In the human digestive system, complex sugar molecules are broken down in the decomposition reactions that they undergo with enzymes present in the digestive tract. The process begins in the mouth, where these complex sugar molecules, otherwise known as carbohydrates, are exposed to the enzymes secreted in saliva.
These salivary enzymes initiate the decomposition of the carbohydrate into shorter chains of polysaccharides. As these polysaccharides make their way to the stomach, they are exposed to stomach acids. The stomach acids destroy the bacteria harbored by the food without further reaction with the polysaccharides. The semi-digested complex sugars arrive at the small intestine, prompting the pancreas to release more sugar-decomposing enzymes. These break down the polysaccharides into simpler disaccharides, molecules consisting of two simple sugar molecules bonded covalently.
The small intestine then produces other enzymes that target the specific disaccharide molecules, breaking them down into simple sugars called monosaccharides. These monosaccharides are then absorbed by the small intestine.
Although the digestive system is efficient, it is not perfect, and some complex sugars are not broken down and utilized in the small intestine. These intact complex sugars are transported to the colon, where they are partly broken down by the bacteria in the large intestine.