In the process of photophosphorylation, the high energy electrons (e-) released from chlorophyll molecules are carried through different carriers, lose energy, become dull and are transferred to another chlorophyll molecule without returning to that chlorophyll molecule is called acyclic photophosphorylation. In 1964 scientists Hill and Bendall explained the reactions of this process by Z diagram. Hence it is called Z scheme reaction. The process of acyclic photophosphorylation is-
(i) Both photosystem-I and photosystem-II also participate in the acyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-680.
(ii) Two high energy electrons (2e-) are emitted from P-680. Two electron carriers are accepted by plastoquinin (PQ).
(iii) Two electrons from plastoquinin come to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(iv) Electrons then pass from two cytochrome-f to photosystem-I through plastocyanin. Because previously two electrons have been released from photosystem-I. As a result there is a shortage of electrons. In this way photosystem-I regains its lost electrons.
(v) Photosystem-I absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(vi) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd). Electrons from ferridoxine enter two NADP-reductases.
(vii) During this period photolysis of H2O molecules takes place in the presence of light to produce O2, 2H+ and 2e-. O2 escapes to the atmosphere. 2e- enters photosystem-II. In this way photosystem-II regains its lost electrons. 2H+ combines with NADP-reductase to produce NADPH+H+.
Both cyclic and acyclic processes can occur simultaneously in higher plants. However, in cyanobacteria, algae and green plants cyclic processes occur when the supply of NADP is cut off. When the water supply is stopped, the acyclic process stops and the cyclic process takes place.
[Applying herbicides to the soil stops the photosynthetic electron flow of the weed, so the weed dies]