Which of the following best describes the composition of cells in the human body?

• A. 20% water
• B. 80% cell organelles
• C. 20% cell organelles
• D. 80% water

Answer: (D)

Cells are mostly water. The cytoplasm of a typical human cell is an aqueous environment in which most biochemical reactions take place, so water accounts for the majority of the cell’s mass—about 70 to 85%, commonly cited around 80%. The rest of the cell’s mass is made up of organelles, membranes, proteins, lipids, nucleic acids, and other biomolecules. Because the water portion vastly outweighs the solid components, 80% water is the best description of cellular composition. The other options imply far less water, or that organelles comprise most of the cell, which isn’t accurate. Water isn’t just a solvent; it enables diffusion, transport, and many chemical reactions essential to cell function.