Figure 1. Mitochondrial structural organization.

Left, electron microscopy image of a representative mitochondrion. Right, schematic of a single mitochondrion. The different structural components of a mitochondrion are denoted. Aerobic cellular respiration. Glycolysis refers to a series of biochemical reactions by which a single molecule of glucose is oxidized to two molecules of pyruvate. In addition to conversion of glucose to pyruvic acid, two molecules of oxidized NAD are reduced to NADH, two molecules of ADP are phosphorylated to ATP, and two molecules of water and two protons are produced. The process of glycolysis occurs within the cytosol of a eukaryotic cell, under either aerobic or anaerobic conditions. When oxygen is present, pyruvate, which serves as a key intersection in several metabolic pathways, moves out of the cytosol and crosses into the mitochondrial matrix. Here, pyruvate is converted into acetyl-coenzyme A, which is the main input for a series of reactions known as the citric acid cycle (also known as the tricarboxylic acid cycle (TCA cycle) or the Krebs cycle). From a single molecule of acetyl-CoA, in the presence of sufficient oxygen, the eight reactions of the citric acid cycle produce three NADH and one flavin adenine dinucleotide (FAD/FADH2), along with one molecule of ATP. NADH and FADH2 molecules act as electron carriers and are used to generate the bulk of ATP during oxidative phosphorylation. Oxidation of NADH and phosphorylation of ADP to form ATP are processes supported by the electron transport chain assembly and ATP synthase, which are integral protein complexes of the inner mitochondrial membrane.