuse carbohydrates in structures. Starchesand sugars are examples of carbohydrates.

Lipids are made mostly from carbonand hydrogen atoms. Fats, oils, and waxesare lipids. Lipids are used in living thingsto store energy. Some lipids are importantparts of biological membranes and water-proof coverings. Lipid molecules are madeup of compounds called fatty acids andglycerol.

Nucleic acids contain hydrogen, oxy-gen, nitrogen, carbon, and phosphorus.Nucleotides are the monomers that makeup nucleic acids. Each nucleotide consistsof a 5-carbon sugar, a phosphate group,and a nitrogenous base. Nucleic acids storeand transmit hereditary, or genetic, infor-mation. There are two kinds of nucleicacids: ribonucleic acid (RNA) and deoxyri-bonucleic acid (DNA).

Proteins contain nitrogen as well as car-bon, hydrogen, and oxygen. Proteins arepolymers of molecules called amino acids.

Some proteins control the rate of reactionsand regulate cell processes. Some are usedto form bones and muscles. Others transport substances into or out of cells orhelp to fight disease.

24 Chemical Reactions andEnzymesA chemical reaction is a process that changesone set of chemicals (reactants) into anotherset of chemicals (products). Chemical reac-tions always involve the breaking of bondsin reactants and the formation of new bondsin products.

Some chemical reactions release energy,and other reactions absorb energy. Chemicalreactions that release energy often occurspontaneously. Every chemical reactionneeds energy to get started, and that startingenergy is called activation energy.

A catalyst is a substance that speeds upthe rate of a chemical reaction. Catalystswork by lowering a reactions activationenergy. Enzymes are proteins that act as bio-logical catalysts. Enzymes speed up chemicalreactions by lowering activation energies. Inan enzyme-catalyzed reaction, the reactantsare known as substrates. The substrates bindto a site on the enzyme called the active site.

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38 Section Summaries/Chapter 2

Name______________________________ Class __________________ Date ______________

Chapter 2 The Chemistry of Life

Pearson Education, Inc. All rights reserved.

Vocabulary Review

Crossword Puzzle Use the clues below to fill in the spaces in the puzzle with the correct words.

Across1. elements or compounds that enter into

a chemical reaction4. process that changes one set of chemi-

cals into another7. positively charged subatomic particle8. substance formed by the chemical com-

bination of two or more elements indefinite proportions

11. positively and negatively chargedatoms

12. carbon compound that stores and trans-mits genetic information

14. the center of an atom16. bond formed when electrons are shared

between atoms17. macromolecule formed when

monomers join together

Down2. negatively charge subatomic particle3. compound that produces hydroxide

ions in solution5. bond formed when one or more elec-

trons are transferred from one atom toanother

6. monomer of nucleic acid9. monomer of protein

10. compound that forms hydrogen ions insolution

13. atom of same element that differs innumber of neutrons compared to otheratoms of the element

15. basic unit of matter

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