In Figure 14–57, we relate these postulated evolutionary pathways to different types of bacteria. By necessity, evolution is always conservative, taking parts of the old and building on them to create something new. Thus, parts of the electron-transport chains that were derived to service anaerobic bacteria 3–4 billion years ago survive, in altered form, in the mitochondria and chloroplasts of today’s higher eukaryotes. A good example is the overall similarity in structure and function between the cytochrome c reductase that pumps H+ in the central segment of the mitochondrial respiratory chain and the analogous cytochrome b-f complex in the electron-transport chains of both bacteria and chloroplasts, revealing their common evolutionary origin (Figure 14–58).
H2S photosynthesis H2O photosynthesis green sulfur bacteria purple sulfur bacteria green flamentous bacteria ˜-proteobacteria (e.g., Bordetella) MITOCHONDRIACHLOROPLASTS EUKARYOTES PROKARYOTES OXIDIZING ATMOSPHERE REDUCING ATMOSPHERE cyanobacteria (e.g., Synechococcus) ancestral fermenting bacteria O2 respiration purple nonsulfur bacteria °-proteobacteria (e.g., Agrobacterium) ˛-proteobacteria (e.g., E. coli)
Figure 14–57 Evolutionary scheme showing the postulated origins of mitochondria and chloroplasts and their bacterial ancestors. the consumption of oxygen by respiration is thought to have first developed about 2 billion years ago. Nucleotide-sequence analyses suggest that an endosymbiotic oxygen-evolving cyanobacterium (cyan) gave rise to chloroplasts (dark green), while mitochondria arose from an α-proteobacterium. the nearest relatives of mitochondria (pink) are members of three closely related groups of α-proteobacteria—the rhizobacteria, agrobacteria, and rickettsias—known to form intimate associations with present-day eukaryotic cells. proteobacteria are pink, purple photosynthetic bacteria are purple, and other photosynthetic bacteria are light green.
Chloroplasts and photosynthetic bacteria have the unique ability to harness the energy of sunlight to produce energy-rich compounds. This is achieved by the photosystems, in which chlorophyll molecules attached to proteins are excited when hit by a photon. Photosystems are composed of an antenna complex that collects solar energy and a photochemical reaction center, in which the collected energy is funneled to a chlorophyll molecule held in a special position, enabling it to withdraw electrons from an electron donor. Chloroplasts and cyanobacteria contain two distinct photosystems. The two photosystems are normally linked in series in the Z scheme, and they transfer electrons from water to NADP+ to form NADPH, generating a transmembrane electrochemical potential. One of the two photosystems—photosystem II—can split water by removing electrons from this ubiquitous, low-energy compound. All the molecular oxygen (O2) in our atmosphere is a by-product of the water-splitting reaction in this photosystem. The three-dimensional structures of photosystems I and II are strikingly similar to the photosystems of purple photosynthetic bacteria, demonstrating a remarkable degree of conservation over billions of years of evolution.