Chapter 2.2
Water
 About 70% of human body weight
 Important internal compound:
 in photosynthesis
 as a solvent for biological reactions
 as a reactant or product in many chemical reactions
 Also an important external environment for many
organisms
Water molecules are polar
 Oxygen is an electron ‘hog’
 Causes an uneven covalent bond
 Oxygen side has a slight – charge
 Hydrogen sides have a slight + charge
 A water molecule forms H bonds with up to four other
water molecules
Because water is polar…
 It is especially good at dissolving other polar
compounds as well as ionic ones
 Hydrophilic (‘water-loving’) substances dissolve
readily in water
 Examples: table sugar (polar) and salt (ionic)
 Hydrophobic (‘water-fearing’) substances don’t
dissolve easily in water and make good structural body
parts and storage units
 Examples: fats, oils, and waxes (all nonpolar)
Hydrogen bonding of water…
 Makes water ‘sticky’
 Cohesion – water molecules stick to each other
 Adhesion – water molecules stick to other surfaces
 These two properties explain:
 Capillary action – the tendency of water to rise in a
narrow tube even against the force of gravity – how
water rises in plants
 Surface tension – makes the top layer of water
‘stronger’
Capillary action
Surface tension
Hydrogen bonding also means…
 Water has a high specific heat
 The amount of energy required to raise the
temperature is very large
 This means that the temperature of water does not
fluctuate as easily as air
 Aquatic environments (both external and internal) do
not change temperatures as quickly as the
surroundings
 Stable internal temperatures are necessary for the
chemical reactions needed for life
Ice formation is related to
hydrogen bonds
 In water vapor, the hydrogen bonds between the water
molecules are broken
 In liquid water, the hydrogen bonds constantly break
and re-form
 In ice, the bonds are firmly fixed, resulting in a regular,
evenly distributed crystalline lattice structure
 This means ice is less dense than liquid water and
floats
 This has important biological consequences
Acids and Bases
 Water molecules have a slight tendency to ionize into
hydrogen ions (H+) and hydroxide ions (OH-)
 In pure water the number of H+ = OH The solution is neutral
 Acid – solution produces more H+ ions
 It is a proton donor
 Base – solution produces more OH- ions
 It is a proton acceptor
pH scale
 The negative logarithm
of the hydrogen ion
concentration
 Neutral = 7
 Acidic < 7
 Basic > 7
 Most living things stay
between 7.2 to 7.4
Buffers
 Resists changes in pH when an acid or base is added
 Includes a weak acid and a weak base
 Important in biological systems in order to maintain
homeostasis
Salt formation
 Acid + base  water + salt
 HCl + NaOH  H2O + NaCl
 Salt = metal and a nonmetal
 A salt, acid or base dissolved in water will conduct an
electric current: electrolytes