A1.1-TheWayofWater
ater–apolarmoleculethatactsastheuniversalmediumforlife;allmajorbiochemicalprocessesin
W
livingorganismsoccurinwater
aterformsaqueoussolutions–mixtureswheresubstances(solutes)aredissolvedinwater(solvent).
W
Theearliestcellsoriginatedintheseaqueousenvironments,oftenreferredtoasthe“primevalsoup”,
whereorganicmoleculesaccumulatedandinteracted.
medium–asubstanceinwhichprocessesoccur;waterservesasthemediumforalleightprocesses
A
oflife,whichiswhy,after~3+billionyearsofevolution,lifestilldependsonwater.
Roleofwaterintheprocessesoflife:
1. Metabolism–mostbiochemicalreactionsoccurinaqueoussolutionbecausemoleculesarefree
tomoveandcollide
→Ex.Reactantsandproductsaredissolvedinwater,allowingefficientenzymeactivityand
chemicalreactions
2. Nutrition–digestionandphotosynthesisbothoccurinaqueousenvironments
→Ex.Digestiveenzymesbreakdownfoodinsolution;photosynthesisreactionsoccurinthe
aqueousstromaofchloroplasts
3. Growth–cytoplasmisanaqueoussolution
→Ex.Cellsincreaseinsizebyabsorbingwaterviaosmosis,allowingexpansionandelongation
4. Reproduction–wateractsasamediumforgametemovement
→Ex.Spermswimthroughfluidtoreachtheegg;mammalianembryosaresupportedby
amnioticfluid
5. Movement–transportofsubstancesdependsonwater
→Ex.Bloodplasma(animals)andsap(plants)movedissolvedsubstances;aquaticorganisms
relyonwaterforlocomotion
6. ResponsetoStimuli–dependsonthemovementofionsinsolution
→Ex.NerveimpulsesinvolvethemovementofNa⁺andK⁺ions;hormonesaretransportedinthe
bloodstream
7. Excretion–wasteremovalrequireswater
→Ex.Urineisanaqueoussolution;gaseslikeCO₂diffuseacrossmoistsurfaces
8. Homeostasis–internalconditionsareregulatedusingaqueousenvironments
→Ex.BloodplasmaandtissuefluidmaintainstablepH,temperature,andsoluteconcentrations
ydrogenbonds–intermolecularattractioninwater
H
Molecule–twoormoreatomsjoinedbycovalentbonds(sharingofelectrons)
Polarcovalentbond–electronsaresharedunequallyduetodifferencesinelectronegativity
→Createspartialcharges(δ⁺andδ⁻)
Water(H₂O)–apolarmolecule
- Oxygenismoreelectronegative→pullselectronscloser
- Oxygenbecomesslightlynegative(δ⁻)
- Hydrogenatomsbecomeslightlypositive(δ⁺)
(2)Hydrogenbond–weakattractionbetween:
● theδ⁺hydrogenofonemolecule
● andtheδ⁻oxygenofanothermolecule
Typesofbondsinwater:
● Intramolecularbonds–covalentbondswithinthemolecule(O–Hbonds)
● Intermolecularbonds–hydrogenbondsbetweendifferentwatermolecules
lthoughindividualhydrogenbondsareweak,largenumbersformsimultaneously,givingwaterstrong
A
collectiveproperties.
