U3 - L2: Light-Dependent Reactions

1st Stage of photosynthesis

Light Dependent reactions

• Electron Transport Chain (in order)

  • PSII

  • Cytochrome complex

  • PSI

  • NADP+ reductase

Overall Reaction

6CO₂ + 6H₂O > C₆H₁₂O₆ + 6O₂

• The main reactions occur in the chloroplast

• The production of actual glucose occurs in the cytoplasm

• Thought of as the reverse of respiration

H₂O

• Used in light dependent reactions

• Enters through veins (xylem) and goes to mesophyll cells

O₂

• Made in light dependent reactions

• Made in the mesophyll cells and expelled as waste through the stomata

Reaction Centers

A special pair of chlorophyll molecules

• When it has received this energy (photon of light), an electron in the reaction centre becoems excited or raised to a higher energy level

• This electron then has enough energy to be passed on to an electron accepting molecule

Pigments and photosystems

Photosystems:

• Clusters of photosynthetic pigmets embedded in the thylakoid membrane

• Absorb photons of particular wavelengths

• This energy is captured in the light reactions to convert ADP to ATP and NADP⁺ to NADPH

• Chloroplasts and algae have 2 photosystems: PSI and PSII

  • The reaction center of PSII is chlorophyll a (P680)~ main pigment of PS

    • The molecule that loses excited electrons to start transport chain

  • The reaction center of PSI s chlorophyll b (P700)

Ultimate goal

Purpose

• Output: To produce Oxygen, ATP, NADPH

• Input: water

• Photons of light

3 General Parts of Light Reactions

• Photoexcitation & Photolysis

  • Absorption of light photons whose energy excites an electron that is trapped by an electron acceptor

  • Energy in light photons ae also used to split water releasing electrons
    

• Electron Transport

  • Harnessing the energy in electrons to form an electrochemical gradient (pump hydrogen ions against its concentration gradient)

• *Photophosphorylation

  • ATP synthesis due to electrochemical gradient and the proton motive force

Commencement of Photosynthesis (Light-dependent reactions)

1. Light hits chlorophyll in PSII

   • Photons energyize electrons in chlorophyll

   • These high-energy electrons are passed into electron transport chain

2. Water splitting (Photolysis)

   • PSII replaces its lst electrons by splitting water

     • 2H₂O > 4H⁺ + 4e^- + O₂

   • This is how/why plants produce oxygen

3. Electron Transport Chain (ETC)

   • Electrons move down the chain

     • Energy is used to pump protons into thylakoid lumen  (cytochrome complex)

     • Creates a proton gradient (high proton concentration inside, low outside)

4. PSI

   • Electrons are now low energy

     • Chlorophyll b (700nm) receives photons and excites electron in photosystem

5. NADP+ Reductase

   • Final destination of electrons

     • NADP+ + E- + H+ > NADPH

6. Photophosphorylation

   • The proton gradient created power ATP Synthase to produce ATP

     • Protons move from Lumen to Stroma

Names of Each Step

1. PSII

   • Receives energy through photons and electrons get excited and move down the ETC

2. Plastoquinone (PQ)

   • Intermediary for electrons to pass through

3. Cytochrome b6f complex

   • Pumps protons to lumen when receiving energy from electrons

4. Plastocyanin

   • Intermediary for electrons to pass through

5. PSI

   • Recharges electrons suing photons of light.

     • Received through chlorophyll b

6. Ferredoxin

   • Intermediary for electrons to pass through

7. NADP+ reductase

   • This is the final end point for electrons. they bind to NADP+

     • NADP+ + 1H+ + 2e- > NADPH

8. ATP Synthase

   •