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Chapter 3 Water and Life
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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 © 2011 Pearson Education, Inc.
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3.1: Polar covalent bonds in water molecules result in hydrogen bonding
The water molecule is a polar molecule: the opposite ends have opposite charges The two hydrogens have a slightly positive charge The oxygen has a slightly negative charge © 2011 Pearson Education, Inc.
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The Properties of Water
Water is a polar molecule: Oxygen side is slightly negative Because it is more electronegative than hydrogen Hydrogen side is slightly positive Polarity allows water molecules to form hydrogen bonds with each other.
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Polarity allows water molecules to form hydrogen bonds with each other
Hydrogen bond + Polar covalent bonds + + + Figure 3.2 Hydrogen bonds between water molecules. 5
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Animation: Water Structure
Right-click slide/select “Play” © 2011 Pearson Education, Inc. 6
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3.2: Four properties of water contribute to Earth’s suitability for life
Four of water’s properties that facilitate an environment for life are Cohesive behavior: Stickiness! Moderates temperature: Water has a strong resistance to change in temperature Expansion upon freezing: Frozen water floats Water is a common solvent for life. © 2011 Pearson Education, Inc.
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Cohesion of Water Molecules
Cohesion is the attraction between molecules of the same substance Hydrogen bonds hold water molecules together by cohesion Adhesion is an attraction between different substances, for example, between water and tiny tubes (xylem) located in tree trunk Cohesion vital for the transport of water from the roots to the leaves of plants. © 2011 Pearson Education, Inc.
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Cohesion & Adhesion help transport water against gravity in plants
Two types of water-conducting cells Cohesion Direction of water movement Figure 3.3 Water transport in plants. 300 m 9
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Animation: Water Transport
Right-click slide/select “Play” © 2011 Pearson Education, Inc. 10
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Surface tension is related to cohesion
Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion Water has a high surface tension due to the hydrogen bonding between water molecules © 2011 Pearson Education, Inc.
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Figure 3.4 Figure 3.4 Walking on water. 12
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3.2: Four properties of water contribute to Earth’s suitability for life
Four of water’s properties that facilitate an environment for life are Cohesive behavior: Stickiness! Moderates temperature: Water has a strong resistance to change in temperature Expansion upon freezing: Frozen water floats Water is a common solvent for life. © 2011 Pearson Education, Inc.
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Moderation of Temperature by Water
Water has a great capacity to absorb and retain heat. Earth’s giant water supply causes temperatures to stay within limits that permit life. the oceans act as heat buffers for the Earth. Hydrogen bonding is why water has a strong resistance to temperature change. © 2011 Pearson Education, Inc.
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Moderation of Temperature by Water
Heat and temperature are related, but different. Heat is the amount of kinetic energy associated with the movement of the atoms and molecules in a body of matter. Temperature measures the intensity of heat due to the average kinetic energy of molecules Water can absorb and store large amounts of heat while only changing a few degrees in temperature. © 2011 Pearson Education, Inc.
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A calorie (cal) is the amount of heat required to raise the temperature of 1 g of water by 1°C
The “calories” on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal © 2011 Pearson Education, Inc.
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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 © 2011 Pearson Education, Inc.
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Water’s high specific heat is due to hydrogen bonding
It can absorb a lot of heat before its temperature goes up This helps animals maintain their body heat and minimizes temperature fluctuations to within limits that permit life © 2011 Pearson Education, Inc.
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Effect of a large body of water on climate.
San Bernardino 100° Burbank 90° Santa Barbara 73° Riverside 96° Los Angeles (Airport) 75° Santa Ana 84° Palm Springs 106° 70s (°F) 80s Pacific Ocean 68° 90s Figure 3.5 Effect of a large body of water on climate. 100s San Diego 72° 40 miles 19
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Evaporative Cooling Evaporation is transformation of a substance from liquid to gas Heat of vaporization is the heat needed to be absorbed for 1 g of a liquid to be converted to gas Water has a high heat of vaporization Evaporative cooling occurs when a substance evaporates and the surface of the liquid remaining behind cools down Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water © 2011 Pearson Education, Inc.
