* Endergonic reactions require energy input to occur. In photosynthesis, the energy input comes from sunlight.

* Photosynthesis uses light energy to convert carbon dioxide and water into glucose (a sugar) and oxygen. This means it's building complex molecules from simpler ones, which requires energy.

Let's contrast this with exergonic reactions:

* Exergonic reactions release energy. A classic example is the burning of wood, which releases heat and light.

In essence, photosynthesis is like building a house. You need to put energy in to create something more complex.