Photophosphorylation (HL) (HL IB Biology)

Types of photophosphorylation

• The photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in photosystem I or photosystem II or both
  • In cyclic photophosphorylation, only photosystem I is involved
  • In non-cyclic photophosphorylation, both photosystem I and photosystem II are involved

Cyclic photophosphorylation

• Cyclic photophosphorylation involves photosystem I (PSI) only
• Light is absorbed by photosystem I (located in the thylakoid membrane) and passed to the photosystem I primary pigment (P700)
• An electron in the primary pigment molecule (i.e. the chlorophyll molecule) is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known as photoactivation
• This excited electron is captured by an electron acceptor, transported via a chain of electron carriers known as an electron transport chain before being passed back to the chlorophyll molecule in photosystem I (hence: cyclic)
• As electrons pass through the electron transport chain they provide energy to transport protons (H⁺) from the stroma to the thylakoid lumen via a proton pump
• A build-up of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (P_i) by the process of chemiosmosis
• Chemiosmosis is the movement of chemicals (protons) down their concentration gradient, the energy released from this can be used by ATP synthase to synthesise ATP
• The ATP then passes to the light-independent reactions

Cyclic photophosphorylation diagram

Cyclic photophosphorylation in photosynthesis involves only photosystem I

Non-cyclic photophosphorylation

• Photophosphorylation is the term for the overall process of using light energy and the electron transport chain to generate ATP from ADP
• During photophosphorylation excited electrons (from Photosystem II) 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
• Thylakoid membranes contain the following structures:
  • Photosystem II
  • ATP synthase
  • A series of electron carriers
  • Photosystem I
• An electron acceptor carries a pair of excited electrons from Photosystem II to the start of a chain of electron carriers
• The electron carriers undergo a series of redox reactions as electrons are gained and lost from each carrier
• Excited electrons gradually release their energy as they pass through the electron carriers which is used to generate a proton gradient
• The excitation of the electrons falls and they are eventually picked up by the reaction centre in Photosystem I
• Finally the pair of electrons are used to reduce NADP (along with protons from the photolysis of water) which is then passed into the light-independent reaction
• The pathway of electrons is linear, photophosphorylation is referred to as non-cyclic photophosphorylation
• ATP and reduced NADP are the main products of photophosphorylation and are immediately passed to the light-independent reaction

Non-cyclic photophosphorylation diagram

Chemiosmosis

• During the light dependent stages of photosynthesis, ATP is synthesizes from ADP + P_i using energy released from the movement of H⁺ protons down an electrochemical gradient

Forming a proton gradient

• 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 intermembrane space (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 contributes to the proton gradient

Synthesis of ATP

• 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

Chemiosmosis diagram

Photophosphorylation and chemiosmosis in photosynthesis

• 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
    • NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺
  • Reduced NADP now carries a pair of electrons and can be passed into the light-independent reactions of photosynthesis

Diagram to show the reduction of NADP in the light dependent stage of photosynthesis

Reduction of NADP in Photosystem I