AP Biology

Why are we studying Why are we studying water?water?

All life occurs in waterAll life occurs in water– inside & outside the cellinside & outside the cell

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Chemistry of waterChemistry of water

Water is polar moleculeWater is polar molecule– remember polar covalent bondsremember polar covalent bonds– + & – poles+ & – poles

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Chemistry of waterChemistry of water HH22O molecules form H bonds with O molecules form H bonds with

each othereach other– + attracted to –+ attracted to –– creates a sticky creates a sticky

molecule molecule

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CohesionCohesion

H bonding between HH bonding between H22O creates O creates cohesioncohesion– water is “sticky”water is “sticky”– surface tensionsurface tension– drinking strawdrinking straw

can you suck can you suck sugar up a sugar up a straw?straw?

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How does HHow does H22O get to O get to top of tree?top of tree? Transpiration Transpiration

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AdhesionAdhesion

HH22O molecules form H bonds with O molecules form H bonds with other substances other substances – capillary actioncapillary action– meniscusmeniscus– water climbs up fiberwater climbs up fiber

ex. paper towelex. paper towel

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Water is the solvent of Water is the solvent of lifelife

HH22O is a good solvent due to its O is a good solvent due to its polaritypolarity– polar Hpolar H22O molecules surround + & – O molecules surround + & –

ionsions– solvents dissolve solutes creating solvents dissolve solutes creating

aqueous solutionsaqueous solutions

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HydrophilicHydrophilic HydrophilicHydrophilic

– substances have affinity for Hsubstances have affinity for H22OO

– polarpolar or or non-polarnon-polar??– ionicionic

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HydrophobicHydrophobic Hydrophobic Hydrophobic

– substances do not have affinity for Hsubstances do not have affinity for H22OO– polarpolar or or non-polarnon-polar??– non-ionicnon-ionic

fat (triglycerol)

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The special case of iceThe special case of ice

Most (all?) substances are more Most (all?) substances are more dense when they are soliddense when they are solid

But not water…But not water… Ice Ice floatsfloats!!

– H bonds form a crystal with H bonds form a crystal with loose structure loose structure

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Ice floatsIce floats

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Why is “ice floats” Why is “ice floats” important?important?

Oceans & lakes don’t freeze solidOceans & lakes don’t freeze solid– if ice sank…if ice sank…

eventually all ponds, lakes & even ocean eventually all ponds, lakes & even ocean would freeze solidwould freeze solid

during summer, only upper few inches during summer, only upper few inches would thawwould thaw

– surface ice insulates water belowsurface ice insulates water below allowing life to survive the winterallowing life to survive the winter

– seasonal turnover of lakesseasonal turnover of lakes cycling nutrientscycling nutrients

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Specific heatSpecific heat HH22O has high specific heatO has high specific heat

– due to H bondingdue to H bonding

HH22O resists changes in temperatureO resists changes in temperature– takes a lot to heat it uptakes a lot to heat it up– takes a lot to cool it downtakes a lot to cool it down

HH22O moderates temperatures on O moderates temperatures on EarthEarth

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Evaporative coolingEvaporative cooling

Organisms rely on Organisms rely on heat of vaporization heat of vaporization to remove heatto remove heat

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Water forms ionsWater forms ions

Hydrogen ion (HHydrogen ion (H++) splits off from water to ) splits off from water to leave a hydroxide ion (leave a hydroxide ion (--OH) OH)

HH220 ----> H0 ----> H++ + + --OHOH

If concentration of 2 ions is equal, If concentration of 2 ions is equal, water is water is neutralneutral

If [If [HH++]] > [> [--OHOH], water is ], water is acidicacidic If [If [--OHOH] > [] > [HH++],], water is water is basicbasic pH scalepH scale = how acidic or basic a solution is = how acidic or basic a solution is

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pH ScalepH Scale

In pure water only 1 water molecule In pure water only 1 water molecule in every 554 million is dissociated.in every 554 million is dissociated.– very small amount of ionsvery small amount of ions– [H[H++] or [OH] or [OH--] is 10] is 10-7-7MM

[H[H++] [OH] [OH--] = 10] = 10-14-14

pH scale is based on this equationpH scale is based on this equation

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pH ScalepH Scale In neutral solution In neutral solution

[H[H++] = 10] = 10-7-7 pH = 7 pH = 7 Values for pH Values for pH declinedecline as [H as [H++] ] increaseincrease

Acids Acids – adding acid adding acid

increases [Hincreases [H+]+] BasesBases

– adding base adding base increases [OHincreases [OH--]]

AP Biology

pH & BiologypH & Biology

pH of a neutral solution = 7pH of a neutral solution = 7 Acidic solutions Acidic solutions == pH < 7 pH < 7

Basic solutionsBasic solutions = = pH > 7pH > 7 Most biological fluids have pH 6 – 8Most biological fluids have pH 6 – 8

– pH values in human stomach can reach 2pH values in human stomach can reach 2 Each pH unit represents a Each pH unit represents a 10-fold10-fold

difference in Hdifference in H++ & OH & OH-- concentrations. concentrations.– small change in pH actually indicates a small change in pH actually indicates a

substantial change in [Hsubstantial change in [H++] & [OH] & [OH--]]

AP Biology

Water is a polar moleculeWater is a polar molecule The special properties of water The special properties of water

make life on Earth possiblemake life on Earth possible The chemical behavior of water The chemical behavior of water

governs how organisms functiongoverns how organisms function

PunchlinePunchline

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Any Questions??Any Questions??