BIOL 600 Lecture Notes - Lecture 8: Hydrogen Bond, Heat Capacity, Celsius
26 May 2018
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AP Bio Chapter 3 Water and the Fitness of the Environment
Lecture Outline
Overview: The Molecule That Supports All of Life
Because water is the substance that makes life possible on Earth, astronomers hope to
find evidence of water on newly discovered planets orbiting distant stars.
Life on Earth began in water and evolved there for 3 billion years before colonizing the
land.
Even terrestrial organisms are tied to water.
o Most cells are surrounded by water.
o Cells are about 70–95% water.
o Water is a reactant in many of the chemical reactions of life.
Water is the only common substance that exists in the natural world in all three physical
states of matter: solid ice, liquid water, and water vapor.
Concept 3.1 The polarity of water molecules results in hydrogen bonding
In a water molecule, two hydrogen atoms form single polar covalent bonds with an
oxygen atom.
o Because oxygen is more electronegative than hydrogen, the region around the
oxygen atom has a partial negative charge.
o The regions near the two hydrogen atoms have a partial positive charge.
A water molecule is a polar molecule in which opposite ends of the molecule have
opposite charges.
Water has a variety of unusual properties because of the attraction between polar water
molecules.
o The slightly negative regions of one water molecule are attracted to the slightly
positive regions of nearby water molecules, forming hydrogen bonds.
o Each water molecule can form hydrogen bonds with up to four neighbors.
Concept 3.2 Four emergent properties of water contribute to Earth’s fitness for life
Organisms depend on the cohesion of water molecules.
The hydrogen bonds joining water molecules are weak, about 1/20 as strong as covalent
bonds.
They form, break, and reform with great frequency. Each hydrogen bond lasts only a few
trillionths of a second.
At any instant, a substantial percentage of all water molecules are bonded to their
neighbors, creating a high level of structure.
Collectively, hydrogen bonds hold water together, a phenomenon called cohesion.
Cohesion among water molecules plays a key role in the transport of water and dissolved
nutrients against gravity in plants.
o Water molecules move from the roots to the leaves of a plant through water-
conducting vessels.
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o As water molecules evaporate from a leaf, other water molecules from vessels in
the leaf replace them.
o Hydrogen bonds cause water molecules leaving the vessels to tug on molecules
farther down.
o This upward pull is transmitted down to the roots.
o Adhesion, clinging of one substance to another, contributes too, as water adheres
to the wall of the vessels.
Surface tension, a measure of the force necessary to stretch or break the surface of a
liquid, is related to cohesion.
Water has a greater surface tension than most other liquids because hydrogen bonds
among surface water molecules resist stretching or breaking the surface.
Water behaves as if covered by an invisible film.
Some animals can stand, walk, or run on water without breaking the surface.
Water moderates temperatures on Earth.
Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to
cooler air.
Water can absorb or release relatively large amounts of heat with only a slight change in
its own temperature.
Atoms and molecules have kinetic energy, the energy of motion, because they are always
moving.
o The faster a molecule moves, the more kinetic energy it has.
Heat is a measure of the total quantity of kinetic energy due to molecular motion in a
body of matter.
Temperature measures the intensity of heat in a body of matter due to the average kinetic
energy of molecules.
o As the average speed of molecules increases, a thermometer will record an
increase in temperature.
Heat and temperature are related, but not identical.
When two objects of different temperatures come together, heat passes from the warmer
object to the cooler object until the two are the same temperature.
o Molecules in the cooler object speed up at the expense of kinetic energy of the
warmer object.
o Ice cubes cool a glass of pop by absorbing heat from the pop as the ice melts.
In most biological settings, temperature is measured on the Celsius scale (°C).
o At sea level, water freezes at 0°C and boils at 100°C.
o Human body temperature is typically 37°C.
While there are several ways to measure heat energy, one convenient unit is the calorie
(cal).
o One calorie is the amount of heat energy necessary to raise the temperature of one
g of water by 1°C.
o A calorie is released when 1 g of water cools by 1°C.
In many biological processes, the kilocalorie (kcal) is more convenient.
o A kilocalorie is the amount of heat energy necessary to raise the temperature of
1000 g of water by 1°C.
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Another common energy unit, the joule (J), is equivalent to 0.239 cal.
Water stabilizes temperature because it has a 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 1°C.
o By definition, the specific heat of water is 1 cal per gram per degree Celsius or 1
cal/g/°C.
Water has a high specific heat compared to other substances.
o For example, ethyl alcohol has a specific heat of 0.6 cal/g/°C.
o The specific heat of iron is 1/10 that of water.
Water resists changes in temperature because of its high specific heat.
o In other words, water absorbs or releases a relatively large quantity of heat for
each degree of temperature change.
Water’s high specific heat is due to hydrogen bonding.
o Heat must be absorbed to break hydrogen bonds, and heat is released when
hydrogen bonds form.
o Investment of one calorie of heat causes relatively little change to the temperature
of water because much of the energy is used to disrupt hydrogen bonds, not speed
up the movement of water molecules.
Water’s high specific heat has effects that range from the level of the whole Earth to the
level of individual organisms.
o A large body of water can absorb a large amount of heat from the sun in daytime
during the summer and yet warm only a few degrees.
o At night and during the winter, the warm water will warm cooler air.
o Therefore, ocean temperatures and coastal land areas have more stable
temperatures than inland areas.
o Living things are made primarily of water. Consequently, they resist changes in
temperature better than they would if composed of a liquid with a lower specific
heat.
The transformation of a molecule from a liquid to a gas is called vaporization or
evaporation.
o This occurs when the molecule moves fast enough to overcome the attraction of
other molecules in the liquid.
o Even in a low-temperature liquid (with low average kinetic energy), some
molecules are moving fast enough to evaporate.
o Heating a liquid increases the average kinetic energy and increases the rate of
evaporation.
Heat of vaporization is the quantity of heat that a liquid must absorb for 1 g of it to be
converted from liquid to gas.
o Water has a relatively high heat of vaporization, requiring about 580 cal of heat to
evaporate 1 g of water at room temperature.
o This is double the heat required to vaporize the same quantity of alcohol or
ammonia.
o This is because hydrogen bonds must be broken before a water molecule can
evaporate from the liquid.
o Water’s high heat of vaporization moderates climate.
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Document Summary
Life on earth began in water and evolved there for 3 billion years before colonizing the land. Even terrestrial organisms are tied to water. o most cells are surrounded by water. o cells are about 70 95% water. o water is a reactant in many of the chemical reactions of life. Water is the only common substance that exists in the natural world in all three physical states of matter: solid ice, liquid water, and water vapor. A water molecule is a polar molecule in which opposite ends of the molecule have opposite charges. Concept 3. 2 four emergent properties of water contribute to earth"s fitness for life organisms depend on the cohesion of water molecules. The hydrogen bonds joining water molecules are weak, about 1/20 as strong as covalent bonds. They form, break, and reform with great frequency. Each hydrogen bond lasts only a few trillionths of a second.
