Electron Transport Chain — Revision Notes

⚡ 30-Second Revision

Non-cyclic ETC (Z-scheme): — PSII $ightarrow$ PQ $ightarrow$ Cyt b6f $ightarrow$ PC $ightarrow$ PSI $ightarrow$ Fd $ightarrow$ NADP+ Reductase.
Products (Non-cyclic): — ATP, NADPH, $O_2$.
Products (Cyclic): — ATP only.
Electron Source: — Water ($H_2O$) for non-cyclic.
Oxygen Evolution: — Only in non-cyclic (from $H_2O$ photolysis at PSII).
Proton Pumping: — By Cyt b6f complex (stroma to lumen).
ATP Synthesis: — Chemiosmosis via ATP synthase (lumen to stroma).
NADP+ Reduction: — By NADP+ reductase (stroma side).

2-Minute Revision

The Electron Transport Chain (ETC) in photosynthesis is the engine of light reactions, converting light energy into chemical energy (ATP and NADPH). It occurs in the thylakoid membranes. There are two main pathways: non-cyclic and cyclic.

Non-cyclic photophosphorylation (Z-scheme) involves both Photosystem II (PSII) and Photosystem I (PSI). Light excites electrons in PSII (P680), which are then passed to plastoquinone (PQ), cytochrome b6f complex, plastocyanin (PC), and finally to PSI (P700).

PSII replenishes its lost electrons by splitting water (photolysis), releasing $O_2$ and $H^+$ into the lumen. Electrons are re-excited in PSI, then transferred via ferredoxin (Fd) to NADP+ reductase, which reduces NADP+ to NADPH.

The passage of electrons through the cytochrome b6f complex pumps protons from the stroma into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthase to produce ATP (chemiosmosis).

Products: ATP, NADPH, $O_2$.

Cyclic photophosphorylation involves only PSI. Electrons from PSI are passed to Fd, then cycle back to the cytochrome b6f complex and PC, before returning to PSI. This pathway only pumps protons, thus generating only ATP. It does not produce NADPH or $O_2$. It's favored when more ATP is needed or NADP+ is scarce.

5-Minute Revision

The photosynthetic Electron Transport Chain (ETC) is the central machinery of the light-dependent reactions, converting solar energy into the chemical energy forms of ATP and NADPH. This process unfolds within the thylakoid membranes of chloroplasts.

Non-cyclic Electron Flow (Z-scheme): This is the predominant pathway. It begins when light energy is absorbed by Photosystem II (PSII), exciting electrons in its reaction center, P680. These high-energy electrons are transferred to a primary acceptor, leaving P680 oxidized (P680+).

To replace these lost electrons, water molecules are split (photolysis) at the Oxygen Evolving Complex associated with PSII ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$). The electrons replenish P680+, protons ($H^+$) are released into the thylakoid lumen, and oxygen is released.

The electrons then move sequentially through a series of carriers: first to plastoquinone (PQ), a mobile lipid-soluble carrier. PQ transfers electrons to the cytochrome b6f complex. As electrons pass through this complex, it actively pumps protons from the stroma into the thylakoid lumen, significantly contributing to the proton gradient. From the cytochrome b6f complex, electrons are carried by plastocyanin (PC), a water-soluble protein in the lumen, to Photosystem I (PSI).

At PSI, light energy re-excites the electrons in its reaction center, P700. These re-energized electrons are then transferred to ferredoxin (Fd), a small iron-sulfur protein. Finally, Fd donates the electrons to the enzyme NADP+ reductase, which catalyzes the reduction of NADP+ to NADPH ($NADP^+ + 2e^- + H^+ \rightarrow NADPH$). NADPH is a crucial reducing agent for the Calvin cycle.

ATP Synthesis (Chemiosmosis): The accumulation of protons in the thylakoid lumen creates a strong electrochemical gradient. Protons flow down this gradient, from the lumen back into the stroma, through the ATP synthase (CF0-CF1 complex). This proton flow drives the rotation of the CF0 unit, inducing conformational changes in the CF1 unit, which synthesizes ATP from ADP and inorganic phosphate (Pi). This light-driven ATP synthesis is called photophosphorylation.

Cyclic Electron Flow: Under certain conditions (e.g., high ATP demand, low NADP+), electrons from PSI are not passed to NADP+ reductase but are instead cycled back to the cytochrome b6f complex via ferredoxin. This cyclic flow continues to pump protons, generating additional ATP, but does not produce NADPH or oxygen.

Summary of Products: Non-cyclic produces ATP, NADPH, and $O_2$. Cyclic produces only ATP. Both ATP and NADPH are then utilized in the light-independent reactions (Calvin cycle) to fix carbon dioxide into sugars.

Prelims Revision Notes

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Location: — Light reactions (ETC) occur in the thylakoid membranes of chloroplasts.
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Photosystems: — Two types – Photosystem II (PSII, P680) and Photosystem I (PSI, P700).
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Non-cyclic Photophosphorylation (Z-scheme):

* Involves both PSII and PSI. * Electron Source: Water ($H_2O$) is split at PSII (photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$. * Electron Flow Sequence: PSII $ightarrow$ Plastoquinone (PQ) $ightarrow$ Cytochrome b6f complex $ightarrow$ Plastocyanin (PC) $ightarrow$ PSI $ightarrow$ Ferredoxin (Fd) $ightarrow$ NADP+ Reductase.

* Proton Pumping: Cytochrome b6f complex pumps $H^+$ from stroma to lumen. * Products: ATP, NADPH, and $O_2$. * ATP Synthesis: Via chemiosmosis, $H^+$ flow through ATP synthase (CF0-CF1) from lumen to stroma.

* NADPH Synthesis: NADP+ Reductase reduces NADP+ to NADPH on the stromal side.

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Cyclic Photophosphorylation:

* Involves only PSI. * Electron Flow: PSI $ightarrow$ Fd $ightarrow$ Cytochrome b6f complex $ightarrow$ PC $ightarrow$ PSI (electrons cycle back). * Products: Only ATP. * **No $O_2$ evolution, no NADPH production.** * Occurs when ATP is needed in excess of NADPH, or when NADP+ is unavailable.

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Proton Gradient Formation:

* Photolysis of water releases $H^+$ into lumen. * PQ/Cytochrome b6f complex pumps $H^+$ into lumen. * Consumption of $H^+$ from stroma during NADPH formation.

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Key Components & Functions:

* PSII (P680): Light absorption, water splitting, electron donation. * PQ: Mobile electron/proton carrier from PSII to Cyt b6f. * Cytochrome b6f: Electron transfer, major proton pump. * PC: Mobile electron carrier from Cyt b6f to PSI.

* PSI (P700): Light absorption, re-excitation of electrons, electron donation to Fd. * Fd: Electron carrier from PSI to NADP+ Reductase. * NADP+ Reductase: Enzyme reducing NADP+ to NADPH.

* ATP Synthase: Uses proton gradient to synthesize ATP.

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Inhibitors: — DCMU inhibits electron flow from PSII to PQ.

Vyyuha Quick Recall

To remember the sequence of electron carriers in the non-cyclic ETC (Z-scheme):

Please Quietly Carry Protons For Nice ATP

PSII
Quietly (Plastoquinone - PQ)
Carry (Cytochrome b6f Complex)
Protons (Plastocyanin - PC)
For (PSI - First Photosystem)
Nice (Ferredoxin - Fd, then NADP+ Reductase)
ATP (Resulting in ATP and NADPH)