Final answer:
The cell is the lowest structural level at which all of life's properties, such as metabolism and reproduction, manifest due to the specialized functions of organelles within the cell.
Step-by-step explanation:
The lowest structural level in which all of life's properties occur is the cell. While atoms and molecules make up the physical substance of living organisms, it is at the cellular level that these components come together to form the basic unit of life capable of carrying out all of life's essential processes. Cells contain organelles, such as the nucleus and mitochondria, which perform specialized functions necessary for life. The hierarchy of biological organization builds from atoms to molecules to macromolecules, which then form organelles within cells, and ultimately cells themselves. This structural level befits the complexity required to sustain life.