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