Class 11 Biology Photosynthesis In Higher Plants Chemiosmotic hypothesis

Chemiosmotic hypothesis

  • The theory which explains how ATP is synthesized in the chloroplast is chemiosmotic hypothesis.
  • Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across the membranes of the thylakoid.
  • In phototsynthesis, proton accumulation is towards the lumen, whereas in respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS.
  • The processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop are the following:
  • Since splitting of the water molecule takes place on the inner side of the membrane, the hydrogen ions that are produced accumulate within the lumen of the thylakoids.
  • As electrons move through the photosystems, protons are transported across the membrane because the primary accepter of electron transfers its electron to an H carrier; hence, this molecule removes a proton from the stroma while transporting an electron.
  • Protons are necessary for the reduction of NADP+ to NADPH+ H+, these protons are also removed from the stroma.
  • Within the chloroplast, protons in the stroma decrease in number, whereas in the lumen there is accumulation of protons, which creates a proton gradient across the thylakoid membrane as well as a decrease in pH in the lumen.
  • The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0 of the ATPase.
  • The ATPase enzyme consists of two parts
  • one is F0 embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane
  • other portion is called F1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
  • The break down of the gradient provides enough energy to cause a conformational change in the F1 particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.
  • Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase.

Class_11_Biology_Photosynthesis_in_Plants_ATP_Synthesis

Fig. ATP synthesis through chemiosmosis

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