Chapter 3: Water and Life

WATER

Objectives Importance of H bonds to properties of water 4 unique properties of water Interpreting a pH scale Importance of buffers in biological systems

Structure of Water Structure key to special properties – Structure dictates function Polarity Hydrogen bonding – Each molecule can form max 4

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

4 Properties of Water Cohesion Moderation of temperature Ice floats on liquid water Water as a solvent

Properties of Water: Cohesive Behavior Cohesion – Water linked to water – Responsible for surface tension – Partial charges make water “sticky” Adhesion – Clinging of one substance to another Transpiration Water moves through xylem to open stomata What roles do cohesion and adhesion play ? How does polarity of water contribute to its cohesive and adhesive properties?

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

Concept Check How does polarity of water contribute to its cohesive and adhesive properties? How might Rainex alter the properties of glass to allow it to shed water. Discuss with a partner then we will discuss as a class.

Properties of Water: Ability to moderate Temperature 2. Moderation of temperature – Specific heat: amount of heat required to change 1g of substance 1 degree C – Water has very high specific heat – Implications to earths climate Oceans Organisms – Heat of vaporization: amount of heat to convert 1g of liquid to gas Water has high heat of vaporization

Evaporative cooling Evaporation is transformation of a substance from liquid to gas As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling helps stabilize temperatures in organisms and bodies of water

Specific Heat Water’s high specific heat can be traced to hydrogen bonding – Heat is absorbed when hydrogen bonds break – Heat is released when hydrogen bonds form The high specific heat of water minimizes temperature fluctuations to within limits that permit life

Properties of Water: Expansion Upon Freezing 3. Ice insulates large bodies of water – Ice floats – Moderates temps by not allowing large bodies of water to freeze solid – What are affects on pond life? – bond angle

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

Properties of Water: Versatility as a Solvent 4. Water as a solvent – Solvent – Solute – Solution – Hydrophobic substances – Hydrophilic substances – Sphere of hydration

Fig. 3-8bc (c) Ionic and polar regions on the protein’s surface attract water molecules. (b) Lysozyme molecule (purple) in an aqueous environment

Deriving pH equation In any aqueous solution at 25°C the product of H + and OH – is constant and can be written as [H + ][OH – ] = 10 –14 The pH of a solution is defined by the negative logarithm of H + concentration, written as pH = –log [H + ] For a neutral aqueous solution [H + ] is 10 –7 = –(–7) = 7

Fig. 3-UN2 Hydronium ion (H 3 O + ) Hydroxide ion (OH – ) 2H 2 O H H H H H H H H O O O O

Acids & Bases Acids and bases alter [H+] pH of pure water is 7 How does HCl increase the acidity ? – Increase [H] How does NaOH decrease acidity ? – Decrease [H] direct bases NH 3 gas Indirect NaOH

Fig. 3-9 Neutral solution Acidic solution Basic solution OH – H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Neutral [H + ] = [OH – ] Increasingly Acidic [H + ] > [OH – ] Increasingly Basic [H + ] < [OH – ] pH Scale Battery acid Gastric juice, lemon juice Vinegar, beer, wine, cola Tomato juice Black coffee Rainwater Urine Saliva Pure water Human blood, tears Seawater 9 10 Milk of magnesia Household ammonia Household bleach Oven cleaner

Importance of Buffers to Biological Systems Carbonic acid and hyperventilation – Rapid breathing – Blood CO 2 concentration decreases – Blood carbonic acid levels decrease – Blood pH changes from normal – Adverse physiological effects Dizziness Visual impairment Fainting Seizures Death

Acids and Bases pH scale – pH = –log [H + ] – 0-14 – Lower more acidic < 7 – Higher more basic > 7 Acids: Excess H+ ions Bases: Excess OH- Pure water is neutral Buffers: minimize changes in pH – Can accept or give H+ ions when needed – Carbonic Acid (H 2 CO 3 ) moderates pH changes in blood plasma and in the ocean

Summary and Review 1.List and explain the four properties of water that emerge as a result of its ability to form hydrogen bonds 2.Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution 3.Define acid, base, and pH 4.Explain how buffers work