Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
30
Molecular and structural formulae
The formula for a hexose can be written as C 6
H 12
O 6
.
This is known as the molecular formula. It is also useful
to show the arrangements of the atoms, which can be
done using a diagram known as the structural formula.
Figure 2.3 shows the structural formula of glucose, a
hexose, which is the most common monosaccharide.
H
H
C
C
H O C
H
H
H
C
C
C
H
O
O
H
O
O
O
H
H
H
H
more
commonly
shown as
Figure 2.3 Structural formula of glucose. –OH is known as a
hydroxyl group. There are five in glucose.
H
HO
H
H
H
C
C
C
C
C
CH 2 OH
O
OH
H
OH
OH
Ring structures
One important aspect of the structure of pentoses and
hexoses is that the chain of carbon atoms is long enough
to close up on itself and form a more stable ring structure.
This can be illustrated using glucose as an example. When
glucose forms a ring, carbon atom number 1 joins to the
oxygen on carbon atom number 5 (Figure 2.4). The ring
therefore contains oxygen, and carbon atom number 6 is
not part of the ring.
You will see from Figure 2.4 that the hydroxyl group,
–OH, on carbon atom 1 may be above or below the
plane of the ring. The form of glucose where it is below
the ring is known as α-glucose (alpha-glucose) and the
form where it is above the ring is β-glucose (beta-glucose).
The same molecule can switch between the two forms.
Two forms of the same chemical are known as isomers,
and the extra variety provided by the existence of α- and
β-isomers has important biological consequences, as
we shall see in the structures of starch, glycogen
and cellulose.
QUESTION
2.1 The formula for a hexose is C 6
H 12
O 6
or (CH 2
O) 6
. What
would be the formula of:
a a triose?
b a pentose?
6
CH 2 OH
OH
H
5
C
O
H
O
H
1
C
O
4 H
C 1
C
OH H
OH
OH
3
C 2
C
or, more
simply
OH
OH
OH
H
HO
2
C
3
C
OH
H
H
OH
α-glucose
OH
H
4
C
OH
6
CH 2 OH
OH
H
5
C
6
CH 2 OH
OH
glucose straight-chain form
with C atoms numbered
H
4
C
OH
5
C
H
OH
3
C
H
O
H
2
C
OH
1
C
OH
H
or, more
simply
β-glucose
OH
OH
O
OH
OH
Figure 2.4 Structural formulae for the straight-chain and ring forms of glucose. Chemists often leave out the C and H atoms from
the structural formula for simplicity.