PLEASE HELP ME - In 1839, Schleiden and Schwann began formulating a theory of cells and their role in living organisms. Over time, cell theory has been updated. Modern theory is summarized below. All known living things are made up of cells. The cell is the structural and functional unit of all living things. All cells come from pre-existing cells by division. The flow of energy occurs within cells. Cell theory explains all of the following except a. the growth of animals. b. how viruses require the cells of living organisms to survive. c. how bacteria reproduce. d. the storage of chemical energy in plant cells.