Photosynthesis Overview:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

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)

Photosynthetic Adaptations:

(a) Photorespiration problem in C3 plants:

RuBisCO's dual function:

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) can catalyze two reactions:

1. Carboxylation (desired): $\text{RuBP} + \text{CO}_2 \xrightarrow{\text{RuBisCO}} 2 \text{ 3-PGA}$ → Enters Calvin cycle → Makes glucose

2. Oxygenation (wasteful): $\text{RuBP} + \text{O}_2 \xrightarrow{\text{RuBisCO}} \text{3-PGA} + \text{2-phosphoglycolate}$ → Photorespiration pathway → Wastes energy

Problem in hot, dry conditions:

• Stomata close to prevent water loss
• CO₂ levels drop inside leaf
• O₂ levels rise (from light reactions)
• RuBisCO binds O₂ instead of CO₂
• Photorespiration increases

Consequences:

• No sugar produced from photorespiration
• Consumes ATP and releases CO₂
• Reduces photosynthetic efficiency by up to 50%
• Evolution of C₄ and CAM as solutions

Photorespiration pathway:

1. 2-phosphoglycolate → peroxisome
2. Converted to glycine → mitochondria
3. 2 glycine → 1 serine + CO₂
4. Serine → 3-PGA (back to Calvin cycle)
5. Net: Wastes 1 CO₂, uses 1 ATP

(b) C4 Photosynthesis - Spatial separation:

Anatomy - Kranz anatomy:

Two cell types in concentric rings:

1. Mesophyll cells (outer):

• Exposed to air spaces
• High CO₂ concentration maintained
• Contains PEP carboxylase

2. Bundle-sheath cells (inner):

• Surround vascular tissue
• Site of Calvin cycle
• Contains RuBisCO
• Low O₂, high CO₂ environment

Biochemistry:

Step 1: CO₂ fixation in mesophyll cells

PEP carboxylase (not RuBisCO!) fixes CO₂:

$\text{PEP} + \text{CO}_2 \xrightarrow{\text{PEP carboxylase}} \text{Oxaloacetate (4C)}$

Key advantage:

• PEP carboxylase has NO oxygenase activity
• High affinity for CO₂ (works even at low [CO₂])
• Not inhibited by O₂

Oxaloacetate → Malate or Aspartate (4-carbon compounds)

Step 2: Transport to bundle-sheath cells

Malate diffuses through plasmodesmata

Step 3: CO₂ release in bundle-sheath

$\text{Malate} \xrightarrow{\text{decarboxylase}} \text{Pyruvate (3C)} + \text{CO}_2$

Step 4: Calvin cycle

Released CO₂ enters Calvin cycle:

• High [CO₂] around RuBisCO
• Low [O₂] (thick bundle-sheath walls)
• Photorespiration minimized

Step 5: Regeneration

Pyruvate returns to mesophyll → regenerates PEP

Energy cost:

• C₃: 18 ATP per glucose
• C₄: 30 ATP per glucose (extra 12 ATP for PEP regeneration)
• Worth it in hot/dry environments!

C4 plants:

• Corn (maize), sugarcane, sorghum
• Crabgrass, many tropical grasses
• ~3% of plant species, but ~25% of terrestrial photosynthesis!

(c) CAM Photosynthesis - Temporal separation:

Crassulacean Acid Metabolism (CAM):

Temporal separation instead of spatial:

• Open stomata at night (cooler, less water loss)
• Close stomata during day (hot, dry)

Night (stomata open):

Step 1: CO₂ uptake $\text{PEP} + \text{CO}_2 \xrightarrow{\text{PEP carboxylase}} \text{Oxaloacetate}$

Step 2: Store as malate $\text{Oxaloacetate} + \text{NADH} \rightarrow \text{Malate}$

Step 3: Accumulate in vacuole

• Malate stored as malic acid
• Vacuole becomes acidic (pH drops)
• Reaches high concentrations

Day (stomata closed):

Step 4: Release malate from vacuole

Step 5: Decarboxylation $\text{Malate} \rightarrow \text{Pyruvate} + \text{CO}_2$

Step 6: Calvin cycle

• Released CO₂ enters Calvin cycle
• Uses light energy from light reactions
• High [CO₂] minimizes photorespiration

Step 7: Regenerate PEP

• Pyruvate → PEP (for night)

Advantages:

• Stomata closed during day → minimal water loss
• CO₂ fixed at night when cooler
• Can survive extreme drought

Disadvantages:

• Slower growth rate
• Large vacuoles needed for malate storage
• Limited by vacuole capacity

CAM plants:

• Cacti, succulents (jade plant)
• Pineapple, agave
• Some orchids
• ~10% of plant species

Comparison:

$\boxed{\text{C4: spatial separation; CAM: temporal separation; Both minimize photorespiration}}$

Optimal conditions:

• C3: Cool, moist, normal light
• C4: Hot, sunny, moderate water
• CAM: Very hot, very dry, desert conditions