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Light-Dependent Reactions

Biology: Cambridge International A Level, topic 13.1.2

Non-Cyclic Photophosphorylation

There are two types of photophosphorylation that take place during the light-dependent reaction: cyclic and non-cyclic photophosphorylation. Here, we will look at non-cyclic photophosphorylation:

Light absorption by PSII

Light absorption by PSII

  • The light-dependent reactions occur in the thylakoid membranes of chloroplasts.
  • Embedded in the thylakoid membranes are two photosystems (PSI and PSII) which are complexes of pigments that capture light energy (light harvesting systems).
  • Non-cyclic photophosphorylation involves both PSII and PSI.
  • The process is initiated when light energy is absorbed by PSII.
  • Light energy excites electrons in the chlorophyll of PSII and the electrons are moved to a higher energy level (they are high-energy electrons).
  • This is photoionisation of chlorophyll.
Electron transport chain

Electron transport chain

  • High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
  • Electron carriers are proteins located in the thylakoid membranes. They are tightly linked to PSI and PSII and transfer electrons.
  • When high-energy electrons are released from PSII they are transferred along a chain of electron carriers to PSI.
  • The series of electron carriers is called the electron transport chain (ETC).
The proton gradient

The proton gradient

  • As the electrons move down the ETC, they lose energy.
  • This energy pumps protons from the stroma into the thylakoids. The protons are being transported against their concentration gradient and this requires energy.
  • As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane because the concentration of protons inside the thylakoids is greater than in the stroma.
Chemiosmosis

Chemiosmosis

  • The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme.
  • As protons diffuse through the ATP synthase, energy is released.
  • This energy converts ADP and inorganic phosphate to ATP.
  • This process is called chemiosmosis.
Reduced NADP

Reduced NADP

  • Non-cyclic photophosphorylation produces reduced NADP and ATP.
  • When light energy is absorbed by PSI, high-energy electrons are released.
  • The electrons are transferred directly to NADP. They are not passed along the ETC.
  • The electrons react with a proton in the stroma to produce reduced NADP.
Photolysis

Photolysis

  • An important feature of non-cyclic photophosphorylation is that the electrons are NOT recycled through the photosystems.
  • Instead, the electrons are replaced in PSII by photolysis.
  • In this process, light energy splits water into protons, electrons and oxygen.
  • The electrons can then replace those released when PSII absorbs light.

Cyclic Photophosphorylation

There are two types of photophosphorylation that take place during the light-dependent reaction: cyclic and non-cyclic photophosphorylation. Here, we will look at cyclic photophosphorylation.

Absorption of light by PSI

Absorption of light by PSI

  • Cyclic photophosphorylation only involves PSI.
  • The process is initiated when light energy is absorbed by PSI.
  • Light energy excites electrons in the chlorophyll of PSI and the electrons are moved to a higher energy level (they are high-energy electrons).
  • This is photoionisation of chlorophyll.
Electron transport chain

Electron transport chain

  • High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
  • The electrons are transferred along a chain of electron carriers in the electron transport chain (ETC).
  • Unlike in non-cyclic photophosphorylation, the electrons are not transferred from PSII to PSI.
  • Instead the electrons cycle continuously through the electron carriers to PSI.
The proton gradient

The proton gradient

  • As the electrons move down the ETC, they lose energy.
  • This energy pumps protons from the stroma into the thylakoids.
  • This is the same as in non-cyclic photophosphorylation.
  • As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane.
Chemiosmosis

Chemiosmosis

  • The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme.
  • As protons diffuse through the ATP synthase, energy is released.
  • This energy converts ADP and inorganic phosphate to ATP.
  • This process is called chemiosmosis.
Comparing cyclic and non-cyclic

Comparing cyclic and non-cyclic

  • In cyclic photophosphorylation:
  • ATP is produced.
  • No reduced NADP is produced.
  • Electrons are continuously recycled.
  • Photolysis does not take place.
  • In non-cyclic photophosphorylation:
  • ATP and reduced NADP are produced.
  • Electrons in PSII are replaced by photolysis.

Test yourself on Light-Dependent Reactions

  • What are the two types of photophosphorylation?

  • Stages in Non-Cyclic Photophosphorylation

  • Which photosystem transfers electrons directly to NADP instead of the electron transport chain?

  • Which of the following statements are true of non-cyclic photophosphorylation?

  • Which photosystem is only involved in cyclic photophosphorylation?

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