Light-Dependent Reactions

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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.