Which term describes the smallest unit of life?

• A. Cells
• B. Organ System
• C. Tissues
• D. Organism

Answer: (A)

Life begins at the cellular level. The cell is the smallest unit that can carry out all the processes that define living things—metabolism, growth, response to the environment, and reproduction. This idea is central to cell theory: all living things are made of cells, and new cells come from existing cells.

Tissues are made of similar cells working together, organs are built from different tissues forming a structure with a specific function, and organ systems group organs to perform broader tasks. These levels depend on cells; they can’t operate as life on their own in multicellular organisms. In single-celled organisms, one cell handles all life processes, highlighting why the cell is the fundamental unit of life.