The human need for nourishment is satisfied by eating and drinking. Humans consume both plant and animal matter, making us, like certain other animals, omnivores. Like all animals, however, our bodies are unable to produce the chemicals out of which our tissues are made. Only green plants and certain bacteria can perform the task of converting water, carbon dioxide and nutrients into sugar, protein and other sources of nutrition. Plants and bacteria that do this are called primary producers.
All animals are consumers.
Animals that consume only plant matter are called herbivores. Animals that consume other animals are called carnivores. Those carnivores that eat primary consumers are called secondary consumers. Animals that eat secondary consumers are called tertiary consumers, which in turn are eaten by quaternary consumers, and so on. This sequence comprises the food chain.