Chemistry Reference
In-Depth Information
Biological buffers
The human body contains many buffer systems, which control the pH of
body compartments and fluids very effectively. Blood plasma is maintained
at a pH of 7.4 by the action of three main buffer systems: first, dissolved
carbon dioxide, which gives carbonic acid (H
2
CO
3
) in solution, and its
sodium salt (usually sodium bicarbonate, NaHCO
3
). This is responsible for
most of the buffering capacity. The other two buffers are dihydrogenphos-
phate (H
2
PO
4
), also with its sodium salt, and protein macromolecules.
Proteins are polymers composed of repeating units called amino acids.
These amino acids (as their name suggests) are compounds containing NH
2
and COOH groups in the same molecule and have the general formula
shown in Figure 1.1.
COOH
H
C
R
NH
2
Figure 1.1
The general formula of amino acids.
Proteins are composed of about 20 different amino acids, which are
connected to each other by
peptide bonds
formed between one amino acid
and its neighbour. The side-chain of the amino acid may be acidic (as in the
case of glutamic and aspartic acids), basic (as in the case of arginine and
lysine) or neutral (as in alanine). A protein, which may be composed of
hundreds of amino acids, is therefore a polyelectrolyte whose properties
depend on the balance of acidic and basic groups on the side-chains. Gener-
ally, most proteins act as weak acids and form buffers with their sodium
salts. Compounds like amino acids, which are capable of acting as both
acids and bases, are known as
amphoteric
, or sometimes,
amphiprotic
. In
solution, free amino acids usually do not exist in the molecular form shown
in Figure 1.1, but instead both the amino and carboxyl groups ionise to
form an internal salt as shown in Figure 1.2.
COO
-
H
C
+
R
N
H
3
Figure 1.2
The structure of a zwitterion.
