Photosynthesis

Light reactions and Calvin cycle in photosynthesis

☀️ Photosynthesis

Overview

Photosynthesis: Process converting light energy into chemical energy

Overall equation:

6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂

Location: Chloroplasts

Thylakoids: light reactions
Stroma: Calvin cycle

Two main stages:

Light-dependent reactions (thylakoids)
Light-independent reactions/Calvin cycle (stroma)

Light-Dependent Reactions

Location: Thylakoid membrane

Key components:

Photosystem II (PSII)
Photosystem I (PSI)
Electron transport chain
ATP synthase

Process:

PSII:
- Light excites electrons in chlorophyll
- Water splitting: 2H₂O → 4H⁺ + O₂ + 4e⁻
- O₂ released as byproduct
- Electrons replace those lost from chlorophyll
ETC between PSII and PSI:
- Electrons move through chain
- Energy pumps H⁺ into thylakoid space
- Creates gradient
PSI:
- Light re-excites electrons
- Electrons transferred to NADP⁺
- Forms NADPH
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

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

📚 Practice Problems

1Problem 1medium

❓ Question:

Compare the light-dependent reactions and Calvin cycle: (a) where each occurs, (b) inputs and outputs of each, (c) the role of ATP and NADPH, and (d) why both are necessary for photosynthesis.

💡 Show Solution

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:

Photosystem II: H₂O → O₂ + 4H⁺ + 4e⁻
Electron transport: Creates H⁺ gradient
ATP synthase: ADP + Pi → ATP
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:

Without light reactions:
- No ATP or NADPH
- Calvin cycle stops
- No glucose produced
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:

| Feature | Light Reactions | Calvin Cycle | |---------|----------------|--------------| | Location | Thylakoid membrane | Stroma | | Needs light? | Yes (directly) | No (indirectly) | | Input | H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH | | Output | O₂, ATP, NADPH | G3P (glucose) | | Purpose | Capture light energy | Fix carbon |

3 phases of Calvin Cycle:

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

2Problem 2medium

❓ Question:

Compare the light-dependent reactions and Calvin cycle: (a) where each occurs, (b) inputs and outputs of each, (c) the role of ATP and NADPH, and (d) why both are necessary for photosynthesis.

💡 Show Solution

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:

Photosystem II: H₂O → O₂ + 4H⁺ + 4e⁻
Electron transport: Creates H⁺ gradient
ATP synthase: ADP + Pi → ATP
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:

Without light reactions:
- No ATP or NADPH
- Calvin cycle stops
- No glucose produced
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:

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

3Problem 3medium

❓ Question:

Compare the light-dependent reactions and Calvin cycle: (a) where each occurs, (b) inputs and outputs of each, (c) the role of ATP and NADPH, and (d) why both are necessary for photosynthesis.

💡 Show Solution

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:

Photosystem II: H₂O → O₂ + 4H⁺ + 4e⁻
Electron transport: Creates H⁺ gradient
ATP synthase: ADP + Pi → ATP
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:

Without light reactions:
- No ATP or NADPH
- Calvin cycle stops
- No glucose produced
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:

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

4Problem 4hard

❓ Question:

Explain C4 and CAM photosynthesis as adaptations to hot, dry environments: (a) describe the problem with photorespiration in C3 plants, (b) explain how C4 plants avoid photorespiration (include anatomy and biochemistry), and (c) explain how CAM plants avoid water loss.

💡 Show Solution

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:

2-phosphoglycolate → peroxisome
Converted to glycine → mitochondria
2 glycine → 1 serine + CO₂
Serine → 3-PGA (back to Calvin cycle)
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:

| Feature | C3 | C4 | CAM | |---------|-------|-------|-------| | CO₂ fixation | RuBisCO | PEP carboxylase | PEP carboxylase | | Separation | None | Spatial (cells) | Temporal (day/night) | | Photorespiration | High | Low | Low | | Water use efficiency | Low | Medium | High | | Energy cost | 18 ATP | 30 ATP | 30 ATP | | Growth rate | Fast | Fast | Slow | | Examples | Rice, wheat, oak | Corn, sugarcane | Cactus, pineapple | | % of species | 85% | 3% | 10% |

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

Optimal conditions:

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

5Problem 5hard

❓ Question:

💡 Show Solution

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:

2-phosphoglycolate → peroxisome
Converted to glycine → mitochondria
2 glycine → 1 serine + CO₂
Serine → 3-PGA (back to Calvin cycle)
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

6Problem 6hard

❓ Question:

💡 Show Solution

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:

2-phosphoglycolate → peroxisome
Converted to glycine → mitochondria
2 glycine → 1 serine + CO₂
Serine → 3-PGA (back to Calvin cycle)
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

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