The Light-Dependent Stage (OCR A Level Biology)

• Photosynthesis occurs in two stages: the light-dependent stage, which takes place in the thylakoids, and the light-independent stage, which takes place in the stroma
• 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

Photophosphorylation & chemiosmosis

• Photophosphorylation is the name for the overall process of using light energy and the electron transport chain to phosphorylate ADP to ATP
  • The light-dependent reaction is sometimes called 'photophosphorylation'

• During photophosphorylation, energetic (excited) electrons are 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
  • The energy for this active transport comes from the excited electrons moving through the electron transport chain

• 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
• 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

Cyclic photophosphorylation

Cyclic photophosphorylation

Non-cyclic photophosphorylation

• Light is absorbed by photosystem 2 (located in the thylakoid membrane) and passed to the photosystem 2 primary pigment (P680)
• Two electrons in the primary pigment molecule (ie. the chlorophyll molecule) are excited to a higher energy level and are emitted from the chlorophyll molecule in a process known as photoionisation
• Each excited electron is passed down a chain of electron carriers known as an electron transport chain, before being passed on to photosystem 1
• During this process chemiosmosis occurs:
  • The energy given by the electrons moving through the electron transport chain enables H­­⁺ ions (protons) to pass from a low concentration in the stroma to a high concentration in the thylakoid lumen 
  • The creation of this proton gradient across the membrane later drives the synthesis of ATP in photophosphorylation

• Photosystem 2 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 2 and are passed on to photosystem 1, they are replaced by electrons from the photolysis of water
• At the same time as photoactivation of electrons in photosystem 2, electrons in photosystem 1 (PSI) also undergo photoionisation
• The excited electrons from photosystem 1 also pass along an electron transport chain, alternatively reducing and oxidising proteins as they are accepted then passed on
• These electrons combine with hydrogen ions (produced by the photolysis of water and transported out of the thylakoid lumen by ATP synthase) 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 carbohydrates
• The electrons lost by photosystem 1 are replaced by the de-energised electrons from photosystem 2

Light-dependent photophosphorylation leads to the production of ATP and NADP

Comparison of Cyclic Photophosphorylation & Non-cyclic Photophosphorylation Table