According to scientific procedure, each discipline first describes the phenomena of its research area, then analyzes them, and finally categorized them in a system. To date, biology has lacked such a system for its smallest building blocks, the cells. Although the theory of evolution explains certain central evolutionary mechanisms of the cell, there existed no generally accepted theory of the organization of the cell. The endocytobiotic cell theory is suggested as a possible basis for a satisfying explanation of the structure, function, information, and evolution of the cell. Furthermore, a hypothetical periodic system of the cell is developed. This system consists of eight groups, including the ecological niches fermentation, respiration, photergy, and photosynthesis (each aerobic and anaerobic) and seven periods with increasing numbers of protein biosynthesis machineries (cytoplasma, mitochondria, plastids, endocytobionts). We find furthermore, a division according to typical animal of plant cells and between these two in fungus-like cells.