Which process consists of chemical reactions that break down nutrient molecules in living cells to release energy?

• A. Ingestion
• B. Respiration
• C. Reproduction
• D. Growth

Answer: (B)

Respiration is the series of chemical reactions that break down nutrient molecules in living cells to release energy. In this process, nutrients such as glucose are oxidized step by step, and the energy released is captured in ATP, the cell’s usable energy. This mainly happens in mitochondria and usually requires oxygen, giving carbon dioxide and water as waste plus a large amount of ATP. When oxygen isn’t available, anaerobic pathways can occur, producing less energy and different byproducts like lactate or ethanol. This is how cells convert the energy in food into a form they can use for work, unlike ingestion (taking in food), reproduction (making offspring), or growth (increasing in size).