Why is photosynthesis an example of a chemical reaction, and why is it classified as endothermic?

The reactants in the process of photosynthesis are carbon dioxide, water, and sunlight. They react to produce sugar and oxygen. The chemical equation reads as follows:

CO2 (gas) + H2O + light → O2 (gas) + C6H12O6 (solid)

Photosynthesis requires energy, making it an endothermic reaction. Light, generally sunlight, is the source of this energy. The process converts the sun’s electromagnetic energy into chemical energy, which is then stored in chemical bonds in the plant.

Sunlight is the primary source of energy on the surface of Earth. Even the energy in fossil fuels and biomass originally comes from sunlight, converted into chemical energy through photosynthesis. This energy is released when fuels are burned.

For plants, the purpose of photosynthesis is to create sugar, the food they need to live and grow. But photosynthesis also has a massive effect on Earth’s atmosphere and oceans because it absorbs carbon dioxide and releases oxygen.

An endothermic process is one that needs to have energy added in order to take place. A simple example of an endothermic process is the melting of ice, because ice doesn't melt unless you put energy into it.

In the case of photosynthesis, nothing much happens until light hits the chlorophyll in the leaves, causing electrons to move around and a whole bunch of other complicated stuff to take place. The specific stuff isn't the important part - what is important is that without the addition of energy in the form of light, nothing will happen.