The smallest autonomous functional unit of the body is the cell. In its fetal form it is undifferentiated, but as growth continues the cell differentiates into specific tissue types, forming organs and systems. The cell wall is a semipermeable membrane that separates the intracellular from the extracellular components, allowing for an exchange of materials through the membrane in an effort for the cell to obtain energy, synthesize complex molecules, participate in electrical events, and replicate. The cell senses signals that help it respond to changes in its environment and adapt accordingly.