Wood is generally made of carbohydrates that can range from simple sugars to complex combinations of sugars. Three common carbohydrates are cellulose, starch, and glycogen.
Cellulose is made from long chains of glucose molecules that are for the most part linear, these help to maintain the structure of the wood.
Image from: https://microbewiki.kenyon.edu/index.php/File:GluoseFormsCellulose.jpg
Starch is made up of very branched glucose molecule chains; the primary purpose of these is to act as energy resource.
Image from: http://www.science-projects.com/Amylase.htm
Glycogen structures are chains of glucose molecules that are extremely branched, these structures like starch is used as energy stores.
Image from: http://biology.tutorvista.com/biomolecules/carbohydrates.html