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  • ICAR and TNAU E-Course Summarized

    Summarized Notes
  • The correct sequence of flow of electrons in the light reaction is

    Question: The correct sequence of flow of electrons in the light reaction is

    Options:

    PSII, plastoquinone, cytochromes, PSI, ferredoxin
    PSI, plastoquinone, cytochromes, PSII, ferredoxin
    PSI, ferredoxin, PSII,
    PSI, plastoquinone, cytochromes, PSII, ferredoxi

    -Photosynthesis is the process by which carbohydrates are synthesized from inorganic materials like CO2 and H2O with the help of light energy which is trapped by pigments like chlorophyll.

    -Oxygen is produced as a by-product of photosynthesis. It is used up by other organisms like humans for essential metabolic activities.
    -Photosynthesis takes place in the chloroplast of the leaves. The chloroplast contains pigments called chlorophyll which absorb the light energy needed for photosynthesis.

    • The process of photosynthesis involves two stages:
    -Light-dependent reaction or Light reaction: ATP and NADPH2 are formed at the end of the light reaction which then enters the next stage of photosynthesis (dark reaction).
    -Light-independent reaction or Dark reaction: It is a carbon-fixing stage, where CO2 from the atmosphere taken up by the plants is fixed into a sugar molecule.

    ✅Explanation:

    ✏️Photophosphorylation:
    -Photophosphorylation takes place during the light reaction of photosynthesis.
    -Oxidative phosphorylation is a process by which energy-rich molecules called ATP are produced from ADP by utilizing the energy derived from the transfer of electrons.
    -The phosphorylation process that takes place in the presence of light is called photophosphorylation.

    ✏️During photosynthesis, this transfer of electrons takes place in two ways:

    • Cyclic Photophosphorylation:
    -Cyclic electron transfer takes place.
    -In this process, only Photosystem I is involved
    -During this process, a high-energy electron released from a reaction center (P700) returns to the same reaction center.
    -Thus the movement of electrons in this process is cyclic.
    -Two ATP molecules are produced at the end of cyclic photophosphorylation.

    ✏️The cyclic phosphorylation process can be summarized as follows:
    -The flow of electrons is as follows: PS-I (P700) → Primary electron acceptor → Ferredoxin → Cytochrome b6 → Cytochrome f  → Plastocyanin → P700.

    • Non-cyclic Phosphorylation:
    -It is a non-cyclic electron transfer process.
    -It involves both the photosystems – Photosystem I and Photosystem II.
    -In this process, the electrons released from a reaction center do not return to the same reaction center.
    -At the end of the process, 1 ATP and 2 NADPH molecules are formed.
    -Oxygen is released into the atmosphere by this process.
    -The non-cyclic phosphorylation process can be summarized as follows: 
    -The flow of electrons is as follows: PS-II (P680) → Primary electron acceptor → Plastoquinone → Cytochrome b6 →Cytochrome f → Plastocyanin → PS-I (P700) → Electron acceptors → Ferredoxin.
    -Using the above-given information, let us now solve the given question.
    -The correct sequence of the flow of electrons in the light reaction that matches the above-given information is Option 1: PS-II → Plastoquinone →  Cytochromes → PS-I → Ferredoxin

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