An onion is a monocot plant in the Phylum Angiospermophyta. Its leaves have long reticulate veins running from the petiole to the apex. Onions are plants used by people for cooking purposes. An onion has two structures.
Root system
Shoot system
Onions have an adventitious root system. The purpose of the roots is to provide firm anchorage of the plant and absorption of water and mineral salts from the soil. The shoot system comprises of the leaves that perform a photosynthetic function. They have numerous chloroplasts. They contain chlorophyll that trap light energy from the sun. Flowers provide reproduction sites. This is where pollen grains are produced by the anthers and ovules by the ovary. Onions develop specialized underground stem to create bulbs. This is a modification of the standard stem that is set to provide support to the plant. The bulbs are usually swollen fleshy leaves found on the underground. This differs from the standard plant structure where the stem is found above the ground.
Animals are important in a given ecosystem. The behavior of different animals ensure ecological balance. A vulture is a scavenger that benefits humans in various ways. They feed on decaying carcasses. When animals die, either naturally or through illness, Vultures feed on the remains and clean the environment. This is imperative to control the spread of diseases that might destroy the human population.
Animals perform certain functions that plants do not. This reason for this is through adaptations that animals possess over plants. (Ford, 1983).
Locomotion. Animals can change places, but animals cannot.
Specialized digestive systems to even digest hard materials
Faster reception to change.

References
Ford, M. J. (1983). "The changing climate: Responses of the natural fauna and flora," The Journal of Ecology 71(3):