The organelle primarily responsible for building the molecule that stores energy is the chloroplast.

Here's why:

* Chloroplasts are found in plant cells and are the sites of photosynthesis.

* Photosynthesis is the process where plants convert light energy into chemical energy stored in the form of glucose, a sugar molecule.

While mitochondria are involved in energy production through cellular respiration, they don't build the molecule that stores energy. They break down glucose to release energy in the form of ATP.