Weak interactions in aqueous
systems
Water is a good solvent for polar solutes with which it
forms hydrogen bonds. Non-polar compounds dissolve
poorly in water. They can not form a hydrogen bond with
the solvent. Numerous weak non covalent interactions
influence the folding of macromolecules. The physical
properties of aqueous solutions are strongly influenced
by the concentration of solutes. Ex: The tendency for
water to move across a semi permeable membrane is the
osmotic pressure.
Ionization of water, weak acids and weak
bases
Pure water is slightly ionized. pH is defined as the negative
logarithm of the molar hydronium ion concentration. An acid that
ionizes completely in water is a strong acid. An acid which is
partially ionized in water is a weak acid. pk provides the measure
of the ease of the dissociation of the proton. The lower the number
the stronger is the acid.
Buffering against pH changes in
biological systems
Buffers are aqueous systems that tend to resist in changes in pH
when small amounts or acids or base are added. It consists of a weak
acid and its conjugate base. As an example a mixture of equal
concentrations of acetic acid and acetate ion is a buffer system.
Every life form is extremely sensitive to slight pH changes. Human
blood for example needs to remain within the pH range of 7.38 to
7.42. The relationship between pH and pk is described by Henderson �
Hasselbach equation. It is used to calculate the pH of biological
fluids. Two especially important biological buffers are the
phosphate and bicarbonate systems. The property of a solution that
depends on the concentration of a dissolved solute but not on its
chemical identity is known as colligative properties. Substances
that do not ionize in water are non electrolytes. Vapor pressure,
boiling point, freezing point and osmosis are colligative
properties.
Water as a reactant
Water is very often a direct participant in the chemical reactions
of living cells. Formation of ATP from ADP and inorganic phosphate
is an example of a condensation reaction in which the elements of
water are eliminated. Two amino acid molecules are covalently joined
through a peptide bond with a loss of water molecule.
Fitness of the aqueous environment for
living organisms
The high specific heat of water is useful to cells and organisms
because it allows water to act as a �heat buffer�, keeping the
temperature of an organism relatively constant as the temperature of
the surroundings fluctuates.
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