While the answer above reviews the process of digestion, the question can be viewed as what type of chemical reaction results in the break down of starch into smaller subunits known as glucose. This process is called hydrolysis. Hydrolysis is a reaction whereby a water molecule is inserted between a disaccharide, causing the glycoside bond that connects the two sugars to break and two monosaccharide molecules to be produced. Starch is a long chain of glucose molecules joined one to the next. Hydrolysis helps to break the chemical bonds that hold the glucose molecules together. Of course, this works because specific enzymes are needed to facilitate this reaction. Starch enzymes are found in both the oral cavity and the small intestine. Once starch is digested, the glucose can be transported via the bloodstream and enter the cells via facilitated diffusion where it can be used for energy.
Starch is considered a polysaccharide or complex carbohydrate. With its many sugars linked together, it can not be absorbed in its polymer state. Therefore, the digestive system must break the bonds that hold the polysaccharide together. It is then that the body can absorb the monosaccharidesugar molecules.
The first part of digestion is mechanical digestion. The grinding action of the teeth will increase the surface area of the food and therefore allow enzymes to more effectively break down the macromolecules.
An enzyme called amylase is present in saliva and begins the break down of starch. However, understand that food spends little time in the mouth and enzymes are very sensitive to pH. Once swallowed and moved to the stomach, the acidic environment will render the amylase useless. This is not enough time to completely digest the starch.
Once the food mixture (called chyme) leaves the stomach it enters the duodenum (first portion of the small intestine). The pancreas releases a proenzyme called trypsinogen which essentially is an inactive form of trypsin (an enzyme that digests proteins and not starch). However, once trypsin is activated, it can activate other enzymes in the intestines. One of those is amylase. Onc e amylase is activated, starch will be further broken down into eventual monosaccharides and then absorbed into the blood stream.