Light Reactions — Revision Notes

⚡ 30-Second Revision

Location: — Thylakoid membranes of chloroplasts.
Inputs: — Light energy, $ext{H}_2\text{O}$, $ext{ADP}$, $ext{Pi}$, $ext{NADP}^+$.
Outputs (Non-cyclic): — $ext{ATP}$, $ext{NADPH}$, $ext{O}_2$.
Outputs (Cyclic): — $ext{ATP}$ only.
Photosystems: — PS-II (P680) and PS-I (P700).
Photolysis: — $ext{2H}_2\text{O} \rightarrow \text{4H}^+ + \text{4e}^- + \text{O}_2$ (occurs near PS-II).
Electron Carriers (Non-cyclic): — Pheophytin $ightarrow$ Plastoquinone (Pq) $ightarrow$ Cytochrome b6f complex $ightarrow$ Plastocyanin (Pc) $ightarrow$ Ferredoxin (Fd) $ightarrow$ NADP+ reductase.
ATP Synthesis: — Chemiosmosis via ATP synthase (CF0-CF1 complex).
Proton Gradient: — Higher $ext{H}^+$ in thylakoid lumen, lower in stroma.

2-Minute Revision

The light reactions are the initial, light-dependent phase of photosynthesis, occurring within the thylakoid membranes of chloroplasts. Their primary goal is to convert light energy into chemical energy in the form of ATP and NADPH.

This process involves two photosystems, PS-II (P680) and PS-I (P700), which capture light energy via chlorophyll and accessory pigments. In non-cyclic photophosphorylation (Z-scheme), electrons from PS-II are excited, passed through an electron transport chain (Pq, cyt b6f, Pc), re-excited by PS-I, and finally reduce NADP+ to NADPH.

Water is split (photolysis) near PS-II to replenish electrons, releasing oxygen and protons. The electron flow and water splitting create a proton gradient across the thylakoid membrane, driving ATP synthesis via chemiosmosis through the ATP synthase enzyme.

Cyclic photophosphorylation, involving only PS-I, generates only ATP and no NADPH or oxygen, typically occurring when more ATP is needed or NADP+ is scarce. Both ATP and NADPH are then used in the Calvin cycle.

5-Minute Revision

Light reactions, the photochemical phase of photosynthesis, are confined to the thylakoid membranes of chloroplasts. This phase harnesses light energy to produce ATP and NADPH, the energy currencies for the subsequent biosynthetic phase. The process begins with light absorption by photosynthetic pigments organized into Photosystem II (PS-II, P680 reaction center) and Photosystem I (PS-I, P700 reaction center).

In non-cyclic photophosphorylation (Z-scheme), light excites electrons in P680 of PS-II. These electrons are accepted by pheophytin, then passed through an electron transport chain (ETC) involving plastoquinone (Pq), cytochrome b6f complex, and plastocyanin (Pc).

During this transfer, the cytochrome b6f complex pumps protons from the stroma into the thylakoid lumen. To replace the electrons lost by P680, water molecules are split (photolysis: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2$), releasing electrons, protons into the lumen, and oxygen.

The electrons then reach PS-I, get re-energized by light in P700, and are transferred via ferredoxin (Fd) to NADP+ reductase, which reduces NADP+ to NADPH using stromal protons. This pathway produces ATP, NADPH, and oxygen.

ATP synthesis occurs via chemiosmosis. The accumulation of protons in the thylakoid lumen (from water splitting and proton pumping) creates a proton gradient. Protons flow back into the stroma through the ATP synthase (CF0-CF1 complex), driving the phosphorylation of ADP to ATP.

Cyclic photophosphorylation involves only PS-I. Excited electrons from P700 are passed to ferredoxin, then cycle back to the cytochrome b6f complex and plastocyanin, eventually returning to P700. This cycle generates only ATP (due to proton pumping by cytochrome b6f) but no NADPH and no oxygen. It is favored when the cell needs more ATP or when NADP+ is limited. Understanding the distinct inputs, outputs, and components of both pathways is critical for NEET.

Prelims Revision Notes

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Site: — Thylakoid membranes of chloroplasts.
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Photosynthetic Pigments:

* Chlorophyll a: Primary pigment, reaction center (P680 in PS-II, P700 in PS-I). * Accessory Pigments (Chlorophyll b, Carotenoids): Light harvesting, broaden absorption spectrum, photoprotection.

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Photosystems:

* Photosystem II (PS-II): P680 reaction center, absorbs 680 nm light. Involved in water splitting. * Photosystem I (PS-I): P700 reaction center, absorbs 700 nm light. Involved in NADPH formation.

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Non-Cyclic Photophosphorylation (Z-scheme):

* Involves: Both PS-II and PS-I. * Electron Source: Water (photolysis). * Electron Flow: Linear, from water $ightarrow$ PS-II $ightarrow$ ETC $ightarrow$ PS-I $ightarrow$ NADP+. * ETC Components: Pheophytin, Plastoquinone (Pq), Cytochrome b6f complex, Plastocyanin (Pc), Ferredoxin (Fd), NADP+ reductase.

* Products: ATP, NADPH, $ext{O}_2$. * Photolysis: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2$. Occurs on lumen side of thylakoid membrane.

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Cyclic Photophosphorylation:

* Involves: Only PS-I. * Electron Flow: Cyclic, electrons return to PS-I. * Products: Only ATP. * **No Photolysis, No $ext{O}_2$ release, No NADPH production.** * Favored: When more ATP is needed, or NADP+ is scarce.

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Chemiosmotic Hypothesis (ATP Synthesis):

* Proton Gradient: $ext{H}^+$ accumulates in thylakoid lumen (high concentration) compared to stroma (low concentration). * **Sources of Lumen $ext{H}^+$:** Water splitting, proton pumping by cytochrome b6f complex. * **Consumption of Stroma $ext{H}^+$:** NADP+ reduction. * ATP Synthase (CF0-CF1 complex): CF0 (transmembrane channel) allows $ext{H}^+$ flow; CF1 (stromal side) catalyzes $ext{ADP} + \text{Pi} \rightarrow \text{ATP}$.

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Final Products & Fate: — ATP and NADPH used in Calvin cycle (biosynthetic phase) in the stroma. $ext{O}_2$ released to atmosphere.

Vyyuha Quick Recall

To remember the electron carriers in the Z-scheme: People Prefer Cold Coffee Packed Freshly. (Pheophytin, Plastoquinone, Cytochrome b6f, Plastocyanin, PS-I, Ferredoxin)