ABSTRACT
INTRODUCTION Sunlight provides a steady, abundant supply to a world increasingly concerned about energy. The cause of this concern is a disconnect between inputs and outputs of energy forms. Forms of energy traditionally used to drive our mechanical devices differ from those that support living systems. Nuclear energy, fraught with waste problems, hydroelectric, geothermal, or wind power, are all forms that can be used for mechanical devices but provide a minor fraction of the total energy input needed in the world economy. The bulk of the available energy has been, and still is, derived from light energy from the sun. Technology has been developed to convert photons to electrons with photovoltaic cells that use absorption of light energy by silicon semiconductor devices to generate electricity as the end product. This approach has great promise as a sustainable energy source but thus far does not compete economically with fossil fuels. Combustion of fossil fuels (coal, oil, and natural gas) is currently the major source of energy used to generate electricity and power machines. Fossil fuels are products derived from ancient photosynthesis but, with few exceptions, are not
directly usable by living systems. Life is also sustained by converting photons to electrons but through contemporary photosynthesis. Except for organisms that live next to thermal vents in the deep oceans and those that can metabolize hydrocarbons, life is sustained by carbohydrates produced by photosynthesis. The major end product of photosynthesis is glucose 6-phosphate, the predominant starting material for storage forms of chemical energy and for synthesis of cellular components. When glucose 6-phosphate is metabolized in cells, the energy that was trapped in the molecule by photosynthesis is released to provide the energy required for synthetic reactions and growth. In contrast to animals, glucose does not occur in the free form to a significant amount in plants. Of fundamental importance to the biosphere however, is the fact that plants are the primary source of the glucose unit, which they store in polymeric form.
