Daniel Arnon and coworkers carried out experiments with intact, salt-washed chlo

Question

Daniel Arnon and coworkers carried out experiments with intact, salt-washed chloroplasts to study photophosphorylation

Label these as either the first reaction or the second reaction

Part A Daniel Amon and coworkers camed out experiments with intact, d chloroplasts to study phat When the chloroplasts were iluminated in the presence ot ADP- Pa, ATP was produced, but oxygen was not produced or norconsumed. ATP formation was not accompanied by a measurable electron transport involving any external electron donor or acceptor The overall reaction for this result is ADP Pi ATP when NADP was inoluded in additio to the ADP EPI. ilumination of the intact chloroplats again resulted in resulted in the photophosphorylation ADP to ATP. In addition, the NADP was reduced to NADPH H and O2 was produced. Moreover, the light. induced reduction of NADP was greatly decreased ADP l Pwere ommitted. The equation forthis reaction is: NADP H20 ADP. P NADPH H ATP Ion Briefly describe the mechanism(s) of these two types of photophosphorylation that explain all of these results. Drag the appropriate ite pective bi Reset Celp Reduced The omission of ADP. P NADPH s not produced door asas the reduction of NADP n transport chain. O2is not produoed.

Explanation / Answer

Answer:

Living beings extracts energy from oxidizable substrates and stores this energy in the form of high energy ATP. The synthesis of ATP from ADP and Pi in the presence of light is called photophosphorylation.

The two types of photophosphorylation mentioned here are Non-cyclic and Cyclic photophosphorylation.

(The underlined words/phrases explains all the results given above in the box)

Non-cyclic photophosphorylation:

When the two photosystems work together in parallel with Photosystem II and then the Photosystem I,it is called non-cyclic photo-phosphorylation. Photosystem I and Photosystem II are connected via an electron transport chain (the Z-scheme) and both ATP and NADPH + H+are produced. It involves the following steps to generate the proton gradient for ATP synthesis:

(a) Splitting of water: The protons which are produced by the splitting of water accumulate within the lumen of the thylakoids. Oxygen is produced in this step for further use.

(b) When electron transport occurs through the photosystems, protons are transported across the membrane of grana.

(c) NADP reductase is located in the stroma of the membrane. Protons which are necessary for the reduction of NADP+to NADPH+ H+are removed from the stroma along with the electrons which comes from the Photosystem I electron acceptors.

This creates a proton gradient with more protons in the lumen of the thylakoid while a decreased number in the stroma. This gradient facilitates the activation of ATP synthase resulting in the synthesis of ATP from ADP and Pi. When ATP is low due to utilization in glycolysis, NADPH accumulates and the system may change from non-cyclic to cyclic. In this case, NADP+does not get reduced to NADPH.

Cyclic-photophosphorylation:

Here, the phosphorylation occurs due to cyclic flow of electrons, that is the electron is circulated within the photosystem and only Photosystem I is functional.

The lamellae of the grana have both Photosystem I and Photosystem II, while the stroma lamellae lacks Photosystem II and NADP reductase. The excited electron is cycled back to the Photosystem I through the electron transport chain and does not reduce NADP+. Thus, only synthesis of ATP takes place in cyclic photophosphorylation, but not of NADPH + H+. Unlike non-cyclic photophosphorylation, oxygen is not produced in cyclic photophosphorylation.

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