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Photosynthesis in Higher Plants - Chemiosmotic hypothesis

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Chemiosmotic Hypothesis, proposed by Peter Mitchell, explains the mechanism of ATPATP synthesis in chloroplasts by the creation of a proton (H+H^+) gradient across the thylakoid membrane.

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Proton accumulation occurs in the thylakoid lumen due to three main processes: the splitting of water on the inner side of the membrane, the transport of protons from the stroma to the lumen by the cytochrome complex, and the consumption of protons in the stroma for NADP+NADP^+ reduction.

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The splitting of water reaction is represented as: 2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-. These protons are released into the lumen.

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The NADPNADP reductase enzyme is located on the stroma side of the membrane. Along with electrons from PS IPS \ I, protons are removed from the stroma to reduce NADP+NADP^+ to NADPH+H+NADPH + H^+.

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The resulting gradient (higher [H+][H^+] in lumen, lower [H+][H^+] in stroma) creates a proton motive force and a measurable pHpH decrease in the lumen.

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The ATPATP synthase enzyme consists of two parts: CF0CF_0, a transmembrane channel that allows facilitated diffusion of protons, and CF1CF_1, which protrudes into the stroma and catalyzes ATPATP synthesis.

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The breakdown of the gradient provides enough energy to cause a conformational change in the CF1CF_1 particle of the ATPATP synthase, making the enzyme synthesize ATPATP from ADPADP and inorganic phosphate (PiPi).

📐Formulae

2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-

NADP++2e−+2H+→NADP reductaseNADPH+H+NADP^+ + 2e^- + 2H^+ \xrightarrow{\text{NADP reductase}} NADPH + H^+

ADP+Pi→ATP synthaseATPADP + Pi \xrightarrow{\text{ATP synthase}} ATP

ΔpH=pHstroma−pHlumen\Delta pH = pH_{\text{stroma}} - pH_{\text{lumen}}

💡Examples

Problem 1:

During active photosynthesis, if the pHpH in the thylakoid lumen is found to be 55 and the pHpH in the stroma is 88, calculate the difference in proton concentration [H+][H^+] between the two compartments.

Solution:

The concentration of protons is given by [H+]=10−pH[H^+] = 10^{-pH}. For the lumen, [H+]lumen=10−5 M[H^+]_{\text{lumen}} = 10^{-5} \text{ M}. For the stroma, [H+]stroma=10−8 M[H^+]_{\text{stroma}} = 10^{-8} \text{ M}. The ratio is 10−510−8=103\frac{10^{-5}}{10^{-8}} = 10^3.

Explanation:

This demonstrates a 10001000-fold difference in proton concentration, which drives the diffusion of protons through the CF0CF_0 channel to synthesize ATPATP.

Problem 2:

Identify the number of protons (H+H^+) required to be transported through ATPATP synthase to produce one molecule of ATPATP according to modern estimates.

Solution:

Approximately 3H+3H^+ to 4H+4H^+ are required for the synthesis of 11 molecule of ATPATP.

Explanation:

The movement of protons down the electrochemical gradient through the CF0CF_0 channel provides the free energy required for the phosphorylation of ADPADP by the CF1CF_1 headpiece.

Chemiosmotic hypothesis Class 11 Notes & Examples