Here's why:

* Cellulose is a polysaccharide: It's a long chain of sugar molecules (glucose) linked together.

* It's the most abundant organic compound on Earth: Plants use it to build their cell walls, providing structural support.

* Humans can't digest it: We lack the necessary enzymes to break down the bonds in cellulose. However, it's still important for digestive health as it adds bulk to our stool.

While other complex carbohydrates like starch are also found in plants, cellulose is unique because it's the primary structural component of plant cell walls.