All organisms are made of cells, the organism's basic unit of structure and function. Theme of emergent properties. Life at the cellular level arises from interactions among cellular components. Correlation of structure and function. Interaction of organisms within their environment.
Cells are excitable responding to environmental stimuli.
In addition, cells are open systems that exchange materials and energy with their environment. Unifying theme of evolution. Evolutionary adaptations are the basis for the correlation between structure and function. The microscope's invention and improvement in the seventeenth century led to the discovery and study of cells. In 1665, Robert Hooke described cells using a light microscope .