# 6.1: Definitions

We start by defining a few terms. Anabolic processes refer to collections of biochemical reactions that make bigger molecules from smaller ones. Examples include the synthesis of fatty acids from acetyl-CoA, of proteins from amino acids, of complex carbohydrates from simple sugars, and of nucleic acids from nucleotides. Just as any construction project requires energy, so, too, do anabolic processes require input of energy. Anabolic processes tend to be reductive in nature, in contrast to catabolic processes, which are oxidative. Not all anabolic processes are reductive, though. Protein synthesis and nucleic acid synthesis do not involve reduction, though the synthesis of amino acids and nucleotides does.

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
heterotrophic organisms is not made in directly in these reactions. Instead, the electrons released by oxidation are collected by electron carriers which donate them, in the mitochondria, to complexes that make ATP (ultimately) by oxidative phosphorylation.

Figure 6.1.1: Redox Reactions

In our tour of metabolism, we will tackle in this chapter processes that are the most oxidative/reductive in nature and in the following chapter those pathways that involve less reduction/oxidation. The aim in this coverage is not to go through the step- by-step reactions of the pathway, but rather to focus on control points, interesting enzymes, molecules common between pathways, and how the metabolic pathways meet the organism’s needs.