Which statement describes respiration in cellular metabolism?

• A. Photosynthesis
• B. The process of nitrogen fixation
• C. The process of protein synthesis
• D. The process by which cells break down simple food molecules to release the energy they contain; release CO2 and water

Answer: (D)

Respiration in cellular metabolism is the process by which cells break down simple nutrients, like glucose, to release usable energy (ATP). This catabolic pathway unfolds in stages and ultimately produces carbon dioxide and water as byproducts. The statement describes exactly that: the breakdown of basic food molecules to liberate energy and yield CO2 and water.

Photosynthesis, by contrast, builds sugars from light energy, carbon dioxide, and water, so it’s the opposite energy flow. Nitrogen fixation is a separate process that converts nitrogen gas into usable forms for biosynthesis, not energy production. Protein synthesis is the assembly of amino acids into proteins and uses energy, but it isn’t respiration.