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Buffers in biological systems
Buffers are designed to resist changes in pH. They should not break down under working
conditions. They should be stable and not oxidise or be affected by the system in which it is being
Biological buffers have an important role to play in the body. An imbalance in pH caused by some
factors can result in health problems. Humans can only function within in the pH range 7.35 to 7.45.
Enzymes are extremely sensitive to solution pH and require buffered solutions to function.
Originally, different substances were used as buffers, for example phosphate and bicarbonate. Later
on, weak organic acids were used as well. However they have lasting effects on the system such as
inhibiting enzymes. The majority of biological buffers used today were produced by NE Good and are
N-substituted taurine or glycine buffers.
The fluid inside and surrounding cells have an almost constant pH. This is maintained through buffer
systems. There are two systems in particular: the phosphate buffer system and the carbonic acid
Phosphate buffer system
This occurs in the internal fluid of all cells. It consists of dihydrogen phosphate ions (H2PO4-) as the acid
and hydrogen phosphate ions (HPO42-) as the base, which are in equilibrium:
If more hydrogen ions enter the fluid, equilibrium shifts to the left. Conversely, adding hydroxide
ions enter the fluid, equilibrium shifts to the right. In mammals, cellular fluid has a pH range of 6.9 to
7.4 and the phosphate buffer is effective in maintaining this pH range.
Carbonic acid system
Carbon dioxide dissolves in body fluids to form carbonic acid and this leads to a reduction in pH. In
blood plasma, the carbonic acid and hydrogen carbonate ion equilibrium buffers the pH, with
carbonic acid acting as the acid and hydrogen carbonate acting as the base.
The pH of arterial blood plasma is 7.40. If the pH falls below this value acidosis (increased acidity in
the blood and other tissues) is produced. If the pH rises above this value then alkalosis (when the
body fluids have excess alkali) occurs. The carbonic acid-hydrogen carbonate ion buffer function to
maintain the pH of blood plasma close to 7.40. This is done by eliminating either the acid or the base.
Increased or decreased respiration can affect changes in carbonic acid concentration. On the other
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hand, changes in the hydrogen carbonate ion concentration need hours through relatively slow
elimination through the kidneys.
Oceans can also act as a buffer system for the carbon dioxide in the atmosphere. Carbon dioxide
dissociates to form hydrogen ions and hydrogen carbonate ions. The hydrogen carbonate ions then
dissociate to form carbonate ions and hydrogen ions. The average pH of the oceans is around 7.8-8.0
so the equilibrium shifts toward the formation of hydrogen carbonate ions.…read more