Why are we studying water in Biology? How does the habitat of a polar bear demonstrate the properties of water? AP Biology

Polar Covalent Bonds Atoms differ in their affinity for electrons Electronegativity AP Biology

Nonpolar vs Polar Covalent Bonds Nonpolar Covalent Bonds Affinity for electrons is the same between identical atoms Electrons are shared equally & bond is nonpolar Polar Covalent Bonds For atoms that differ greatly in electronegativity, electrons are not shared equally. Shared electrons will likely be closer to the atom with greater electronegativity Molecule is still electrically neutral but regions of partial (δ) negative & positive charges exist AP Biology

Chemistry of water Polar molecule polar covalent bonds δ+ and δ- poles AP Biology

Water molecules form H bonds with each other δ+ attracted to δ–   Hydrogen bond  + Polar covalent bonds    +    +    + Figure 3.2 Hydrogen bonds between water molecules.

Properties of Water Properties exist because of the polarity of water Cohesion Adhesion High Specific Heat High Heat of Vaporization Lower Density of Ice Solubility AP Biology

Cohesion Water molecules are attracted to one another H bonding between water creates cohesion Individual H bonds are transient & last only 100 billionth of a second Cumulative effects of large numbers of H bonds are enormous! Surface tension AP Biology

Adhesion Water molecules form H bonds with other substances Capillary action Meniscus AP Biology

How does water get to top of tree? Transpiration (cohesion and adhesion) AP Biology

Water is the solvent of life Water is a good solvent due to its polarity Polar water molecules surrounds + and – ions Solvents dissolve solutes creating aqueous solutions Hydration shell Sphere of water molecules around each dissolved ion Nonionic compounds are also water soluble if they have ionic and/or polar regions on their surface AP Biology

Hydrophilic Hydrophilic Substances have affinity for water Polar or nonpolar? ionic (a) Lysozyme molecule in a nonaqueous environment (b) Lysozyme molecule (purple) in an aqueous environment (c) Ionic and polar regions on the protein’s surface attract water molecules. AP Biology

Hydrophobic Substances do not have affinity for water Polar or nonpolar? Non-ionic AP Biology

The special case of ice Most substances are more dense when they are solid But not water… Ice floats H bonds form a crystal with loose structure AP Biology

Ice floats Liquid water: Hydrogen bonds break and re-form Ice: Fig. 3-6a Ice floats Liquid water: Hydrogen bonds break and re-form Ice: Hydrogen bonds are stable AP Biology

Why is the density of ice important? Oceans and lakes don’t freeze solid If ice sank… All ponds, lakes and oceans would eventually freeze solid During summer, only upper few inches would thaw Surface ice insulates water below Allows life to survive winter Seasonal turnover of lakes Cycling nutrients AP Biology

AP Biology

Specific heat Amount of heat 1g of a substance must absorb or lose to change its temperature by 1°C Water has high specific heat Due to H bonding Water resists changes in temperature Takes a lot to heat it up Takes a lot to cool it down Water moderates temperatures on earth AP Biology

High Specific Heat Water will change its temperature less when it absorbs or loses a given amount of heat In order to change water from solid to a liquid or liquid to a gas: Hydrogen bonds between water molecules must be broken--heat must be absorbed AFTER hydrogen bonds are broken, then water molecules can begin to move faster, thus increasing their kinetic energy The reverse is also true…. AP Biology

Heating curve for 1.00 mol of ice at -25° C AP Biology

High Heat of Vaporization Amount of energy required to change 1g of substance from liquid to gas Requires lots of heat energy (586 cal) to accomplish this change in water Facilitates Cooling Organisms rely on heat of vaporization to remove heat Evaporative cooling Results from high heat of vaporization Tendency for water molecules to move fast enough to overcome attraction to one another and depart the liquid and enter the air as gas Molecules with greatest energy tend to vaporize first Reduces average speed of remaining molecules of water When substance evaporates, surface of liquid remaining behind cools Cools land dwelling animals and plants, cools bodies of water etc… AP Biology

Why is this important to living things? Large bodies of water absorb and store huge amount of heat in daytime and during warm seasons, without a dramatic increase in temperature At night and during cooler seasons, gradual cooling of water warms the air Results in coastal areas having milder climates than areas further away from the ocean Tends to stabilize ocean temperatures Because organisms are made mostly of water, they are able to resist changes in their own temperatures AP Biology

San Bernardino Burbank Santa Barbara 73° 100° 90° Riverside 96° Fig. 3-5 San Bernardino 100° Burbank 90° Santa Barbara 73° Riverside 96° Los Angeles (Airport) 75° Santa Ana 84° Palm Springs 106° 70s (°F) 80s Pacific Ocean 90s 100s San Diego 72° 40 miles AP Biology

Water forms ions Hydrogen ion (H+) splits off from water to leave a hydroxide ion (OH-) If concentration of 2 ions is equal, water is neutral If [H+] > [OH-], water is acidic If [OH-] > [H+], water is basic pH scale Acidity or basicity of a solution Relative concentrations of H+ and OH+ AP Biology

Acidic [H+] > [OH] Neutral [H+] = [OH] Basic [H+] < [OH] Figure 3.UN05 Acidic [H+] > [OH] Acids donate H+ in aqueous solutions. Neutral [H+] = [OH] 7 Bases donate OH or accept H+ in aqueous solutions Figure 3.UN05 Summary figure, Concept 3.3 Basic [H+] < [OH] 14 24

pH scale In pure water, only 1 water molecule in every 554 million is dissociated Very small amount of ions pH = -log [H+] [H+] or [OH-] is 10-7 M pH scale based on this equation AP Biology

pH and biology Most biological fluids 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+] and [OH-] AP Biology

Buffers Living organisms must maintain pH within a fairly narrow range Substance that minimizes changes in [H+] & [OH-] in a solution Buffers accept H+ when they are in excess & donate H+ when they are depleted AP Biology

Acid Deposition More acidic 1 2 3 Acid rain Acid rain 4 5 Normal rain Fig. 3-10 Acid Deposition More acidic 1 2 3 Acid rain Acid rain 4 5 Normal rain 6 7 8 9 10 11 12 13 More basic 14 AP Biology

Ocean Acidification Carbonic Acid Bicarbonate ion CO2 Ocean Acidification Carbonic Acid CO2 + H2O H2CO3 H2CO3 H+ + HCO3 Bicarbonate ion H+ + CO32 HCO3 Reduced amounts of CO32- available for calcification Figure 3.11 Atmospheric CO2 from human activities and its fate in the ocean. CO32 + Ca2+ CaCO3 29

Data Analysis 40 (mmol CaCO3/m2 • day) Calcification rate 20 200 250 Figure 3.UN06 Data Analysis 40 (mmol CaCO3/m2 • day) Calcification rate 20 Figure 3.UN06 200 250 [CO32] (mol/kg) Researchers measured rate of calcification by reef organisms and examined how calcification rate changed with differing amounts of dissolved carbonate ions in seawater 30