Biology Properties of Water

Matter consists of chemical elements in pure form and in combinations called compounds Organisms are composed of matter Matter is anything that takes up space and has mass Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Matter is made up of elements An element is a substance that cannot be broken down to other substances by chemical reactions A compound is a substance consisting of two or more elements in a fixed ratio A compound has characteristics different from those of its elements

Table 2-1

Fig. 2-UN3 Nucleus Protons (+ charge) determine element Neutrons (no charge) determine isotope Atom Electrons (– charge) form negative cloud and determine chemical behavior

Fig. 2-UN6 Ionic bond Electron transfer forms ions Na Sodium atom Cl Chlorine atom Na + Sodium ion (a cation) Cl – Chloride ion (an anion)

Fig. 2-9 Hydrogen 1 H Lithium 3 Li Beryllium 4 Be Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Helium 2 He Atomic number Element symbol Electron- distribution diagram Atomic mass 2 He 4.00 First shell Second shell Third shell Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar

Electron-distribution diagram (a) (b) Separate electron orbitals Neon, with two filled shells (10 electrons) First shellSecond shell 1s orbital2s orbitalThree 2p orbitals (c) Superimposed electron orbitals 1s, 2s, and 2p orbitals x y z Fig

Hydrogen Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom In living cells, the electronegative partners are usually oxygen or nitrogen atoms Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Fig    ++ ++    ++ ++ ++ Water (H 2 O) Ammonia (NH 3 ) Hydrogen bond

Overview: The Molecule That Supports All of Life Water is the biological medium on Earth All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water The abundance of water is the main reason the Earth is habitable Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-2 Hydrogen bond  – – H  + + H O — —  + +  + +  + +  – –  – –  – –

Four emergent properties of water contribute to Earth’s fitness for life Four of water’s properties that facilitate an environment for life are: –Cohesive behavior –Ability to moderate temperature –Expansion upon freezing –Versatility as a solvent Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cohesion Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances, for example, between water and plant cell walls Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-3 Water-conducting cells Adhesion Cohesion 150 µm Direction of water movement

Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-4

Water’s High Specific Heat The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC The specific heat of water is 1 cal/g/ºC Water resists changing its temperature because of its high specific heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insulation of Bodies of Water by Floating Ice Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense Water reaches its greatest density at 4°C If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20 Heat Capacity Water has a high heat capacity  Temperature = rate of vibration of molecules  Apply heat to liquid ­Molecules bounce faster ­Increases temperature  But, when heat applied to water ­Hydrogen bonds restrain bouncing ­Temperature rises more slowly per unit heat ­Water at a given temp. has more heat than most liquids Thermal inertia – resistance to temperature change  More heat required to raise water one degree than most other liquids (1 calorie per gram)  Also, more heat is extracted/released when lowering water one degree than most other liquids

21 Properties of Water: Heat of Vaporization High heat of vaporization  To raise water from 98 to 99 ºC; ~1 calorie  To raise water from 99 to 100 ºC; ~1 calorie  However, large numbers of hydrogen bonds must be broken to evaporate water  To raise water from 100 to 101 ºC; ~540 calories! This is why sweating (and panting) cools  Evaporative cooling is best when humidity is low because evaporation occurs rapidly  Evaporative cooling works poorest when humidity is high because evaporation occurs slowly

22 Evaporative Cooling of Animals

23 Heat of Fusion Heat of fusion (melting) –To raise ice from -2 to -1 ºC; ~1 calorie –To raise water from -1 to 0 ºC; ~1 calorie –To raise water from 0 to 1 ºC; ~80 calories! This is why ice at 0 ºC keeps stuff cold MUCH longer than water at 1 ºC This is why ice is used for cooling –NOT because ice is cold –But because it absorbs so much heat before it will warm by one degree

24 Heat Content of Water at Various Temperatures

25 Properties of Water: Uniqueness of Ice Frozen water less dense than liquid water  Otherwise, oceans and deep lakes would fill with ice from the bottom up  Ice acts as an insulator on top of a frozen body of water  Melting ice draws heat from the environment

26 A Pond in Winter

27 Density of Water at Various Temperatures

Fig. 3-6a Hydrogen bond Liquid water Hydrogen bonds break and re-form Ice Hydrogen bonds are stable

Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds A colloid is a stable suspension of fine particles in a liquid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Basic Chemistry 30 Properties of Water: Water as a Solvent Solutions consist of:  A solvent (the most abundant part) and  A solute (less abundant part) that is dissolved in the solvent Polar compounds readily dissolve; hydrophilic Nonpolar compounds dissolve only slightly; hydrophobic Ionic compounds dissociate in water  Na + ­Attracted to negative (O) end of H 2 O ­Each Na + completely surrounded by H 2 O  Cl - ­Attracted to positive (H 2 ) end of H 2 O ­Each Cl - completely surrounded by H 2 O

Basic Chemistry 31 pH of Water: Acids Acids  Dissociate in water and release hydrogen ions (H + )  Sour to taste  Hydrochloric acid (stomach acid) is a gas with symbol HCl ­In water, it dissociates into H + and Cl - ­Dissociation of HCl is almost total, therefore it is a strong acid

Basic Chemistry 32 pH of Water: Bases Bases:  Either take up hydrogen ions (H + ) or release hydroxide ions (OH - )  Bitter to taste  Sodium hydroxide (drain cleaner) is a solid with symbol NaOH ­In water, it dissociates into Na + and OH - ­Dissociation of NaOH is almost total, therefore it is a strong base

Basic Chemistry 33 pH Scale pH scale used to indicate acidity and alkalinity of a solution.  Values range from 0-14 ­0 to <7 = Acidic ­7 = Neutral ­>7 to 14 = Basic (or alkaline)  Logarithmic Scale ­Each unit change in pH represents a change of 10X ­pH of 4 is 10X as acidic as pH of 5 ­pH of 10 is 100X more basic than pH of 8

Basic Chemistry 34 The pH Scale

Basic Chemistry 35 Buffers and pH When H + is added to pure water at pH 7, pH goes down and water becomes acidic When OH - is added to pure water at pH 7, pH goes up and water becomes alkaline Buffers are solutes in water that resist change in pH  When H + is added, buffer may absorb, or counter by adding OH -  When OH - is added, buffer may absorb, or counter by adding H +

Basic Chemistry 36 Buffers in Biology Health of organisms requires maintaining pH of body fluids within narrow limits  Human blood normally 7.4 (slightly alkaline)  Many foods and metabolic processes add or subtract H + or OH - ions ­Reducing blood pH to 7.0 results in acidosis ­Increasing blood pH to 7.8 results in alkalosis ­Both life threatening situations  Bicarbonate ion ( - HCO 3 ) in blood buffers pH to 7.4

Basic Chemistry 37Review Chemical Elements  Atoms  Isotopes  Molecules and Compounds Chemical Bonding  Ionic and Covalent  Hydrogen Properties of Water Acids and Bases