The organelle that forms glucose sugar in plant cells for energy purposes is the chloroplast.

Here's why:

* Photosynthesis: Chloroplasts are the sites of photosynthesis. This process uses sunlight, water, and carbon dioxide to produce glucose (a type of sugar) and oxygen.

* Energy Storage: Glucose is a primary source of energy for plant cells. It can be used directly for cellular processes or stored as starch for later use.

Note: While mitochondria are involved in energy production within plant cells (through cellular respiration), they don't actually create the glucose. They break down glucose to release energy in the form of ATP.