All organisms must be able to maintain the proper balances necessary for life in a somewhat hostile world. Every organism must be able to coordinate its life processes, acquire food and oxygen, circulate that food and oxygen, eliminate waste, and reproduce. As organisms have evolved from single-celled to multicellular, they have developed increasingly sophisticated organ systems to accomplish these crucial tasks. Each organ system, such as the nervous system or circulatory system, is made up of a number of organs that work in concert to carry out one or a number of vital body processes. Each organ, in turn, is made up a number of tissues. A tissue is a conglomeration of specialized cells that all perform a common function necessary to an organ’s larger efforts. During the billions of years that life has existed on Earth, different organisms have developed various systems of meeting the needs of their bodies. Oak trees and alligators, for instance, have very different ways of getting food and nutrients and circulating those nutrients within their bodies.

The SAT II Biology Test focuses its many questions about organismal biology on the structure and function of humans and plants. This chapter on organismal biology is therefore split into two parts. The first deals with the structure and function of animals, the second with plants.