This chapter discusses the process of oxygenic photosynthesis, which first evolved in a water-borne cyanobacterium. Cyanobacteria are ultimately responsible for the oxygen-rich air that we breathe. Our body essentially reverses the process of oxygenic photosynthesis, recombining oxygen and organic matter to liberate carbon dioxide, water, and energy in the process of aerobic respiration. Organic matter can be broken down in other ways – if you are really exerting yourself, and your breathing and heartbeat cannot supply enough oxygen to your muscles, you can switch to anaerobic metabolism – transforming one organic compound into another without the use of oxygen. However, the energy yields are miserable in comparison. Aerobic metabolism yields an order of magnitude more energy per molecule of organic matter broken down compared to anaerobic pathways such as fermenting lactic acid.
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