Ch.3 Life on Earth is Aqueous!

Predominance of Water -3/4 of earth covered with water (liquid & solid) -cells are 70-95% water -all organisms require water for survival ~ 1 week survival time for human without water!

LE 3-2 Hydrogen bonds

Key properties of water defines behavior -Polarity: partial positive and negative charges -Hydrophilic nature: attracted to other water molecules and charged particles

Four of water’s properties - Cohesive behavior - Ability to moderate temperature - Expansion upon freezing - Versatility as a solvent

LE 3-3 Water-conducting cells 100 µm Cohesion & Adhesion During Transpiration H2OH2O >100 ft

Cohesion Water molecules hold together through H-bonds to other water molecules Example Cohesion helps transport water against gravity in plants from roots to stems during transpiration Adhesion Water’s attraction to other charged surfaces Example Water’s attraction to cell walls helps upward transport against gravity

Surface Tension: Strong ordered film-like structure at interface of water and atmosphere Held together through H-bonds Strength creates surface for small organisms to move across

Consequence –Lessens temperature fluctuations to within limits that permit life –Heat is absorbed to hydrogen bonds break –Heat is released when hydrogen bonds form High specific heat 1 cal/g/ o C Amount of heat gained or lost to change the temperature of 1g of water by 1ºC Compare to alcohol: specific heat of 0.6 cal/g/ o C

Evaporative Cooling –transformation of a substance from liquid to gas –Heat of vaporization The amount of heat 1 g of liquid must absorb to be converted to gas (water: ~580 cal/g at 25 o C) remaining surface cools during evaporation, a process called evaporative cooling Consequence Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water Perspiration: sensation?

Solid water (ice) : less dense than liquid because H-bonds more stable and ordered; expansion occurs Consequence: Ice floats on liquid water Insulates; prevents temperature fluctuations Example: ponds and lakes in wintertime aquatic organisms survive in the liquid water beneath ice

Polar solvent Dissolves other polar or charged solutes Examples: salts, polar proteins, nucleic acids Creates an aqueous solution -through Hydration shells H-bonds

LE 3-6 Na + Cl – – – – – – – – – – – – Hydration shells form around cations and anions Causes salt crystals to dissolve In H 2 O

LE 3-7a Lysozyme molecule (protein) in a nonaqueous environment.

LE 3-7b Lysozyme molecule in a aqueous environment. Can you deduce what regions on lysozyme are positive and negative?

Hydrophilic substance –Attracted to water due to charged or polar nature e.g. salts (ionic) Hydrophobic substance -Repelled by water due to nonpolar nature e.g. oils, fats (nonpolar) Important when considering the plasma membrane. Concepts of Hydrophilic and Hydrophobic

Most biochemical reactions occur in water Most reactions are highly sensitive to pH Aqueous chemistry in biological systems What is pH and how does it relate to water? Reactant-1 + Reactant-2 Product Enzyme

LE 3-UN53 Hydronium ion (H 3 O + ) Hydroxide ion (OH – ) Simplified to H+ Water occasionally produces protons (H+) and hydroxide ions (OH-)

Results: –molecule with the extra proton is now a hydronium ion (H 3 O + ) –The molecule that lost the proton is now a hydroxide ion (OH - ) Water Dissociation - Hydrogen involved in H-bonds in H 2 O can lose electron -H+ (proton) can bond with another H 2 O molecule

Dissociation of water molecules Rare in pure water (25 o C) » [H+]=10^-7 M »[OH-]=10^-7 M Changes in concentrations of H + and OH - drastically affect the chemistry of a cell Such changes alter the pH

pH -reflects the molar concentration of H+ in a solution pH= -log[H+] -increases in [H+] increase acidity e.g. HCl (hydrochloric acid) readily dissociates into H+ and Cl- -increases in [OH-] raises alkalinity, decreases in acidity e.g. the base NaOH (sodium hydroxide) readily dissociates into Na+ and OH-

The pH Scale pH 7 occurs when [H+] =[OH-] Acidic solutions pH [OH-] Basic solutions pH > 7, [H+] < [OH-] Most biological fluids: pH 6-8

LE 3-8 pH Scale Oven cleaner Household bleach Household ammonia Milk of magnesia Seawater Pure water Human blood Urine Rainwater Black coffee Tomato juice Vinegar, beer, wine, cola Digestive (stomach) juice, lemon juice Battery acid Neutral [H + ] = [OH – ] Increasingly Acidic [H + ] > [OH – ] Increasingly Basic [H + ] < [OH – ]

Given: pH= -log[H+] Constant: Water ion product 10^-14 M^2= [H+][OH-] Calculating pH What is the pH of a solution containing 10^-7 M H+? 10^-4 M For the same solutions, what is the concentration of OH-? Determine the concentration of H+ and OH- at pH 3.

Apparent small changes in pH value are really LARGE Exponential! Calculate the difference between pH 7 and pH 4 [H+] is10^3 x larger

Buffers pH of most living cells must remain close to pH 7 Buffers minimize changes in [H +] and [OH - ]in a solution Most buffers consist of an acid-base pair that reversibly combines with H + H 2 CO 3 HCO H+ carbonic acid bicarbonate

The Damage of Acid Precipitation Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6 Caused by the mixing of different pollutants with water in the air e.g. sulfur and nitrogen oxides Main source: combusted fossil fuels Acid precipitation can damage life in lakes and streams –Leaches geological buffers from soils –Solubilizes toxic heavy metals e.g. aluminum

LE More basic Normal rain More acidic Acid rain