Essential Cell Biology 5th edition
68PANEL 2–2 THE CHEMICAL PROPERTIES OF WATERHYDROGEN BONDSBecause they are polarized, twoadjacent H 2 O molecules can forma noncovalent linkage known as ahydrogen bond. Hydrogen bondshave only about 1/20 the strengthof a covalent bond.Hydrogen bonds are strongest whenthe three atoms lie in a straight line.δ +δ +HHδ _ δ +OH O δ _H hydrogenδ + bondHHbond lengthsOhydrogenbond0.17 nmHO0.10 nmcovalent bondWATERTwo atoms connected by a covalent bond may exert different attractions forthe electrons of the bond. In such cases, the bond is polar, with one endslightly negatively charged (δ _ ) and the other slightly positively charged (δ + ).WATER STRUCTUREMolecules of water join together transientlyin a hydrogen-bonded lattice.H δ+electropositiveregionOδ +Hδ _ δ _electronegativeregionAlthough a water molecule has an overall neutral charge (having the samenumber of electrons and protons), the electrons are asymmetrically distributed,making the molecule polar. The oxygen nucleus draws electrons away fromthe hydrogen nuclei, leaving the hydrogen nuclei with a small net positive charge.The excess of electron density on the oxygen atom creates weakly negativeregions at the other two corners of an imaginary tetrahedron. On these pages,we review the chemical properties of water and see how water influences thebehavior of biological molecules.The cohesive nature of water isresponsible for many of its unusualproperties, such as high surface tension,high specific heat capacity, and high heatof vaporization.HYDROPHILIC MOLECULESSubstances that dissolve readily in water are termed hydrophilic. They includeions and polar molecules that attract water molecules through electrical chargeeffects. Water molecules surround each ion or polar molecule and carry itinto solution.HH HOH OOδ _ Oδ _ HHδ + HHδ +δ _ Na + H Hδ + Cl _Oδ _ δ _ O O H Hδ + δ +HH OH O HHO HHIonic substances such as sodium chloridedissolve because water molecules areattracted to the positive (Na + ) or negative(Cl _ ) charge of each ion.HOOHHHHHOHOCOHHHNNHHHOHOHHHPolar substances such as ureadissolve because their moleculesform hydrogen bonds with thesurrounding water molecules.HYDROPHOBIC MOLECULESSubstances that contain a preponderanceof nonpolar bonds are usually insolublein water and are termed hydrophobic.Water molecules are not attracted to suchhydrophobic molecules and so have littletendency to surround them and bring theminto solution.HH OH HH HHOHCHCCHHHHydrocarbons, which contain manyC–H bonds, are especially hydrophobic.OHOOHHHHOOHHH
69WATER AS A SOLVENTMany substances, such as household sugar (sucrose), dissolve in water. That is, theirmolecules separate from each other, each becoming surrounded by water molecules.watermoleculesugar crystalsugardissolvessugar moleculeWhen a substance dissolves in aliquid, the mixture is termed a solution.The dissolved substance (in this casesugar) is the solute, and the liquid thatdoes the dissolving (in this case water)is the solvent. Water is an excellentsolvent for hydrophilic substancesbecause of its polar bonds.ACIDSSubstances that release hydrogen ions (protons) into solutionare called acids.HYDROGEN ION EXCHANGEPositively charged hydrogen ions (H + ) can spontaneouslymove from one water molecule to another, thereby creatingtwo ionic species.HClhydrochloric acid(strong acid)Many of the acids important in the cell are not completelydissociated, and they are therefore weak acids—for example,the carboxyl group (–COOH), which dissociates to give ahydrogen ion in solution.COOHcarboxyl group(weak acid)H +hydrogen ionNote that this is a reversible reaction.H + ++Cl –chloride ionCH HO H OHoften written as:OO –Because the process is rapidly reversible, hydrogen ions arecontinually shuttling between water molecules. Pure watercontains equal concentrations of hydronium ions andhydroxyl ions (both 10 –7 M).H+OHhydronium ionhydroxyl ionH 2 O H + + OH –hydrogenionH +OHhydroxylionpHThe