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  • 5/13/2013

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    Water & Life Chapter 3

    You should 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

    4. Explain how buffers work

    Water: The Molecule That Supports

    All of Life o Water is the biological medium on Earth

    o All organisms require more water than any other substance

    o Most cells are surroundedby water & cells themselves are ~7095% water

    o The abundance of water is the main reason the Earth is habitable

    o Ghandi fasted for 21 days while

    in his 70s

    o Most Dr.s agree that healthy

    humans can live up to 8 weeks

    w/o food

    o However, can only live 3-5 days

    w/o water

    The Blue Planet

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    The polarity of water molecules results

    in hydrogen bonding

    o Oxygen is more electronegative than Hydrogen

    o The electrons orbiting the nuclei will spend more

    time around the Oxygen than the two Hydrogens

    o The water molecule is a polar molecule: The

    opposite ends have opposite charges

    o Polarity allows water molecules to form hydrogen

    bonds with each other

    Animation: Water Structure

    Hydrogen

    bond

    Polar covalent

    bonds

    +

    +

    +

    +

    The charged regions of a polar water

    molecule are attracted to oppositely charged

    parts of neighboring molecules. Each

    molecule can hydrogen bond to multiple

    partners, and these associations are

    constantly changing

    Four emergent properties of water

    contribute to Earths fitness for life

    o Based on its polarity

    o Four of waters properties that facilitate an environment for life are:

    1. Cohesive behavior

    2. Ability to moderate temperature

    3. Expansion upon freezing

    4. Versatility as a solvent

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    Cohesion

    o Collectively, hydrogen bonds hold water

    molecules together, a phenomenon called

    cohesion

    Liquid Water: The molecules are sticking together

    1.

    Water-conducting

    cells

    Adhesion

    Cohesion

    150 m

    Direction

    of water

    movement

    oCohesion helps

    the transport of

    water against

    gravity in plants

    oAdhesion is an

    attraction between

    different

    substances,

    for example, between water

    and plant cell walls

    Cohesion

    Animation: Water Transport

    o Surface tension is a measure of how hard it is to break the surface of a liquid, because the molecules that are at the surface are sticking together

    o Surface tension is related to cohesion

    Note the top layer of

    molecules is like a solid

    layer/floor because the

    molecules are sticking

    together

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    Moderation of Temperature

    o Water itself is very resistant to changes in temperature

    o Water absorbs heat from warmer air and releases stored heat to cooler air

    o Where much water is, temperatures will be subject to fewer extremes because water can absorb or release a large amount of heat with only a slight change in its own temperature

    o Ice can also resist prevent (much) heat loss from adjacent areas

    o Some living things actively use waters properties to maintain temperature

    2.

    Fig. 3-5

    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

    Despite having similar environmental factors, land

    close to much water has more moderate temperatures.

    Heat and Temperature

    o Atoms and Molecules are always moving.

    o Even in solid objects, the particles are moving a little bit in (usually) random directions.

    o Kinetic energy is the energy of motion

    o Heat is a measure of the total amount of kinetic energy due to molecular motion

    o Temperature measures the intensity of heat due to the average kinetic energy of molecules

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    o The Celsius scale is a measure of temperature using

    Celsius degrees (C) (Water freezes at 0C and boils at

    100C, body temperature ~ 37C, and comfortable room

    temperature ~20-25C)

    o A calorie (cal) is the amount of heat (heat energy)

    required to raise the temperature (average kinetic

    energy/motion) of 1 g (~1 ml) of water by 1C

    o The calories on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal

    o The joule (J) is another unit of energy where

    1 J = 0.239 cal, or 1 cal = 4.184 J

    510 kcal = 510,000 calories!!!!!!!!

    Waters High Specific Heat

    o 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 1C

    o The specific heat of water is 1 cal/g/C whereas the specific heat of ethyl alcohol (the type of alcohol in alcoholic beverages) is 0.6 cal/g/C.

    o Water resists changing its temperature because of its high specific heat

    o It takes a lot of (absorbed) heat to make water hotter, and a lot of heat loss (making other things warmer) to make water colder.

    o Waters high specific heat can be traced to hydrogen bonding

    o Heat is absorbed when hydrogen bonds break o As water heats, hydrogen bonds break and energy (heat) is

    absorbed

    o Heat is released when hydrogen bonds form o (The molecules are now moving less (stuck to their

    neighbors), so that kinetic energy is released)

    o The high specific heat of water minimizes temperature fluctuations to within limits that permit life o Absorbs excess heat

    o Releases heat when air temperature drops

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    Evaporative Cooling

    o Evaporation is transformation of a substance from

    liquid to gas

    o It is the individual molecules at the surface finally

    breaking free and jumping into the air.

    o Heat is absorbed by water molecules when hydrogen

    bonds break

    2b.

