Associated with light-absorbing molecules and electron transport systems of thylakoids. They involve the splitting of water and the release of O 2. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophylla. The cyclic electron pathway begins after PS I pigment complex absorbs solar energy. Before they return, the electrons enter and travel down an electron transport system.
Electrons pass from a higher to a lower energy level.
It is possible that in plants, the cyclic flow of electrons is utilized only when CO 2 is in such limited supply that carbohydrate is not being produced. I initiates regulatory mechanism by which enzymes of the light-independent reactions are turned on. CO 2 becomes CH 2 O within a carbohydrate molecule. The reduction of CO 2 occurs in the stroma of a chloroplast by series of reactions called the Calvin cycle. Hydrocarbon skeleton of PGAL is used to form fatty acids and glycerol of plant oils, and amino acids.