Page 43 - Biology_F5

Biology for Advanced Level Secondary Schools

Open chain and ring forms of pentose structures. For example glucose can exist
and hexose sugars in two isomers of six membered rings
Pentose and hexose sugars can exist in (α- glucose or β-glucose). The formation
both open chains (straight structures) and of ring form is when the oxygen atom of
ring structures. For example, furanose or carbon number 5 joins to the carbon number

furan ring has five membered ring, and 1 bearing the aldehyde and transfer its
FOR ONLINE READING ONLY
pyranose or pyran ring has six membered hydrogen to it and break the bond to form
ring structures. These aromatic ring forms
polysaccharides. The ring form occurs in aqueous solution and can form two isomers w
the OH either above or below the ring. hich
are the most stable and are used to form
are alpha (α) and beta (β) isomers. The α form is when the hydroxyl (OH) group on ca
The oxygen is part of the ring and carbon rbon
disaccharides and polysaccharides. The ring
number 6 sticks up out of the ring. The
atom number 1 projects below the ring while β form is when the OH group on carbon atom
form occurs in aqueous solution and can
number 1projects above the ring. Pentose sugars may form the five membered ring when
same process occurs in galactose. The only
form two isomers, which are either alpha
their carbon atom number 1 joins with the oxygen atom of carbon number 4, an example is as
difference between galactose and glucose
(α) or beta (β) isomers. The α form is when
shown in ribose and deoxyribose sugars (Figure 1.21 a and b). The only difference that exists
is the different orientation of their hydroxyl
the hydroxyl (OH) group on carbon atom
between ribose and deoxyribose sugar is that deoxyribose sugar lacks oxygen in carbon
group in carbon number 4 (Figure 1.20 c
number 1 projects below the ring while β
number 2. Hexoses can form both six and five membered ring. For example glucose can exist
and d). In fructose, the carbonyl group is
form is when the OH group on carbon atom
in two isomers of six membered rings (α- glucose and β-glucose). The formation of ring form
in carbon number 2, hence ring is formed
number 1 projects above the ring. Pentose
is when the oxygen atom of carbon number 5 joins to the carbon number 1 bearing the
by the oxygen atom of carbon number 5
sugars may form the five membered ring
aldehyde and transfer its hydrogen to it and break the bond to form the OH either above or
joining with carbon number 2, leading to
when their carbon atom number 1 joins
below the ring. The oxygen is part of the ring and carbon number 6 sticks up out of the ring.
with the oxygen atom of carbon number 4, the formation of furan ring. When the OH
The same process occurs in galactose. The only difference between galactose and glucose is
an example is as in ribose and deoxyribose in carbon atom number 1 is projected below
the different orientation of their hydroxyl group in carbon number 4 (Figure 1.21 c and d). In
sugars (Figure 1.20 a and b). The only
the ring, it becomes α-fructose while when
fructose, the carbonyl group is in carbon number 2, so the ring is formed by the oxygen atom
difference that exists between ribose and
OH in carbon atom number 2 is projected
of carbon number 5 joining with carbon number 2, leading to the formation of furan ring.
deoxyribose sugar is that deoxyribose sugar
above the ring it becomes β-fructose (Figure
When the OH in carbon atom number 1 is projected below the ring, it becomes α-fructose
lacks oxygen in carbon number 2. Hexoses
1.20 e). Fructose can also form the pyronose
while when OH in carbon atom number 2 is projected above the ring is β-fructose (Figure
can form both six and five membered ring as in glucose.
1.21 e). Fructose can also form the pyronose as in glucose

(a) (b) (b)
(a)

Figure 1.21 Open chain and ring form of (a) ribose and (b) deoxyribose
Figure 1.20: Open chain and ring form of (a) ribose and (b) deoxyribose

Form Five Student’s Book
36