Provide
a ready source of energy for the body (4 kilocalories/gram) and structural
constituents for the formation of cells
Carbohydrates are molecules composed of carbon, hydrogen,
and oxygen that provide energy to the body. The major food sources of
carbohydrates are milk, grains, fruits, and starchy vegetables, like potatoes.
Non-starchy vegetables also contain carbohydrates, but in lesser quantities.
Carbohydrates are broadly classified into two forms based on their chemical
structure: simple carbohydrates (often called simple sugars) and complex
carbohydrates.
Simple carbohydrates consist of one or two basic sugar units linked together. Their scientific names are "monosaccharides" (1 sugar unit) and disaccharides (2 sugar units). They are broken down and absorbed very quickly in the digestive tract and provide a fast burst of energy to the body. Examples of simple sugars include the disaccharide sucrose, the type of sugar you would have in a bowl on the breakfast table, and the monosaccharide glucose, the most common type of fuel for most organisms including humans. Glucose is the primary sugar that circulates in blood to provide energy to cells. The terms "blood sugar" and "blood glucose" can be substituted for each other.
Complex carbohydrates are long chains of sugars units that can link in a straight chair or a branched chain. During digestion, the body breaks down digestible complex carbohydrates into simple sugars, mostly glucose. Glucose is then absorbed into the bloodstream and transported to all our cells where it is stored, used to make energy, or used to build macromolecules. Fiber is also a complex carbohydrate, but it cannot be broken down by digestive enzymes in the human intestine. As a result, it passes through the digestive tract undigested unless the bacteria that inhabit the colon or large intestine break it down.
One gram of digestible carbohydrates yields 4 kilocalories of energy for the cells in the body to perform work. In addition to providing energy and serving as building blocks for bigger macromolecules, carbohydrates are essential for proper functioning of the nervous system, heart, and kidneys. As mentioned, glucose can be stored in the body for future use. In humans, the storage molecule of carbohydrates is called glycogen, and in plants, it is known as starch. Glycogen and starch are complex carbohydrates.
