Which of the following is a principle of cell theory?

The principle that cells must arise from preexisting cells is a fundamental concept in cell theory, highlighting the continuity of life. This principle asserts that new cells are formed only from the division of existing cells, emphasizing the idea that life propagates through cellular reproduction rather than spontaneous generation. This concept is crucial as it provides a framework for understanding biological development, growth, and reproduction at the cellular level.

In the context of the other options, the assertion that all living organisms are made of tissues is too narrow, as cell theory focuses on the role of cells rather than tissues. While tissues consist of groups of cells, the fundamental unit of life is the cell itself. The idea that cells have no role in metabolism is incorrect; in fact, cells are the site of metabolic processes essential for life. Lastly, the statement that every tissue has its own unique cell type overlooks the fact that many tissues can contain multiple cell types, emphasizing diversity in function rather than uniqueness. Understanding these distinctions reinforces the significance of the correct answer as a core component of biological study.