Photosynthesis in Higher Plants

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Electron transport chain

Excited

electrons (2e )

–

Electron carrier

Chlorophyll

Light

Energy for

ATP production

ATP

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The electron is circulated within the photosystem and the ATP synthesis

occurs due to cyclic flow of electrons.

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The lamellae of grana have PS I & PS II. The stromal lamellae membranes

lack PS II and NADP reductase.

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The electron does not pass on to NADP⁺ but is cycled back to PS I complex

through electron transport chain.

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Here, only ATP is synthesised (no NADPH⁺ + H⁺).

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Cyclic photophosphorylation also occurs when only light of wavelength

beyond 680 nm are available for excitation.

4. ÇHËMÏØSMØTÏÇ HÝPØTHËSÏS

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It explains mechanism of ATP synthesis in chloroplast and mitochondria.

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ATP synthesis is linked to development of a proton gradient across

thylakoid membranes.

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Cause of proton gradients across the membrane:

(a) Splitting of water occurs on the inner side of the membrane.

So the protons accumulate in the lumen of thylakoids.

(b) As electrons move through the photosystems, protons

are transported across the membrane. The primary

electron acceptor is located towards the outer side of

the membrane and transfers its electron to a H⁺ carrier.

So this molecule removes a proton from the stroma while

transporting an electron. When this molecule passes

on its electron to the electron carrier on the inner side of

the membrane, proton is released into the lumen of the

membrane.

(c) The NADP reductase enzyme is located on the stroma side

of the membrane. Along with electrons coming from PS I,

protons are necessary to reduce NADP⁺. These protons are

also removed from the stroma.