Here's why:

* Photosynthesis: Plants use sunlight, water, and carbon dioxide to create glucose (a simple sugar) during photosynthesis.

* Storage: Glucose is then converted into starch, a complex carbohydrate, for long-term energy storage.

* Breakdown: When the plant needs energy, it breaks down the starch back into glucose, which is then used in cellular respiration.

While other molecules like lipids and proteins also store energy, starch is the primary storage form in most plants.