Photosynthesis – Light Reactions & Calvin Cycle Vocabulary

Potential vs. Kinetic Energy in the Chloroplast

• Pumping protons (H⁺) into the thylakoid lumen stores potential energy.
  • Comparable to pumping water up a hill in a hydro-electric dam.
  • When protons flow back through ATP synthase, potential → kinetic energy (rotation of the rotor).
• Hydroelectric analogy: water potential drives turbines → electrical energy; proton potential drives ATP synthase → chemical energy (ATP).

Light-Dependent Reactions Overview

• Occur on the thylakoid membrane; include Photosystem II (PS II), cytochrome b6f complex, Photosystem I (PS I), ferredoxin, and ATP synthase.
• Primary outputs: \text{ATP} and \text{NADPH} (energy & reducing power for the Calvin cycle).
• Overall energy flow:
  1. Photons excite pigments.
  2. Electron transport chain (ETC) pumps H⁺ into lumen.
  3. Proton-motive force drives ATP synthase.
  4. Electrons eventually reduce \text{NADP}^+ \;\rightarrow\; \text{NADPH}.

Photosystem II (PS II)

• First complex hit by light (though discovered second).
• Only biological system able to oxidize water.
  • Splits \text{H}2\text{O} → 4\,e^- + 4\,H^+ + O2.
• Special reaction-center pair: P680 (absorbs 680\,\text{nm}).
• Primary electron acceptor: pheophytin → plastoquinone (PQ).
• PQ shuttles electrons to cytochrome b6f and simultaneously pumps H⁺.

Cytochrome b6f Complex & Plastoquinone (PQ)

• Acts like mitochondrial Complex III.
• PQ cycle translocates additional H⁺ from stroma to lumen (builds gradient).

Plastocyanin (PC)

• Soluble Cu-protein in lumen; transfers electrons from cytochrome b6f to PS I.
• Physical bridge linking PS II and PS I.

Photosystem I (PS I)

• Special pair P700 (absorbs 700\,\text{nm}).
• Photons re-energize incoming electrons (they lost energy traveling downhill from PS II).
• Electrons passed to ferredoxin (Fd) → NADP⁺ reductase (FNR) → \text{NADPH}.

Z-Scheme (Energy Diagram)

• Illustrates the two “ups” in redox potential: one at PS II, one at PS I.
• Older view treated systems as separate; modern “Z-scheme” shows simultaneous, continuous flow.

Cyclic vs. Non-Cyclic Electron Flow

• Non-cyclic (described above): net production of \text{ATP}, \text{NADPH}, O_2.
• Cyclic (PS I only): electrons cycle back to cytochrome b6f → extra \text{ATP}, no \text{NADPH}, no O_2; helps adjust ATP/NADPH ratio.

ATP Synthase & Proton Gradient

• ~3\,H^+ moving down gradient → rotation → phosphorylation \text{ADP} + P_i \;\rightarrow\; \text{ATP}.
• Same rotary mechanism as mitochondrial ATP synthase.

Why Make ATP & NADPH?

• Fuel the Calvin cycle (light-independent reactions).
• PS II → majority of ATP; PS I → all NADPH.

Calvin Cycle (C₃ Pathway)

• Location: chloroplast stroma.
• Discovered by Melvin Calvin; initial acceptor: ribulose-1,5-bisphosphate (RuBP) (5 C).
• Enzyme: RuBisCO (ribulose bisphosphate carboxylase/oxygenase).
  • Most abundant protein on Earth; slow, large, smells “plant-y”.
  • Catalyzes addition of CO2 or O2 (photorespiration).

Three Phases

1. Carbon Fixation
   • \text{RuBP} (5\,C) + CO_2 \xrightarrow[{RuBisCO}]{} 2\,\times\,3\text{PGA} (3\,C).
2. Reduction
   • Uses 6\,ATP + 6\,NADPH to convert 3\text{PGA} → glyceraldehyde-3-phosphate (G3P).
3. Regeneration
   • 5\,\text{G3P} + 3\,ATP → regenerate 3\,\text{RuBP}.
   • For every 1 G3P exported, 5 are recycled.

Net Stoichiometry (per G3P exported)

3\,CO2 + 9\,ATP + 6\,NADPH \;\rightarrow\; G3P + 9\,ADP + 8\,Pi + 6\,NADP^+

Fate of G3P

• 1 G3P (3 C) ⇒ combine two G3P → glucose/fructose (6 C).
• Glucose + fructose → sucrose; storage as starch in chloroplast.

Stomata, Gas Exchange & Water Loss

• Leaf surface coated by lipid cuticle to limit evaporation.
• Stoma (sing.) / Stomata (pl.): pore formed by two guard cells.
  • Open → CO2 in, O2 + water vapor out.
  • Close to conserve water or when CO_2 sufficient.
• Guard-cell dynamics regulate photosynthesis vs. transpiration.

