Chemical Reactions in Photosynthesis

• The main chemical reaction that takes place during photosynthesis is:
  
  (light)
  6CO2 + 6H2O--------> (C6H12O6) + 6O2
  
  In words, 6 molecules of carbon dioxide, plus 12 molecules of water, in the presence of light energy, creates carbohydrate (glucose), plus oxygen.
  
  You may have heard the expression, "Plants breathe in carbon dioxide and breathe out oxygen". This is the overall chemical formula that shows how that works.
  
  This reaction, however, is really the result of two separate processes. The first process is the splitting of the water molecule. The second process is the conversion of carbon dioxide to glucose.
  
  

Splitting the Water Molecule

• The chemical equation that shows the splitting of the water molecule is shown below:
  (light energy)
  12 H2O -----------------------> 6 O2 + 24 H+ + 24e-
  
  For every 12 molecules of water, 6 oxygen molecules are released. This is accompanied by the release of 24 hydrogen atoms and 24 electrons. Since the water molecule is losing electrons in this reaction, the water molecule is said to be oxidized.
  
  

Conversation of Carbon Dioxide to Glucose

• The chemical equation that shows the conversation of carbon dioxide to glucose is shown below:
  CO2 + 24 H+ + 24 e- ------> C6H12O6 + 6 H2O
  
  In this reaction, carbon dioxide, in the presence of hydrogen ions and electrons is converted to glucose and water. Since carbon dioxide is adding electrons, it is reduced.
  
  

More Oxidation Reduction Reactions

• While these are the overall net reactions for converting carbon dioxide and water into glucose and oxygen during photosynthesis, these are not the only oxidation-reduction reactions that occur.
  
  When water is converted to hydrogen ions and electrons, a chemical by the name of Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is formed. NADPH is an electron carrier capable of donating electrons to a wide variety of reactions within a cell.
  
  NADP+ + H20 -------> NADPH + H+ + O
  
  NADP+ is the oxidized form of NADPH. NADP+ is reduced to NADPH by accepting the electrons that are given off by the splitting of the water molecule.
  Therefore, during photosynthesis, in addition to carbon dioxide becoming reduced, NADP+ is also reduced.
  
  As NADP+ is reduced to NADPH, another compound is created. The molecule adenosine triphosphate (ATP) is formed. ATP is formed when adenosine diphosphate (ADP) is phosphorylated (designated Pi for inorganic phosphate) during the light reaction (adds another phosphate group).
  
  ADP + Pi ------------> ATP
  
  While ADP is phosphorylated to create ATP, this is not an oxidation/reduction reaction. In fact, the term used for this reaction is photophosphorylation, as ADP is phosphorylated using the energy of the sunlight.
  
  

Summary

• Photosynthesis occurs in two steps. The first step splits water using the energy from light into oxygen and hydrogen molecules plus electrons. In the process, the water molecule is said to be oxidized. This occurs in the light-dependent phase of photosynthesis, as it depends on the energy from light to split the water molecules.
  
  The electrons released from this reaction reduce NADP+, creating NADPH. The energy released from that reaction is used to phosphorylate ADP using Pi creating ATP, a major energy carrier for the cell. In the process, carbon dioxide is reduced resulting in the formation of glucose.