/ PI~\~\\I\'QN- L\. .'

CHAPTER 3

WATER AND THE FITNESS OFTHE ENVIRONMENT

FRAMEWORK

Water makes up 70% to 95% of the cell content of liv-ing organisms and covers 75% of the Earth's surface.Its unique properties make the external environmentfit for living organisms and the internal environmentof organisms fit for the chemical and physical process-es of life.

Hydrogen bonding between polar water moleculescreates a cohesive liquid with a high specific heat andhigh heat of vaporization, both of which help to regu-late environmental temperature. The polarity of watermakes it a versatile solvent. An organism's pH may beregulated by buffers. Acid precipitation poses a seri-ous environmental threat.

CHAPTER REVIEW

Water's Polarity and its Effects

. The polarity of water moleculesresults inhydrogen bonding (37-38)

A water molecule consists of two hydrogeI\ atomseach covalently bonded to a more electronegative oxy-gen atom. This polar molecule has an L shape with aslight positive charge on each hydrogen atom and aslight negative charge associated with the oxygen.Hydrogen bonds form between the hydrogen atoms ofone water molecule and the oxygen atoms of twoother molecules, creating a higher level of structuralorganization and leading to the emergent propertiesof water.

12

. INTERACTIVEQUESTION 3.1

Draw the four water molecules that can hydrogen-bond to this water molecule. Indicate the slight nega-tive and positive charges that account for the forma-tion of hydrogen bonds.

. Organismsdepend on the cohesionof watermolecules (38-39)

Liquid water is unusually cohesive due to the con-stant forming and re-forming of hydrogen bonds thathold the molecules together. This cohesion creates amore structurally organized liquid and enables waterto move against gravity in plants. The adhesion ofwater molecules to the walls of plant vessels also con-tributes to water transport. Hydrogen bondingbetv.'een water molecules produces a high surface ten-sion at the interface between water and air.

@

. Water moderates temperatures on Earth (DO-DO)

Heat and Temperature In a body of matter, heat is ameasure of the total quantity of kinetic energy, theenergy associated with the movement of atoms andmolecules. Temperature measures the average kineticenergy of the molecules ina substance.

Temperature is measured using a Celsius scale.Water at sea level freezes at O°C and boils at 100°C.

Heat is measured by the calorie (cal). A calorie is theamount of heat energy it takes to raise 1 g of water1°C. A kilocalorie (kcal) is 1000 calories, the amountof heat required to raise 1 kg of water 1°C. A joule (J)equals 0.239 cal; a calorie is 4.184 J.

Water's High Specific Heat Specific heat is theamount of heat absorbed or lost when 1 g of a sub-stance changes its temperature by 1°C. Water's specif-ic heat of 1 call g/OC is unusually high comparedwith that of other common substances; water mustabsorb or release a relatively large quantity of heat inorder for its temperature to change. Heat must beabsorbed to break hydrogen bonds before water mole-cules can move faster and the temperature can rise,and conversely, heat is released when hydrogenbonds form as the temperature of water drops. Theability of large bodies of water to stabilize air temper-ature is due to the high specific heat of water. Thehigh proportion of water in the environment andwithin organisms keeps temperature fluctuationswithin limits that permit life.

Evaporative Cooling The transformation from a liq-uid to a gas is called vaporization or evaporation andhappens when molecules with sufficient kinetic ener-gy overcome their attraction to other molecules andescape into the air as gas. The heat of vaporization isthe quantity of heat that must be absorbed for 1 g of aliquid to be converted to a gas. Water has a high heatof vaporization (580 call g) because a large amount ofheat is needed to break the hydrogen bonds holdingwater molecules together. This property of waterhelps moderate the climate on Earth as solar heat isdissipated from tropical seas during evaporation andheat is released when moist tropical air condenses toform rain. .

As a substance vaporizes, the liquid left behindloses the kinetic energy of the escaping molecules andcools down. Evaporative cooling helps to protect ter-restrial organisms from overheating and contributesto the stability of temperatures in lakes and ponds.

