Important
words and concepts from Chapter 3, Campbell & Reece, 2002 (1/14/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
words
are found below |
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(1) Chapter title: Water and the Fitness of
the Environment
(a)
Found at this site are additional pages of possibly related interest
including: [water]
(b)
[water and the fitness of the
environment (Google Search)] [atoms, molecules, water, and
pH (Biology at Clermont College)]
[index]
(a)
Water is the most important substance
necessary to the existence of life (Carbon fills the #2
slot).
(b)
“Water is so common that it is easy to overlook the fact that it is an
exceptional substance with many extraordinary qualities. Following the theme of
emergent properties,
we can trace water’s unique behavior to the structure and interactions of its
molecules.”
(c)
Life evolved in water and all
metabolically active life either lives in water
or carries water around with it, such
that a high fraction of the make up of an organism ([e.g., ]50%) usually
consists simply of liquid water. Water is a chemically unique compound to
which life is both fully and irreversibly adapted. Outside of the cell,
nutrients are dissolved in water,
which facilitates their passage through cell membranes. And inside the cell, water is the medium in which most
chemical reactions take place. (note, I’m betting the above is a quote but I’m
not-yet certain from where or of whom)
(d)
The emergent properties that will concern us include: cohesion, adhesion, surface
tension, high specific heat, high heat of vaporization, water is a liquid, evaporative cooling,
ice
floats, water as a solvent, hydrophilicity, hydrophobicity, hydration shells,
hydrophobic exclusion,
pH,
and pH
buffering (the italicized terms are either not found in the chapter
or not described by that name in the chapter).
(e)
This is the first chapter of your text to begin focusing on the details
of life processes, in this case the noncovalent interactions that are
exemplified by water. Why water? Water, first, is absolutely essential to the
existence of life on earth and understanding water’s properties give us a means
of starting to understand not just the more-complex molecules of life but also
how those more-complex molecules interact with water (as they most-assuredly
do) and with each other. But besides all of that, by studying water we begin
our study of liquids, and life, at the level of molecular (and intramolecular)
interactions is, for the most part, liquid. What does that mean? It means that
living things are complex but simultaneously dynamic. In the scheme of the
phases of physical matter, it is not difficult to understand that solids are
capable of exhibiting significant complexity. However, solids, pretty much by
definition, are not terribly dynamic. That is, they tend to stay fixed in their
form (though culture, e.g., human technology, employs particularly the solid
phase, building dynamic machines mostly from macroscopic, solid-phase
components). At the other end of the spectrum are gasses. Gasses are inherently
dynamic – amazingly so. But gasses, in their chaotic disorder, are inherently
non-complex. In the middle, of course, are liquids, which can be significantly
dynamic (as any sailor may tell you) but simultaneously are capable of
significant complexity, especially complexity that exists at the level of
noncovalent inter- and intra-molecular interactions. Living things embody the
complexity that comes from intermolecular interactions (noncovalent bonds)
existing basically within a liquid. Even the non-liquids of living things are
forged, molecule-by-molecule, within a liquid milieu. Thus, living things, by
and large, are molecular machines that operate within and interact with a
watery environment.
(f)
“Your objective in this chapter is to develop a conceptual
understanding of how water contributes to the fitness of Earth for life.”
(h)
[water and life, water and biology (Google Search)] [index]
PROPERTIES OF WATER
(a)
Water is a polar molecule by two related criteria
(i)
Water contains two polar covalent bonds
(both are O-H bonds)
(ii)
One end of water possesses a partial positive charge (the 2x H end)
while the other end of water possesses a partial negative charge (the O
end—particularly important are the two electron pairs found on oxygen)
(b)
The attraction of opposite partial charges within liquid (or solid)
water combined with the attraction between water’s partial charges and the
partial or full charges associated with other compounds underlie many of the
unusual properties associated with water, including:
(i)
High cohesion
(ii)
High adhesion
(iii)
High surface tension
(iv)
High specific heat
(vi)
The fact that water is a liquid at room temperature
(vii)
Evaporative cooling
(viii)
The fact that ice floats
(ix)
Water serves as an excellent polar solvent
(c)
The dissociation of water that we study as pH is an additional
emergent property of water
(d)
See Figure 2.13, Polar
covalent bonds in a water molecule
(e)
[water is a polar molecule
(Google Search)] [structure of water
(Online Biology Book)]
[water shockwave animation (John Kyrk)] [index]
(a)
Each water molecule can hydrogen bond with
a maximum of four neighboring water
molecules
(b)
Liquid water possesses some structure
due to this hydrogen bonding
(c)
See Figure 3.1, Hydrogen
bonds between water molecules
(d)
Hydrogen bonds in water are ~1/20th as strong as covalent bonds
(e)
Hydrogen bonds, in water, have only brief
durations
(f)
However, hydrogen bonds in water are extremely abundant, breaking and reforming continuously
(g)
[hydrogen bond, hydrogen bonding (Google Search)] [index]
(a)
Surface tension is an emergent property of water that results from the
tendency of water molecules to stick to each other (by hydrogen
bonding) better than they adhere to air molecules
(b)
Surface tension “makes water behave as though it were coated with an
invisible film.”
