Chemistry Reference
In-Depth Information
H
C
If starch is mixed with saliva and left to stand for
a few minutes, it will break down to maltose, a
disaccharide
(that is two joined monosaccharides).
The enzyme present in the saliva, called amylase,
catalyses this hydrolysis reaction.
starch + water in saliva
amylase
maltose
2(C
6
H
10
O
5
)
n
(
s
) +
n
H
2
O(
l
)
n
C
12
H
22
O
11
(
aq
)
Enzymes are very effi cient natural catalysts present
in plants and animals. They do not require high
temperatures to break down the starch to maltose. In
humans, a salivary amylase breaks down the starch in
our food. If you chew on a piece of bread for several
minutes, you will notice a sweet taste in your mouth.
The above hydrolysis reactions are summarised in
Figure 15.21.
R
H
H
H
H
2
N
C
COOH
H
2
N C COOH
H
2
N C COOH
H
H
H
general
structure
glycine
alanine
Amino acids are the building blocks of
proteins
.
Similar to nylon (see p. 241) proteins are
polyamides, as they contain the -CONH- group,
which is called the amide or, in the case of
proteins, the peptide link. Proteins are formed by
condensation polymerisation.
glycine
alanine
H
H
H
2
N C
COOH
H
2
N
C
COOH
starch
O
O
O
O
O
O
O
H
CH
3
acid
hydrolysis
enzyme
hydrolysis
H
O
H
H
2
N
C
C
N
C
COOH
H
2
O
H
H
CH
3
HO
OH HO
OH
HO
O
OH
HO
OH HO
OH
HO
O
OH
a
dipeptide
(composed of two amino acids joined together)
HO
OH HO
OH
HO
O
OH
glucose
(C
6
H
12
O
6
)
maltose
(C
12
H
22
O
11
)
Protein chains formed by the reaction of many
amino acid molecules have the general structure
shown below.
Figure 15.21
Starch produces glucose or maltose depending on the type
of hydrolysis used. Using chromatography it is possible to separate and
identify the products of the different styles of hydrolysis.
H
H
O
R
H
H
O
N
C
C
N
C
C N
C
C
Questions
1
In the hydrolysis of starch, how, using a chemical
test, could you tell whether all the starch had been
hydrolysed?
2
Describe a method you could possibly use to identify the
products of the different types of hydrolysis.
R
H
H
O
R
Further reaction with many more amino acids takes
place at each end of each molecule to produce
the fi nal protein (Figures 15.22 and 15.23). For
a molecule to be a protein, there must be at least
100 amino acids involved. Below this number, they
are called polypeptides. Proteins make up some 15%
of our body weight.
Amino acids
There are 20 different amino acids and they each
possess two functional groups. One is the carboxylic
acid group, -COOH. The other is the amine group,
-NH
2
. The two amino acids shown at the top of the
next column are glycine and alanine.
H
O
H
O
N
CN
CN C
H
O
Figure 15.22
General structure of a protein.
