Carbohydrates are the most plentiful energy source in the human diet and are available in the following forms:
- Monosaccharides: These simple sugars, such as glucose and fructose, are found in fruits and honey.
- Disaccharides: These carbohydrates, such as sucrose, lactose, and maltose, are composed of two monosaccharide units linked via a glycosidic bond. Sucrose is found in table sugar and fruits, lactose is found in milk, and maltose is found in malt sugar.
- Oligosaccharides: These carbohydrates are composed of from 3 to 10 monosaccharide units and are found in vegetables. Oligosaccharides are also found attached to proteins and lipids (e.g., glycoproteins and glycolipids, respectively) on the outer surface of cellular membranes in animals.
- Polysaccharides: These carbohydrates are composed of more than 10 monosaccharide units and are exemplified by glycogen (from animal products) and starch (from plant products).
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| Classification of Carbohydrates |
Digestion and Absorption
Various enzymes that contribute to the digestion and absorption of carbohydrates as they pass from the mouth to the intestines are as follows:
- α-Amylase: The breakdown of polysaccharides into oligosaccharides and disaccharides is catalyzed by salivary α-amylase (in the oral cavity) and pancreatic α-amylase (in the duodenum).
- Lactase, sucrase, maltase: Oligosaccharides and disaccharides are hydrolyzed into monosaccharides by the enzymes lactase (which targets lactose), sucrase (which targets sucrose), and maltase (which targets maltose). These enzymes are produced and secreted by intestinal cells. The resulting monosaccharides (e.g., glucose, galactose, and fructose) are absorbed by epithelial cells of the intestinal mucosa and transported by the portal circulation to target tissues.
Glucose, the most important molecule for energy, is stored in the liver and muscle as glycogen, which can be degraded when energy is needed. Carbohydrates are also important components of cellular membranes (as components of glycolipids and glycoproteins), tissues (as components of bones and cartilage), and genetic material (as the sugar molecules in nucleic acids).
