What is the main function of the stroma in chloroplasts?

The stroma in chloroplasts is the fluid-filled space that surrounds the thylakoid membranes. Its primary function is to serve as the site for the light-independent reactions of photosynthesis, which are also known as the Calvin cycle. During these reactions, carbon dioxide is fixed into carbohydrates using the energy produced from the light-dependent reactions, where ATP and NADPH are generated.

In the stroma, enzymes catalyze the conversion of inorganic carbon into organic compounds, making it a critical location for synthesizing glucose and other carbohydrates that plants use as energy sources. The stroma also contains enzymes that facilitate these biochemical reactions, as well as DNA and ribosomes, which indicate that it has its own genetic material and can produce some proteins independently.

Other choices do not accurately describe the primary role of the stroma. For example, the initiation of light-dependent reactions takes place in the thylakoid membranes, while chlorophyll is embedded within these membranes rather than stored in the stroma. Additionally, gas exchange occurs at the leaf level through structures called stomata, rather than being a function of the chloroplasts themselves.