, (3)Cohesion–watermoleculesstickingtoeachother
Cohesion–attractionbetweenwatermoleculesduetohydrogenbonding
Biologicalimportance:
● Createscontinuouscolumnsofwaterinplantxylem
→Allowswatertobepulledfromrootstoleaves(transpirationstream)
→Columnsrarelybreakduetostrongcohesiveforces
● Producessurfacetension
→Watersurfaceresistsbreakingbecausehydrogenbondsmustbedisrupted
→AllowsorganismslikeGerrislacustris(waterstrider)tostandonwater
Surfacetension–cohesivelayeratthesurfaceofwaterthatresistsexternalforce
(4)Adhesion–waterstickingtoothersubstances
Adhesion–attractionbetweenwaterandothermaterials
Occurswhensubstancesarehydrophilic(water-attracting):
● Polarsubstances(e.g.,cellulose)
● Chargedsubstances(ions)
Biologicalimportance:
● Wateradherestocelluloseinplantcellwalls
→Helpsmaintainwatermovementthroughtissues
● Enablescapillaryaction
→Watermovesthroughnarrowspaces(e.g.,xylemvessels,soilpores)
→Movementoccursagainstgravityduetocombinedadhesion+cohesion
● Soilparticles(humus,sand,clay)arehydrophilic
→Drawwaterupwardandretainmoisture
(5)Waterasasolvent–“universalsolvent”properties
➔ Waterdissolvesawiderangeofsubstancesduetoitspolarity
Howsubstancesdissolve:
(1) Polarmolecules(e.g.,urea)
→Formhydrogenbondswithwater
(2) Ions(chargedparticles)
→Positiveions(e.g.,Na⁺)attractedtoδ⁻oxygen
→Negativeions(e.g.,Cl⁻)attractedtoδ⁺hydrogen
→Formshydrationshellsaroundions
Example:NaCldissolving
→Na⁺andCl⁻ionsseparateandbecomesurroundedbywatermolecules
Biologicalimportance:
(1) Allowsenzymereactionstooccur
→Enzymesandsubstratescanmovefreely→morecollisions
(2) Enablestransportsystems
→Bloodplasma,xylemsap,andphloemsapcarrydissolvednutrients,gases,andwastes
(3) Supportsmetabolicreactions
→Mostreactionsoccurinaqueouscytoplasm
HydrophilicvsHydrophobic:
● Hydrophilic–attractedtowater,dissolveseasily
● Hydrophobic–non-polar,doesnotdissolve
→Watermoleculesbondmorestronglytoeachotherthantothesesubstances
ater–apolarmoleculethatactsastheuniversalmediumforlife;allmajorbiochemicalprocessesin
W
livingorganismsoccurinwater
aterformsaqueoussolutions–mixtureswheresubstances(solutes)aredissolvedinwater(solvent).
W
Theearliestcellsoriginatedintheseaqueousenvironments,oftenreferredtoasthe“primevalsoup”,
whereorganicmoleculesaccumulatedandinteracted.
medium–asubstanceinwhichprocessesoccur;waterservesasthemediumforalleightprocesses
A
oflife,whichiswhy,after~3+billionyearsofevolution,lifestilldependsonwater.
Roleofwaterintheprocessesoflife:
1. Metabolism–mostbiochemicalreactionsoccurinaqueoussolutionbecausemoleculesarefree
tomoveandcollide
→Ex.Reactantsandproductsaredissolvedinwater,allowingefficientenzymeactivityand
chemicalreactions
2. Nutrition–digestionandphotosynthesisbothoccurinaqueousenvironments
→Ex.Digestiveenzymesbreakdownfoodinsolution;photosynthesisreactionsoccurinthe
aqueousstromaofchloroplasts
3. Growth–cytoplasmisanaqueoussolution
→Ex.Cellsincreaseinsizebyabsorbingwaterviaosmosis,allowingexpansionandelongation
4. Reproduction–wateractsasamediumforgametemovement
→Ex.Spermswimthroughfluidtoreachtheegg;mammalianembryosaresupportedby
amnioticfluid
5. Movement–transportofsubstancesdependsonwater