. Oceansand lakes don't freezesolid becauseice floats (40-41)

As water cools below 4°C, it expands. By O°C, eachwater molecule becomes hydrogen-bonded to four

Clwpter 3: Water and the Fitness of the Emnronment l ', ..)

other molecules, creating a crystalline lattice thatspaces the molecules apart. Ice is less dense than liq-uid water, and therefore, it floats. The floating iceinsulates the liquid water below.

. INTERACTIVE QUESTION 3.2

The following concept map is one way to show howthe breaking and forming of hydrogen bonds is relat-ed to temperature regulation. Fill in the blanks andcompare your choice of concepts to those given in theanswer section. Or, better still, create your own mapto help you understand how water stabilizes tempera-ture.

I a.

between1

T

results in

organismsand

environment

as when as when

i.

leads to

Eseas absorb

It \Ih.

'a'

. Water is the solvent of life (41-43) .

A solution is a liquid homogeneous mixture of two ormore substances; the dissolving agent is called thesolvent and the substance that is dissolved is the

solute. An aqueous solution is one in which water is

rr-;

14 Unit One: The Chemistry ofLife

the solvent. The positive and negative regions ofwater molecules are attracted to oppositely chargedions or partially charged regions of polar molecules.Thus, solute molecules become surrounded by watermolecules and dissolve into solution.

Hydrophilic and Hydrophobic Substances Ionic andpolar substances are hydrophilic; they have an affini-ty for water due to electrical attractions and hydrogenbonding. Nonpolar and non-ionic compounds arehydrophobic; they will not mix with or dissolve inwater.

. INTERACTIVE QUESTION 3.3

Indicate whether the following are hydrophilic orhydrophobic. Do these substances contain ionic, polar,or nonpolar bonds?

a. olive oil

b. sugarc. salt

d. candle wax

Solute Concentration in Aqueous Solutions Most ofthe chemical reactions of life take place in water. Amole (mol) is the amount of a substance that has amass in grams numerically equivalent to its molecu-lar weight (sum of the weight of all atoms in the mol-ecule) in daltons. A mole of any substance has exactlythe same number of molecules-6.02 x 1023,calledAvogadro's number. The molarity of a solution(abbreviated M) refers to the number of moles of asolute dissolved in 1 liter of solution.

. INTERACTIVE QUESTION 3.4

a. How many grams of lactic acid (C3H6O3) are in a0.5 M solution of lactic acid? e2C, 1H, 160)

b. How many grams of salt (NaCl) must be dis-solved in water to make 2 liters of a 2 M saltsolution? (23Na, 34Cl)

Dissociation of Water Molecules

A water molecule can dissociate into a hydrogen ion,H+ (which binds to another water molecule to form ahydronium ion, H3O+), and a hydroxide ion, OH-.

Although reversible and statistically rare, this dissoci-ation into the highly reactive hydrogen and hydroxideions has important biological consequences.

. Organismsare sensitiveto changesin pH (43-45)

-Acids and Bases In pure water, the concentrations ofH+ and OH- ions are the same; both are equal to 10-7M. When acids or bases dissolve in water, the H+ andOH- balance shifts. An acid adds H+ to a solution,whereas a base reduces H+ in a solution by acceptinghydrogen ions or by adding hydroxide ions (whichthen combine with H+ and thus remove hydrogenions). A strong acid or strong base dissociates com-pletely when mixed with water. A weak acid or basereversibly dissociates, releasing or binding H+. A solu-tion with a higher concentration of H+ than of OH- isconsidered acidic. A basic solution has a higher con-centration of OH- than of H+.

The pH Scale In any solution, the product of the[H+] and [OH-J is constant at 10-14 M. Brackets, [ ],indicate molar concentration. If the [H+] is higher,then the [OH-] is lower, due to the tendency of excesshydrogen ions to combine with the hydroxide ions insolution and form water. Likewise, an increase in

[OH-] causes an equivalent decrease in [H+]. If [OH-]is equal to 10-10M, then [H-] will equal 10-4 M.

