Cellulose. Computer-generated molecular model of cellulose. Cellulose is composed of carbon (green), hydrogen (white-grey) and oxygen (red). Cellulose is a polysaccharide and is the most abundant organic substance in the world. It is the principal component of cell walls in plants. Saccharides consist of rings of four or five carbon atoms and one oxygen atom. Attached around the ring are hydroxyl (OH) groups. Polysaccharides comprise chains of hundreds of monosaccharide units. Cellulose consists of chains of between 5000 and 10,000 glucose units.