* Primary Metabolites: These are essential for the basic life processes of an organism, such as growth, development, and reproduction. They are directly involved in the organism's core metabolism. Examples include sugars, amino acids, and fatty acids.

* Secondary Metabolites: These are not directly involved in the basic life processes of an organism. They are often produced under specific environmental conditions or in response to stress. They often have ecological functions, such as defense mechanisms against predators, competition with other organisms, or attracting pollinators.

Cellulase is an enzyme that breaks down cellulose, a complex carbohydrate found in plant cell walls. While cellulose is a primary component of plant matter, cellulase is not essential for the basic life processes of the organism producing it.

Instead, cellulase is produced by certain fungi, bacteria, and some animals (like ruminants) for the following reasons:

* Breakdown of Cellulose: To access the energy stored in cellulose for their own growth and metabolism.

* Defense Mechanism: To break down cellulose in the cell walls of other organisms, providing a source of nutrients or preventing the growth of competitors.

Therefore, because cellulase is produced for specific ecological purposes and is not directly involved in the basic life processes of the organism, it is categorized as a secondary metabolite.