Carbohydrate metabolism refers to the production, storage and use of carbohydrates within organisms. It is essentially the metabolism of sugars and the control of blood sugar levels - we need to maintain these levels in an internal homeostasis. Carbohydrate metabolism is highly conserved and can be observed even within bacteria- the prime example being the Lac Operon of E.coli.
Carbohydrates all consist of carbon, hydrogen and oxygen (CnH2nOn), yet not all forms of carbohydrate can be metabolised. The primary example of a carbohydrate that is used almost universally within living organisms is glucose, which is a monosaccharide. However, larger molecules (disaccharides, oligosaccharides and polysaccharides) are also metabolised within our bodies. Other examples include fructose and galactose.
Numerous processes within the body are dependent upon carbohydrate metabolism: Cellular respiration to produce ATP involves catabolic reactions that often rely upon the use of sugars, for example. In particular, the brain relies heavily on the metabolism of glucose.
After we have taken in carbohydrates (the main sources being foods such as breads, cereals, pastas, potatoes, etc.) metabolism begins within the gastrointestinal tract - namely the small intestine. This is where simple monosaccharides are absorbed through the thin epithelium into the bloodstream. Larger molecules are broken down by enzymes called glycoside hydrolases. These enzymes include amylase, which is produced by the pancreas and generally converts starch into disaccharides and trisaccharides, to later be hydrolysed into glucose. Not all carbohydrates are digested at this stage (particularly not polysaccharides) and many pass straight into the large intestine.
While alpha-glucose (which makes up starch) is metabolised in a relatively straight-forward manner, beta-glucose cannot be digested and this is the reason why some polysaccharides such as cellulose are never digested in humans. Ruminants, such as cattle, are able to digest cellulose.