Answer: c6h12o6+6o2→6co2+6h2o= C6H12O6 + 6O2 -> 6CO2 + 6H2O Yields 2755 kJ/mole of glucose. The reverse of this reaction – combing carbon dioxide and water to make sugar is known as photosynthesis. Photosynthesis is the process responsible for storing all the energy we extract from fossil fuels, crops, and all of our food. We will also see that it is part of a globally important cycle affected by our consumption of fossil fuels. Photosynthesis How is photosynthesis able to run the reaction above in the reverse direction? Somehow it must come up with 2755 kJ of energy to make each mole of glucose. Where does that energy come from? The short answer: photons of sunlight. The long answer: When the pigment chlorophyll inside the chloroplasts of a photosynthetic organism (phytoplankton, trees, other plants) absorbs sunlight, it becomes energetically ‘excited’ and grabs the hydrogen atoms away from a water molecule, leaving the oxygen atoms to escape as O2 gas. This is called ‘splitting water.’ The hydrogen atoms are then split into their component protons and electrons. The electrons are used to reduce carbon dioxide, in a series of many steps requiring more absorption of sunlight by chlorophyll, to glucose. When carbon dioxide receives those electrons, the extra negative charge attracts protons from elsewhere, creating hydrogen atoms attached to the carbon atom. This process is called reduction. When those reduced carbon dioxide molecules are combined together in a larger molecule, the result is glucose. This ‘combing together’ of small molecules requires an input of energy, which is provided by the ATP molecules made by the protons diffusing through the membrane of the chloroplast. The ATP molecule is simply a molecule that biology uses to store energy for later use. In this case, the mechanical energy created by the protons diffusing across the membrane turns a sort of molecular turbine that smashes together its precursors.
