Name: Light and the Dark Reactions of Photosynthesis
Uploaded: 2019-07-19
Duration: 4 min 38 s
Description: A big plan is to show all the reactions involved in photosynthesis, the light reactions as well as the dark reactions leading to the synthesis of carbohydrates.

Question

Where does non-cyclic photophosphorylation take place? Describe this process. Why is this process so called?

Accepted Answer

The synthesis of ATP from ADP by using light energy in the chloroplast of the cell is called photophosphorylation. At the reaction centre, energy-rich electrons are expelled on receiving light energy and there is an electron transfer. There are two types of photophosphorylation. i) Non-cyclic Photophosphorylation and ii) Cyclic Photophosphorylation. Non-cyclic photophosphorylation takes place in the thylakoid membrane in the grana. It is also called Z-scheme of electron transport.

Non-cyclic photophosphorylation is completed by both the photosystems, PS I and PS II. PS II absorbs light of wavelength up to 680nm, hence it is called P₆₈₀. In this reaction, a pair of electrons are released by P₆₈₀, are taken up by plastoquinone (PQ). The energy-rich electrons from PQ roll down the chain of various electron carriers like cytochrome b₆, cytochrome f, and plastocyanin (PC). This movement of electrons is downhill according to the redox potential scale.

The reduced plastocyanin transfers the de-energised electrons to the reaction centre of PS I, i.e., P₇₀₀. During the transfer of electrons through the electron transport system, the energy released is used in the synthesis of ATP. From P₇₀₀, the energy-rich electrons are expelled and are taken up by iron-containing red protein called ferredoxin (Fd). From this, these electrons are taken up by coenzyme NADP. NADP accepts 2H⁺ ions obtained from the hydrolysis of water and converts into NADPH₂.

P₆₈₀ acts as a strong oxidising agent and brings about the splitting of water molecules to release oxygen. The four electrons generated in the break-down of four water molecules are accepted by oxidised P₆₈₀. During non-cyclic photophosphorylation, the electron released from PS II does not return back to it. Hence, it is so called.

Where does non-cyclic photophosphorylation take place? Describe this process. Why is this process so called?

Important Questions on Photosynthesis in Higher Plants

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