Cellulose is important as it is the major component of every plant cell wall and is therefore the most abundant organic compound on earth. Cellulose is a large molecule, composed of over 3,000 glucose units each, and is very difficult to break down chemically. Together with pressurized water vacuoles in plant cells, cellulose cell walls are crucial to maintaining and protecting plant structures.
Plants are far and away the most important producers on land. The cellulose that covers each plant cell makes most parts of plants inedible to any animals without specialized digestive functions. Indeed, animals cannot digest cellulose at all without the aid of special microorganisms that live in their guts. For animals that eat leaves or wood, such as termites or cows, cellulose is the primary source of energy. These animals are often the main food sources for all the predators in an ecosystem. Cellulose is the molecule that is the fiber in any plant-based food and is the reason that only plants have fiber.
For human use, cellulose is the major component of paper and wood and is used in many other products as well. In the right configurations, its toughness is important for many structural purposes, while its ability to be burned has made it a crucial fuel source for thousands of years.