The oxygen is given off in the form of O 2 gas. It takes the hydrogen atoms to the dark reaction. Light energy is absorbed by the pigment chlorophyll . The absorbed energy causes some electrons to be boosted from a low energy state to a high energy state. This energy is then used to produce ATP, which, like the hydrogen atoms from the water molecules, is used in the dark reaction.
Water is split.
Oxygen is given off. This reaction is called a dark reaction because it doesn't use light directly. 12 O 6 is produced. All the time that a plant is in the light, both the light and dark reactions are occurring. As soon as the light is removed, the light reaction stops.