Topic 3: Water Properties (PDF)

Topic 3 Properties of Water, pH, Acid/bases PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Learning objectives (LOBs) 1. Describe the properties of water and...

Topic 3 Properties of Water, pH, Acid/bases PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Learning objectives (LOBs) 1. Describe the properties of water and its role in living organisms. 2. Define hydrophilic/hydrophobic molecules, acid/bases, pH and buffers. Reading: Campbell Biology Chapter 3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water: The Molecule That Supports All of Life Life on earth began in the oceans The conditions that existed billions of years ago determined the chemistry of living beings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Life is based on the properties of water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water: The Molecule That Supports All of Life Water is the biological medium on Earth All living organisms require water more than any other substance All living organisms are mostly made of water Most cells are surrounded by water Cells themselves consist of about 70–95% water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water (Η2Ο) is the basis of all life Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water 75% of the Earth’s surface is submerged in water The abundance of water is the main reason the Earth is habitable Figure 3.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Most cellular reactions take place in water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water is a polar molecule The water molecule is a polar molecule: The opposite ends have opposite charges. The polarity of water molecules – Allows them to form hydrogen bonds with each other – Contributes to the various properties water exhibits – Hydrogen + bonds H + – H – + + – Figure 3.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Polar covalent bond example: H20 Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen. – This results in a partial negative charge on the oxygen and a O partial positive charge on the hydrogens. Figure 2.12 H H + + H2O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Electronegativity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Hydrogen Bonds A hydrogen bond forms when the slightly negatively charged oxygen of one water molecule is attracted to – the slightly positively charged hydrogen of a nearby molecule Hydrogen + bond H O – + H – + – + Fig. 3-2 Copyright © 2005 Pearson Education, In c. publishing as Benjamin Cummings Four properties of water contribute to Earth’s fitness for life 1. Cohesion 2. Temperature moderation 3. Evaporative cooling 4. Versatility as a solvent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  1. Cohesion Water molecules exhibit cohesion Cohesion: – the bonding of water molecules to neighboring molecules – occurs due to hydrogen bonding Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Cohesion Helps pull water up through the microscopic vessels of plants Related to the properties of adhesion and surface tension Water conducting cells Figure 3.3 100 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Cohesion: Water transport in plants Adhesion of water molecules to plant cell walls due to H-bonds helps transfer water upwards against gravity Cohesion of water molecules helps them stick together during transfer Water-conducting cells Direction of water movement Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Surface tension A measure of how hard it is to break the surface of a liquid Related to cohesion Water has higher surface tension compared to other liquids due to hydrogen bonds Figure 3.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  2. Temperature Moderation Water moderates air temperature in 2 ways: – By absorbing heat from air that is warmer – By releasing the stored heat to air that is cooler Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Heat and Temperature Kinetic energy: – the energy of motion – atoms and molecules are always moving → thus have kinetic energy – the faster a molecule moves → kinetic energy Heat: a measure of the total amount of kinetic energy due to molecular motion in a body of matter Temperature: measures the intensity of heat When two objects of different temperature are brought together, heat passes from the warmer to the cooler object Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Specific Heat The specific heat of a substance: – the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC Water has a high specific heat, which allows it to resist changes in temperature Water’s specific heat = 1 cal/g/ºC Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water has a high specific heat Reason: H-bonding – Heat is absorbed when hydrogen bonds break – Heat is released when hydrogen bonds form – Much of the heat water absorbs is used in breaking hydrogen bonds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  3. Evaporative Cooling Evaporation: the transformation of a substance from a liquid to a gas Heat of vaporization: the quantity of heat a liquid must absorb for 1 gram of it to be converted to a gas Water has a high heat of vaporization due to H-bonding – same reason that water has a high specific heat Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Evaporative cooling Due to water’s high heat of vaporization Water is a great coolant The ‘hottest’ molecules escape as gas and leave the ‘cooler’ molecules behind Allows water to cool a surface Organisms keep their body temperature constant by sweating Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Insulation of Bodies of Water by Floating Ice Ice = solid water - Less dense than liquid water - Floats in liquid water → Insulates the bodies of water underneath Since ice floats in water → Life can exist under the frozen surfaces of lakes and polar seas Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Hydrogen bonds in water vs ice The hydrogen bonds in ice are more “ordered” than in liquid water →Ice has lower density than water Hydrogen bond Ice Liquid water