Why is the sun considered the primary source of energy for most ecosystems?

• A. Because it never changes position.
• B. Because it is always visible.
• C. Because plants use it to make glucose, starting the energy flow.
• D. Because it is the hottest object.

Answer: (C)

Sunlight provides the energy that starts almost all energy flow in ecosystems. Plants use that light energy in photosynthesis to make glucose, which is a form of chemical energy stored in plant tissues. This stored energy is passed along when organisms eat plants or other organisms, creating the energy transfer through the food chain. Without sunlight, producers wouldn’t capture energy, and the whole web of life would run out of energy.

The other ideas don’t explain where the energy in living things comes from. Being in a fixed position or always visible isn’t what starts energy flow, and being the hottest object isn’t the energy source for ecosystems. The crucial idea is that plants convert light into chemical energy in glucose, starting the energy flow through the ecosystem.