Question

what is " chemiosmotic hypothesis "?

Answer 1

suggest that the action of atp synthese is coupled with that of  a proton gradient.

Answer 2

NCERT based Answer for Class 11 Biology Students

Chemiosmotic Hypothesis:

 In photosynthesis, ATP synthesis is linked to development of proton (H+) gradient across thylakoid membranes.

 Splitting of water molecule occurs inside thylakoid. As a result, protons accumulate within lumen of thylakoids.

 Photoinduced electrons when flow downhill through chain of electron carriers connecting PS II and PS I, release some energy which moves protons across the thylakoid membrane, from the stromal side to the thylakoid lumen. This further increases concentration of protons inside the lumen and generates proton gradient across thylakoid membrane between stroma and thylakoid lumen. This proton gradient plays very important role in synthesis of ATP.

 NADP reductase enzyme is located on stromal side of thylakoid membrane. Along with electron that comes from PS I, protons are also required for reduction of NADP to NADPH.  

 Dark reactions occur in stroma which require input of hydrogen which is provided by NADPH.

 Hence within chloroplast, protons in stroma decrease in number while in lumen of thylakoid there is accumulation of protons.

 The gradient is broken down by movement of protons from lumen of thylakoid to stroma through transmembrane channel of F0 unit of ATPase.  

 ATPase enzyme consists of two parts:

• F0 unit is embedded in thylakoid membrane and forms transmembrane channel that carries out facilitated diffusion of protons across the membrane.

• F1 unit protrudes on the outer surface of thylakoid membrane towards stroma. F1 unit acts as ATP synthase enzyme which catalyzes conversion of ADP to ATP. Energy for this conversion is provided by breakdown of proton gradient across membrane.  

Note: Chemiosmosis requires- a membrane, proton pump, proton gradient and ATPase.

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