The logarithmic pH scale compresses the range ofhydrogen and hydroxide ion concentrations, whichcan vary in different solutions by many orders ofmagnitude. The pH of a solution is defined as thenegative log (base 10) of the [H+]: pH = -log [H+]. Fora neutral solution, [H+] is 10-7 M, and the pH equals7. As the [H+] increases in an acidic solution, the pHvalue decreases. The difference between each unit of

the pH scale represents a tenfold difference in the con-centration of [H+] and [OH-].

. INTERACTIVE QUESTION 3.5

Complete the following table to review your under-standing of pH

II

[H+] [OW] pH Acidic, Basic,or Neutral?

10-11 3 acidic

10-8

B 10-7

1

Buffers Most cells have an internal pH close to 7.Buffers within the cell maintain a constant pH byaccepting excess H+ ions or donating H+ ions whenH+ concentration decreases. Weak acid-base pairs thatreversibly bind hydrogen ions are typical of mostbuffering systems.

. INTERACTIVE QUESTION 3.6

The carbonic acid/bicarbonate system is an importantbiological buffer. Label the molecules and ions in thisequation and indicate which is the H+ donor andacceptor.

In which direction will this reaction proceed(a) when the pH of a solution begins to fall?

(b) when the pH rises above normal level?

H2CO3 ~ HC°:3+ H+

. Acid precipitation threatens the fitness of theenvironment (45-46)

Acid precipitation, with a pH lower than normal pH5.6, is due to the reaction of water in the atmospherewith the sulfur oxides and nitrogen oxides released bythe combustion of fossil fuels. Lowering the pH of thesoil solution affects the solubility of minerals needed

Chapter 3: Water and the Fitness of the Environment l'::>

by plants. In lakes and ponds, a lowered pH and theaccumulation of minerals leached from the soil bv

acid rain harm many species of fishes, amphibians,and aquatic invertebrates,

STRUCTURE YOUR KNOWLEDGE

1. Fill in the table below that summarizes the prop-erties of water that contribute to the fitness of theenvironment for life.

2. To become proficient in the use of the conceptsrelating to pH, develop a concept map to organizeyour understanding of the following terms: pH,[H+], [OH-], acidic, basic, neutral, buffer, 1-14,acid-base pair. Remember to label connectinglines and add additional concepts as you needthem. A suggestedconceptmapis given in theanswersection, but rememberthat your conceptmap shouldrepresentyour own understanding The value of thisexerciseis il1organizing theseconceptsfor yourse~f.

TEST YOUR KNOWLEDGE

MULTIPLE CHOICE: Choosetheonebestanswer.

1. Each water molecule is capable of forminga. one hydrogen bond.b. three hydrogen bonds.c. four hydrogen bonds.d. one covalent bond and two hydrogen bonds.e. one covalent bond and three hydrogen bonds.

-,Property Explanation of Property Example of Benefit to Life

i!

a. Hydrogen bonds hold molecules together b.

Iand adhere them to hydrophilic surfaces.I

High specific heat c. Temperature changes in environment and org-Ianisms are moderated.

d. Hydrogen bonds must be broken for water e.to-evaporate:

f. Water molecules with high kinetic energy evap- g.orate; remaining molecules are cooler.

Ice floats h. i. !

j. k. Most chemical reactions in life involve solutesdissolved in water.

'i

, 16 Unit One: The Chemistry of Life

2. The polarity of water moleculesa. promotes the fom,ation of hydrogen bonds.b. helps water to dissolve nonpolar solutes.c. lowers the heat of vaporization and leads to

evaporative cooling.d. creates a crystalline structure in liquid water.e. does all of the above.

3. What accounts for the movement of water upxylem vessels in a plant?a. cohesionb. hydrogen bondingc. adhesiond. hydrophilic vessel wallse. all of the above

4. Climates tend to be moderate by large bodies ofwater because

a. a large amount of solar heat is absorbed by thegradual rise in temperature of the water.

b. the gradual cooling of the water releases heatto the environment.

c. the high specific heat of water helps to regulateair temperatures.

d. a great deal of heat is absorbed and released bythe breaking and forming of hydrogen bonds.

e. of all of the above.

