What is cellulose primarily known for?

Cellulose is primarily known as a structural polysaccharide found in the cell walls of plants. It plays a crucial role in providing rigidity and strength to plant cells, helping them maintain their shape and structure. This rigidity is vital for plants as it allows them to stand upright and grow towards sunlight, which is essential for photosynthesis.

Cellulose is made up of long chains of glucose molecules linked together, forming a strong and insoluble fiber that is not digestible by humans but can be broken down by certain microorganisms. This characteristic allows cellulose to serve as a significant component in the diet of herbivores, which possess the necessary enzymes to digest it.

The other provided options do not accurately describe cellulose. It is not found in animal tissues and does not serve as a direct source of energy in cellular respiration; instead, glucose derived from other carbohydrates is typically used for that purpose. Additionally, it is not an enzyme and does not assist in protein synthesis, as enzymes are proteins that catalyze biochemical reactions, which is a distinct function from that of cellulose.