What is the substance that cell walls in green plants are made of?

The substance that green plant cell walls are chiefly made of is cellulose. Cellulose is a tough carbohydrate built from long, straight chains of glucose molecules. These chains group into strong microfibrils, giving plant cells structural support and helping stems, leaves, and roots maintain their shape.

Cellulose is produced by enzymes in the plant cell membrane and deposited outside the cell membrane as the wall develops. Plant cell walls also contain hemicellulose, pectin, and, in many tissues, lignin, but cellulose is the principal reinforcing material and the standard answer to this trivia question.

Humans cannot digest cellulose because we lack the enzymes needed to break its beta-linked glucose chains apart. It therefore acts as dietary fiber. Animals such as cows and termites rely on microorganisms in their digestive systems to help break it down. Cellulose is also the main component of cotton and a major ingredient in wood, paper, cardboard, rayon, and cellophane.