What Happens to Water Molecules in the Light-Dependent Reaction?

During the stage of photosynthesis referred to as the light-dependent reaction, 12 molecules of water begin the process of reacting with six molecules of carbon dioxide to produce one molecule of glucose, six molecules of oxygen and six molecules of water. The presence of light and enzymes within the plant's chloroplasts act as the catalysts for the light-dependent reaction and begin the process by splitting the water molecules into electrons, protons and oxygen. The complete reaction, which also includes the light-independent stage of photosynthesis, can be expressed as 6CO2 + 12H2O + light and plant enzymes €”> C6H12O6 + 6O2 + 6H2O.

The electrons released from the water molecules in the light-reaction stage travel along the electron transport system within the chloroplast's thylakoid membranes and lose energy as they pass each transport point along the chain of proteins. The energy lost by the electrons is stored in the high-energy phosphate bonds in the plant cell's adenosine triphosphate, or ATP. Some of the protons released by the water molecules react with NADP, which is another energy-transport agent found within the plant cell. The reaction forms NADP-H, which then stores the energy lost by the protons.

The light energy absorbed by the chloroplast is stored in both ATP and NADP-H in the form of chemical energy and transported to the plant cell's stroma. The energy directed to the stroma will be converted again and stored within the chemical bonds that comprise the newly-formed sugar molecules that the plant requires for an energy source. The sugar-producing stage taking place within the stroma is the light-independent portion of photosynthesis and is also referred to as the Calvin cycle.