8.10Topology of Mitochondria and Chloroplasts
As outlined in Chapter 5, mitochondria and chloroplasts originate from certain eubacteria*11 that lived symbiotically in eukaryotic cells (endosymbiotic theory: see the Column in 5.4). Unlike other organelles, mitochondria and chloroplasts are therefore surrounded by double membranes - one inner and one outer *12. The inner membrane of mitochondria forms cristae, through which H+ is transported outside the membrane by electron transport in the respiration chain. In chloroplasts, on the other hand, the photosynthetic system (including chlorophyll) does not exist in the inner envelope. Rather, it is found in the thylakoid membrane further inside, and transports H+ into this membrane by photosynthetic electron transport. The directions of this transport seem opposite to each other; however, since the matrix of mitochondria and the stroma of chloroplasts are homologous compartments, the two directions are in fact the same when it is considered that H+ is emitted from them (i.e., they are topologically homologous). The matrix of mitochondria (in which citric-acid-cycle enzymes work) and the stroma of chloroplasts (in which Calvin-cycle enzymes work) are homologous, and H+ movement in electron transport is directed outward from these compartments. Coupling with H+ transport based on this H+ concentration gradient, F-ATP synthase therefore synthesizes ATP in the same compartment as the citric acid cycle and the Calvin cycle.
Eubacteria: A type of cyanobacteria (a blue-green algae), which are ancestors of chloroplasts. The major candidate ancestors of mitochondria are rickettsiae (intracellular parasitic pathogens and a type of α-proteobacterium), but due to the marked specialization of mitochondria, their ancestral relationship has not been determined.
Inner and outer membranes: In chloroplasts, these are also referred to as the inner and outer envelopes. In both chloroplasts and mitochondria, the outer membrane has non-specific carriers and therefore does not block material transport. Endosymbiotic theory suggests that the outer membrane is derived from a host cell, but another well-accepted theory is that the outer membrane is homologous to a special outer membrane of cyanobacteria, which are ancestors of chloroplasts.