The Light Dependent Reaction - Photosynthesis Explained

Photosynthesis, as you will have learnt about in GCSE Biology, is a chemical reaction carried out by plants through which they utilise light energy to produce their own food in the form of glucose.

Photosynthesis is a complex series of reactions and is represented by the overall equation:

Carbon dioxide + water → oxygen + glucose

This reaction occurs inside chloroplasts and requires the photosynthetic pigment, chlorophyll, to absorb light energy.

In A-level Biology, we learn about the complex stages of the reaction, with the first stage being the light dependent reaction. This stage takes place inside chloroplasts in the thylakoids. 

The thylakoids are disc shaped structures surrounded by the thylakoid membranes, which collectively have a very large surface area. This increases the surface area over which this metabolic reaction can occur. A stack of thylakoid discs is called a granum. The thylakoids are surrounded by the stroma of the chloroplasts.

The Chloroplast Structure:

On the thylakoid membranes there are photosystems I and II which contain light absorbing (photosynthetic) pigments. 

At the centre of the funnel-shaped photosystems, in the primary pigment reaction centre, is the pigment chlorophyll A. There are also other pigments such as chlorophyll B and carotenoids, known as accessory pigments. Having a variety of pigments is an advantage to the plant as they can absorb a range of different wavelengths of light, utilising more of the available light, which helps to increase the rate of photosynthesis. Green light is mostly reflected by the pigments which is why leaves appear green. Red and blue light are preferentially absorbed by the pigments - the energy from this light is used to drive the light dependent reactions.

The Light Dependent Reaction explained

There are 4 main stages:

1. Photo-ionisation of chlorophyll

2. Photophosphorylation of ADP to make ATP

3. Photolysis of water

4. Reduction of NADP

Stage 1 - Photo-ionisation of chlorophyll:

A photon of light strikes a photosystem.

The light energy excites a pair of electrons inside chlorophyll.

These excited electrons leave the chlorophyll molecule and are taken up by an electron carrier on the thylakoid membrane. 

The chlorophyll molecule is oxidised and now has a positive charge. This is called photo-ionisation.

This is the first of a sequence of redox reactions.

Stage 2 - Photophosphorylation of ADP to make ATP:

Next, the electrons are passed along a series of electron carriers, in a stage known as the electron-transport chain.

The electron carriers are all proteins and can be called electron carriers or electron acceptors.

Non-cyclic photophosphorylation involves both photosystem I and photosystem II. The light energy excites electrons in photosystem II and they pass along the electron carriers before being passed to photosystem I and continuing down the electron transport chain.

Cyclic photophosphorylation only involves photosystem I as light energy excites electrons in photosystem I before they continue along the electron transport chain.

Energy that is released as the electrons pass down the electron transport chain is used to pump protons (H+) across the thylakoid membrane and into the thylakoid lumen.

As protons accumulate in the thylakoid lumen, a proton gradient is created.

Protons will then diffuse down their concentration gradient through protein channels associated with ATP synthase enzymes back into the stroma.

As they diffuse through the channels, the ATP synthase enzymes join ADP and Pi to produce ATP

ADP + Pi → ATP

This process is known as chemiosmosis - the movement of protons down a proton gradient to drive the formation of ATP.

As the initial energy was derived from light, the formation of ATP in this way is called photophosphorylation.

Stage 3 - Photolysis of water:

When light struck chlorophyll, 2 electrons were lost.

These need to be replaced.

In non-cyclic photophosphorylation they are replaced using water in the process of photolysis.

Light energy splits water into protons, electrons and oxygen. Oxygen is a waste product and will diffuse out of the leaf (some oxygen may be used in respiration).

The electrons are used to replace the electrons lost by chlorophyll in photosystem II.

The protons are used to help create the proton gradient as they build up inside the thylakoids.

In cyclic photophosphorylation the electrons that left photosystem I pass back to photosystem I at the end of the electron transport chain to replace the electrons that were lost. No photolysis of water occurs.

Stage 4- Reduction of NADP:

When the electrons reach the end of the electron transport chain in non-cyclic phosphorylation they are passed to NADP (a co-enzyme)

NADP also accepts protons that have diffused through the ATP synthase enzymes.

When NADP accepts electrons and protons it is reduced. Forming NADPH or reduced NADP.

Therefore, at the end of the light dependent stage the important products are ATP and NADPH. These products will diffuse into the stroma of the chloroplasts where they can be used in the light independent reaction.

As the thylakoid membranes have a large surface area there are many electron transport chains and many ATP synthase enzymes, this increases the rate of the light dependent reaction and enables the plant to make more ATP and NADPH.

 

Now you know the light dependent reaction that takes place in photosynthesis! Make sure to think of the light dependent reaction in 4 chunks, this will make it much easier to remember:

1. Photo-ionisation of chlorophyll

2. Photophosphorylation of ADP to make ATP

3. Photolysis of water

4. Reduction of NADP