What function does the enzyme Rubisco primarily serve?

Rubisco, or ribulose-1,5-bisphosphate carboxylase/oxygenase, plays a crucial role in the process of photosynthesis, specifically in the fixation of atmospheric carbon dioxide. Its primary function is to catalyze the reaction where carbon dioxide is attached to ribulose bisphosphate (RuBP), resulting in the formation of two molecules of 3-phosphoglycerate (3-PGA). This reaction is a key step in the Calvin cycle, which is essential for converting carbon dioxide into organic compounds that plants can use for energy and growth.

The other options pertain to different processes that are not directly related to Rubisco's function. The breakdown of glucose is part of cellular respiration, while the absorption of sunlight is primarily the role of chlorophyll and other pigments in the chloroplasts. Nutrient absorption through roots involves different mechanisms, such as active and passive transport, and does not directly involve Rubisco. Thus, the choice highlighting Rubisco's role in fixing atmospheric CO2 during photosynthesis accurately captures the enzyme's primary function in plant biology.