What two substances provide rigidity to the plant cell wall?

The correct answer identifies cellulose and lignin as the two substances that provide rigidity to the plant cell wall.

Cellulose is a polysaccharide that forms a major structural component of plant cell walls. It consists of long chains of glucose molecules linked together, creating a rigid framework that helps maintain the plant's structure and integrity. The high tensile strength of cellulose allows plant cells to withstand internal pressures from water, enabling plants to grow upright.

Lignin, on the other hand, is a complex organic polymer that adds additional strength and rigidity to the cell wall, particularly in woody plants. It fills the spaces between cellulose fibers, reinforcing the wall and contributing to the water-resistant properties of aquatic and terrestrial plants. This combination allows plants to develop a robust structure capable of supporting their height and withstanding various environmental stresses.

In contrast, while thylakoids are part of the chloroplasts and are involved in photosynthesis, they do not contribute to the rigidity of the cell wall. Grana and chlorophyll are associated with the photosynthetic process but serve different functions related to energy capture rather than structural support. Therefore, the correct pairing for providing rigidity to the plant cell wall is indeed cellulose and lignin.