5. Temperature is a measure ofa. specificheat.b. average kinetic energy of molecules.c. total kinetic energy of molecules.d. Celsius degrees.e. joules.

6. Evaporative cooling is a result ofa. a low heat of vaporization.b. a high heat of melting.c. a high specificheat.d. a reduction in the average kinetic energy of the

liquid remaining after molecules enter thegaseous state.

e. release of heat caused by the breaking ofhydrogen bonds when water molecules escape.

7. Ice floats becausea. air is trapped in the crystalline lattice.b. the formation of hydrogen bonds releases heat;

warmer objects float.c. it has a smaller surface'area than liquid water.d. it insulates bodies of water so they do not

freeze from the bottom up.e. hydrogen bonding spaces the molecules farther

apart, creating a less dense structure.

8. The molarity of a solution is equal toa. Avogadro's number of molecules in 1 liter of

solvent.b. the number of moles of a solute in 1 liter of

solution.

c. the molecular weight of a solute in 1 liter ofsolution.

d. the number of solute particles in 1 liter of sol-vent.

e. 342 g if the solute is sucrose.

9. Some archaebacteria are able to live in lakes with

pH values of 11. How does pH 11 compare withthe pH 7 typical of your body cells?a. It is four times more acidic than pH 7.b. It is four times more basic than pH 7.c. It is a thousand times more acidic than pH 7.d. It is a thousand times more basic than pH 7.e. It is ten thousand times more basic than pH 7.

10. A buffer

a. changes pH by a magnitude of 10.b. absorbs excess OH-.c. releases excess H+.

d. is often a weak acid-base pair.e. maintains pH at a value of 7.

11. Which of the following is least soluble in water?a. polar moleculesb. nonpolar compoundsc. ionic compoundsd. hydrophilic moleculese. anions

12. Which would be the best method for reducingacid precipitation?a. Raise the height of smoke stacks so that

exhaust enters the upper atmosphere.b. Add buffers and bases to bodies of water

whose pH has dropped.c. Use coal burning generators rather than

nuclear power to produce electricity.d. Increase emission control standards for facto-

ries and automobiles.

e. Reduce the concentration of heavy metals inindustrial exhaust.

13. What bonds must be broken for water to vapor-ize?

a. polar covalent bondsb. nonpolar covalent bondsc. hydrogen bondsd. ionic bonds

e. polar covalent and hydrogen bonds

14. How would you make a 0.1 M solution of glucose(C6H1206)? The mass numbers for these elementsare C = 12,0 = 16,H = 1.a. Mix 6 g C, 12 g H, and 6 g 0 in 1 liter of water.b. Mix 72 g C, 12 g H, and 96 g 0 in 1 liter of

water.

c. Mix 18 g of glucose with enough water to make1 liter of solution.

d. Mix 29 g of glucose with enough water to yield1 liter of solution.

e. Mix 180 g of gLcose with enough water toyield 1 liter of solution.

15. How many molecules of glucose would be in thesolution in question 14?a. 0.1 c. 60b. 6 d. 6 X 1023

e. 6 x 1022

16. Why is water such an excellent solvent?a. As a polar molecule, it can surround and dis-

solve ionic and other polar molecules.b. It forms ionic bonds with ions, hydrogen bonds

with polar molecules, and hydrophobic inter-actions with nonpolar molecules.

c. It forms hydrogen bonds with itself.d. It has a high specific heat and high heat of

vaporization.e. It is wet and has a great deal of surface tension.

17. Adding a base to a solution woulda. raise the pH.b. lower the pH.c. decrease [H+].d. do both a and c.e. do both b and c.

Chupter 3: ~Vaterand the Fitness of the Environment 1,

,18. The following are pH values: cola-2; orangejuice-3; beer-4; coffee-5; human blood-7.4. Whichof these liquids has the highest molar concentra-tion of OH?

a. cola

b. orange juicec. beer -

d. coffee.

e. human blood

19. Comparing the [H+] of orange juice and coffee,

a. the [H+] of coffee is two times higher.b. the [H+] of coffee is 100 times higher.c. the [H+] of orange juice is 10 times higher.d. the [H+] of orange juice is 100 times higher.e. the [H+] of orange juice is 1000 times higher.

