The chemical level is the simplest level of organization.An atom is the smallest unit of a chemical element (fundamental substance) that retains the characteristic properties of that element.At the cellular level we find that many diverse molecules may associate with one another to form omp1ex and highly specialized structures within cells called organelles.Each major group of biological functions is performed by a coordinated group of tissues and examples of organs, called an organ system.Tissues in turn, are arranged into functional structures called organs, such as the heart and stomach in animals or roots and leaves in punts.