Glucose can be both a product and a substrate, depending on the specific metabolic process being considered.

Here's why:

* Product: Glucose is a product of photosynthesis in plants. During photosynthesis, plants use sunlight, carbon dioxide, and water to create glucose and oxygen.

* Substrate: Glucose is a substrate for cellular respiration. In cellular respiration, glucose is broken down to release energy (ATP) that cells can use.

In summary:

* Photosynthesis: CO2 + H2O + Sunlight → Glucose + O2 (Glucose is a product)

* Cellular Respiration: Glucose + O2 → CO2 + H2O + ATP (Glucose is a substrate)