Photorespiration

• RuBisCO oxygenase activity: RuBP + O_2 \rightarrow 3\text{PGA} + 2\text{C glycolate}.
• Consumes O2, releases previously fixed CO2; wastes ATP & NADPH.
• Hypothesized regulatory/photoprotective role under high light, low CO_2.

Regulation of Photosynthesis

• Light triggers synthesis/activation of photosynthetic proteins.
• High sucrose/glucose in cell → repress photosynthetic genes, induce storage pathways.
• RuBisCO activated by light-linked signals; inhibited when stromal CO_2 low.
• Balance of ATP/NADPH fine-tuned by switching between cyclic & non-cyclic flow.

Global Significance

• Carbon fixation in Calvin cycle supplies nearly all organic carbon for ecosystems.
• Every carbon atom in your body once passed through RuBisCO.

Transition to Cellular Respiration

• Plants: perform both photosynthesis (chloroplast) and cellular respiration (mitochondria).
• Animals & other heterotrophs: rely solely on respiration; glucose ultimately comes from plants (or organisms that ate plants).

Intro to Cellular Respiration (Preview)

• Overall equation (aerobic):
  C6H{12}O6 + 6\,O2 \;\rightarrow\; 6\,CO2 + 6\,H2O + \text{energy (ATP + heat)}
• Sequence of pathways:
  1. Glycolysis (cytosol) → 2 pyruvate, small ATP, NADH.
  2. Pyruvate oxidation (mitochondrial matrix).
  3. Citric Acid (TCA) Cycle → more NADH, FADH2, CO2.
  4. Electron Transport Chain & Oxidative Phosphorylation → large ATP yield via proton gradient (chemiosmosis).
• Analogy to automobile engine: glucose ≈ gasoline; O_2 as oxidant; ATP ≈ mechanical work; heat lost.
• Fermentation: backup ATP production without O_2; lower energy yield; end products retain potential energy (e.g., lactate, ethanol).

Hydroelectric Dam Analogy (Revisited)

• Upper reservoir water ≈ proton gradient.
• Turbine rotation ≈ ATP synthase.
• Pumping water uphill ≈ ETC-driven H⁺ pumping (respiration or photosynthesis).

Study & Exam Tips Mentioned by Instructor

• Redraw diagrams yourself (track carbons/electrons).
• Understand, don’t just memorize; focus on inputs/outputs of each phase.
• Expect quiz on energetics & photosynthesis; final exam soon after.

Key Equations & Numbers (LaTeX Style)

• Photosynthesis summary:
  6\,CO2 + 6\,H2O + \text{light} \;\rightarrow\; C6H{12}O6 + 6\,O2
• Cellular respiration summary:
  C6H{12}O6 + 6\,O2 \;\rightarrow\; 6\,CO2 + 6\,H2O + \text{ATP}
• Proton-to-ATP ratio (chloroplast): \sim 3\,H^+ / \text{ATP}.

Connections to Previous & Future Topics

• Chemiosmosis concept shared across chloroplasts and mitochondria.
• Redox coupling: exergonic electron flow drives endergonic ATP/NADPH synthesis.
• Calvin cycle parallels gluconeogenesis & glycogenesis in animals (anabolic sugar formation).
• Cellular respiration will mirror photosynthesis in reverse redox flow.

Ethical / Ecological Implications

• Plant carbon fixation mitigates atmospheric CO_2—critical for climate regulation.
• Blocking RuBP regeneration would halt global food webs (comic-villain thought experiment).
• Understanding photorespiration aids crop engineering for higher yields under climate stress.

Example & Metaphors Used by Lecturer

• Hydroelectric dam = thylakoid lumen gradient.
• Breaking Hershey bar into two = glycolysis splitting glucose.
• Money analogy: spend 1 G3P (dinner), bank 5 G3P (savings) to keep cycle running.
• NADPH = rechargeable battery; ATP = arcade token (spendable currency).

Numerical Highlights

• Calvin cycle cost per G3P: 9\,ATP + 6\,NADPH.
• Light reactions ATP/NADPH ratio adjustable via cyclic flow.
• Cells store only \approx 30!\text{–}!60\,\text{s} worth of ATP – must regenerate continuously.

Vocabulary Recap

• Potential vs. kinetic energy, chemiosmosis, proton-motive force, resonance energy transfer, redox, oxidized vs. reduced, electron carrier, photorespiration, cuticle, guard cells, stoma/stomata, Z-scheme, RuBP, RuBisCO, G3P, 3-PGA, ferredoxin, plastocyanin, plastoquinone, cytochrome b6f, ATP synthase, cyclic electron flow, non-cyclic electron flow, Calvin cycle phases, glycolysis, citric acid cycle, oxidative phosphorylation.