Photosynthesis

🌞 Overview

Photosynthesis is a two-stage process in which cells convert light energy into chemical energy, ultimately synthesizing organic molecules (e.g., glucose).

• Stage 1: Light-dependent reactions (in thylakoid membranes)

• Stage 2: Light-independent reactions / Calvin Cycle (in the stroma)

🌿 Stage 1: Light-Dependent Reactions (The Light Reactions)

📍 Where It Happens:

• Location: Thylakoid membranes of the chloroplast

⚡ Purpose:

• Convert light energy into ATP and NADPH

• Split water molecules (H₂O) to release O₂ and protons

• Build a proton gradient to drive ATP synthesis

🔋 Key Processes of the Light Reactions

1. Photon Absorption & Electron Excitation

• Chlorophyll a, the principal photoreceptor in green plants, absorbs light.

• Light excites electrons in pigment molecules to higher energy states.

• Excited electrons are transferred to nearby electron acceptors.

2. Photosystem II (PSII)

• Reaction center: P680

• Function: Splits water (photolysis), releasing:

  • Electrons (to replace those lost by chlorophyll)

  • Protons (into the thylakoid lumen)

  • Oxygen (as a byproduct)

• Initiates electron transport chain to Photosystem I

3. Electron Transport & Proton Pumping

• Electrons travel through the cytochrome b₆f complex.

• Protons are pumped from the stroma into the thylakoid lumen.

• This creates a proton gradient (high H⁺ in lumen, low in stroma).

4. Photosystem I (PSI)

• Reaction center: P700

• Absorbs another photon of light → excites electrons again.

• Electrons are passed to ferredoxin, then to ferredoxin-NADP⁺ reductase, reducing NADP⁺ to NADPH.

5. ATP Synthesis

• ATP synthase uses the proton gradient to generate ATP (photophosphorylation).

• ATP and NADPH are both produced on the stromal side of the membrane.

🔁 Optional: Cyclic Photophosphorylation

• If NADPH is abundant, PSI can cycle electrons back to cytochrome bf.

• Produces ATP without making NADPH or O₂.

• Useful when more ATP is needed relative to NADPH.

🎨 Accessory Pigments & Protection

• Chlorophyll b, carotenoids (e.g., β-carotene) help capture light and funnel energy to reaction centers.

• Also serve a protective role, using non-photochemical quenching (NPQ) to safely dissipate excess light energy as heat.

• Prevent formation of reactive oxygen species (ROS), which can damage cells.

☠ Herbicides and Inhibition

• Diuron, atrazine: block Photosystem II

• Paraquat: disrupts Photosystem I, causes ROS buildup

🧬 Evolution and Endosymbiosis

• Chloroplasts originated from cyanobacteria via endosymbiosis.

• Chloroplasts contain DNA but depend on nuclear-encoded proteins.

• Photosynthesis evolved ~2 billion years ago in bacteria (no archaea perform it).

🌱 Stage 2: Light-Independent Reactions (Calvin Cycle)

📍 Where It Happens:

• Location: Stroma of the chloroplast

🎯 Purpose:

• Use ATP and NADPH from the light reactions to convert CO₂ into glucose

🔄 The Calvin Cycle: 3 Stages

1. Carbon Fixation

• Enzyme RuBisCO attaches CO₂ to ribulose-1,5-bisphosphate (RuBP)

• Produces two molecules of 3-phosphoglycerate (3-PGA)

2. Reduction

• ATP and NADPH reduce 3-PGA to glyceraldehyde-3-phosphate (G3P)

• G3P is the precursor to glucose and other carbohydrates

3. Regeneration

• Some G3P is used to regenerate RuBP, allowing the cycle to continue

🧬 Structural Components of Photosynthesis

🌳 Chloroplast Structure:

• Double membrane organelle

• Stroma: Site of Calvin Cycle

• Thylakoid membranes: Site of light reactions

  • Contain photosystems, ATP synthase, electron transport chains

🔌 Electron Flow Summary (Linear Pathway)

H₂O → PSII → Cyt b₆f → PSI → Ferredoxin → NADP⁺ → NADPH

This flow:

• Generates O₂

• Builds proton gradient → drives ATP synthesis

• Reduces NADP⁺ to NADPH

🔁 Photosynthesis vs. Cellular Respiration