Carbohydrates are organic compounds. They consist of three main elements: carbon, hydrogen and oxygen. Common carbohydrates include starch and sugars.
There are three types of carbohydrate molecule:
- Monosaccharides, which contain a single sugar unit
- Disaccharides, which are made up of two monosaccharides
- Polysaccharides, which are multiple monosaccharides linked together
Monosaccharides have the same general molecular formula: C6H12O6. Because of the number of carbon atoms, each is a hexose. Glucose, galactose and fructose are monosaccharides. Glucose is the main source of energy for many organisms.
Sucrose, lactose and maltose are three common disaccharides.
- Sucrose (common table sugar) = glucose + fructose
- Lactose (major sugar in milk) = glucose + galactose
- Maltose (product of starch digestion) = glucose + glucose
Starch is the most abundant polysaccharide. It is a mixture of the two compounds amylose and amylopectin. Amylose is made up of unbranched chains of several hundred glucose residues which are linked by a glycosidic bond between their α(1,4) carbon atoms. Amylopectin is a highly branched structure which could consist of several hundred glucose residues.
The main thing to remember is that carbohydrates can only be absorbed in the form of monosaccharides. Complex carbohydrates in the food are reduced to disaccharides (mainly maltose) by salivary and pancreatic amylase. Brush border enzymes convert disaccharides into monosaccharides (glucose, galactose).