Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods  of producing ATP: aerobic and anaerobic. 
In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules.
In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP  continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.
There are several different types of carbohydrates: polysaccharides (e.g., starch, amylopectin, glycogen, cellulose), monosaccharides (e.g., glucose, galactose, fructose, ribose) and the disaccharides (e.g., sucrose, maltose, lactose).
[https://www.ncbi.nlm.nih.gov/books/NBK459280/ Monosaccharides], also known as simple sugars, are the most basic, fundamental unit of a carbohydrate. These are simple sugars with the general chemical structure of C6H12O6.
[https://www.ncbi.nlm.nih.gov/books/NBK459280/ Disaccharides] are a type of carbohydrate. Disaccharides consist of compound sugars containing two monosaccharides with the elimination of a water molecule with the general chemical structure C12H22O11.
[https://www.ncbi.nlm.nih.gov/books/NBK459280/ Oligosaccharides] are carbohydrates that consist of a polymer that contains three to ten monosaccharides linked together by glycosidic bonds.
Glucose reacts with oxygen in the following reaction, CHO + 6O → 6CO + 6HO. Carbon dioxide and water are waste products, and the overall reaction is exothermic.
The reaction of glucose with oxygen releasing energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms.