Which of the following is NOT a part of the cell theory?

The principle that all cells contain chloroplasts is not part of cell theory because chloroplasts are specific organelles found only in plant cells and some protists, which means not all cells have them. Cell theory encompasses three main points: that all living organisms are made up of cells, that cells are the basic structural and functional units of life, and that all cells arise from pre-existing cells. These points highlight the universality of cells in living organisms and their roles as the fundamental units of life. Therefore, the assertion that all cells contain chloroplasts is inaccurate and does not reflect the broad applicability or inclusiveness of the established tenets of cell theory.