13.1.7 Photophosphorylation

• The thylakoid membrane is the site of the light-dependent stage of photosynthesis
• During the light-dependent stage of photosynthesis:
  • Light energy is used to breakdown water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
  • A proton gradient is formed due to the photolysis of water resulting in a high concentration of hydrogen ions in the thylakoid lumen
  • Electrons travel through an electron transport chain of proteins within the membrane
  • Reduced NADP (NADPH) is produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
  • ATP is produced during a process known as photophosphorylation (ADP + P_i → ATP) using the proton gradient between the thylakoid lumen and stroma to drive the enzyme ATP synthase

• 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

• Photosystems are collections of photosynthetic pigments that absorb light energy and transfer the energy onto electrons, each photosystem contains a primary pigment
  • Photosystem II has a primary pigment that absorbs light at a wavelength of 680nm and is therefore called P680
  • Photosystem II is at the beginning of the electron transport chain and is where the photolysis of water takes place
  • Photosystem I has a primary pigment that absorbs light at a wavelength of 700nm and is therefore called P700
  • Photosystem I is in the middle of the electron transport chain

• The energy carried by the ATP is then used during the light-independent reactions of photosynthesis

The light-dependent stage of photosynthesis takes place in the thylakoid membrane and thylakoid lumen

• 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 (ie. 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

• Non-cyclic photophosphorylation involves both photosystem I (PSI) and photosystem II (PSII)

Photosystem II

• Light is absorbed by photosystem II (located in the thylakoid membrane) and passed to the photosystem II primary pigment (P680)
• An electron in the primary pigment molecule (ie. 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 passed down a chain of electron carriers known as an electron transport chain, before being passed on to photosystem I
• During this process to ATP is synthesised from ADP and an inorganic phosphate group (P_i) by the process of chemiosmosis
• The ATP then passes to the light-independent reactions
• Photosystem II contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light:

H₂O → 2H⁺ + 2e^- + ½O₂

• As the excited electrons leave the primary pigment of photosystem II and are passed on to photosystem I, they are replaced by electrons from the photolysis of water

Photosystem I

• At the same time as photoactivation of electrons in photosystem II, electrons in photosystem I also undergo photoactivation
• The excited electrons from photosystem I also pass along an electron transport chain
• These electrons combine with hydrogen ions (produced by the photolysis of water) and the carrier molecule NADP to give reduced NADP:

2H⁺ + 2e^- + NADP → reduced NADP

• The reduced NADP (NADPH) then passes to the light-independent reactions to be used in the synthesis of carbohydrate

Non-cyclic photophosphorylation

• During photophosphorylation, energetic (excited) electrons are captured by an electron acceptor in a thylakoid membrane
• These excited electrons are then passed along a chain of electron carriers (known as the electron transport chain)
• The electron carriers are alternately reduced (as they gain an electron) and then oxidised (as they lose the electron by passing it to the next carrier)
• The excited electrons gradually release their energy as they pass through the electron transport chain
• The released energy is used to actively transport protons (H⁺ ions) across the thylakoid membrane, from the stroma (the fluid within chloroplasts) to the thylakoid lumen (the space within thylakoids)
• A ‘proton pump’ transports the protons across the thylakoid membrane, from the stroma to the thylakoid lumen
• This creates a proton gradient, with a high concentration of protons in the thylakoid lumen and a low concentration in the stroma
• Protons then return to the stroma (moving down the proton concentration gradient) by facilitated diffusion through transmembrane ATP synthase enzymes in a process known as chemiosmosis
• This process provides the energy needed to synthesise ATP by adding an inorganic phosphate group (P_i) to ADP (ADP + P_i → ATP)
• The whole process is known as photophosphorylation as light provides the initial energy source for ATP synthesis
• After being passed down the electron transport chain, the de-energised electrons from photosystem II are taken up by photosystem I

The process of photophosphorylation first involves the active transport of protons across the thylakoid membrane. These protons then move back across the thylakoid membrane via a process of facilitated diffusion known as chemiosmosis