The role of sugar in photosynthesis is to serve as a primary energy source and structural component for plants. During photosynthesis, plants convert sunlight, carbon dioxide, and water into glucose, a simple sugar, and oxygen. This glucose can be utilized immediately for energy through cellular respiration or stored as starch for later use. Additionally, sugars are fundamental in synthesizing other organic compounds, contributing to cell structure and function, thus playing a vital role in the overall growth and development of the plant.