8.3.2 Photophosphorylation

• Photophosphorylation is a process that uses light energy and the electron transport chain to generate ATP from ADP
  • Photo = light
  • Phosphorylation = addition of phosphate to ADP
• Photophosphorylation involves the following sequence of events:
  1. Light causes electrons in photosystem II to gain energy and become excited
     • The excitement of electrons can be referred to as photoactivation
  2. The excited electrons leave photosystem II and are passed down a series of electron carriers that form the electron transport chain
     • The electron transport chain occurs on the thylakoid membranes within the chloroplast
     • One of the electron carriers on the thylakoid membrane is known as plastoquinone; plastoquinone accepts a pair of electrons from photosystem II
  3. The electron carriers undergo a series of redox reactions as electrons are gained and lost from each carrier
     • Remember that:
       • Reduction = gain of electrons
       • Oxidation = loss of electrons
  4. As the electrons pass along the electron transport chain energy is released; this energy is used to pump protons across the thylakoid membrane, generating a proton gradient between the thylakoid space and the stroma (see below)
     • This proton gradient enables the phosphorylation of ADP to produce ATP during chemiosmosis (also below)
  5. The pair of electrons pass from the electron transport chain to photosystem I
  6. The electrons in photosystem I can also be excited by light energy, at which point they are used in the reduction of NADP (also covered in more detail below)

• The photophosphorylation process described here is referred to as non-cyclic as the electrons do not return to the location at which they started
• ATP and reduced NADP are the main products of photophosphorylation and are immediately passed to the light-independent reaction

• Electrons are passed from carrier to carrier in the electron transport chain
• As they do so they release energy which is used to pump protons from the stroma across the thylakoid membrane and into the thylakoid space
  • The thylakoid space is also known as the the thylakoid lumen
• The protons move via a proton pump
• A high concentration of protons builds inside the intermembrane space creating a concentration gradient
• Photolysis of water releases hydrogen ions which contribute to the proton gradient

• The proton gradient within the intermembrane space of the thylakoid powers the synthesis of ATP
  • The protons travel down their concentration gradient through the membrane protein ATP synthase
  • Energy is released by the movement of protons and is used to make ATP from the phosphorylation of ADP
• This process is called chemiosmosis
• The ATP produced is used in the light-independent reaction

Photophosphorylation and chemiosmosis

• Photosystem I is involved in the reduction of NADP which is a key molecule used in the light-independent reaction
  • Chlorophyll molecules in the reaction centre absorb photons of light energy
  • Electrons within the reaction centre are photoactivated to a higher energy level
  • They are passed to a protein on the outside of the thylakoid membrane called ferredoxin and reduce it
  • The reduced ferredoxin, along with protons that have passed through ATP synthase during chemiosmosis, are used to reduce NADP to NADPH (reduced NADP)
    • NADP + 2H⁺ + 2e⁻ → NADPH
  • The ferredoxin is now oxidised and free to be reused in this reaction again
  • Reduced NADP now carries a pair of electrons and can be passed into the light-independent reactions of photosynthesis

Reduction of NADP in Photosystem I