Biology XI:13 Photosyn in higher plants(3 Chemiosmotic Hypothesis)

Chemiosmotic Hypothesis Chemiosmotic Hypothesis Chemiosmotic hypothesis explains how ATP is synthesised in chloroplast. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Membrane: Thylakoid membrane; protons accumulate towards the inside of the membrane; in the lumen Proton gradient develops because of the following reasons: Water splitting reaction takes place on the inner side of the membrane. So H+ produced in the reaction accumulates within the lumen. During the photo-phosphorylation steps when the electron-carrier molecule passes on its electrons to the electron-carriers on the inner side of the membrane, protons are released into the inner side or the lumen of the membrane. NADP reductase enzyme is located on the stroma side of the membrane. Protons are removed from the stroma and provided to the lumen because protons are necessary for the reduction of NADP+ to NADPH + H+, along with the electrons that come from the electron- accepter located in PSI. As a result, the protons in the stroma decrease in number, while the protons in the lumen, increase in number, thus creating a proton gradient across the thylakoid membrane. The proton gradient is broken down due to the movement of H+ from the lumen towards the stroma through the transmembrane channel to the F0 of ATPase. ATPase: This enzyme has two parts F0 − embedded in the membrane and carry out facilitated diffusion of H+ F1 − protrudes towards the stroma Conformational changes are induced in the F1 particle of the ATPase. This makes ATPase synthesise molecules of ATP.