→Ex.Bloodplasma(animals)andsap(plants)movedissolvedsubstances;aquaticorganisms
relyonwaterforlocomotion
6. ResponsetoStimuli–dependsonthemovementofionsinsolution
→Ex.NerveimpulsesinvolvethemovementofNa⁺andK⁺ions;hormonesaretransportedinthe
bloodstream
7. Excretion–wasteremovalrequireswater
→Ex.Urineisanaqueoussolution;gaseslikeCO₂diffuseacrossmoistsurfaces
8. Homeostasis–internalconditionsareregulatedusingaqueousenvironments
→Ex.BloodplasmaandtissuefluidmaintainstablepH,temperature,andsoluteconcentrations
ydrogenbonds–intermolecularattractioninwater
H
Molecule–twoormoreatomsjoinedbycovalentbonds(sharingofelectrons)
Polarcovalentbond–electronsaresharedunequallyduetodifferencesinelectronegativity
→Createspartialcharges(δ⁺andδ⁻)
Water(H₂O)–apolarmolecule
- Oxygenismoreelectronegative→pullselectronscloser
- Oxygenbecomesslightlynegative(δ⁻)
- Hydrogenatomsbecomeslightlypositive(δ⁺)
(2)Hydrogenbond–weakattractionbetween:
● theδ⁺hydrogenofonemolecule
● andtheδ⁻oxygenofanothermolecule
Typesofbondsinwater:
● Intramolecularbonds–covalentbondswithinthemolecule(O–Hbonds)
● Intermolecularbonds–hydrogenbondsbetweendifferentwatermolecules
lthoughindividualhydrogenbondsareweak,largenumbersformsimultaneously,givingwaterstrong
A
collectiveproperties.
, (3)Cohesion–watermoleculesstickingtoeachother
Cohesion–attractionbetweenwatermoleculesduetohydrogenbonding
Biologicalimportance:
● Createscontinuouscolumnsofwaterinplantxylem
→Allowswatertobepulledfromrootstoleaves(transpirationstream)
→Columnsrarelybreakduetostrongcohesiveforces
● Producessurfacetension
→Watersurfaceresistsbreakingbecausehydrogenbondsmustbedisrupted
→AllowsorganismslikeGerrislacustris(waterstrider)tostandonwater
Surfacetension–cohesivelayeratthesurfaceofwaterthatresistsexternalforce
(4)Adhesion–waterstickingtoothersubstances
Adhesion–attractionbetweenwaterandothermaterials
Occurswhensubstancesarehydrophilic(water-attracting):
● Polarsubstances(e.g.,cellulose)
● Chargedsubstances(ions)
Biologicalimportance:
● Wateradherestocelluloseinplantcellwalls
→Helpsmaintainwatermovementthroughtissues
● Enablescapillaryaction
→Watermovesthroughnarrowspaces(e.g.,xylemvessels,soilpores)
→Movementoccursagainstgravityduetocombinedadhesion+cohesion
● Soilparticles(humus,sand,clay)arehydrophilic
→Drawwaterupwardandretainmoisture
(5)Waterasasolvent–“universalsolvent”properties
➔ Waterdissolvesawiderangeofsubstancesduetoitspolarity
Howsubstancesdissolve:
(1) Polarmolecules(e.g.,urea)
→Formhydrogenbondswithwater
(2) Ions(chargedparticles)
→Positiveions(e.g.,Na⁺)attractedtoδ⁻oxygen
→Negativeions(e.g.,Cl⁻)attractedtoδ⁺hydrogen
→Formshydrationshellsaroundions
Example:NaCldissolving
→Na⁺andCl⁻ionsseparateandbecomesurroundedbywatermolecules
Biologicalimportance:
(1) Allowsenzymereactionstooccur
→Enzymesandsubstratescanmovefreely→morecollisions
(2) Enablestransportsystems
→Bloodplasma,xylemsap,andphloemsapcarrydissolvednutrients,gases,andwastes
(3) Supportsmetabolicreactions
→Mostreactionsoccurinaqueouscytoplasm
HydrophilicvsHydrophobic:
● Hydrophilic–attractedtowater,dissolveseasily
● Hydrophobic–non-polar,doesnotdissolve
→Watermoleculesbondmorestronglytoeachotherthantothesesubstances
