1) Energy from the sun powers photosynthesis in plants and algae, producing carbohydrates that form the base of most ecosystems' food chains.
2) Some deep sea ecosystems use chemosynthetic bacteria that derive energy from sulfur compounds rather than the sun, as the base of their food chains.
3) Energy is transferred between organisms as one eats another, but about 90% is lost as heat or waste at each transfer, making long food chains unsustainable in most ecosystems.