chemical reactions
How Light-Dependent Reactions Work The overall purpose of the light-dependent reactions is to convert light energy into chemical energy. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon, a quantity or packet of light energy, at a time. A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become excited. The energy given to the electron allows it to break free from an atom of the chlorophyll molecule. Chlorophyll is therefore said to donate an electron Figure 5.12 . To replace the electron in the chlorophyll, a molecule of water is split. This splitting releases an electron and results in the formation of oxygen O2 and hydrogen ions H+ in the thylakoid space. Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons.