Explain the three pathway of breakdown in living organisms.

Living organisms employ three main pathways for breakdown: glycolysis, the Krebs cycle (or citric acid cycle), and oxidative phosphorylation. Glycolysis, occurring in the cytoplasm, breaks down glucose into pyruvate, generating ATP and NADH. The Krebs cycle, located in the mitochondria, further oxidizes pyruvate, producing more ATP, NADH, and FADH2. These electron carriers shuttle electrons to the electron transport chain (ETC) in oxidative phosphorylation, also in the mitochondria. The ETC uses these electrons to create a proton gradient, driving ATP synthesis. Together, these pathways convert organic molecules into usable energy for cellular functions.