The six limbs shown in the image have a similar bone arrangement, but the size and shape of the bones vary among the organisms. The structures in the image serve different functions for each organism. Flippers helps whales swim, and arms help humans use tools. Horses and lions use their legs for support and movement, and a frog uses its legs to jump. Wings help birds fly.
Living systems can be defined from various perspectives, from the broad (looking at the entire earth) to the minute (individual atoms). Each perspective provides information about how or why a living system functions:
At the chemical level, atoms, molecules (combinations of atoms), and the chemical bonds between atoms provide the framework upon which all living activity is based. The cell is the smallest unit of life. Organelles within the cell are specialized bodies performing specific cellular functions. Cells themselves may be specialized. Thus, there are nerve cells, bone cells, and muscle cells.