Figure 3.5 Hydrogen bonds are stable Hydrogen bonds constantly break and re-form Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  4. The Solvent of Life Water is a versatile solvent due to its polarity → forms hydrogen bonds easily It can form aqueous solutions The different regions of the polar water molecule can interact with ionic compounds called solutes and dissolve them Solution = solvent + solute e.g. sodium chloride aqueous solution (NaCl) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Solute, solvent and solutions A solution is a liquid that is a homogeneous mixture of substances A solvent is the dissolving agent of a solution The solute is the substance that is dissolved An aqueous solution is one in which water is the solvent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Solute, solvent and solutions sugar (solute) sugar solution (solvent) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The Solvent of Life Sodium chloride aqueous solution (NaCl) Negative oxygen regions (δ-)of – polar water molecules are Na+ attracted to sodium cations (Na+). + – + + – – Positive hydrogen regions (δ+) Na+ – of water molecules cling to + + chloride anions (Cl–). Cl– Cl – – + – + – + – – Figure 3.6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ionic and hydrogen bonds in NaCl solution Ιοnic bonds formed between Na+ and Cl- Η2Ο (water molecule) Hydrogen bonds formed between water molecules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The Solvent of Life Water can also interact with polar molecules such as proteins and dissolve them – + (a) Lysozyme molecule (b) Lysozyme molecule (c) Ionic and polar regions on the in a non-aqueous (purple) in an aqueous protein’s surface attract water environment environment such as tears molecules. or saliva Figure 3.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Hydrophilic and Hydrophobic Substances A hydrophilic substance: – Has an affinity for water – e.g. polar solutions, ionic molecules A hydrophobic substance: – Does not have an affinity for water – e.g. non-polar solutions, non ionic molecules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Solute Concentration in Aqueous Solutions Since most biochemical reactions occur in water → It is important to calculate the concentration of solutes in an aqueous solution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Mole, Molecular mass, Molarity A mole: – Represents an exact number of molecules of a substance in a given mass – 1 mole of a substance has the same number of molecules as 1 mole of any other substance = 6.02 x 1023 molecules = Avogadro’s number – 1 mole of a substance has the same mass in grams as its molecular mass Molecular mass (molecular weight, MW): – the sum of the atomic masses in a molecule Molarity (M) – Is the number of moles of solute per liter of solution (moles/L) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Example 1 mole of a substance is equal to the molecular mass in g Sucrose: C12H22O11 Molecular mass (M.M or M.W.) = 342 Daltons C atomic mass= 12 H atomic mass=1 12x12C + 1x22H + 11x16O = 342 O atomic mass=16 1 mole of sucrose = 342 g 1 M = 1 mole/L = 342 g/L Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water dissociation: Acids and Bases Dissociation of water molecules leads to acidic and basic conditions that affect living organisms Water dissociation: hydronium ions + hydroxide ions Changes in the concentration of these ions can have a great affect on living organisms + – H H H H + Figure of water H H H H dissociation Hydronium Hydroxide ion (H3O+) ion (OH–) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Acids and Bases An acid: – any substance that increases the hydrogen ion concentration [H+] of a solution → by releasing H+ A base: – any substance that reduces the hydrogen ion concentration of a solution → by accepting H+ or by releasing OH- Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Effects of Changes in pH Concentrations of H+ and OH– are equal in pure water Addition of acids and bases modifies the concentrations of H+ and OH– pH scale: describes whether a solution is acidic or basic Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The pH Scale The pH of a solution: – a measure of the acidity of a solution – determined by the relative concentration of hydrogen ions ([H+]) – low in an acid (pH7) Acidic solutions: pH values < 7 Basic solutions: pH values > 7 Most biological fluids have pH values in the range of 6 to 8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The pH scale and pH values of various aqueous solutions pH Scale 0 1 Battery acid Increasingly Acidic 2 Digestive (stomach) [H+] > [OH–] juice, lemon juice 3 Vinegar, beer, wine, cola 4 Tomato juice 5 Black coffee Rainwater 6 Urine Neutral 7 Pure water [H+] = [OH–] Human blood 8 Increasingly Basic Seawater [H+] < [OH–] 9 10 Milk of magnesia 11 Household ammonia 12 Household bleach 13 Oven cleaner Figure 3.8 14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  0 Acidic [H+] > [OH–] Acids donate H+ in aqueous solutions Neutral [H+] = [OH–] 7 Bases donate OH– or accept H+ in Basic aqueous solutions [H+] < [OH–] 14 Copyright © 200 5 Pearson Education, Inc. publishing as Benjamin Cummings  Buffers The internal pH of cells must remain close to pH 7 for the functionality and viability of the cells Buffers: – substances that minimize changes in the concentrations of hydrogen (H+) and hydroxide (OH–) ions in a solution – consist of an acid-conjugate base pair (or vice versa) that reversibly combines with hydrogen ions Living organisms have their own buffers that prevent dramatic changes of pH and keep it close to 7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Summary Properties of water: 1. Cohesion 2. Temperature moderation 3. Evaporative cooling 4. Versatility as a solvent Hydrophilic/hydrophobic molecules Acid/bases, pH, buffers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  SBA example What is the cellular content of water in living organisms? A. 10% B. 20% C. 50% D. 70% E. 100% Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings