Chapter 13: Mitochondria, Chloroplasts, & Peroxisomes

Mitochondria, Chloroplasts, & Peroxisomes begins by exploring the double-membrane structure of mitochondria, identifying the inner membrane's cristae as the critical site for the electron transport chain and oxidative phosphorylation, while the matrix houses the enzymes for the citric acid cycle. The summary explains the endosymbiotic origin of mitochondria, evidenced by their circular genomes and distinct genetic code which utilizes extreme wobble rules for translation. Key focus is placed on protein sorting, detailing how positively charged N-terminal presequences direct proteins through the Tom and Tim complexes, a process driven by the electrochemical gradient and ATP-dependent chaperones like Hsp70. The text then transitions to chloroplasts, the photosynthetic organelles of plants, distinguishing their three-membrane architecture which includes the thylakoid system responsible for generating the proton motive force required for ATP synthesis. It covers the chloroplast genome, which uses the universal genetic code, and the complex import pathways involving Toc and Tic translocons, as well as the specialized Sec and Tat systems for targeting proteins to the thylakoid lumen. The discussion expands to the broader family of plastids, such as amyloplasts and chromoplasts, and their developmental interconversions. Finally, the chapter characterizes peroxisomes as single-membrane compartments essential for fatty acid oxidation, hydrogen peroxide decomposition by catalase, and lipid biosynthesis, including plasmalogens. It elucidates peroxisome biogenesis, describing how peroxins (Pex proteins) mediate the import of folded proteins via PTS1 and PTS2 signals and how these organelles form both de novo from the endoplasmic reticulum and through growth and division, with defects leading to severe pathologies like Zellweger spectrum disorders.