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Photosynthesis in Higher Plants - Cyclic and non-cyclic photophosphorylation

Grade 11CBSEBiology

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

🔑Concepts

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Photophosphorylation is the process of synthesizing energy-rich ATPATP molecules from ADPADP and inorganic phosphate (PiP_i) in the presence of light and chlorophyll.

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Non-cyclic photophosphorylation (Z-scheme) involves both Photosystem II (PS IIPS\,II or P680P_{680}) and Photosystem I (PS IPS\,I or P700P_{700}). It occurs in the grana lamellae.

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Photolysis of water occurs on the inner side of the thylakoid membrane associated with PS IIPS\,II, releasing electrons, protons, and oxygen: 2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-.

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In non-cyclic flow, the electrons lost by PS IIPS\,II are replaced by water, and the electrons lost by PS IPS\,I are used to reduce NADP+NADP^+ to NADPH+H+NADPH + H^+.

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Cyclic photophosphorylation occurs when only PS IPS\,I is functional (usually when light wavelength is >680 nm> 680\text{ nm}). It takes place in the stroma lamellae which lacks PS IIPS\,II and NADPNADP reductase enzyme.

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The Chemiosmotic Hypothesis states that ATPATP synthesis is driven by a proton gradient (accumulation of H+H^+ in the thylakoid lumen) and the action of F0−F1F_0-F_1 ATPATP synthase.

📐Formulae

2H2O→Light/Oxygen Evolving Complex4H++4e−+O22H_2O \xrightarrow{Light/Oxygen\,Evolving\,Complex} 4H^+ + 4e^- + O_2

ADP+Pi→Light energyATPADP + P_i \xrightarrow{Light\,energy} ATP

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

Net Non-cyclic: 2H2O+2NADP++3ADP+3Pi→O2+2NADPH+2H++3ATP\text{Net Non-cyclic: } 2H_2O + 2NADP^+ + 3ADP + 3P_i \rightarrow O_2 + 2NADPH + 2H^+ + 3ATP

💡Examples

Problem 1:

Identify the primary products of non-cyclic photophosphorylation and state where the protons (H+H^+) accumulate within the chloroplast during this process.

Solution:

The primary products are ATPATP, NADPHNADPH, and O2O_2. Protons (H+H^+) accumulate in the lumen of the thylakoid.

Explanation:

In non-cyclic electron transport, ATPATP and NADPHNADPH are formed for the dark reaction (Calvin cycle), and O2O_2 is evolved as a byproduct. Protons accumulate in the lumen due to the photolysis of water and the pumping of protons from the stroma via the cytochrome b6fb_6f complex.

Problem 2:

Why does the stroma lamellae perform only cyclic photophosphorylation?

Solution:

Because stroma lamellae membranes lack both PS IIPS\,II and the enzyme NADPNADP reductase.

Explanation:

Without PS IIPS\,II, water cannot be split to provide electrons, and without NADPNADP reductase, NADP+NADP^+ cannot be reduced to NADPHNADPH. Thus, electrons are cycled back to PS IPS\,I through the electron transport chain, resulting only in ATPATP synthesis.

Cyclic and non-cyclic photophosphorylation Revision - Class 11 Biology CBSE