20. The ability of water molecule~ to form hydrogenbonds accounts for water's

a. high specific heat.b. evaporative cooling.c. high heat of vaporization.d. cohesiveness and surface tension.

e. All of the above result from water's hydro-gen-bonding capacity.

j

, .. "

CHAPTER 4,'",

CARBON AND THE MOLECULAFDIVERSITY OF LIFI

FRAMEWORK

I CARBON I./" "-

essential to forms

can form can bind with

structural andfunctionaldiversity

straight orbranching chains

and rings

increase may eXlst as

~isomer~

CHAPTER REVIEW

The Importance of Carbon

. Organicchemistryis the study of carboncompounds (48-49)

Organic chemistry is the study of carbon-containingmolecules. Early organic chemists could not synthe-size the complex molecules found in living organismsand therefore attributed the existence of life and the

formation of these molecules to a life force indepen-dent of physical and chemicallaw5, a belief known asvitalism. Mechanism, the philosophy underlyingmodern organic chemistry, holds that physical andchemical laws and explanations are sufficient to

lR

account for all natural phenomena, even the evolutiorof life.

. Carbonatoms are the most versatile buildingblocks of molecules (49-51)

Carbon has six electrons. To complete its valenceshell, carbon forms four covalent bonds with otheratoms. This tetravalence is at the center of carbon's

ability to form large and complex molecules withcharacteristic three-dimensional shapes and proper-ties. When carbon forms four single covalent bonds,it's hybrid orbitals create a tetrahedral shape. Whentwo carbons are joined by a double bond, the othercarbon bonds are in the same plane, forming a flatmolecule,

. Variation in carbon skeletons contributes tothe diversity of organic molecules (51-53)

Carbon atoms readily bond with each other, produc-ing chains or rings of carbon atoms. These molecularbackbones can vary in length, branching, placementof double bonds, and location of atoms of other ele-ments. The simplest organic molecules are hydrocar-

. INTERACTIVE QUESTION 4.1

From what you know about the valences of C, H, and0, sketch the structural formulas for the followingmolecules: C3H8O (propanol) and CzH4 (ethene).

c

bons, consisting of only carbon and hydrogen. Thenonpolar C-H bonds in hydrocarbon chains accountfor their hydrophobic behavior.

Isomers Isomers are compounds with the same mol-ecular formula but different structural arrangementsand, thus, different properties. Structural isomers dif-fer in the arrangement of atoms and often in the loca-tion of double bonds. Geometric isomers have the

same sequence of covalently bonded atoms but differin spatial arrangement due to the inflexibility of dou-ble bonds. Enantiomers are left- and right-handedversions of each other and can differ greatly in theirbiological activity. An asymmetric carbon is one thatis covalently bound to four different kinds of atoms orgroups of atoms. Due to the tetrahedral shape of theasymmetric carbon, the four groups can be attachedin spatial arrangements that are not superimposableon each other.

. INTERACTIVE QUESTION 4.2

Identify the structural isomers, geometric isomers,and enantiomers from the following compounds.

COOHI

H3C-C-HI

OH

HHI I

H-C-C-OHI I

HH

I-lactic acid ethanol

COOHI

H-C-CH3IOH

0II

H-C-C-OHII

HO-C-C-HII0

d -lactic acid fumaric acid

H HI I

H-C-O-C-HI I.

H H

0II

H-C-C-OHII

H-C-C-OHII0

dimethyl ether maleic acid

Chapter 4: Carbon and the Molewlar Diversity of Life 1\

Functional Groups

. Functional groups also contribute to themolecular diversity of life (53-55)

The properties of organic molecules are determinedby groups of atoms, known as functional groups,which bond to the carbon skeleton and behave consis-

tently from one carbon-based molecule to another.Because the functional groups considered here arehydrophilic, they increase the solubility of organiccompounds in water.

