Water & life

Campbell and ReeceChapter 3Water & LifeHydrogen Bonding2 characteristics contribute to H-bondspolar covalent bond between H & Omolecule is bent (V-shaped)

Water in Liquid StateH-bonds ~ 1/20th as strong as covalent bondbonds constantly being formed/broken (each lasts a few trillionth of a sec.)@ any given moment a substantial % of all water molecules are part of a H-bond H-bonds in Water

4 Emergent Properties of Waterall 4 contribute to suitability for life on EarthCohesionModeration of Temperature by WaterIce FloatsWater is Universal SolventCohesion of Water MoleculesH-bonds hold water molecules together = cohesion (like molecules sticking together)contributes (with adhesion) to the transport of water & nutrients against gravity in plants

Water & Nutrients Move up the Xylem

Surface Tensiona measure of how difficult it is to stretch or break the surface of a liquidWater has greater surface tension than most other liquids due to H-bonding

Moderation of Temperature by WaterWater moderates temperature by absorbing heat from air that is warmer and releasing stored heat energy to air that is cooler.Water can absorb or release a relatively large amount of heat with only a slight change in its own temperature.Citrus Trees Sprayed with Water to Prevent Frost Damage

Heat & Temperatureheat: a measure of matters total KE due to motion of its molecules (so heat depends on volume of matter)temperature: a measure of heat intensity; represents the average KE of its molecules regardless of volume

Calorie the amt of heat it takes to raise the temperature of 1 g water 1 C orthe amt of heat that 1 g water releases when it cools by 1C

Kilocalories a kilocalorie (kcal): quantity of heat required to raise temperature of 1,000 g of water by 1CFood packaging calls a kcal calorie

1 joule (J) = 0.239 cal1 cal = 4.184 JSpecific Heatis the amt of heat that must be absorbed or lost for 1 g of the substance to change its temperature by 1C

specific heat of water is high which explains its ability to stabilize temperature Specific Heat of Water1 cal/g x C specific heat of water high compared to other materials, water changes its temperature less when it absorbs or loses a given amt of heatSpecific Heat of Waterdue to H-bondingabsorbed heat must 1st break all the H-bonds b/4 it increases the average KE of its moleculeswhen temperature drops slightly, many additional H-bonds form releases additional energy in form of heatWaters High Specific Heatallows water to absorb and store a large amt of heat from Sun & warm up only a few degrees@ night & during winter the gradually cooling water warms the airWaters High Specific Heatthe water that covers most of Earth keeps temperature fluctuations on land & water w/in limits to permit life on Earth

Moderating Effects of Large Bodies of Water on Temperatures

Evaporative Coolingtemperature is average KE of moleculessome evaporization of water occurs @ any temp because some individual molecules of water my gain enough KE to gas phase

Heat of Vaporization= quantity of heat a liquid must absorb for 1 g liquid phase gas phasewater has high heat of vaporization relative to most other liquids because of H-bonding: absorbed energy must 1st break H-bonds, then increase KE of molecules

High Heat of Vaporization of WaterGlobal scale: helps moderate Earths climate

Evaporative Cooling= as liquid evaporates the surface of the liquid that remains cools downcontributes to stability of temperature in lakes & pondsprovides mechanism to keep terrestrial organisms from overheatingEvaporative Cooling

Ice Floatswater one of few substances that is less dense in its solid phase than in its liquid phasewater expands when goes from liquid solid> 4C water acts like other liquids: expands as it warms up, contracting as it coolsIce Floatsimportant factor in suitability of Earths environment to lifeif ice sank: lakes & ponds (even oceans) could freeze solid (from bottom to top)with summer temps only top upper meter or so would meltIce Floatsso ice insulates water below it, preventing it from freezing & allowing life to exist under frozen surface

Global Warmingaverage air temperature in Arctic has increased by 1.4C in the 50 years from 1961 -2011ice forms later in fall and melts earlier to cover smaller area

Water: Solvent of Lifeaqueous solution: one in which water is the solventwater is best solvent on Earth because

water ispolarWater as Solventsalt in water dissociates and each cation and anion is surrounded by water molecules called a hydration shell

Water as Solventsea water & cytosol in cells contain variety of ions all in hydration shellspolar molecules dissolve in water sugarsproteins with ionic or polar regions on surface

Hydrophilic Substancesany substance that has an affinity for watersubstances too large to dissolve but still hydrophilic remain suspended in aqueous liquid (a colloid)

Hydrophobic Substancesnonionic, nonpolar substances repel water

Molecular Mass/Molessum of masses of all the atoms in a moleculenot practical to use #s of molecules so use moles (mol)

1 mole = 6.02 x 10

Molar Massmolecular mass of glucose is 144 daltons6.02 x 10 daltons in 1 gso molecular mass of glucose can be written as 144g in 1 mole of glucose

measuring in moles allows you to combine substances in fixed ratios of moleculesMolarity1 mole of glucose in 1 liter water = 1-molar or 1M solution of glucose

Molarity: the # of moles of solute per liter of water (for aqueous solutions)

Water on MarsAstrobiologists have looked for life elsewhere based on: Is water present?

Mars has polar ice caps, ice under surface and enough water vapor in atmosphere to form froststill looking for life.

Dissociation of Water

Dissociation of Water Moleculesonly 1/554 million molecules of water are dissociated (in pure water)@ 25C the concentration of H+ or OH-is 10- Mconcentration of H+ or OH- is very low but they are each very reactivepH Scale Acid a substance that increases the [H+]overall has more H+ than OH-Base a substance that reduces the [H+]overall has more OH- than H+pH Scalebases:1. reduce [H+] by accepting H+or2. reduce [H+] indirectly by contributing OH-

Weak Acids Form Reversible Reactions

pH ScaleIn any aqueous solution @ 25C, the product of [H+] x [OH-] = 10

the pH scale compresses the range of H+ and OH concentrations by using logarithmspH = the negative logarithm of [H+] or pH = -log [H+]pH ScalepH < 7.0 = acidpH > 7.0 = basemost biological solutions are pH 6-8exceptions: stomach acid is pH

Each pH unit = 10x differences in [H+] or [OH] (pH of 4 has 100x more H+ than pH of 6)Buffersmolecules that are able to resist changes in pHconsists of an acid-base pair that combines reversibly with H+ when H+ in excess & donate H+ when they have been depletedBuffers

Buffers

Acidification burning fossil fuels increases CO2 in atmosphere

CO2 dissolves in water

fresh water & sea water pH decreasesAcidification of Oceans

Acid Precipitationrain, snow, sleet, fog with a pH < 5.2normal rain has pH 5.2adversely affects: life in ponds & streamsland plantssoil chemistryAcid Precipitation