Process:

1. PSII:

   • Light excites electrons in chlorophyll
   • Water splitting: 2H₂O → 4H⁺ + O₂ + 4e⁻
   • O₂ released as byproduct
   • Electrons replace those lost from chlorophyll

2. ETC between PSII and PSI:

   • Electrons move through chain
   • Energy pumps H⁺ into thylakoid space
   • Creates gradient

3. PSI:

   • Light re-excites electrons
   • Electrons transferred to NADP⁺
   • Forms NADPH

4. Chemiosmosis:

   • H⁺ flows through ATP synthase
   • Produces ATP (photophosphorylation)

Products:

• ATP (energy)
• NADPH (reducing power)
• O₂ (byproduct from water)

Light-Independent Reactions (Calvin Cycle)

Location: Stroma

Three phases:

1. Carbon Fixation

• CO₂ combines with RuBP (5C)
• Catalyzed by RuBisCO enzyme
• Forms 2 molecules of 3-PGA (3C each)

2. Reduction

• 3-PGA reduced to G3P (glyceraldehyde-3-phosphate)
• Uses ATP and NADPH from light reactions
• Some G3P exits to make glucose

3. Regeneration

• Remaining G3P regenerates RuBP
• Uses ATP
• Cycle continues

For one G3P (½ glucose):

• 3 CO₂ fixed
• 9 ATP used
• 6 NADPH used

For one glucose:

• 6 CO₂
• 18 ATP
• 12 NADPH

C4 and CAM Plants

Problem: Hot, dry conditions cause stomata to close

• Less CO₂ available
• O₂ builds up
• Photorespiration: RuBisCO uses O₂ instead of CO₂ (wasteful)

C4 Plants

• Separate CO₂ fixation from Calvin cycle
• Mesophyll cells: fix CO₂ → 4C compound
• Bundle sheath cells: Calvin cycle occurs
• Concentrates CO₂ around RuBisCO
• Examples: corn, sugarcane

CAM Plants

• Temporal separation
• Night: open stomata, fix CO₂ → 4C compound
• Day: close stomata, use stored CO₂ for Calvin cycle
• Conserves water
• Examples: cacti, pineapple

Key Concepts

1. Light reactions: convert light → ATP and NADPH
2. Water is split: source of O₂
3. Chemiosmosis: H⁺ gradient drives ATP synthesis
4. Calvin cycle: uses ATP/NADPH to fix CO₂ → glucose
5. RuBisCO: enzyme that fixes CO₂
6. C4 and CAM: adaptations to reduce photorespiration
7. Photosynthesis is reverse of cellular respiration

Two stages: Light reactions + Calvin cycle

(a) Locations:

Light-Dependent Reactions:

• Location: Thylakoid membrane (and lumen)
• Requires: Light
• Part of chloroplast: grana (stacked thylakoids)

Calvin Cycle (Light-Independent Reactions):

• Location: Stroma (fluid-filled space around thylakoids)
• Doesn't require: Direct light (but needs ATP/NADPH from light reactions)
• Also called "dark reactions" (misleading - actually occur during day)

(b) Inputs and Outputs:

Light-Dependent Reactions:

Inputs:

• Light energy (photons)
• H₂O (12 molecules)
• ADP + Pi
• NADP⁺

Outputs:

• O₂ (6 molecules) - from water splitting
• ATP (18 molecules)
• NADPH (12 molecules)
• H⁺ gradient (powers ATP synthesis)

Key reactions:

1. Photosystem II: H₂O → O₂ + 4H⁺ + 4e⁻
2. Electron transport: Creates H⁺ gradient
3. ATP synthase: ADP + Pi → ATP
4. Photosystem I: NADP⁺ + H⁺ + 2e⁻ → NADPH

Calvin Cycle (1 turn makes ½ glucose):

Inputs:

• CO₂ (3 molecules per turn, 6 for glucose)
• ATP (9 molecules per turn, 18 for glucose)
• NADPH (6 molecules per turn, 12 for glucose)

Outputs:

• G3P (glyceraldehyde-3-phosphate) - 1 per turn, 2 for glucose
• ADP + Pi
• NADP⁺

Net for 1 glucose: $6\text{CO}_2 + 18\text{ATP} + 12\text{NADPH} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 18\text{ADP} + 18\text{P}_i + 12\text{NADP}^+$

(c) Role of ATP and NADPH:

In Light Reactions:

ATP:

• Produced by chemiosmosis
• H⁺ gradient (lumen → stroma) drives ATP synthase
• Provides energy currency

NADPH:

• Produced by Photosystem I
• Electron carrier (reducing power)
• Carries high-energy electrons

In Calvin Cycle:

ATP:

• Phase 2 (Reduction): Phosphorylates 3-PGA → 1,3-bisphosphoglycerate
• Phase 3 (Regeneration): Phosphorylates RuMP → RuBP
• Provides energy for endergonic reactions
• 3 ATP per CO₂ fixed

NADPH:

• Phase 2 (Reduction): Reduces 1,3-BPG → G3P
• Donates electrons (and H⁺)
• Reduces CO₂ to carbohydrate level
• 2 NADPH per CO₂ fixed

Ratio: 3 ATP : 2 NADPH (per CO₂)

(d) Why both are necessary:

Light Reactions provide energy:

• Cannot make glucose without energy input
• ATP provides chemical energy
• NADPH provides reducing power (electrons + H⁺)

Calvin Cycle fixes carbon:

• Light reactions produce O₂, not carbohydrates
• Calvin cycle incorporates CO₂ into organic molecules
• Builds glucose from CO₂

Interdependence:

Light Reactions  →  ATP + NADPH  →  Calvin Cycle
                                        ↓
                    ADP + NADP⁺  ←  G3P (glucose)

Cannot function independently:

1. Without light reactions:

   • No ATP or NADPH
   • Calvin cycle stops
   • No glucose produced

2. Without Calvin cycle:

   • ATP and NADPH accumulate
   • Feedback inhibition
   • Light reactions slow/stop
   • No regeneration of NADP⁺, ADP

Both needed for complete photosynthesis: $\boxed{\text{Light reactions: energy capture → Calvin cycle: carbon fixation}}$

Summary Table:

3 phases of Calvin Cycle:

1. Carbon fixation: CO₂ + RuBP → 2(3-PGA) (via RuBisCO)
2. Reduction: 3-PGA → G3P (uses ATP + NADPH)
3. Regeneration: G3P → RuBP (uses ATP)