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

 

 

Course-external links are in brackets

Click [index] to access site index

Click here to access text’s website

Vocabulary words are found below

 

 

(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]

(2) Introduction to water

(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.”

(g)                    [“Exactly where all of Earth’s water came from is still debated, but in one scenario, some planetary embryos were water-rich, endowing any growing planet they happened to hit with extra water. Because the most important impacts were the last few hits by the largest remaining bodies… a planet’s water allotment might be determined by a few very large impacts, making the difference between a wet and a dry planet even more of a roll of the dice… ‘Although water is what we would call a minor constituent [of a planet] it seems to play an important role in determining how a planet works.’ On Earth, it appears to act as a kind of lubricant. Ocean plates sinking into the mantle carry traces of water that lower the melting temperature of mantle rock, helping to fire overlying volcanoes. The subducted water also seems to soften the layer of mantle rock on which the planets glide. ‘If there were no water… you might not have plate tectonics’ on Earth. Venus lacks plate tectonics even though it is nearly Earth’s twin in size and has similar reserves of internal heat. Its lack of water… may be the crucial difference” Richard A. Kerr, 1999, Making new worlds with a throw of the dice. Science 286:68-69]

(h)                    [water and life, water and biology (Google Search)] [index]

 

PROPERTIES OF WATER

 

(3) Water is a polar molecule

(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

(v)                    High heat of vaporization

(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

(x)                    Hydrophobic exlusion

(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]

(4) Hydrogen bonding

(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]

(5) Surface tension

(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]

(6) High specific heat

(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]

(7) High heat of vaporization

(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]

(8) Water is a liquid

(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)

(9) Evaporative cooling

(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]

(10) Ice floats

(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]

(11) Water as a solvent

(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]

(12) Hydration shells

(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

(13) Cohesion

(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]

(14) Adhesion

(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]

(17) Hydrophobic exclusion

(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)