(c)
FAQ: Is cohesion responsible for surface tension? Yes, but also
involved is the fact that water molecules don't bond very well with air
molecules.
(d)
[surface tension (Google Search)] [index]
(a)
Water is able to absorb heat – without increasing much in temperature –
better than many substances
(b)
This is because for water to increase in temperature, water molecules
must be made to move faster within the water; this requires breaking hydrogen bonds, and the breaking of hydrogen
bonds absorbs heat
(c)
Water is also able to retain heat better than
many substances (i.e., resist cooling)
(d)
This is because for water to decrease in temperature, water molecules
must be made to move more slowly within the water; this requires the forming of
hydrogen bonds, and the forming of hydrogen
bonds gives off heat (hence counteracting cooling tendencies as heat is lost
from liquid water)
(e)
Note again the concept that if forming something requires energy, then
breaking that now-formed thing probably releases energy, in this case as heat
(f)
Water’s high specific heat serves to buffer the internal temperature of
organisms, the temperatures of bodies of water, and the temperatures of the
entire biosphere, all things that enhance the ability of life to survive on
this planet
(g)
[specific heat water
(Google Search)] [index]
(a)
Water resists evaporating (i.e., vaporizing) because hydrogen bonds must be broken in order for
water to transition from liquid to the gas state
(b)
This high heat of vaporization contributes to the ability of water to
serve as local heat sinks (e.g., organisms, lakes, ponds) and
as a global heat sink (i.e., oceans) – these are regions (volumes) that retain
heat for longer than surrounding substances (such as air or rocks)
(c)
[heat of vaporization of water
(Google Search)] [index]
(a)
Water’s high heat of vaporization, resulting from
hydrogen bonding, also is responsible for water being a liquid at typical
ambient temperatures
(b)
That is, most molecules which are of similar molecular weight to water
are gasses at typical ambient temperatures rather than liquids
(c)
The molecular weight (MW) of water is 18, but unlike water, all of the
following are gasses at room temperature:
(i)
CO2 (MW=44)
(ii)
O2 (MW=32)
(iii)
CO (MW=28)
(iv)
N2 (MW=28)
(v)
CH4 (MW=16)
(vi)
H2 (MW=2)
(a)
The vaporization of water is a consequence of individual water
molecules escaping the liquid state for the gas (or vapor) state
(b)
Those water molecules that are most energetic (i.e., moving fastest)
are most likely to escape liquid water
(c)
Faster moving water molecules carry more heat than slower
moving ones (heat actually is simply a measure of degree of molecular motion)
(d)
“It is as if the 100 fastest runners at a college transferred to
another school; the average speed of the remaining students would decline.” (Campbell et al.,
1999)
(e)
This results in the average temperature of liquid water declining with
the loss of each more-energetic water molecule to the vapor phase
(f)
Evaporative cooling contributes to water’s ability to serve as a
temperature buffer
(g)
We employ evaporative cooling when we sweat
(h)
(Evaporative cooling is an example of a system that is perturbed from a
dynamic equilibrium.
When the air about water is saturated with water—100% relative humidity—water
molecules leave the liquid phase as fast as water molecules in the vapor phase
enter the liquid phase. Thus there is no net movement of water molecules in and
out of the liquid or vapor phases, but there still is continuous movement
between the two phases. At times like this you sweat like a pig but don’t cool
down at all because there is no net movement of water molecules to effect
cooling and, assuming constant temperature between the phases, no difference
on-average in the temperature between those water molecules leaving and those
entering the liquid phase.)
