You MUST learn the overall equation for photosynthesis and what it means. Respiration and photosynthesis each involve a series of chemical reactions. Water is split into hydrogen ions, electrons, and oxygen. 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.