☀️ 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:

1. Light-dependent reactions (thylakoids)
2. 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:

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