(i)
[evaporative cooling
(Google Search)] [index]
(a)
Unlike most substances, solid water (ice) has a lower density than
liquid water
(b)
As a consequence, solid water floats upon liquid water, rather than
sinking beneath it
(c)
The lower density of ice is a result of the water solid phase
containing on average more hydrogen bonds per
water molecule (i.e., approaching 4) than does liquid water at any given moment
(d)
See Figure 3.5, The
structure of ice
(e)
More hydrogen bonds results in more structure
which, in water’s case, results in more unoccupied space, i.e., a lower density
upon freezing
(f)
Because ice floats, bodies of water freeze from the top down rather
than the bottom up
(g)
Since ice serves as an insulator, this property of water assures that
the complete freezing of bodies of water is far less likely, thus further
explaining why so much liquid water exists on this planet
(h)
[ice floats (Google Search)] [Wilson A. Bentley, photographer
of snow crystals (Jericho Historical Society)]
[index]
(a)
The most important property of water to the existence of life has to do
with the ability of water to dissolve some substances and exclude others
(b)
Water dissolves substances to which it can readily hydrogen bond (or is
otherwise attracted to typically because the substance contains a full or
partial charge)
(c)
Solute
= a substance that dissolves in another substance
(d)
Solvent = the substance the solute dissolves in
(e)
Solution = a solvent in which solutes are dissolved
(f)
Aqueous solution = a solution in which water is the solvent
(g)
[water solvent (Google Search)] [index]
(a)
For a substance to dissolve in water, the substance must displace water-to-water
hydrogen bonds
(b)
Consequently, for a substance to readily dissolve in water, it must be
something that water will hydrogen bond to at least as well as water hydrogen
bonds to itself
(c)
Furthermore, the substance must also more-readily hydrogen bond to
water than it interacts with molecules of its own kind; that way the molecule
tends to leave the solid state and enter into solution
(d)
Within solution, such a substance will be surrounded by water molecules
which are hydrogen bonded to it
(e)
See Figure 3.7, A crystal of
table salt dissolving in water
(f)
This surrounding array of water molecules is called a hydration shell
(g)
[hydration shell (Google Search)] [index]
INTERACTIONS WITH WATER
(a)
Cohesion refers to the tendency of water molecules to hydrogen bond to each other
(b)
Cohesion contributes to a number of water’s properties
(c)
These properties include the ability of water to be siphoned as well
the related property of transport of water from the roots to the leaves of
plants
(d)
[cohesion and water
(Google Search)] [is cohesion responsible for surface tension?
(MicroDude)] [index]
(a)
Adhesion is the tendency of water to stick to substances other than
water
(b)
We will discuss adhesion below in terms of hydrophilicity and hydration shells
(c)
[adhesion and water
(Google Search)] [index]
(15)
Hydrophilicity (hydrophilic)
(a)
A substance that either readily dissolves in water or, if it is very
large, is readily wetted by water is said to be hydrophilic
(b)
Hydrophilic substances are ones with which water readily hydrogen bonds, forming a hydration
shell
(c)
Examples include ions, carbohydrates (sugars, starch, cellulose, etc.), etc.
