Here's why:

* Light energy is used to power the chemical reactions. Plants use light energy to convert carbon dioxide and water into glucose (sugar) and oxygen. This process is called photochemical reaction.

* Heat energy is a byproduct. While some heat is generated as a byproduct of photosynthesis, it's not the primary energy source.

Think of it like this:

* The sun is like a giant light bulb, providing the energy needed for photosynthesis.

* Heat is like the warmth you feel from the light bulb, but it's not what makes the bulb work.