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Topic 4: Carbohydrates-Structure & Functions
TLO: Identify the building blocks of carbohydrates.
Carbon, hydrogen and oxygen are the elements found in carbohydrates. The ratio of hydrogen to oxygen atoms is usually 2:1, the same as in water. Although there are exceptions, carbohydrates generally contain one water molecule for each carbon atom. This is the reason why they are called carbohydrates which means “watered carbon”. Carbohydrates include sugar, glycogen, starches and cellulose. They only represent about 2-3% of body mass. In human and animal, carbohydrate functions mainly as a source of chemical energy for generating ATP needed for metabolic reactions.
There are three major groups of carbohydrates, based on their sizes including monosaccharides, disaccharides and polysaccharides.
4.1 Monosaccharides and disaccharides
Monosaccharides and disaccharides are known as simple sugars. Monosaccharides, the monomers of carbohydrates contain from three to seven carbon atoms. They are designated by suffices “-ose” at the end and prefix that indicates the number of the carbon atoms. For example, monosaccharides with three carbons are called trioses (three-carbon sugar). There are also tetroses (four-carbon sugar), pentoses (five-carbon sugar), hexoses (six-carbon sugar) and heptoses (seven-carbon sugar). Cells throughout our body utilize hexose glucose to produce ATP. Example of triose, pentose and hexoses sugar are shown in the following diagram.
Figure 1 Sugar are varied in structures even they have same numbers of carbon. Yet another difference can come in their reactive group on the end, being either ketone (ketoses) or aldehyde group (aldoses).
A disaccharide is a molecule formed from the combination of two monosaccharides by dehydration synthesis. For example, molecules of the monosaccharides glucose and galactose combine to form a molecule of the disaccharide lactose.
Figure 2 Molecules of monosaccharide galactose and glucose forms the disaccharide lactose.
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