Answer;
Cellulose
Nearly all of plant tissue called cellulose is made up of cell walls
Step-by-step explanation;
Plant cell walls are composed of cellulose, which is a structural carbohydrate and a complex sugar that is used in both protection and structure.
Cellulose is the most abundant polysaccharide on earth. It gives plants cell wall a rigid structure that would resist changes in the outside environment such as osmotic pressures, and pH, and also makes the plant to be upright due to turgidity.