# Photosynthesis: A Comprehensive Overview

## Definition
Photosynthesis is the biological process by which photoautotrophic organisms convert light energy into chemical energy, storing it in the bonds of organic molecules (primarily glucose) while using carbon dioxide and water as raw materials.

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## Overall Equation

**6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂**

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## Where It Occurs

- **Primarily in plants, algae, and cyanobacteria**
- Within the **chloroplasts** of eukaryotic cells
- Specifically in the **thylakoid membranes** (light reactions) and **stroma** (Calvin cycle)

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## Key Pigments

- **Chlorophyll a** – primary pigment, absorbs red and blue light
- **Chlorophyll b** – accessory pigment, broadens absorption spectrum
- **Carotenoids** – absorb blue-green light, also provide photoprotection
- **Phycobilins** – found in cyanobacteria and red algae
- **Xanthophylls** – yellow pigments involved in photoprotection

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## The Two Main Stages

### 1. Light-Dependent Reactions (Thylakoid Membranes)

**Purpose:** Convert light energy into chemical energy (ATP and NADPH) and split water.

#### Components:
- **Photosystem II (PSII, P680)**
  - Absorbs light at 680 nm
  - Oxidizes water (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂
  - This is the source of oxygen released during photosynthesis
  - Electrons are excited and passed to the electron transport chain

- **Electron Transport Chain (ETC)**
  - Plastoquinone (Pq)
  - Cytochrome b6f complex
  - Plastocyanin (Pc)
  - Protons are pumped into the thylakoid lumen, creating a proton gradient

- **Photosystem I (PSI, P700)**
  - Absorbs light at 700 nm
  - Re-energizes electrons
  - Passes electrons through ferredoxin to NADP⁺ reductase

- **NADP⁺ Reductase**
  - Reduces NADP⁺ + H⁺ → NADPH

- **ATP Synthase (Chemiosmosis)**
  - The proton gradient drives H⁺ through ATP synthase
  - ADP + Pi → ATP (photophosphorylation)

#### Types of Electron Flow:
- **Non-cyclic electron flow** – involves both PSI and PSII; produces ATP, NADPH, and O₂
- **Cyclic electron flow** – involves only PSI; produces only ATP (electrons cycle back through cytochrome b6f); no NADPH or O₂ produced

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### 2. Light-Independent Reactions (Calvin Cycle / Dark Reactions / Carbon Fixation)

**Location:** Stroma of the chloroplast

**Purpose:** Use ATP and NADPH to fix CO₂ into organic molecules

#### Three Phases:

1. **Carbon Fixation**
   - CO₂ is attached to ribulose-1,5-bisphosphate (RuBP, a 5-carbon sugar)
   - Enzyme: **RuBisCO** (ribulose-1,5-bisphosphate carboxylase/oxygenase)
   - Produces two molecules of 3-phosphoglycerate (3-PGA, 3 carbons each)

2. **Reduction**
   - 3-PGA is phosphorylated by ATP and reduced by NADPH
   - Produces glyceraldehyde-3-phosphate (G3P)
   - For every 3 CO₂ fixed, 6 G3P are produced, but only 1 net G3P exits the cycle

3. **Regeneration of RuBP**
   - 5 of the 6 G3P molecules are rearranged using ATP to regenerate 3 RuBP
   - This allows the cycle to continue

#### Stoichiometry (per glucose):
- 6 turns of the cycle
- 18 ATP consumed
- 12 NADPH consumed

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## Photorespiration

- Occurs when **RuBisCO** fixes O₂ instead of CO₂ (oxygenase activity)
- Produces 2-phosphoglycolate, which must be recycled at an energy cost
- More prevalent at high temperatures, low CO₂, high O₂
- Reduces photosynthetic efficiency by ~25-50% in C3 plants

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## Alternative Carbon Fixation Pathways

### C3 Photosynthesis
- Most common (85% of plants: rice, wheat, soybeans)
- Initial fixation product is 3-PGA (3 carbons)
- Vulnerable to photorespiration

### C4 Photosynthesis
- Found in corn, sugarcane, many tropical grasses
- **Spatial separation** of initial fixation and Calvin cycle
- CO₂ is first fixed by **PEP carboxylase** in mesophyll cells → oxaloacetate (4 carbons)
- Converted to malate, transported to **bundle sheath cells**
- CO₂ is released and concentrated around RuBisCO, minimizing photorespiration
- More efficient at high temperatures and high light

