Which process is a classic example of a catabolic pathway releasing energy?

• A. Photosynthesis
• B. Protein synthesis
• C. DNA replication
• D. Cellular respiration

Answer: (D)

Catabolic pathways release energy by breaking down complex molecules into simpler ones, with that energy often captured as ATP or in carrier molecules like NADH and FADH2. Cellular respiration is the classic example because it stepwise oxidizes glucose to carbon dioxide and water, releasing energy at each stage. glycolysis yields a small amount of ATP and generates NADH; the citric acid cycle produces more NADH and FADH2; and oxidative phosphorylation uses those carriers to drive a proton gradient that powerfully makes ATP. The overall process is exergonic, providing the energy that powers the cell’s activities.

In contrast, processes like photosynthesis, protein synthesis, and DNA replication are energy-requiring builds-up reactions. Photosynthesis uses energy to assemble glucose from CO2 and water, while protein synthesis and DNA replication construct new macromolecules, consuming energy rather than releasing it.