We now turn to another reaction sequence in which the flow of electrons is coupled to the synthesis of ATP: light-driven phosphorylation. The capture of solar energy by photosynthetic organisms and its conversion to the chemical energy of reduced organic compounds is the ultimate source of nearly all biological energy.
Photosynthetic and heterotrophic organisms live in a balanced steady state in the biosphere. Photosynthetic organisms trap solar energy and form ATP and NADPH, which they use as energy sources to make carbohydrates and other organic compounds from CO2 and H2O; simultaneously, they release O2 into the atmosphere.
Aerobic heterotrophs (humans, for example, as well as plants during dark periods) use the O2 so formed to degrade the energy-rich organic products of photosynthesis to CO2 and H2O, generating ATP. The CO2 returns to the atmosphere, to be used again by photosynthetic organisms. Solar energy thus provides the driving force for the continuous cycling of CO2 and O2 through the biosphere and provides the reduced substrates—fuels, such as glucose— on which nonphotosynthetic organisms depend.
Photosynthesis occurs in a variety of bacteria and in unicellular eukaryotes (algae) as well as in vascular plants. Although the process in these organisms differs in detail, the underlying Mechanisms are remarkably similar, and much of our understanding of photosynthesis in vascular plants is derived from studies of simpler organisms. The overall equation for photosynthesis in vascular plants describes an oxidation-reduction reaction in which H2O donates electrons (as hydrogen) for the reduction of CO2 to carbohydrate (CH2O.
Photosynthetic and heterotrophic organisms live in a balanced steady state in the biosphere. Photosynthetic organisms trap solar energy and form ATP and NADPH, which they use as energy sources to make carbohydrates and other organic compounds from CO2 and H2O; simultaneously, they release O2 into the atmosphere.
Aerobic heterotrophs (humans, for example, as well as plants during dark periods) use the O2 so formed to degrade the energy-rich organic products of photosynthesis to CO2 and H2O, generating ATP. The CO2 returns to the atmosphere, to be used again by photosynthetic organisms. Solar energy thus provides the driving force for the continuous cycling of CO2 and O2 through the biosphere and provides the reduced substrates—fuels, such as glucose— on which nonphotosynthetic organisms depend.
Photosynthesis occurs in a variety of bacteria and in unicellular eukaryotes (algae) as well as in vascular plants. Although the process in these organisms differs in detail, the underlying Mechanisms are remarkably similar, and much of our understanding of photosynthesis in vascular plants is derived from studies of simpler organisms. The overall equation for photosynthesis in vascular plants describes an oxidation-reduction reaction in which H2O donates electrons (as hydrogen) for the reduction of CO2 to carbohydrate (CH2O.
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