Earlier in this course you learned about the Sun, its origin and lifespan. As you know, the Sun generates tremendous amounts of heat and energy. This energy makes life on Mercury and Venus difficult or impossible. However, because of Earth's distance from the Sun, the "Goldilocks" position, there is enough energy to sustain life. In fact, most of the energy for Earth's ecosystem comes from the Sun.

But the energy from the Sun isn't processed by all the biotic factors in the ecosystem directly. Instead most energy from the Sun is processed through photosynthesis. The next few pages will explain photosynthesis.