Figure 4.4 Diagrammatic representation of the non-cyclic process (Z scheme)

b) Photoexcitation of PS II : Similarly, on receiving photons of light, the chl-a (680) of PS II also gets activated and expels electrons. As a result, it becomes ionized chl-a⁺. The high energy containing electrons from PS II are first accepted by an electron acceptor PQ (plastoquinone) and are then transferred along stet Electron Transport System consisting of plastoquinone, cytochrome b6, Cytochrome f and plastocyanin. Finally, the electrons are accepted by the ionized chl-a⁺ of PS I. In this way, ionized PS I comes back to the ground state and can again participate in the process.

(c) Photolysis of water : The ionized chlorophyll-a⁺ of PS II is brought back to the ground state with the help of the electrons made available through the photolysis of water.

(e) Non-Cyclic electron transfer : In this process, the movement of electrons is considered to be non-cyclic, or unidirectional. This is because, the final electron acceptor is different from the initial electron donor.

Initial electron donor Final electron acceptor

In this way, the light-induced electron flow is maintained continuously in the non-cyclic manner from water to NADP as follows

Because of the zig-zag path of the electrons (Figure 4.4), the non cyclic process is also called ‘Z’ scheme electron transport.

(f) ATP formation : During the non-cyclic transfer of energy-rich electrons through the Electron Transport System from PS II to PS I, energy from the electrons is released. This is used for the formation of ATP from ADP and H₃PO₄. This is called non-cyclic photophosphorylation.

End products of non-cyclic process and the assimilatory power : The end products are ATP, NADPH₂ and O₂. Of these, O₂ is liberated from the green plants. ATP and NADPH₂ are used in the following dark reactions (phase II) for carbon assimilation( i.e., reduction of CO₂ to form carbohydrates). ATP and NADPH₂ of light reaction are called ‘assimilatory power.

Chapter 5