### CAM Photosynthesis (Crassulacean Acid Metabolism)
- Found in cacti, succulents, pineapples, some orchids
- **Temporal separation** – stomata open at night, close during day
- CO₂ fixed at night by PEP carboxylase → stored as malic acid in vacuoles
- During the day, malate is decarboxylated, CO₂ enters Calvin cycle
- Minimizes water loss in arid environments

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## Chloroplast Structure

- **Outer membrane** – permeable to small molecules
- **Inner membrane** – selective permeability
- **Intermembrane space**
- **Stroma** – fluid-filled interior; contains Calvin cycle enzymes, chloroplast DNA, ribosomes
- **Thylakoids** – flattened membrane sacs
  - **Grana** (singular: granum) – stacks of thylakoids
  - **Stroma lamellae** – connect grana
- **Thylakoid lumen** – interior of thylakoids; site of proton accumulation

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## Factors Affecting Photosynthesis

- **Light intensity** – increases rate up to a saturation point
- **CO₂ concentration** – increases rate up to a saturation point
- **Temperature** – optimal range; too high denatures enzymes, too low slows reactions
- **Water availability** – deficit causes stomatal closure, limiting CO₂
- **Wavelength of light** – red and blue most effective; green least absorbed
- **Mineral nutrition** – Mg²⁺ (chlorophyll), Fe (ETC), Mn (water splitting)

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## Evolutionary History

- **Origin:** ~3.5 billion years ago in ancestral cyanobacteria
- **Anoxygenic photosynthesis** evolved first (using H₂S or other donors, not producing O₂)
- **Oxygenic photosynthesis** evolved in cyanobacteria, leading to the **Great Oxidation Event** (~2.4 billion years ago)
- **Endosymbiosis:** Chloroplasts originated from an ancient cyanobacterium engulfed by a eukaryotic ancestor (evidence: double membrane, own DNA, 70S ribosomes)
- Plants inherited photosynthesis through primary endosymbiosis
- Some algae (e.g., brown algae, dinoflagellates) acquired chloroplasts through secondary or tertiary endosymbiosis

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## Types of Photosynthesis

### Oxygenic Photosynthesis
- Uses water as electron donor
- Releases O₂
- Found in plants, algae, cyanobacteria

### Anoxygenic Photosynthesis
- Uses H₂S, Fe²⁺, H₂, or organic molecules as electron donor
- Does NOT release O₂
- Found in purple sulfur bacteria, green sulfur bacteria, heliobacteria
- Uses bacteriochlorophylls
- Typically has only one photosystem (Type I or Type II, but not both)

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## Importance of Photosynthesis

- **Produces oxygen** for aerobic life
- **Foundation of food chains** – provides organic carbon for nearly all ecosystems
- **Carbon sequestration** – removes CO₂ from atmosphere
- **Fossil fuels** are ancient photosynthetic products
- **Maintains atmospheric composition**
- Produces approximately **130 terawatts** of energy equivalent globally

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## Regulation and Adaptation

- **State transitions** – redistribution of light-harvesting complexes between PSI and PSII
- **Non-photochemical quenching (NPQ)** – excess energy dissipated as heat via xanthophyll cycle
- **Rubisco activase** – regulates RuBisCO activity
- **Light activation of Calvin cycle enzymes** via thioredoxin system
- **Stomatal regulation** – balances CO₂ uptake with water loss

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## Artificial Photosynthesis and Applications

- Research into **artificial leaves** and **solar fuel cells**
- Biomimetic systems to split water and produce hydrogen fuel
- Genetic engineering to improve crop photosynthetic efficiency (e.g., introducing C4 traits into C3 crops, engineering more efficient RuBisCO)
- **RIPE project** (Realizing Increased Photosynthetic Efficiency)

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## Key Scientists

- **Jan Ingenhousz** (1779) – discovered plants need light and produce oxygen
- **Julius von Sachs** (1864) – showed starch production in chloroplasts
- **T.W. Engelmann** (1882) – action spectrum experiments with algae
- **C.B. van Niel** (1931) – proposed water (not CO₂) is split
- **Robert Hill** (1937) – Hill reaction; showed O₂ comes from water
- **Melvin Calvin** (1950s) – mapped the Calvin cycle using ¹⁴C (Nobel Prize 1961)
- **Peter Mitchell** – chemiosmotic hypothesis (Nobel Prize 1978)
- **Rudolph Marcus** – electron transfer theory (Nobel Prize 1992)

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This covers the major aspects of photosynthesis, from molecular mechanisms to ecological significance and evolutionary history. Would you like me to go deeper into any particular aspect?
