Final answer:
The bulk of a plant's mass is made up mostly of cellulose, a structural polysaccharide that forms the plant cell wall and is comprised of glucose units.
Step-by-step explanation:
The molecule that makes up the bulk of the mass of the plant comes in mostly from cellulose, a polysaccharide that is the main structural component of the plant cell wall and the most abundant organic compound on Earth. Plant matter is rich in cellulose, which is a fibrous carbohydrate made up of glucose monomers linked together. Carbon that plants take from atmospheric CO₂ composes the majority of the dry mass within most plants, which is then incorporated into sugars and structural molecules like cellulose. Cellulose not only contributes to the rigidity and structure of the plant, but is also a significant component of wood, paper, and textile productions, exemplified by its extensive use in the manufacture of products such as paper and cotton textiles.