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3.2: Four properties of water contribute to Earth’s suitability for life
Four of water’s properties that facilitate an environment for life are Cohesive behavior: Stickiness! Moderates temperature: Water has a strong resistance to change in temperature Expansion upon freezing: Frozen water floats Water is a common solvent for life. © 2011 Pearson Education, Inc.
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The Biological Significance of Ice Floating
Water expands when it freezes to ice A chunk of ice has fewer water molecules than an equal volume of liquid water. Hence, Ice is less dense than liquid water and floats on water. Most other substances become denser when they change from liquid to solid © 2013 Pearson Education, Inc. 22
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Floating of Ice on Liquid Water
Ponds and lakes freeze from the top down and never freeze completely to the bottom If ice did not float, ponds, lakes, and even the oceans would freeze solid. Many plants and fish are therefore saved from freezing © 2011 Pearson Education, Inc.
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Hydrogen bond Liquid water: Hydrogen bonds break and re-form Ice:
Figure 3.6 Ice: crystalline structure and floating barrier. Ice: Hydrogen bonds are stable 24
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3.2: Four properties of water contribute to Earth’s suitability for life
Four of water’s properties that facilitate an environment for life are Cohesive behavior: Stickiness! Moderates temperature: Water has a strong resistance to change in temperature Expansion upon freezing: Frozen water floats Water is a common solvent for life. © 2011 Pearson Education, Inc.
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Water: The Solvent of Life
A solution is a liquid that is a homogeneous mixture of two or more 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 © 2011 Pearson Education, Inc.
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Water is a powerful solvent, with the ability to dissolve more compounds in greater amounts than any other liquid. Water is so versatile as a solvent due to its polarity, which allows it to form hydrogen bonds easily When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell © 2011 Pearson Education, Inc.
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Na Na Cl Cl Figure 3.7
Figure 3.7 Table salt dissolving in water. 28
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Water can also dissolve compounds made of nonionic polar molecules
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions © 2011 Pearson Education, Inc.
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A water-soluble protein.
+ + Figure 3.8 A water-soluble protein. 30
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Hydrophilic and Hydrophobic Substances
A hydrophilic substance is one that has an affinity for water, dissolves in water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds © 2011 Pearson Education, Inc.
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Solute Concentration in Aqueous Solutions
Most of life’s biochemical reactions occur in water Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution © 2011 Pearson Education, Inc.
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Molecular mass Molecular mass is the sum of all masses of all atoms in a molecule We can calculate the molecular mass of a molecule by adding up the mass of each atom in the molecule (Periodic Table of Elements), Na=23 Da Cl=35 Da Molecular mass of NaCl = 58 Daltons (Da) © 2011 Pearson Education, Inc.
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Moles & Molarity A mole (mol) is equal to the molecular weight of a substance but scaled up from Daltons to grams. 1 mole of NaCl = 58 g (Na=23g; Cl=35g) Note: 1 mole is also = 6.02 x 1023 molecules (Avogadro’s number) Molarity (M) is the number of moles of solute per liter of solution 1 molar NaCl = 58g/liter 1 molar NaCl =5.8g/100 ml © 2011 Pearson Education, Inc.
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Moles & Molarity A mole of glucose (C6H12O6) contains 6.02 × molecules and weighs 180 g A mole of sodium chloride (NaCl) also contains × 1023 molecules but weighs only 58 g because the molecules are smaller! © 2011 Pearson Education, Inc.
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3.3: Acidic and basic conditions affect living organisms
Water-based solutions can be acidic, basic, or neutral In pure water, a small fraction of water molecules break apart into an equal amount of hydrogen ions (H+) and hydroxyl ions (OH–) H2O OH– + H+ © 2011 Pearson Education, Inc.
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The pH Scale The pH of a solution is the negative logarithm of H+ concentration, written as pH = –log [H+] For a neutral aqueous solution [H+] is 10–7 M So pH= –(–7) = 7! Also, for aqueous solutions, the product of H+ and H- (written as [H+][OH–]) is constant and is equal to 10–14 © 2011 Pearson Education, Inc.
