What is the primary insoluble substance that constitutes plant cell walls?

Cellulose is the primary insoluble substance that constitutes plant cell walls. It is a polysaccharide composed of long chains of glucose units, which provides structural rigidity and strength to the plant. Cellulose fibers are arranged in a way that makes them particularly rigid and resistant to degradation, allowing plants to maintain their shape, support their structure, and withstand various environmental stresses.

Plant cell walls serve several essential functions, including protecting the plant, providing structural support, and regulating the passage of substances in and out of the cell. Cellulose is a major component of these walls, making it a fundamental part of the plant's anatomy and physiology.

In contrast, while starch is a storage polysaccharide found in plants, it is not a structural component of cell walls. Pectin, another component of plant cell walls, provides some flexibility and is more soluble. Lignin occurs in the cell walls of certain plants, particularly in wood, where it serves to reinforce the cell walls and make them more rigid, but it is not the primary substance that constitutes all plant cell walls. Thus, cellulose is the most significant and widely recognized component contributing to the structural integrity of plant cell walls.