Complex organisms have specialized tissues, such as nerves, organs or flowers. In contrast, simple organisms, such as amoeba or yeast, exist as a single cell. Larger organisms tend to have more specialized cells that smaller ones.
Some single cell organisms bind together to form a colony. In the colony, the organisms sometimes begin to specialize in the tasks they perform. However, in complex organisms, the cells adhere and develop specialized duties. If a cell dies, the organism replaces it with an identical one. In plants, transport material moves water from the roots to the leaves and food from the leaves to the roots. In humans, the various organ systems each have a specific duty dependant on specialized cells.