Photosynthesis occurs in two phases in the chloroplast - the light reaction and the dark reaction. In the light reaction, which takes place in the grana, light energy is converted to chemical energy stored as ATP molecules. Water is split into hydrogen and oxygen, with oxygen being released and hydrogen used in the dark reaction. The dark reaction, which occurs in the stroma, uses the ATP and hydrogen to add hydrogen to carbon dioxide to form glucose without the use of sunlight.