Single-celled organisms must provide for all of their own needs, but the cells of multicellular organisms can evolve specializations to carry out certain functions exclusively and efficiently because they can depend on other cells to carry out other functions. Thus the cells of a multicellular organism can have different developmental fates. Similar cell types can develop together into tissues that accomplish tasks that a single cell cannot. For example, muscle cells develop cellular mechanisms for generating force. A single muscle cell cannot generate much force, but many cells of a muscle tissue can work together to generate considerable force, and working with structural tissues such as bones, they can produce large movements. Different tissue types develop together into organs that accomplish specific functions. The heart, brain, and stomach are each composed of several types of tissues, as are the roots, stems, and leaves of plants. Organs whose functions are interrelated can be grouped into organ systems; the esophagus, stomach, and intestines, for example, are all part of the digestive system. The hierarchy of biological organization from atom to organism is shown in Figure 1.10A. The biology of the two major groups of multicellular organisms, plants and animals, is discussed in detail in Parts Eight and Nine, respectively.