Food undergoes digestion where food is broken down into small enough molecules that can be absorbed by the body. This is accomplished by enzymes through hydrolysis.
Digestive system diagram By Mariana Ruiz Villarreal(LadyofHats) - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2898771
Digestion of carbohydates
The breakdown of carbohydrates starts in the mouth where the enzyme amylase breaks down carbohydrates into smaller disaccharides. In the small intestine these a futher broken down into monosaccharides. This process involves a hydrolysis reaction. In these reactions, water is used to break the glycosidic linkage.
Humans are unable to digest cellulose as they do not have the right enzymes that will break the glycosidic links. Some animals can digest cellulose - these animals harbour microbes which help to digest it.
In humans cellulose acts as fibre to help clean out the intestines.
Digestion of fats
Fat is mainly acted upon when it reaches the small intestine. There, it is mixed with bile from the gall bladder. Bile contains surfactants which convert fats into an emulsion of smaller fat globules. Enzymes from the pancreas (Lipase) catalyse the hydrolysis of fat by reversing the condensation reaction and making glycerol and fatty acids. Once these have been absorbed by the body, they are reassembled into triglycerides via condensation reactions, where they can be stored or released as energy.
Digestive enzymes table By No machine-readable author provided. Stephaniegreenwood assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims)., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1883768
Maltose is broken down to glucose via hydrolysis. Diagram by OpenStax Download for free at http://cnx.org/contents/ea44b8fa-e7a2-4360-ad34-ac081bcf104f@12.
Lipid hydrolysis diagram By JenCom - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11940676