Carbohydrates are compounds composed of single or multiple sugars. The name means "carbon and water," and a chemical shorthand for carbohydrate is CHO, signifying carbon (C), hydrogen (H), and oxygen (O).
Carbohydrates contain the sun's radiant energy, captured in a form that living things can use to drive the processes of life. Green plants make carbohydrate through photosynthesis in the presence of chlorophyll and sunlight. In this process, water (H2O) absorbed by the plant's roots donates hydrogen and oxygen. Carbon dioxide gas (CO2) absorbed into its leaves donates carbon and oxygen. Water and carbon dioxide combine to yield the most common of the sugars, the single sugar glucose. Scientists know the reaction in the minutest detail but have never been able to reproduce it. Green plants are required to make it happen.
Light energy from the sun drives the photosynthesis reaction. The light energy becomes the chemical energy of the chemical bonds that hold six atoms of carbon atoms together in the sugar glucose. Glucose provides energy for the work of all the cells of the stem, roots, flowers, and fruits of the plant.
Plants do not use all of the energy stored in their sugars, so it remains available for use by the animal or human being that consumes the plant. Thus, carbohydrates make the first link in the food chain that supports all life on earth. Carbohydrate-rich foods come almost exclusively from plants; milk is the only animal-derived food that contains significant amounts of carbohydrates.
Forms of Carbohydrates: Monosaccharides and Oligosaccharides
Oligosaccharides are composed of 2 to 10 minosaccharides linked together by a special chemical form called glycosidic. Six sugar molecules are important in nutrition. Three of these are single sugars, or monosaccharides (glucose, fructose, and galactose). The other three are double sugars, or disaccharides (lactose, maltose, and sucrose). All disaccharides contain glucose. All of their chemical names end in ose which means "sugar." When you eat a food containing monosaccharides, you can absorb them directly into your blood. When you eat disaccharides, though, you must digest them first. Disaccharides are the most common form of oligosaccharides.
Forms of Carbohydrates: Polysaccharides
In addition to occurring in sugars, glucose in foods also occurs in long strands of thousands of glucose units. These are the polysaccharides. Starch is a polysaccharide, as are glycogen and most of the fibers. Starch is a plant's storage form of glucose.
Just as plant tissues store glucose in long chains of starch, animal bodies store glucose in long chains of glycogen. Glycogen resembles starch in that it consists of glucose molecules linked together to form chains, but its chains are longer and more highly branched. Glycogen can be broken down quickly, releasing glucose for energy as needed. Liver glycogen stores are used to maintain normal blood glucose levels and account for about one-third of the body's total glycogen stores. The body can only store only limited amounts of glycogen, usually enough to last from a few hours to one day, depending on activity level.
Some of the fibers of a plant form the supporting structures of its leaves, stems, and seeds. Like starch, most fibers are polysaccharides, but they differ from starch in that the sugar units are held together by bonds that human digestive enzymes cannot break. Most fibers therefore pass through the human body without providing energy for its use.
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- Discovering Nutrition. Paul Insel, R. Elaine Turner, Don Ross
- Advanced human nutrition. Robert E. C. Wildman, Denis M. Medeiros