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Acids and Bases A neutral solution has a pH of 7
An acid is any substance that increases the H+ concentration of a solution Acidic solutions have pH values less than 7 A base is any substance that reduces the H+ concentration of a solution Basic solutions have pH values greater than 7 Most biological fluids have pH values in the range of 6 to 8 © 2011 Pearson Education, Inc.
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Examples of Strong Acids and Bases
Hydrochloric acid (HCl) is a strong acid & dissociates completely when mixed with water. HCl Sodium hydroxide (NaOH) is a strong base and dissociates completely when mixed with water. NaOH Na+ + OH− NaOH functions by reducing the H+ concentration indirectly by dissociating to OH−, which then combines with H+ to form water. H+ + Cl-
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pH Scale 1 2 Increasingly Acidic [H+] > [OH] 3 4 Acidic solution 5
1 Battery acid 2 Gastric juice, lemon juice H+ H+ OH H+ Vinegar, wine, cola H+ 3 Increasingly Acidic [H+] > [OH] OH H+ H+ H+ H+ 4 Acidic solution Tomato juice Beer Black coffee 5 Rainwater 6 Urine OH OH Neutral [H+] = [OH] Saliva OH 7 H+ H+ Pure water OH OH Human blood, tears H+ H+ H+ 8 Seawater Neutral solution Inside of small intestine 9 Figure 3.10 The pH scale and pH values of some aqueous solutions. 10 Increasingly Basic [H+] < [OH] Milk of magnesia OH OH 11 OH H+ OH OH Household ammonia OH H+ OH 12 Basic solution Household bleach 13 Oven cleaner 14 40
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Buffers The internal pH of most living cells must remain close to pH 7
Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution Most buffers consist of an acid-base pair that reversibly combines with H+ © 2011 Pearson Education, Inc.
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Buffers Buffers resist changes in the pH of a solution when an acid or base are added to the solution helps maintain a relatively constant pH. Buffers accept hydrogen ions from the solution when they are in excess and donates hydrogen ions when they have been depleted © 2011 Pearson Education, Inc.
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Buffers The carbonic acid/bicarbonate buffer system found in our bloodstream helps prevents pH changes Carbonic acid reversibly dissociates to yield a bicarbonate ion (HCO3-) and a hydrogen ion (H+). H2CO HCO H+ Carbonic acid Bicarbonate ion Hydrogen ion © 2011 Pearson Education, Inc.
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bicarbonate hydrogen ion carbonic acid
If the blood becomes too acidic, bicarbonate accepts (and absorbs) H+ to make carbonic acid HCO3– H H2CO3 bicarbonate hydrogen ion carbonic acid If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH– to form water H2CO OH– HCO3– H2O carbonic acid hydroxide ion bicarbonate water
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Acidification: A Threat to Water Quality
Human activities such as burning fossil fuels threaten water quality CO2 is the main product of fossil fuel combustion About 25% of human-generated CO2 is absorbed by the oceans CO2 dissolved in sea water forms carbonic acid; this process is called ocean acidification © 2011 Pearson Education, Inc.
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As seawater acidifies, H+ ions combine with carbonate ions to produce bicarbonate
Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals © 2011 Pearson Education, Inc.
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Atmospheric CO2 from human activities and its fate in the ocean.
CO2 + H2O H2CO3 H2CO3 H+ + HCO3 H+ + CO32 HCO3 Figure 3.11 Atmospheric CO2 from human activities and its fate in the ocean. CO32 + Ca2+ CaCO3 47
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The Threat of Ocean Acidification to Coral Reef Ecosystems
Figure 3.12 IMPACT: The Threat of Ocean Acidification to Coral Reef Ecosystems (a) (b) (c) 48
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Acid precipitation is rain, fog, or snow with a pH lower than 5.2
The burning of fossil fuels is also a major source of sulfur oxides and nitrogen oxides These compounds react with water in the air to form strong acids that fall in rain or snow Acid precipitation is rain, fog, or snow with a pH lower than 5.2 Acid precipitation damages life in lakes and streams and changes soil chemistry on land © 2011 Pearson Education, Inc.
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