acidity of asolution is definedby the concentration (conc.)of hydronium ions (H 3 O + ) itpossesses, generallyabbreviated as H + .For convenience, weuse the pH scale, wherepH = _ log 10 [H + ]For pure water[H + ] = 10 _ 7 moles/literpH = 7.0H +conc.moles/literACIDICBASIC1 010 _ 110 _ 210 _ 310 _ 410 _ 510 _ 610 _ 710 _ 810 _ 910 _ 1010 _ 1110 _ 1210 _ 1310 _ 14pH1234567891011121314BASESSubstances that reduce the number of hydrogen ions insolution are called bases. Some bases, such as ammonia,combine directly with hydrogen ions.NH 3ammonia+ H +hydrogen ionNH 4+ammonium ionOther bases, such as sodium hydroxide, reduce the number ofH + ions indirectly, by producing OH – ions that then combinedirectly with H + ions to make H 2 O.NaOH Na + +sodium hydroxidesodium(strong base)ionOH –hydroxylionMany bases found in cells are partially associated with H + ionsand are termed weak bases. This is true of compounds thatcontain an amino group (–NH 2 ), which has a weak tendencyto reversibly accept an H + ion from water, therebyincreasing the concentration of free OH – ions.–NH 2 + H + + –NH 3Panel 2.02b
- Page 51 and 52: The Eukaryotic Cell17nucleusnuclear
- Page 53 and 54: The Eukaryotic Cell19chloroplastsch
- Page 55 and 56: The Eukaryotic Cell21lysosomenuclea
- Page 57 and 58: The Eukaryotic Cell23duplicatedchro
- Page 59 and 60: PANEL 1-2 CELL ARCHITECTURE 25ANIMA
- Page 61 and 62: Model Organisms27(C)(D)(A) (B) (E)
- Page 63 and 64: Model Organisms29Figure 1-34 Drosop
- Page 65 and 66: Model Organisms31INTRODUCE FRAGMENT
- Page 67 and 68: Model Organisms33(A) (B) (C)50 µm
- Page 69 and 70: Model Organisms35TABLE 1-2 SOME MOD
- Page 71 and 72: Questions37• Free-living, single-
- Page 73 and 74: CHAPTER TWO2Chemical Components of
- Page 75 and 76: Chemical Bonds41number of protons.
- Page 77 and 78: Chemical Bonds43+atoms+SHARING OFEL
- Page 79 and 80: Chemical Bonds45Four electrons can
- Page 81 and 82: Chemical Bonds47Because of the favo
- Page 83 and 84: Chemical Bonds49Some Polar Molecule
- Page 85 and 86: Small Molecules in Cells51with othe
- Page 87 and 88: Small Molecules in Cells53optical i
- Page 89 and 90: Small Molecules in Cells55can be re
- Page 91 and 92: Small Molecules in Cells57O _O _ ph
- Page 93 and 94: Macromolecules in Cells59Figure 2-3
- Page 95 and 96: Macromolecules in Cells61ultracentr
- Page 97 and 98: Macromolecules in Cells63BBAAthe su
- Page 99 and 100: Questions65KEY TERMSacid electrosta
- Page 101: 67C-O COMPOUNDSMany biological comp
- Page 105 and 106: 71ELECTROSTATIC ATTRACTIONSElectros
- Page 107 and 108: 73α AND β LINKSThe hydroxyl group
- Page 109 and 110: 75LIPID AGGREGATESFatty acids have
- Page 111 and 112: 77ACIDIC SIDE CHAINSNONPOLAR SIDE C
- Page 113 and 114: 79NOMENCLATUREThe names can be conf
- Page 115 and 116: CHAPTER THREE3Energy, Catalysis, an
- Page 117 and 118: The Use of Energy by Cells83(A) (B)
- Page 119 and 120: The Use of Energy by Cells85ABraise
- Page 121 and 122: The Use of Energy by Cells87H 2 OPH
- Page 123 and 124: Free Energy and Catalysis89thermody
- Page 125 and 126: Free Energy and Catalysis91lake wit
- Page 127 and 128: Free Energy and Catalysis93FOR THE
- Page 129 and 130: Free Energy and Catalysis95REACTION
- Page 131 and 132: Free Energy and Catalysis97to the c
- Page 133 and 134: Free Energy and Catalysis99Figure 3
- Page 135 and 136: Activated Carriers and Biosynthesis
- Page 137 and 138: Activated Carriers and Biosynthesis
- Page 139 and 140: Activated Carriers and Biosynthesis
- Page 141 and 142: Activated Carriers and Biosynthesis
- Page 143 and 144: Activated Carriers and Biosynthesis
- Page 145 and 146: Activated Carriers and Biosynthesis
- Page 147 and 148: Essential Concepts113ESSENTIAL CONC
- Page 149: Questions115a kilocalorie (kcal) is
69
WATER AS A SOLVENT
Many substances, such as household sugar (sucrose), dissolve in water. That is, their
molecules separate from each other, each becoming surrounded by water molecules.
water
molecule
sugar crystal
sugar
dissolves
sugar molecule
When a substance dissolves in a
liquid, the mixture is termed a solution.
The dissolved substance (in this case
sugar) is the solute, and the liquid that
does the dissolving (in this case water)
is the solvent. Water is an excellent
solvent for hydrophilic substances
because of its polar bonds.
ACIDS
Substances that release hydrogen ions (protons) into solution
are called acids.
HYDROGEN ION EXCHANGE
Positively charged hydrogen ions (H + ) can spontaneously
move from one water molecule to another, thereby creating
two ionic species.
HCl
hydrochloric acid
(strong acid)
Many of the acids important in the cell are not completely
dissociated, and they are therefore weak acids—for example,
the carboxyl group (–COOH), which dissociates to give a
hydrogen ion in solution.
C
O
OH
carboxyl group
(weak acid)
H +
hydrogen ion
Note that this is a reversible reaction.
H + +
+
Cl –
chloride ion
C
H H
O H O
H
often written as:
O
O –
Because the process is rapidly reversible, hydrogen ions are
continually shuttling between water molecules. Pure water
contains equal concentrations of hydronium ions and
hydroxyl ions (both 10 –7 M).
H
+
O
H
hydronium ion
hydroxyl ion
H 2 O H + + OH –
hydrogen
ion
H +
O
H
hydroxyl
ion
pH
The acidity of a
solution is defined
by the concentration (conc.)
of hydronium ions (H 3 O + ) it
possesses, generally
abbreviated as H + .
For convenience, we
use the pH scale, where
pH = _ log 10 [H + ]
For pure water
[H + ] = 10 _ 7 moles/liter
pH = 7.0
H +
conc.
moles/liter
ACIDIC
BASIC
1 0
10 _ 1
10 _ 2
10 _ 3
10 _ 4
10 _ 5
10 _ 6
10 _ 7
10 _ 8
10 _ 9
10 _ 10
10 _ 11
10 _ 12
10 _ 13
10 _ 14
pH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
BASES
Substances that reduce the number of hydrogen ions in
solution are called bases. Some bases, such as ammonia,
combine directly with hydrogen ions.
NH 3
ammonia
+ H +
hydrogen ion
NH 4
+
ammonium ion
Other bases, such as sodium hydroxide, reduce the number of
H + ions indirectly, by producing OH – ions that then combine
directly with H + ions to make H 2 O.
NaOH Na + +
sodium hydroxide
sodium
(strong base)
ion
OH –
hydroxyl
ion
Many bases found in cells are partially associated with H + ions
and are termed weak bases. This is true of compounds that
contain an amino group (–NH 2 ), which has a weak tendency
to reversibly accept an H + ion from water, thereby
increasing the concentration of free OH – ions.
–NH 2 + H + + –NH 3
Panel 2.02b
Change language