Which system converts nutrients to energy?

• A. Digestive System
• B. Respiratory System
• C. Endocrine System
• D. Circulatory System

Answer: (A)

Energy for the body's activities comes from nutrients that are broken down and absorbed; the digestive system handles this breakdown, turning food into simple components that cells can use to generate energy. Once these nutrients—like glucose, amino acids, and fatty acids—are absorbed into the bloodstream, cells can metabolize them to produce ATP, the cell’s usable energy. Oxygen, provided by the respiratory system, supports this energy production, but the actual conversion of nutrients into usable energy starts with digestion. The other systems play supporting roles (delivery of nutrients and oxygen, hormonal regulation), but the step that converts food into the forms cells can use for energy is the digestive system.