Untitled Flashcards Set

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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.