AP Biology Chemistry of Life Properties of Water

AP Biology The amazing molecule: Water

AP Biology More about Water Why are we studying water? All life occurs in water  inside & outside the cell All life occurs in water  inside & outside the cell

AP Biology Chemistry of water  H 2 O molecules form H-bonds with each other  + attracted to –  creates a sticky molecule

AP Biology Elixir of Life  Special properties of water  cohesion & adhesion  surface tension, capillary action  good solvent  many molecules dissolve in H 2 O  hydrophilic vs. hydrophobic  lower density as a solid  ice floats!  high specific heat  water stores heat  high heat of vaporization  heats & cools slowly I like the part about the ice!

AP Biology Cohesion & Adhesion  H bonding between H 2 O molecules is cohesion  water is “sticky”  surface tension  drinking straw  H bonding between H 2 O & other substances is adhesion  capillary action  meniscus  water climbs up paper towel or cloth Can you suck sugar up a straw?

AP Biology How does H 2 O get to top of trees? Transpiration built on cohesion & adhesion How tall can a tree get?

AP Biology Water is the solvent of life  Polarity makes H 2 O a good solvent  polar H 2 O molecules surround + & – ions  solvents dissolve solutes creating solutions

AP Biology Do you dissolve in water?  Hydrophilic  substances have attraction to H 2 O  polar or non-polar?

AP Biology Or don’t you?  Hydrophobic  substances that don’t have an attraction to H 2 O  polar or non-polar? fat (triglycerol)

AP Biology The special case of ice  Most (all?) substances are more dense when they are solid, but Not water…  Ice floats!  H bonds form a crystal And this has made all the difference!

AP Biology Ice floats

AP Biology Why is “ice floats” important?  Oceans & lakes don’t freeze solid  surface ice insulates water below  allowing life to survive the winter  if ice sank…  ponds, lakes & even oceans would freeze solid  in summer, only upper few inches would thaw  seasonal turnover of lakes  cycling nutrients in autumn

AP Biology Specific heat  H 2 O resists changes in temperature  high specific heat  takes a lot to heat it up  takes a lot to cool it down  H 2 O moderates temperatures on Earth

AP Biology Heat of vaporization Evaporative cooling Organisms rely on heat of vaporization to remove body heat

AP Biology Ionization of water & pH  Water ionizes  H + splits off from H 2 O, leaving OH –  if [H + ] = [ - OH], water is neutral  if [H + ] > [ - OH], water is acidic  if [H + ] < [ - OH], water is basic  pH scale  how acid or basic solution is  1  7  14 H 2 O  H + + OH –

AP Biology pH Scale  In pure water only 1 water molecule in every 554 million is dissociated.  very small amount of ions  [H + ] or [OH - ] is M [H + ] [OH - ] =  pH scale is based on this equation

AP Biology pH Scale 10 –1 H + Ion Concentration Examples of Solutions Stomach acid, Lemon juice 1 pH 10 0 Hydrochloric acid0 10 – –3 Vinegar, cola, beer 3 10 –4 Tomatoes 4 10 –5 Black coffee, Rainwater 5 10 –6 Urine, Saliva 6 10 –7 Pure water, Blood 7 10 –8 Seawater 8 10 –9 Baking soda 9 10 –10 Great Salt Lake –11 Household ammonia –12 Household bleach –13 Oven cleaner –14 Sodium hydroxide14

AP Biology Amount of base added Buffering range 452 pH Buffers & cellular regulation  pH of cells must be kept ~7  pH affects shape of molecules  shape of molecules affect function  pH affects cellular function  Control pH by buffers  reservoir of H +  donate H+ when [H + ] falls  absorb H+ when [H + ] rises

AP Biology pH & Biology  pH of a neutral solution = 7  Acidic solutions =pH 7  Most biological fluids have pH 6 – 8  pH values in human stomach can reach 2  Each pH unit represents a 10-fold difference in H + & OH - concentrations.  small change in pH actually indicates a substantial change in [H + ] & [OH - ]

AP Biology  Water is a polar molecule  The special properties of water make life on Earth possible  The chemical behavior of water governs how organisms function Punchline

AP Biology He’s gonna earn a Darwin Award! Do one brave thing today…then run like hell!