Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

Which level of biological organization is the most complex?

1. Cells

2. Organs

3. Organ systems

4. Molecules

The correct answer is: Organ systems

The most complex level of biological organization is organ systems. This level represents a higher order of organization where multiple organs work together to perform specific functions necessary for the survival of an organism. For example, the circulatory system includes the heart, blood vessels, and blood, all of which collaborate to transport nutrients and oxygen throughout the body. In contrast, while cells are fundamental units of life, they are considered basic organizational structures. Molecules represent even simpler compositions, being the fundamental components that make up cells, but do not demonstrate the functional complexities seen at higher organizational levels. Organs, while more complex than cells, are still individual structures that serve specific functions rather than working in concert as part of a broader system. Thus, organ systems integrate multiple organs to facilitate complex biological processes, making them the highest level of biological organization among the choices provided.