(d)
[hydrophilic, hydrophilicity (Google Search)] [index]
(16)
Hydrophobicity (hydrophobic)
(a)
Many substance don’t hydrogen bond well with water
(b)
Such substances tend not to enter into water solutions
(c)
They are termed hydrophobic
(d)
Examples include oils and fats, which, in
biological systems, collectively are known as lipids
(e)
[hydrophobic, hydrophobicity (Google Search)] [index]
(a)
Because hydrophobic substances tend to like to interact with one
another and not with water, hydrophobic substances tend to both not dissolve
into water and to clump together when placed in water
(b)
For example, water and oils don’t mix
(c)
This clumping actually is a result of the area of contact between the
water and the hydrophobic substance being reduced to some minimal amount
(d)
In terms of water’s structure, what is going on is that water is
arranging itself so that only a minimal number of water-to-water hydrogen bonds are lost to water’s interacting
with the hydrophobic substance
(e)
The more the hydrophobic substance clumps, the lower its
surface-to-volume ratio, and the fewer hydrogen bonds that are displaced
(f)
See Figure 3.8, A
water-soluble protein
(g)
(h)
(i)
FAQ: Could you explain what hydrophobic exclusion is? Hydrophobic
exclusion is ery similar to surface tension. In both cases water molecules
prefer to interact with themselves rather than interacting with a less-polar
substance. This failure to interact significantly with a less-polar substance,
however, reduces the number of hydrogen bonds water molecules are able to
participate in. Losing hydrogen bonds is energetically unfavorable, and the
number of hydrogen bonds lost is directly proportional to the area of contact
between the water molecules and the hydrophobic substance. The smaller the area
of contact, the fewer hydrogen bonds lost. Two volumes will have a smaller
combined surface area if they become squished together rather than remaining
separated. Consequently, hydrophobic substances, suspended in water, tend to
pool together, away from water. For example, if you place two drops of oil on
the surface of a water solution, the oil drops will tend to coalesce into a
single drop (see Figures, above). Why care? Lipid bilayer membranes spontaneous
form as a consequence of hydrophobic exclusion. Globular proteins spontaneous
fold so that their hydrophobic parts are inside, away from water (i.e.,
hydrophobic exclusion). Hydrophobic exclusion is also one of things that makes
lipids better long-term (and compact) storage molecules than carbohydrates. In
fact, to life, the two most important roles played by water are (i) water
dissolves hydrophilic substances and (ii) water excludes hydrophobic
substances, i.e., hydrophobic exclusion.
(j)
[hydrophobic exclusion
(Google Search)] [the hydrophobic effect does
not mean that nonpolar molecules are not attracted to water! (Bad Science Home Page)] [hydrophobic exclusion (MicroDude)]
[index]
DISSOCIATION OF WATER
(18) Hydrogen ions (hydroxyl
ions)
(a)
Water molecules can disassociate into charged compounds (i.e., ions)
called H+ (hydrogen ion) and
(b)
There can only be so many of these ions in water (note: just accept
this)
(i)
The more hydroxyl ions there are around, the fewer hydrogen ions there
can be (because excess hydroxyl ions will tend to combine with whatever
hydrogen ions are around, forming water, significantly reducing the number of
hydrogen ions around—this, by the way, is an example of a dynamic chemical
equilibrium)
(ii)
Similarly, excess hydrogen ions tend to mop up hydroxyl ions
(c)
A solution containing an excess of hydrogen
ions is said to be acidic and tastes
sour
(d)
A solution containing an excess of hydroxyl
ions is said to be basic and tastes
bitter; note that an excess of hydroxyl ions has a corresponding dearth of
hydrogen ions
(e)
[hydrogen ion, hydroxyl ion (Google Search)] [index]
(a)
pH is a measure of hydrogen ion (H+) concentration in water
solution
(b)
pH actually is equal to the inverse log of the Molar hydrogen ion
concentration
(c)
The more hydrogen ions present in a water solution, the higher the
Molar concentration, and therefore the lower
the pH
(d)
See Figure 3.9, The pH of
some aqueous solutions
(e)
Solutions containing excessive hydrogen ions are termed acidic and have
a pH that is less than 7
(f)
Solutions containing excess of hydroxide ions (
(g)
(Note that the dissociation of water, H2O
<==>
(h)
[pH (Google Search)] [index]
(a)
Various aspects of biological systems work best at well-defined pHs
(b)
In order to keep pHs within well-defined ranges, biological systems
employ solutes which together are able to mop up excessive hydrogen ions or hydroxide ions
(c)
The reason for the need to keep pHs within well-defined ranges has to
do with enzymes typically functioning optimally only within narrow pH
ranges, and if enzymes stop working well, whole system
falls apart (e.g., organisms die)
(d)
(Yes, I know that hydrogen ions really exist as hydronium ions, and
that even that is a simplification, but this is biology so don’t worry about
it)
(e)
[pH buffer (Google Search)] [buffer lessons
(author unknown)] [index]
VOCABULARY
(21)
Vocabulary [index]
(b)
Cohesion
(f)
Hydration
shells
(g)
Hydrogen ions
(h)
Hydroxyl ions
(i)
Hydrogen bonding
(j)
Hydrophilic
(k)
Hydrophilicity
(l)
Hydrophobic
(n)
Hydrophobicity
(o)
Ice floats
(p)
pH
(q)
pH buffer
(r)
Surface tension