Chapter 5: Aerobic Respiration & the Mitochondrion

Mitochondria are highly organized, dynamic structures capable of fusion and fission, processes often regulated by contact with endoplasmic reticulum tubules. The organelle is defined by a permeable outer membrane, featuring porins, and an inner membrane that folds into specialized sheets called cristae, which house the machinery for energy conversion. Inside is the matrix, which contains essential components, including mitochondrial DNA (mtDNA)—a relic encoding a small number of critical proteins—and the enzymes of the metabolic pathways. The process of extracting energy begins with glycolysis in the cytosol, which produces pyruvate and NADH. Pyruvate then moves into the matrix, where it is converted to acetyl CoA and fed into the Tricarboxylic Acid (TCA) Cycle (also known as the Krebs cycle or citric acid cycle). The TCA cycle acts as a central metabolic funnel, accepting breakdown products from carbohydrates, fatty acids, and amino acids, generating the high-energy coenzymes NADH and FADH2. These reduced coenzymes transfer electrons to the electron-transport chain (ETC), a series of carriers (including flavoproteins, iron-sulfur proteins, cytochromes, and ubiquinone) embedded in the inner membrane. As electrons move down the ETC in energy-releasing steps, they are ultimately transferred to molecular oxygen, reducing it to water. This energy release is coupled to the active translocation of protons from the matrix into the intermembrane space, creating an electrochemical gradient known as the proton-motive force (Δp). This force, which is primarily represented by an electrical potential component in mammalian cells, provides the energy necessary for oxidative phosphorylation. ATP synthesis is catalyzed by the ATP synthase (F1Fo complex), which functions as a rotary nanomachine. Proton movement through the membrane-embedded Fo portion drives the rotation of the asymmetric gamma (γ) subunit relative to the fixed F1 head. This mechanical rotation induces conformational changes (loose, tight, and open states) in the catalytic sites on the beta (β) subunits, driving the spontaneous condensation of ADP and inorganic phosphate and facilitating the release of the newly formed ATP, consistent with the binding change mechanism. The chapter also examines peroxisomes, multifunctional organelles that utilize enzymes like catalase to degrade toxic hydrogen peroxide and oxidize very-long-chain fatty acids (VLCFAs), and specialized plant glyoxysomes involved in fat-to-carbohydrate conversion. Finally, the chapter addresses clinical relevance, detailing disorders resulting from mitochondrial dysfunction (such as Complex I deficiencies) and peroxisomal defects (like X-linked adrenoleukodystrophy), alongside engineering applications like pulse oximetry for non-invasive blood oxygen monitoring.