What is defined as a unit of cell substructure?

An organelle is considered a unit of cell substructure because it refers to specialized structures within a cell that perform distinct functions essential for the cell's life and operation. Each organelle plays a critical role; for instance, mitochondria are involved in energy production, while the endoplasmic reticulum is essential for protein and lipid synthesis. This definition captures the essence of organelles as integral components that collectively contribute to the cell's overall functionality.

In contrast, other options such as the cell membrane represent a boundary rather than a subunit that performs separate tasks within the cytoplasm. Micromolecule refers to small molecules that may participate in various biochemical processes but do not constitute significant structural components of a cell on their own. Complex, while it may suggest a collection of elements, doesn’t specifically indicate a defined substructure associated with cellular functionality. Thus, the choice of organelle accurately encapsulates the concept of a unit of cell substructure.