Chapter 2: Biological molecules
Roles of monosaccharides in living organisms Monosaccharides have two major functions. First, they are commonly used as a source of energy in respiration. This is due to the large number of carbon–hydrogen bonds. These bonds can be broken to release a lot of energy, which is transferred to help make ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and phosphate. The most important monosaccharide in energy metabolism is glucose. Secondly, monosaccharides are important as building blocks for larger molecules. For example, glucose is used to make the polysaccharides starch, glycogen and cellulose. Ribose (a pentose) is one of the molecules used to make RNA (ribonucleic acid) and ATP. Deoxyribose (also a pentose) is one of the molecules used to make DNA (Chapter 6).
a
monosaccharide
disaccharide
6 CH2OH 5
O
6 CH2OH 5
O
–H2O (condensation)
4
1
OH
OH
4
1
OH OH
2
OH
OH
OH
O
4
1 OH 2
3
OH OH
O
3 OH
OH
disaccharide
OH
6 CH2OH
4
5 5
2
OH
2
3
maltose
monosaccharide
1 CH2OH
OH
O
2
31
1
OH glycosidic bond
OH
6 CH2OH
α-glucose
OH
O
4
1
α-glucose
monosaccharide
OH
+H2O (hydrolysis)
5
O
3
H2O
α-glucose
5
OH
2
6 CH2OH
4
3
3
b
Disaccharides, like monosaccharides, are sugars. They are formed by two monosaccharides joining together. The three most common disaccharides are maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose). Sucrose is the transport sugar in plants and the sugar commonly bought in shops. Lactose is the sugar found in milk and is therefore an important constituent of the diet of young mammals. The joining of two monosaccharides takes place by a process known as condensation. Two examples are shown in Figure 2.5. In Figure 2.5a two molecules of α-glucose combine to make the disaccharide maltose. In Figure 2.5b α-glucose and β-fructose combine to make the disaccharide sucrose. Notice that fructose has a different ring structure to glucose.
monosaccharide
6 CH2OH 5
Disaccharides and the glycosidic bond
–H2O (condensation)
6 CH OH +H2O (hydrolysis) 2
H2O
β-fructose
OH O
4 OH
1 CH2OH
2
4 5
2
1 OH
3
O O
3
6 CH OH 2
OH glycosidic bond
sucrose
Figure 2.5 Formation of a disaccharide from two monosaccharides by condensation. a Maltose is formed from two α-glucose molecules. This can be repeated many times to form a polysaccharide. Note that in this example the glycosidic bond is formed between carbon atoms 1 and 4 of neighbouring glucose molecules. b Sucrose is made from an α-glucose and a β-fructose molecule. A monosaccharide is a molecule consisting of a single sugar unit with the general formula (CH2O)n. A disaccharide is a sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.
A polysaccharide is a polymer whose subunits are monosaccharides joined together by glycosidic bonds.