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Chapter 2.2
WaterAbout 70% of human body weightImportant internal compound:
in photosynthesisas a solvent for biological reactionsas a reactant or product in many chemical
reactionsAlso an important external environment for
many organisms
Water molecules are polar
Oxygen is an electron ‘hog’Causes an uneven covalent bondOxygen side has a slight – chargeHydrogen sides have a slight + chargeA 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 onesHydrophilic (‘water-loving’) substances
dissolve readily in waterExamples: table sugar (polar) and salt (ionic)
Hydrophobic (‘water-fearing’) substances don’t dissolve easily in water and make good structural body parts and storage unitsExamples: fats, oils, and waxes (all nonpolar)
Hydrogen bonding of water…Makes water ‘sticky’Cohesion – water molecules stick to each otherAdhesion – water molecules stick to other
surfacesThese 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 heatThe amount of energy required to raise the
temperature is very largeThis means that the temperature of water
does not fluctuate as easily as airAquatic 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 bondsIn water vapor, the hydrogen bonds between
the water molecules are brokenIn liquid water, the hydrogen bonds
constantly break and re-formIn 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 BasesWater 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 neutralAcid – solution produces more H+ ions
It is a proton donorBase – solution produces more OH- ions
It is a proton acceptor
pH scaleThe negative
logarithm of the hydrogen ion concentration
Neutral = 7Acidic < 7Basic > 7Most living things
stay between 7.2 to 7.4
BuffersResists changes in pH when an acid or base
is addedIncludes a weak acid and a weak baseImportant in biological systems in order to
maintain homeostasis
Salt formationAcid + base water + saltHCl + NaOH H2O + NaClSalt = metal and a nonmetal
A salt, acid or base dissolved in water will conduct an electric current: electrolytes