    o Heat of vaporization is the heat energy a liquid

    must absorb for 1 g to be converted to gas

    o As a liquid evaporates, its remaining surface

    cools, a process called evaporative cooling

    o Because the breaking of the H- (and other)

    bonds took away some of the heat energy

    o The hottest molecules, those with the greatest kinetic energy, are the most likely to leave as

    gas

    o Evaporative cooling of water helps stabilize

    temperatures in organisms (sweat) and bodies of water

    Insulation of Bodies of Water by

    Floating Ice

    o As heat energy is removed (substance gets

    colder), the kinetic energy of molecular movement goes away. The molecules come closer

    together

    o Ice floats in liquid water because hydrogen bonds

    in ice are more ordered, making ice less dense (While other materials contract when they solidify,

    water expands due to hydrogen bonds!!!!!)

    o The Hs & Os push away the adjacent Hs & Os, eventually making a crystal shape

    o Water reaches its greatest density at 4C

    3.

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    Fig. 3-6a

    Hydrogen

    bond Liquid water

    Hydrogen bonds break and re-form Ice

    Hydrogen bonds are stable

    If ice sank, all bodies of water

    would eventually freeze solid,

    making life impossible on Earth

    The Solvent of Life

    o A solution is a liquid that is a homogeneous (evenly-mixed) mixture of substances

    o A solvent is the dissolving agent of a solution

    o The solute is the substance that is dissolved

    o An aqueous solution is one in which water is the solvent

    o Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily

    4.

    o When an ionic compound is dissolved in water,

    each ion (atom/molecule) is surrounded by a

    sphere of water molecules called a hydration

    shell

    Ionic compounds

    DISSOLVE in water!!!!!!

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    o Water can also dissolve compounds made

    of nonionic polar molecules

    o Even large polar molecules such as proteins

    can dissolve in water if they have ionic and

    polar regions

    (a) Lysozyme molecule in a nonaqueous environment

    (b) Lysozyme molecule (purple) in an aqueous environment

    (c) Ionic and polar regions on the proteins surface attract water molecules.

    Hydrophilic and Hydrophobic

    Substances

    o A hydrophilic substance is one that has an affinity for water (interacts with the polar bonds)

    o Water loving solutions include ionic compounds (salts) and polar molecules

    Hydrophilic =

    water loving!

    oA colloid is a stable suspension of fine particles in a liquid

    oToo big to dissolve, but can hang in place because of the solvent (water) molecules around them oColloids usually appear opaque or murky

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    Oil and Water

    o A hydrophobic substance is one that does not have an affinity for water

    oOil molecules are hydrophobic because they have relatively nonpolar bonds

    o They will be pushed away by the interactions of water

    Mr. Hydrophobic

    I do

    NOT like

    water!!!

    Water Video

    http://www.youtube.com/watch?v=DVCYlST6mYQ

    Solute Concentration in Aqueous

    Solutions o Most biochemical reactions occur in water--some

    with the water itself, some between 2+ other chemicals dissolved in same water

    o Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution (how likely it is that one chemical will come in contact with another).

    o This likelihood is based on

    1. How many atoms/molecules of each chemical are in the solution

    2. The volume (space) the chemicals have to move around in o 1 + 2 = Concentration

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    o Numbers of molecules are usually measured in

    moles, where 1 mole (mol) = 6.02 x 1023 particles

    o 602,000,000,000,000,000,000,000 atoms/molecules

    o Avogadros number and the unit dalton were defined such that 6.02 x 1023 daltons = 1 g

    o Molecular mass is the sum of all masses of all atoms in

    a molecule

    o Each element/compound has a particular mass (grams)

    = 1 mole

    o 1 mole of oxygen = ~16g

    o 1mole of NaCl = ~58g

    o 1 mole of water = ~18g

    o Molarity (M) is the number of moles of solute per

    liter of solution (1M = 1 mole/liter)

    How many grams are in 6.02 x 1023 particles of Oxygen?

    16g = 1 mole of Oxygen

    Therefore, 16g of Oxygen contain 6.02 x 1023 particles

    How many moles are there in 32 g of Oxygen?

    1 mole Oxygen = x moles

    16 g 32 g

    X = 2

    o The remarkable properties of water support life

    on Earth in many ways

    o Astrobiologists seeking life on other planets are

    concentrating their search on planets with water

    o To date, more than 200 planets have been found

    outside our solar system; one or two of them

    contain water

    o In our solar system, Mars has been found to

    have water

    Possible Evolution of Life on Other

    Planets with Water

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    Acidic and basic conditions affect living

    organisms o A hydrogen atom in a hydrogen bond between two water

    molecules can shift from one to the other:

    o The hydrogen atom leaves its electron behind and is

    transferred as a proton, or hydrogen ion (H+)

    o The molecule with the extra proton is now a hydronium ion

    (H3O+), though it is often represented as H+

    o The molecule that lost the proton is now a hydroxide ion

    (OH)

