Jan 07, 2016
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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