Cells are what make up organisms. Cells make tissues, tissues make organs, organs make organ systems, and organ systems make organisms. An organism is essentially just a giant collection of various cells that work together for two primary reasons. The proximate (immediate) reasoning for this is for survival. The ultimate (long-term) reasoning is for continuation of the species.
Cells Make Up Organisms. It's Matter Of One Makes Up To Create The Other. Cells Create Tissues. Tissues Makes Up Organs. Organs Makes Up A System Like Circulatory