The two photosystems and the cytochrome b6-f complex reside in the thylakoid membrane, a separate membrane system in the central stroma compartment of the chloroplast that is differentiated into stacked grana and unstacked stroma thylakoids. Electron-transport processes in the thylakoid membrane cause protons to be released into the thylakoid space. The backflow of protons through the chloroplast ATP synthase then generates ATP. This ATP is used in conjunction with the NADPH produced by photosynthesis to drive a large number of biosynthetic reactions in the chloroplast stroma, including the carbon-fixation cycle, which generates large amounts of carbohydrates from CO2.
In the early evolution of life, cyanobacteria overcame a major obstacle in devising a way to use solar energy to split water and fix carbon dioxide. Cyanobacteria produced both abundant organic nutrients and molecular oxygen, enabling the rise of a multitude of aerobic life-forms. The chloroplasts in plants have evolved from a cyanobacterium that was endocytosed long ago by an aerobic eukaryotic host organism.
Figure 14–58 A comparison of three electron-transport chains discussed in this chapter.
Bacteria, chloroplasts, and mitochondria all contain a membrane-bound enzyme complex that resembles the cytochrome c reductase of mitochondria. these complexes all accept electrons from a quinone carrier (Q) and pump h+ across their respective membranes. Moreover, in reconstituted in vitro systems, the different complexes can substitute for one another, and the structures of their protein components reveal that they are evolutionarily related. Note that the purple nonsulfur bacteria use a cyclic flow of electrons to produce a large electrochemical proton gradient that drives a reverse electron flow through NaDh dehydrogenase to produce NaDh from NaD+ + h+ + e –.
As we discussed in Chapter 1, mitochondria and chloroplasts are thought to have evolved from endosymbiotic bacteria (see Figures 1–29 and 1–31). Both types of organelles still contain their own genomes (Figure 14–59). As we will discuss shortly, they also retain their own biosynthetic machinery for making RNA and organellar proteins.
Like bacteria, mitochondria and chloroplasts proliferate by growth and division of an existing organelle. In actively dividing cells, each type of organelle must double in mass in each cell generation and then be distributed into each daughter cell. In addition, nondividing cells must replenish organelles that are degraded as part of the continual process of organelle turnover, or produce additional organelles as the need arises. Organelle growth and proliferation are therefore carefully controlled. The process is complicated because mitochondrial and chloroplast proteins are encoded in two places: the nuclear genome and the separate genomes harbored in the organelles themselves. The biogenesis of mitochondria and chloroplasts thus requires contributions from two separate genetic systems, which must be closely coordinated.
Most organellar proteins are encoded by the nuclear DNA. The organelle imports these proteins from the cytosol, after they have been synthesized on cytosolic ribosomes, through the mitochondrial protein translocases of the outer and inner mitochondrial membrane—TOM and TIM. In Chapter 12, we discussed how this happens. Here, we describe the organelle genomes and genetic systems, and consider the consequences of separate organelle genomes for the cell and the organism as a whole.
the Genetic Systems of Mitochondria and Chloroplasts resemble those of prokaryotes