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 . Modern light microscopy is based upon the same principles as microscopy first used by Renaissance scientists. Visible light is focused on a specimen with a condenser lens . Light passing through the specimen is refracted with an objective lens and an ocular lens .