What are the two main roles of polysaccharides in organisms?

Polysaccharides play critical roles in living organisms primarily through energy storage and providing structural support.

As energy storage molecules, polysaccharides such as starch in plants and glycogen in animals serve as reservoirs for energy that can be easily mobilized when needed. These complex carbohydrates can be broken down into simpler sugars like glucose, which can then be utilized by cells for metabolic processes.

In terms of structural support, polysaccharides like cellulose in plants contribute to the rigidity and strength of cell walls, allowing for the maintenance of structure in plant cells. Chitin, another polysaccharide found in the exoskeletons of arthropods and the cell walls of fungi, also serves a similar function in providing support.

The other options focus on roles that polysaccharides do not usually fulfill. For instance, while genetic material involves nucleic acids rather than polysaccharides, signal transduction is often mediated by proteins and lipids instead of polysaccharides. Building immune responses is more associated with proteins and the immune system's complex cellular interactions. Therefore, the choice highlighting energy storage and structural support accurately represents the essential functions of polysaccharides in biological systems.