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Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce, and maintain homeostasis
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Three-quarters of the Earth’s surface is submerged in water
The abundance of water is the main reason the Earth is habitable
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The first cells evolved in water
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Living things are 70-90% water
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Electronegativity An atom’s attraction for the shared electrons of a covalent bond The more electronegative an atom is, the more strongly the shared electrons are pulled toward it Oxygen is more electronegative than hydrogen, so the electrons are pulled closer to oxygen and it becomes negatively charged, and hydrogen becomes positively charged
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Polar covalent bonds occur when electrons are shared unequally
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Water is a polar molecule (contains polar covalent bonds)
Water forms hydrogen bonds because it is a polar molecule
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Hydrogen Bonds Occur when charged regions of one molecule are attracted to oppositely charged regions of neighboring molecules forming weak chemical bonds
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Water forms Hydrogen Bonds between the positive Hydrogen (electropositive) ends of one molecule and the negative Oxygen (electronegative) ends of another water molecule
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Hydrogen bonding is not unique to water
Many biological molecules contain polar covalent bonds between electropositive hydrogen and electronegative oxygen or nitrogen Hydrogen bonds can form between the positive hydrogen and the negative oxygen or nitrogen
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Water’s Polarity and Hydrogen Bonds Give it Unique Properties
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Water exists in all three states
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Liquid Water Hydrogen bonding in liquid water makes liquid water cohesive Fewer bonds which are constantly breaking and reforming This makes water fluid
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Solid Water Water molecules in ice have less energy, so they form maximum number of hydrogen bonds (4) Water molecules become more evenly spaced out forming regular, crystalline structure
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Maximum number of H-bonds causes molecules to be more evenly spaced out
Solid water (ice) is less dense than liquid water because of this
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The hydrogen bonds in ice
Are more “ordered” than in liquid water, making ice less dense
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Solid water is less dense and floats in liquid water
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Life can exist under the frozen surfaces of lakes and polar seas
Since ice floats in water Life can exist under the frozen surfaces of lakes and polar seas Ice insulates water below, prevents freezing
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Cohesion Water molecules exhibit cohesion
Cohesion is the ability of water molecules to cling to each other due to hydrogen bonding. Water flows freely
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Cohesion Water molecules stick together because of their hydrogen bonding This is apparent because water flows freely but does not separate from each other
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Surface tension results from the property of cohesion
Surface of water molecules stick together Is a measure of how hard it is to break the surface of a liquid
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Copyright Pearson Prentice Hall
Surface Tension The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed. Copyright Pearson Prentice Hall
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Adhesion Water sticks to or adheres to polar surfaces as a result of hydrogen bonding
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Cohesion and adhesion helps move water up through the microscopic vessels of plants
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Water has property of capillarity
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Capillarity The ability of water to move upwards in small tubes
Due to cohesion and adhesion
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Cohesion/capillary action helps move water up plants from roots to leaves
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Water is a good moderator of temperature-it resists temperature changes (coastal vs. inland)
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Moderation of Temperature
Water moderates air temperature By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler
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Water’s High Heat Capacity
Water has a high heat capacity (specific heat), which allows it to absorb large amounts of heat without a great change in temperature, this minimizes temperature fluctuations to within limits that permit life The temperature of water rises and falls slowly.
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High Heat of vaporization
Hydrogen bonding prevents evaporation Water must absorb large amounts of heat to vaporize (evaporate) because of hydrogen bonding between water molecules (0-100 C)
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Heat of Vaporization High heat of vaporization
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!
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Evaporative cooling Allows water to cool a surface, because calories (heat) to evaporate water comes from the surface of object water is evaporating from
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Evaporative Cooling Evaporating substances cools surface they evaporate from
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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
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Water is a good solvent Dissolves (surrounds and separates) ionic and or polar molecules Water acts as solvent in cytoplasm, blood, sap Water is a versatile solvent due to its polarity
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Hydrophilic and Hydrophobic Substances
A hydrophilic substance Has an affinity for water and dissolves easily Polar and ionic compounds A hydrophobic substance Does not have an affinity for water, therefore does not dissolve in water
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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
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The different regions of the polar water molecule can interact with ionic compounds called solutes and dissolve them
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Water can also interact with polar molecules such as proteins
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Can have a great affect on living organisms
Water can dissociate or break apart to form hydronium (hydrogen) ions [H+] H3O+ and hydroxide ions [OH-] Changes in the concentration of these ions Can have a great affect on living organisms
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2.4 Acids and Bases
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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
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Buffers and pH Buffers are solutes in water that resist change in pH
When H+ is added, buffer may absorb, or counter by adding OH-
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Buffers in Biology Health of organisms requires maintaining pH of body fluids within narrow limits (homeostasis) Human blood normally 7.4 (slightly alkaline) Bicarbonate ion (-HCO3) in blood buffers pH to 7.4 If blood pH drops below 7.0, acidosis results. If blood pH rises above 7.8, alkalosis results. Both are life-threatening situations. Body has built-in mechanisms to prevent pH changes. Example: carbonic acid buffer dissociates and re-forms to reduce changes in pH
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Acid Precipitation Rain, snow, fog pH < 5.6
Destroys lakes, kills trees, ruins buildings, smog, kill amphibians O2 combines with sulfur, nitrogen to form oxides-oxides react with H2O to form sulfuric, nitric acids Sulfur, nitrogen come from burning fossil fuels (coal, oil, gas)
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