Respiration: 

Food +  oxygen ->  carbon dioxide + water +  energy 

Food is not only a source of energy, but also the nutrients used to build up the organisms' bodies. So we see that nutrients and energy are combined into one entity, food, during photosynthesis. This food may be taken by the other organisms from green plants. So  far we have seen that, green plants are called producers because they produce their own food. Plants live and die, they may be eaten up by  animals who also eventually die. There are organisms  -  the decomposers which act on dead plants and animals and for their existence  they decompose  the substances which form these dead bodies. In other words, the decomposers, such as bacteria, fungi and worms, convert the material of  the plants and animals back to their elementary forms. Besides the producers and decomposers,  there is another category of organisms  -  the consumers. These organisms cannot make their own food, and they depend directly or indirectly on producers,  for energy and nutrients. Consumers include herbivores, animals  that eat plants; carnivores, animals that eat other animals; and omnivores, animals that eat both plants and animals. Deer, duck, goat, and cow are examples of herbivores. Some human beings who are vegetarian by choice, could also be considered as herbivores. Wolf, tiger, cat, dragonfly, and eagles are all carnivores; rats and most human beings are examples of omnivores.