Structure and function of water

1. Water: Biomedical Importance
Water is the predominant chemical component of living organisms. Its
unique physical properties, which include the ability to solvate a wide
range of organic and inorganic molecules, derive from water's dipolar
structure and exceptional capacity for forming hydrogen bonds.
The manner in which water interacts with a solvated biomolecule
influences the structure of each. An excellent nucleophile, water is a
reactant or product in many metabolic reactions.
Water has a slight propensity to dissociate into hydroxide ions and
protons. The acidity of aqueous solutions is generally reported using the
logarithmic pH scale.
Bicarbonate and other buffers normally maintain the pH of extracellular
fluid between 7.35 and 7.45. Suspected disturbances of acid–base
balance are verified by measuring the pH of arterial blood and the CO2
content of venous blood.

2. Water, an ideal Biological Solvent
• A water molecule is an irregular, slightly skewed tetrahedron with
oxygen at its center. The two hydrogens and the unshared electrons
of the remaining two sp 3-hybridized orbitals occupy the corners of
the tetrahedron.
• The 105-degree angle between the hydrogens differs slightly from
the ideal tetrahedral angle, 109.5 degrees. Ammonia is also
tetrahedral, with a 107-degree angle between its hydrogens. Water is
a dipole, a molecule with electrical charge distributed asymmetrically
about its structure.
• The strongly electronegative oxygen atom pulls electrons away
from the hydrogen nuclei, leaving them with a partial positive
charge, while its two unshared electron pairs constitute a
region of local negative charge.

14. The ability of water to ionize, while slight, is of central importance for life.
Since water can act both as an acid and as a base, its ionization may be
represented as an intermolecular proton transfer that forms a hydronium
ion (H3O+) and a hydroxide ion (OH – )
As its name suggests, the ion product Kw is numerically equal to the product of the
molar concentrations of H+ and OH–

20. Importance of pH
Only a narrow range of pH change can be sustained by a living organism, any
further change in pH can make the living difficult. For example: in the case of acid
rain, the pH of water is less than 7. As it flows into a river, it lowers the pH of river
water which makes the survival of aquatic life difficult.
We know that our stomach contains hydrochloric acid which helps in the digestion
of food. When the stomach produces too much of hydrochloric acid during
indigestion, we feel a lot of pain and irritation. Hence, we generally use antacids
or a mild base which increases the pH of the acidic stomach and thus decreases
the pain.
Bacteria present in our mouth sometimes lower the pH of our mouth by
producing acids through degradation of the food particle. Hence, we are
instructed to clean our mouths with toothpaste (which is generally basic) to
prevent their decay by maintaining the pH.
We experience a lot of pain in case of bee-sting as the bee injects the methanoic
acid through its sting. Hence, we are generally advised to apply baking soda or
other mild bases on the surface as it helps in maintaining the pH of the surface.

24. Strong Acids and Strong Bases
The stronger the acid, the more readily it donates H+.
For example, hydrochloric acid (HCl) is highly acidic and completely
dissociates into hydrogen and chloride ions, whereas the acids in tomato
juice or vinegar do not completely dissociate and are considered weak acids
Conversely, strong bases readily donate OH– and/or react with hydrogen
ions.
Sodium hydroxide (NaOH) and many household cleaners are highly basic and
give up OH– rapidly when placed in water; the OH– ions react with H+ in
solution, creating new water molecules and lowering the amount of free H+
in the system, thereby raising the overall pH.
An example of a weak basic solution is seawater, which has a pH near 8.0,
close enough to neutral that well-adapted marine organisms thrive in this
alkaline environment.

25. BUFFERS
• How can organisms whose bodies require a near-neutral pH ingest acidic
and basic substances (a human drinking orange juice, for example) and
survive?
• Buffers are the key. Buffers usually consist of a weak acid and its
conjugate base; this enables them to readily absorb excess H+ or OH–,
keeping the system’s pH within a narrow range.
• Maintaining a constant blood pH is critical to a person’s well-being.
• The buffer that maintains the pH of human blood involves carbonic acid
(H2CO3), bicarbonate ion (HCO3
–), and carbon dioxide (CO2).
When bicarbonate ions combine with free hydrogen ions and become
carbonic acid, hydrogen ions are removed, moderating pH changes.
Similarly, excess carbonic acid can be converted into carbon dioxide gas and
exhaled through the lungs; this prevents too many free hydrogen ions from
building up in the blood and dangerously reducing its pH; likewise, if too
much OH– is introduced into the system, carbonic acid will combine with it to
create bicarbonate, lowering the pH.
Without this buffer system, the body’s pH would fluctuate enough to
jeopardize survival.

26. Key Points
A basic solution will have a pH above 7.0, while an acidic solution will
have a pH below 7.0.
Buffers are solutions that contain a weak acid and its a conjugate base; as
such, they can absorb excess H+ ions or OH–ions, thereby maintaining an
overall steady pH in the solution.
pH is equal to the negative logarithm of the concentration of H+ ions in
solution: pH = – log[H+].
Key Terms
alkaline: having a pH greater than 7; basic
acidic: having a pH less than 7
buffer: a solution composed of a weak acid and its conjugate base that
can be used to stabilize the pH of a solution