The hydroxyl group consists of an oxygen andhydrogen (-OH) covalently bonded to the carbonskeleton. Organic molecules with hydroxyl groups arecalled alcohols, and their names often end in -oJ.

Carbonyl groups consist of a carbon double-bond-ed to an oxygen (::CO). If the carbonyl group is at theend of the carbon skeleton, the compound is called analdehyde. Otherwise, the compound is called aketone.

A carboxyl group consists of a carbon double-bonded to an oxygen and also attached to a hydroxylgroup (-COO H). Compounds with a carboxyl groupare called carboxylic acids or organic acids becausethey tend to dissociate to release H+.

An amino group consists of a nitrogen atom bond-ed to two hydrogens (-NH2). Compounds with anamino group, called amines, can act as bases. Thenitrogen, with its pair of unshared electrons, canattract a hydrogen ion, becoming -NH3+.

The sulfhydryl group consists of a sulfur atombonded to a hydrogen (-SH). Thiols are compoundscontaining sulfhydryl groups.

A phosphate group is bonded to the carbon skele-ton by its oxygen attached to the phosphorus atomthat is bonded to three other oxygen atoms (-QPOi2).The group is an anion.

. The chemicalelements of life:a review (55)

Carbon, oxygen, hydrogen, nitrogen, and smallerquantities of sulfur and phosphorus, all capable offorming strong covalent bonds, are combined into thecomplex organic molecules of living matter. The ver-satility of carbon in forming four covalent bonds, link-ing readily with itself to produce chains and rings,and binding with other elements and functionalgroups makes possible the incredible diversity oforganic molecules. .

(f!

20 Unit Of/e: The Chemistry ofLife,

STRUCTURE YOUR KNOWLEDGE

1. Construct a concept map that illustrates yourunderstanding of the characteristics and signifi-cance of the three types of isomers. A suggestedmap is in the answer section. Comparing and dis-cussing your map with that of a study partner wouldbe most helpful.

2. Fill in the following table on the functionalgroups.

TEST YOUR KNOWLEDGE

MULTIPLE CHOICE: Choose the one best answer.

1. The tetra valence of carbon most directly resultsfrom

a. its tetrahedral shape.b. its very slight electronegativity.c. its four electrons in the valence shell that can

form four covalent bonds.d. its ability to form single, double, and triple

bonds.

e. its ability to form chains and rings of carbonatoms.

2. Hydrocarbons are not soluble in water becausea. they are hydrophilic.

b. the C-H bond is very nonpolar.c. they do not ionize.d. they store energy in the many C-H bonds

along the carbon backbone.e. they are lighter than water.

3. Which of the following is not true of an asymmet-ric carbon atom?

a. It is attached to four different atoms or groups.b. It results in right- and left-handed versions of a

molecule.c. It can create enantiomers.d. Its configuration is in the shape of a tetrahe-

dron.e. It can create geometric isomers.

4. A reductionist approach to considering the struc-ture and function of organic molecules would bebased ona. mechanism.b. holism.c. determinism.d. vitalism.e. emergent properties.

5. The functional group that can cause an organicm0lecule to act as a base isa. -COOH. c. -SH. e. -OPO3-2.b. -OH. d. -NH2.

6. The functional group that confers acidic proper-ties on organic molecules isa. -COOH. c. -SH.b. -OH. d. -NH2.

e. :;C=O

7. Which is not true about structural isomers?a. They have different chemical properties.b. They have the same molecular formula.c. Their atoms and bonds are arranged in differ-

ent sequences.d. They are a result of restricted move men I

around a carbon double bond.e. Their possible numbers increase as carbor

skeletons increase in size.

8. How many asymmetric carbons are there in thtsugar galactose?a. 1 c. 3 e. 5b.2 d.4

H 0'-./

CI

H- C-OHI

H- C-OHI

H-C-OHI

H-C-OHI

Hu

Names and CharacteristicsFunctional Molecular of Organic Compounds

Group Fonnula Containing Functional Group

-OH

Aldehyde or ketone; polargroup

Carboxyl

-NH2

Thiols; cross-links stabilizeprotein structure

Phosphate