    Protons=8

    Electrons=8

    Protons=8

    Electrons=8

    Protons=9

    Electrons=8

    Protons=7

    Electrons=8

    Hydrogen ion

    o Water is in a state of dynamic equilibrium in

    which water molecules dissociate at the

    same rate at which they are being reformed

    Hydronium ion (H3O

    +) Hydroxide ion (OH)

    2H2O

    H

    H

    H

    H

    H

    H

    H

    H

    O O O O

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    o Though statistically rare (1 in 554,000,000

    molecules), the dissociation of water

    molecules has a great effect on organisms

    o Changes in concentrations of H+ and OH

    can drastically affect the chemistry of a cell

    Effects of Changes in pH

    o Concentrations of H+ and OH are equal in pure water

    o Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH (and their ratio)

    o Acid: Increases H+ concentration (and/or removes OH-)

    o Base: Reduces H+ concentration (and/or add OH-)

    o Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic)

    The pH Scale

    o In any aqueous solution at 25C the product of H+ and OH is constant and can be written as

    [H+][OH] = 1014 (10-7 x 10-7)

    o The pH of a solution is defined by the negative logarithm (exponent of 10) of H+ concentration, written as:

    pH = log [H+]

    o For a neutral aqueous solution [H+] is 107 = (7) = 7

    o When H+ = OH-, pH=7

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    Neutral solution

    Acidic solution

    Basic solution

    OH

    OH OH

    OH

    OH OH OH

    H+

    H+

    H+

    OH

    H+ H+

    H+ H+

    OH

    OH

    OH OH

    H+

    OH

    H+

    H+

    H+

    H+ H+

    H+ H+

    OH

    Neutral [H+] = [OH]

    pH Scale

    0

    1

    2

    3

    4

    5

    6

    7

    8

    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

    11

    12

    13

    14

    Each pH unit is 10x

    different than the

    next

    pH=4 has 10x more

    H+ than pH=5, &

    100x more H+ than

    pH=6

    o Acidic solutions have pH values less than 7

    o Basic solutions have pH values greater than

    7

    o Most biological fluids have pH values in the

    range of 6 to 8

    Buffers

    o The internal pH of most living cells must remain close to

    pH 7 (blood pH ~7.4, a person cannot survive for more

    than a few minutes if the blood pH drops to 7 or rises to

    7.8!!!!)

    o Buffers are substances that minimize changes in

    concentrations of H+ and OH in a solution

    o Most buffers consist of an acid-base pair that reversibly

    combines with H+

    H2CO3 HCO3

    - + H+ In your blood Those protons can absorb extra OH-

    s in basic solutions OR contribute H+s to a base that absorbs them

    The HCO3- can absorb extra H+s in

    acidic solutions Carbonic acid H+ acceptor

    Weak base

    Response to rise in pH

    Response to drop in pH

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    o Human activities such as burning fossil fuels

    aquatic organisms

    o CO2 is released by fossil fuel combustion

    and contributes to: o A warming of earth called the greenhouse effect

    o Acidification of the oceans; this leads to a

    decrease in the ability of corals to form calcified

    reefs

    H2CO3 HCO3- + H+

    Weak Base

    Acid (H+ acceptor)

    CO2

    H2O

    Current Event

    http://abcnews.go.com/GMA/video/ocean-acid-10445789

    Polar vs. Non-polar - Review

    Polar molecule A molecule such as water with opposite charges on different ends of the molecule electrons spend more time around the electronegative atom

    Ionic compound A compound formed by an ionic bond

    Non-polar molecule A molecule such as a hydrocarbons in which electrons are shared equally between the atoms which have

    similar electronegativity

    When an ionic compound such as NaCl is placed into water, what

    occurs?

    +

    +

    -

    Polar vs. Non-polar - Review

    Polar molecule A molecule such as water with opposite charges on different ends of the molecule electrons spend more time around the electronegative atom

    Ionic compound A compound formed by an ionic bond

    Non-polar molecule A molecule such as a hydrocarbons in which electrons are shared equally between the atoms which have

    similar electronegativity

    When an non-polar molecule such as oil is placed into water, what

    occurs?

    +

    +

    -

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    Polar vs. Non-polar - Review

    Ionic compounds (salts) can readily be dissolved in water

    Large non-ionic polar molecules can be dissolved in water (if they have ionic and polar regions)

    Non-polar substances do not readily dissolve in water

    Question?????? A mole of of salt (NaCl) weighs 58 g (Na ~23 g, Cl ~35 g). How many

    grams of salt are needed to produce 1 L of a 3-molar (3M) solution?

    Recall:

    Molecular mass is the sum of all masses of all atoms in a molecule

    Each element/compound has a particular mass (grams) = 1 mole

    Molarity (M) is the number of moles of solute per liter of solution (1M =

    1 mole/liter)

    Molecular mass of NaCl = 58 g = 1 mole of NaCl

    And

    3M means 3 moles/L

    So in 1L there are 3 moles of NaCl

    If 1 mole of NaCl = 58 g, 3 moles of Nacl = 174 g