Light-Dependent Reaction

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Photoionisation of Chlorophyll

The first step of the light-dependent reaction is the absorption of light energy by chlorophyll. This causes photoionisation, which is the release of electrons.

Light absorption

Light energy is absorbed by chlorophyll in the photosystems. This is the first step in photosynthesis.
Absorbing light energy excites the electrons within the photosystem.
The electrons are now in a more excited state. This causes them to be released from the chlorophyll.
Chlorophyll is now considered to be photoionised.

Energy from photoionisation

The photoionisation of chlorophyll also results in a release of energy.
This energy is used in photosynthesis to drive three reactions:
- Photophosphorylation - production of ATP from ADP and inorganic phosphate.
- Reduction - production of reduced NADP from NADP.
- Photolysis - splitting of water into protons, electrons and oxygen.
These three reactions are necessary for photosynthesis to take place.

Production of ATP and Reduced NADP

During the light-dependent reaction, ATP and reduced NADP are produced. In this process electrons are transferred down the electron transfer chain and protons pass across the membranes of chloroplasts.

Electron transport chain

Light energy excites electrons in chlorophyll and the electrons are moved to a higher energy level (they are high-energy electrons).
High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
Electron carriers are proteins located in the thylakoid membranes.
When high-energy electrons are released they are transferred along a chain of electron carriers.
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

When light energy is absorbed, high-energy electrons are released.
Some 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

Electrons can be replaced by photolysis.
In this process, light energy splits water into protons, electrons and oxygen.
The electrons can then replace those released when light is absorbed.

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.