Why must large organisms be multicellular?
Simply because as organisms get bigger their surface area to volume ratio decreases. This means it becomes more and more difficult to get nutrients and excrete wastes on a cellular level.
Multicellular organisms are organisms with more than one cell, the cells of which are usually specialized. A large colonial organism is an organism of many cells that are loosely attached to each other and that show little or no specialization among themselves. Multicellular organisms are capable of surviving on their own while colonial organisms are not.
Multicellular organisms require specialized organs and systems because in multicellular organisms, different jobs are done by different cells that are specialized, e.g. A blood cell carries oxygen. A nerve cell sends and receives signals. In multicellular organisms different functions are divided among different cells. The cells in multicellular organisms are organized in ways the enables them to survive and reproduce. For any multicellular organism to survive, different cells must work together. The right type of…