What foundational principle states that all living organisms are composed of cells?

The foundational principle that states that all living organisms are composed of cells is known as cell theory. This theory posits that cells are the basic unit of life, which means that all living organisms, whether they are unicellular or multicellular, consist of one or more cells. The significance of cell theory lies in its comprehensive description of the cellular basis for life and the understanding that cells are the building blocks of all living things.

Cell theory also includes additional key concepts, such as the notion that all cells come from pre-existing cells and that the functions of organisms are facilitated by the interactions of their cells. This principle is crucial because it underlines the importance of cellular structure and function in the study of biology and helps to unify various biological disciplines under the common understanding that life emerges from cellular processes.

In contrast, the other theories mentioned pertain to different aspects of biology. Gene theory focuses on heredity and the role of genes in the transmission of traits. Evolution theory explains the processes that lead to changes in species over time through natural selection and adaptation. Energy theory relates to how living organisms utilize and transform energy. Therefore, the cell theory is the most accurate choice for representing the principle that all living organisms are composed of cells.