# 6.1: Definitions

Catabolic processes are the primary sources of energy for heterotrophic organisms and they ultimately power the anabolic processes. Examples include glycolysis (breakdown of glucose), the citric acid cycle, and fatty acid oxidation. Reductive processes require electron sources, such as NADPH, NADH, or $$\text{FADH}_2$$. Oxidative processes require electron carriers, such as $$\text{NAD}^+$$, $$\text{NADP}^+$$, or FAD. Catabolic processes are ultimately the source of ATP energy in cells, but the vast majority of ATP i