Water and pH: importance of water

Most cells are surrounded by water, and cells themselves are about 70–95% water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 3.2: Water properties 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 Animation: Water Transport Animation: Water Transport Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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

Surface Tension 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

Regulation of Temperature Water absorbs heat from warmer air and releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature because it has a high specific heat. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings San Diego 72° 40 miles Pacific Ocean 70s (°F ) 80s 90s 100s Santa Barbara 73° Los Angeles (Airport) 75° Burbank 90° San Bernardino 100° Riverside 96° Santa Ana 84° Palm Springs 106°

Evaporative Cooling As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water- Universal Solvent A solution is a liquid that is a homogeneous mixture of substances (Most biological chemical reactions take place in water) A solvent is what things dissolve in The solute is the substance that is dissolved Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cl – Na Cl – – – – – – – – – Na + – – – + Water is a versatile solvent due to its polarity, it dissolves many things for the body (vitamins, hormones, waste) When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydrophilic vs. Hydrophobic 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insulation by Ice Ice floats in liquid water because hydrogen bonds in ice create a crystal like structure. So, ice is less dense than water. 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

Concept 3.3: Acids, Bases and pH – Water is in a state of dynamic equilibrium in which water molecules can break up and reform. – The molecule with the extra hydrogen is now a hydronium ion (H 3 O + ), though it is often represented as H + (Acids donate H + ) – The molecule that lost the hydrogen is now a hydroxide ion (OH – ) (Bases accept H + ) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hydronium ion (H 3 O + ) Hydroxide ion (OH – ) 2H 2 O H H H H H H H H O O O O

pH measures the H+ concentration in a solution. Acidic solutions have pH values less than 7 (lots of H+) Basic solutions have pH values greater than 7 (little H+ but lots of OH-) Most biological fluids have pH values in the range of 6 to 8 Water has a pH of 7 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The pH Scale

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

pH in the Body The internal pH of most living cells must remain close to pH 7 Water helps to regulate pH level in the body (Kidneys) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Threats to Water Quality on Earth Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6 (from fossil fuels) Acid precipitation can damage life in forests, lakes, streams, and oceans (coral reefs have lack of calcification) Effects of acid precipitation on soil chemistry are contributing to the decline of some forests Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig More acidic 0 Acid rain Acid rain Normal rain More basic

Fig. 3-11a

You should now be able to: 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings