Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321


Water, of its very nature, as it occurs automatically in the process of cosmic evolution, is fit, with a fitness no less marvelous and varied than that fitness of the organism which has been won by the process of adaptation in the  course of organic evolution. 

L. J. Henderson
The Fitness of the Environment
, 1913

I.  Water is absolutely essential for all living organisms
The evidence:

  1. Most organisms are comprised of at least 70% or more water.  Some plants, like  head of lettuce, are made up of nearly 95% water;

  2. When organisms go dormant, they loose most of their water.  For example, seeds and buds are typically less than 10% water, as are desiccated rotifers, nematodes and yeast cells;

  3. Earth is the water planet (that's why astronomers get so excited about finding water in space).

  4. Water is the limiting resource for crop productivity (see text)

Tom Robbins, author of Even Cowgirls Get the Blues, has very eloquently stated the importance of water:

                �Water - the ace of elements.  Water dives from the clouds without parachute, wings or safety net.  Water runs over the steepest precipice and blinks not a lash.  Water is buried and rises again; water walks on fire and fire gets the blisters.  Stylishly composed in any situation - solid, gas or liquid - speaking in penetrating dialects understood by all things - animal, vegetable or mineral - water travels intrepidly through four dimensions, sustaining, destroying, and creating.  Always in motion, ever-flowing (whether at stream rate or glacier speed), rhythmic, dynamic, ubiquitous, changing and working its changes, a mathematics wrong side out, a philosophy in reverse, the ongoing odyssey of water is virtually irresistible.�

Now, let's change our perspective for a minute and put ourselves in the place of water.  According to Robbins, water is so important that �[i]t has even been suggested that life evolved as a means to transport water.�  

II.  Water is important because it it polar and readily forms hydrogen bonds

A.  Water is Polar. 
In other words, the water molecule has a positively-charged (hydrogen side) and negatively-charged side (oxygen).  This occurs because:

  1. The hydrogen atoms are arranged at an angle of about 105 degrees; 

  2. The covalent bond between O‑H is polarized.  This is caused by an unequal sharing of electrons between these atoms which, in turn, results in a slight negative charge on the oxygen atom (electronegative) and slight positive charge on the hydrogen.

B.  Hydrogen Bonds.  
    The 'fancy' definition of a hydrogen bond is that it is a weak bond that forms between a hydrogen atom that is covalently bonded to an electronegative atom (like oxygen) and another electronegative atom.  In other words a positively-charged hydrogen atom is attracted to a negatively-charged oxygen.

    The end result is that water readily forms hydrogen bonds with itself and other polar molecules.   When likes attract it is termed cohesion (i.e., hydrogen bonds between water molecules).  When unlikes attract, it is called adhesion (i.e., when a paper towel absorbs water, water and cellulose adhere to one another).  Cohesion and adhesion are responsible for capillary action, the movement of water up a thin tube.

    In liquid water, hydrogen bonds between water molecules are continuously made and broken.  The molecules can even form temporary "quasi-crystalline" areas.  Individually, each hydrogen bond is weak (20 kJ mol-1), but collectively they give water many unique properties (a Marxist molecule!).   

III.  The Properties of Water

A.  Water is a liquid at physiological temperatures (i.e., between 0 - 100 C).   
In other words, water has a high boiling point and a high melting point when compared to other similar-sized molecules such as ammonia, carbon dioxide, hydrogen sulfide.  These other molecules are gases at room temperature.  This is important because if life exists anywhere, we predict that it occurs between approx. 0 and 100 C.  Temperatures much below 0 are too cold to permit significant chemistry for metabolism;  temperatures above 100 tend to disrupt bonds.

B.  Water has a high heat of vaporization 
In other words, it takes a lot of energy (ca. 44 kJ mol-1) to convert water from a liquid to a gas; or stated another way, Water resists evaporation.  This property is responsible for water's use in evaporative cooling systems, hence the reason dog's pant, people perspire, and leaves transpire.    

C.  Water has a high specific heat (heat capacity) 
It takes a lot of energy (4.184 J g-1 C-1; to convert this to a less desired unit, 1 cal = 4.184 J) to raise the temperature of water (because it requires a lot of energy to break/make hydrogen bonds).  Thus, water is slow to heat up and cool down, or stated another way, water resists temperature changes.   This is why you can swim in the Sag in late fall but not the spring.  In contrast, a sidewalk has low specific heat - it heats up quickly (try walking barefoot on a sidewalk in summer on a sunny day), but cools down quickly.  This property is important in water's role as a thermal buffer.  It's not surprising that desert plants are succulent - to help resist temperature fluctuations.

D.  Water has a high heat of fusion.  
It takes a lot of energy to convert water from a solid to a liquid, or put another way, water resists freezing.  Energy is required to break the collective hydrogen bonds holding water in its solid configuration.  Conversely, a lot of energy (6 kJ mol-1) must be released by water to freeze.  This property is used by citrus growers ‑ prior to a light freeze they spray fruit with water; ice forms releasing the heat of fusion which will help protect the crop from serious damage.

E.  Water has a high surface tension.  
It takes a lot of energy to break through the surface of water, because water molecules at the surface are attracted (cohesion) to others within the liquid much more than they are to air.  Thus, water acts as though it has a skin.  This phenomenon is important at air/water interfaces and explains why: (1) a belly-whopper into a pool of ammonia would not hurt nearly as much as one into water; (2) raindrops are round (the molecules at the surface attract one another); (3) water striders and other bugs can "walk on water"; (4)  a meniscus forms; and (5) water rises up a thin tube (capillary action).

F.  The density of water decreases on crystallization.  
Good thing too, or else ice fishing would be a moist business.  This occurs because when ice forms, each water molecule is hydrogen bonded to exactly four others.  At four degrees, water is it's densest, and each water molecule is attracted to slightly more than four others.  Thus, as water cools it gets denser and denser until it reaches 4 C, then, it gets less dense.  And ice floats.

G.  Water is a universal solvent.  
Water dissolves more different kinds of molecules than any other solvent.  Hydrophilic (water-loving) molecules dissolve readily in water (likes dissolve likes), hydrophobic (water-fearing) ones do not.  

H.  Water has high tensile strength and incompressibility.  
In other words, if you put water in a tube and put a piston on either end, you won�t be able to push the pistons together.  Thus, water is good for hydraulic systems because when it is squeezed it doesn't compress and produces positive pressures (hydrostatic pressures).  This pressure provides the driving force for cell growth and other plant movements.  The pressure is measured in units of Pascals (or actually MegaPascals, MPa).  One MPa is approximately equal to ten atmospheres.

    In a similar fashion, if you put water in the tube and pulled the pistons away from one another to form suction on the water column (like putting your finger on the end of a syringe and pulling back the plunger), the water column would resist breaking.  Negative pressures (tensions) can develop in the water column.  Very sizable tensions can be generated in a thin water column.  

I.  Water is transparent to light.  
This is important because chloroplasts (inside a cell) are obviously surrounded by water.  Ecologically speaking, aquatic plants, and aquatic ecosystems, couldn't exist if water were opaque.

J.  Water is chemically inert.  
    It doesn't react unless designed to do so.

K.  Water dissociates into protons and hydroxide ions
    This serves as the basis for the pH system (see below).

L.  Water affects the shape and stability of biological molecules.
For example, many molecules are hydrated (water hydrogen bonded to them such as DNA) and have a hydration shell surrounding them.

IV.  Functions of Water
.  In addition to the functions mentioned above, water:

  1. is a major component of cells

  2. is a solvent for the uptake and transport of materials

  3. is a good medium for biochemical reactions

  4. is a reactant in many biochemical reactions (i.e., photosynthesis; but remember it is inert and doesn�t react unless it is designed to do so)

  5. provides structural support via turgor pressure (i.e., leaves)

  6. is the medium for the transfer of plant gametes (sperms swim to eggs in water, some aquatic plants shed pollen underwater)

  7. offspring dispersal (think �coconut�)

  8. plant movements are the result of water moving into and out of those parts (i.e., diurnal movements, stomatal opening, flower opening)

  9. cell elongation and growth

  10. thermal buffer

  11. perhaps most importantly, water has directed the evolution of all organisms.   You can think of morphological features of organisms as a consequence of water availability.  For example, think about organisms growing in xeric (dry), mesic (moderate) and hydric (aquatic) environments.

V.  Acids and Bases

A.  General
    Water ionizes to a small degree to form a hydrogen ion (or proton) and hydroxide ion (OH-).  In reality, two waters form a hydronium ion (H30+) and hydroxide ion.

In pure water,
[H+] [OH-] This solution is neutral
[H+] > [OH-] Then, the solution is acidic
[H+] [OH-] Then the solution is basic (alkaline)

     Thus, an acid is a substance that increases the [H+], or as the chemists say, is an acid is a proton donor.  

e.g.,  HCl H+  +  Cl-

     A base is a substance that increases the [OH-]; or from the perspective of a proton, a base is a substance that decreases the proton concentration; or in other words, a base is a proton acceptor. 

            e.g.,   NaOH  Na+  + OH- (accepts protons to make water) 

            e.g.,   NH3 (ammonia) + H+  NH4+ (ammonium ion)

B.  pH Scale.
pH is the scale to express the degree of acidity (or alkalinity) of a solution.  The scale ranges from 0 to 14 where 1 is highly acidic, 7 is neutral, and 14 is highly alkaline. 

    Points to note: 

1.      the pH scale is based on proton concentration; and  

2.      the pH scale is logarithmic, there is a 10-fold difference in concentration between each pH unit.


at pH 7:

[H+] [OH-]
0.0000001 mol H+ liter-1 = 10-7 H+ 0.0000001 OHliter-1 
pH = log [H+]
= - (-7)

    The products of [H+] x [OH-] always equals 10-14. Thus, you can always determine concentration of one if you know the other. For example, if the [H+] = 10-2, then the [OH-] is 10-12.

VI.  Living systems are very sensitive to pH
- organisms must maintain pH within tolerable ranges.  This is a good example of homeostasis.

    A buffer is a solution that resists fluctuations in pH when additional OH- or H+ are added.  A buffer maintains a constant pH and usually consists of a proton donor and a proton acceptor.

    e.g.,  blood pH must be between 7.36 (venous) and 7.41 (arterial).   The carbonic acid/bicarbonate buffer helps to maintain pH: 

 H2O + CO2   H2CO3 (carbonic acid; proton donor)   H+  +  HCO3- (bicarbonate ion; proton acceptor)

Any given buffer has a limited range in which it is useful.  For example, if protons were continually added to the bicarbonate buffer above eventually the system would run out of bicarbonate to accept them.  At this point, additional protons would lower the pH.


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Last updated: July 14, 2009     � Copyright by SG Saupe