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Acids, bases and buffers - Biochemistry
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Here are lecture notes for a biochemistry lecture on acids, bases and buffers
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BIOCHEMISTRY – LECTURE TWO PART TWOacids, bases and buffers
An acid is a molecule that can act as a proton (H+) donor. A base is a molecule
that can act as a proton (H+) acceptor. A buffer is a solution that can resist
changes in pH when small amounts of acid or base are added. Its made from a
weak acid and its conjugate base.
o Acid H+ + Base
Water can dissociate in a reversible reaction:
o H2O H+ + OH-
If an acid is added, it dissociates in solution and the equilibrium of the reaction is
moved to the left – the excess protons react with the OH - to form water, but
there would still be an excess of H+ in the solution. If a base is added to the
solution, protons are accepted by it and the number of protons in solution
decreases. In an aqueous solution, H + and OH- are always present but in an
acidic solution H+ are in excess, in a basic solution OH - are in excess.
pH is a measurement of hydrogen ion concentration. The scale is based on a
logarithmic scale (log10) of the proton concentration (H+) concentration. pH scale
is useful as the range of concentrations that need to be referred to can vary
greatly. The range of pH in the blood which is compatible with life is 7.3-7.5 but
more so 7.35-7.45. Outside of this range conformation of proteins are altered
and lose their function through denaturation. So, the control of pH is vital for
health. Although H+ is the smallest ion and is in very low concentrations, the
range of H+ in the blood which is compatible with life is very narrow.
There is an important relationship between carbon dioxide and pH
o CO2 + H2O H2CO3 H+ + HCO3-
As the amount of carbon dioxide increases, there is an increase in the formation
of carbonic acid. This dissociates to form bicarbonate and a proton. Therefore,
when the concentration of carbon dioxide increases, there is an increase in the
concentration of protons and therefore a decrease in pH. This relationship can
cause problems in relation to health, but it is physiological and, in many tissues,
catalysed by an enzyme carbonic anhydrase (CA).
In a red blood cell, the reaction catalysed by carbonic anhydrase is vital
for the transport of carbon dioxide from respiring tissue to the lungs.
Carbon dioxide produced by respiring tissues diffuses into red blood cells.
It reacts with water to form carbonic acid, which dissociates into
bicarbonate and a proton. The bicarbonate shifts out of the cell and
another anion (Cl-) moves in to keep the anionic balance. This is known as
the chloride shift. The drop in pH caused by the formation of carbonic acid
causes a shape change in oxyhaemoglobin, which causes it to release
oxygen to the respiring tissues.
At the lungs the reverse happens, oxygen diffuses into red blood cells due
to the low concentration of carbon dioxide which causes a change in
haemoglobin which increases its affinity for oxygen. Cl - shifts out of the
cell and HCO3- moves in. It combines with H+ to form carbonic acid which
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