Sugar cubes contain the disaccharide known as sucrose which is made up of one molecule of the monosaccharide most common in fruit called fructose in addition to one molecule of the monosaccharide called glucose which is essential for energy production within the body and brain.
The cellulose portion of trees is made of long fairly straight chains of glucose with no fructose, so trees and sugar cubes aren’t really alike. The bonds between table sugar and tree fiber are at slightly different angles which means a hungry person or animal would require different enzymes in order to be able to break them down during digestion into smaller molecules and atoms for further use as an energy source.
The straighter angle between the simple sugars of plant fiber allow the linked chains of glucose to line up with each other. When lined up the fibers then can form layers, which might seem a little like sheets of paper stacked on top of each other in a book, except it would be a round cylinder doughnut shaped book. Cellulose is one type of plant fiber, it and other types of plant fiber are found in the cell walls throughout the plant in the leaves, stems and roots.
Chitin is similar strong chain of the simple sugar N-acetylglucosamine. The simple sugars in chitin and cellulose both have the slightly straighter beta angle than the bonds found in energy storage starches or polysaccharides. Termites  and the bacteria found in the stomach of grazing animals are able to digest the stronger beta bonds of cellulose.
Humans and most other animals can’t digest the strong beta bonds of cellulose because a specific enzyme is needed. The termites and bacteria in the stomach of grazing animals can make the enzyme from other chemicals but humans and the grazing animals themselves can’t make it.
Energy rich plant starches have alpha type bonds between the simple sugars. Alpha bonds connect at an angle that might twist into a spiral chain similar to the double helix spiral of DNA.
The angled alpha bonds are also found in branching shapes of storage starches like glycogen or amylose. The sugar molecule at the end of each ‘branch’ is available for rapid digestion. Glycogen is the energy storage polysaccharide of glucose in animals and humans and amylose is the form of glucose storage used in plants. Glycogen is slightly more branched than amylose.
Tree bark and tree sap both contain glucose but the bark contains cellulose and the sap would have amylose or a similar alpha bonded energy storage starch. A shiny insect shell or seashells also are a type of sugar but not glucose. Shells contain N-acetyl-glucosamine in the form of chitin.
Supplements of glucosamine may be helpful for reducing joint pain. Clinical research studies with patients have found 1500 mg/day may be beneficial. 
Disclaimer: Opinions are my own and the information is provided for educational purposes within the guidelines of fair use. While I am a Registered Dietitian this information is not intended to provide individual health guidance. Please see a health professional for individual health care purposes.
- S.A. Brooks, M. V. Dwek, U. Schumacher, Functional and Molecular Glycobiology, (BIOS Scientific Publishers, Ltd., 2002), Amazon.
- “Questions and Answers: NIH Glucosamine/Chondroitin Arthritis Intervention Trial Primary Study,” National Institutes of Health, National Center for Complementary and Alternative Medicine [nccam.nih.gov]
- Nakashima K, Watanabe H, Saitoh H, Tokuda G, Azuma JI.,”Dual cellulose-digesting system of the wood-feeding termite, Coptotermes formosanus Shiraki.” Insect Biochem Mol Biol. 2002 Jul;32(7):777-84. [ncbi.nlm.nih.gov]