Which substance are cells of all living things composed of?

• A. Cytoplasm
• B. Mitochondria
• C. Protoplasm
• D. Nucleus

Answer: (C)

The living content inside a cell—the material that makes it alive and drives its life processes—is protoplasm. Protoplasm refers to the living substance of the cell, encompassing the entire interior that carries out metabolism, growth, and other vital activities. Within protoplasm, the cytoplasm forms the jelly-like background where most organelles reside and where many reactions occur, while the nucleus sits inside as the control center that houses genetic material and directs cellular activities. Mitochondria are important energy-producing parts within the cytoplasm, but they are just one component of the whole living material. So protoplasm best describes the substance cells are composed of because it denotes the living contents of the cell as a complete, functioning material.