Many reactions of digestion are called hyrolysis reactions because they involve the reaction of a molecule of carbohydrate with a molecule of water, resulting in the carbohydrate, protein or fat molecule being broken into two new molecules. Hydrolysis is a Greek term that translates to "water separation."
Biological molecules tend to be relatively large, so before they can be absorbed by the intestines, they need to be broken into smaller pieces. The digestive system does this with the help of enzymes, special proteins that cause targeted chemical reactions to occur much more quickly. The reason many people cannot digest lactose is because they lack the enzyme to do it. The pieces of the molecules that can be absorbed are different for each type of molecule.
Carbohydrates, such as starches, are made up of smaller sugar units such as glucose and fructose. The bond that is broken through hydrolysis is called the glycosidic bond. The units of proteins are known as amino acids, and they are joined by peptide bonds. Fats are actually made of two different types of units, and are broken down into fatty acids and glycerol, a type of alcohol. Once they are broken down, these molecules are carried to the bloodstream to the cells where they are transformed further and used for energy.