1. All living organisms are composed of one or more cells. This means that cells are the fundamental unit of life. Everything from the smallest bacteria to the largest whales is made up of cells.

2. The cell is the basic unit of structure and function in living organisms. This means that cells are not just building blocks, but also the smallest unit that can carry out all the processes necessary for life, such as metabolism, reproduction, and response to stimuli.

3. All cells arise from pre-existing cells. This means that cells do not spontaneously generate, but instead are produced through the division of other cells.

These three tenets form the foundation of our understanding of biology and are fundamental to all areas of biological study.