Light harvesting and the two photosystems

A leaf is a solar panel

A solar panel has many small cells that catch sunlight, and wires that carry the energy away. A thylakoid membrane works in a similar way. It holds many pigment molecules that catch light, and chains of carriers that pass the energy on as moving electrons.

The light reactions are the steps that need light. They include:

• the absorption of light by pigments,
• the splitting of water and the release of oxygen,
• the formation of two energy carriers, ATP and NADPH.

Everything happens in the thylakoid membranes. They hold the two photosystems, a carrier called the cytochrome b6-f complex, and the enzyme ATP synthase.

Two photosystems

Pigments in the thylakoid are not scattered. They sit in groups called photosystems. There are two, PS I and PS II. They are named in the order in which they were discovered. In the light reactions, PS II works first and PS I second.

Each photosystem is built in two parts.

• The antenna, also called the light harvesting complex (LHC), is made of hundreds of pigment molecules bound to proteins. These are chlorophyll a, chlorophyll b and the accessory pigments. They absorb light of different wavelengths, so a wider range of light is used.
• The reaction centre is a single molecule of chlorophyll a. The antenna passes the energy it absorbs to this one molecule.

The reaction centres of the two photosystems differ.

• Photosystem II has the reaction centre P680. It absorbs best at $680\ \text{nm}$.
• Photosystem I has the reaction centre P700. It absorbs best at $700\ \text{nm}$.

The letter P stands for pigment, and the number is the wavelength of the best absorption.

Look at where they lie. The grana membranes carry both photosystems. The stroma lamellae carry PS I, but they lack PS II and the enzyme NADP reductase. This will matter when you reach cyclic flow.

Water refills photosystem II

When light excites the reaction centre of PS II, it gives up an electron. It is now short of an electron, and it will not work again until the electron is replaced. The electron comes from water. Water is split in the lumen of the thylakoid, by a complex that is attached to PS II.

From two water molecules you get four electrons, four protons and one oxygen molecule. The electrons refill PS II. The protons collect in the lumen, and they will be put to use in the third lesson. The oxygen leaves the leaf. This is the oxygen that you breathe.

You should find that light of $650\ \text{nm}$ can excite both photosystems. At $690\ \text{nm}$ only PS I can still be excited. At $720\ \text{nm}$ neither can. Light of a longer wavelength carries less energy in each photon.

Try it in Python

The program gives the energy of a photon and says which photosystems the light can excite.

You should see that PS II stops being excited above $680\ \text{nm}$, and PS I above $700\ \text{nm}$. Light of $660\ \text{nm}$ excites both. The photon energies run from about $2.76\ \text{eV}$ at $450\ \text{nm}$ down to about $1.72\ \text{eV}$ at $720\ \text{nm}$.

Practise with new numbers

Common mistakes

• Thinking PS I works first. It was discovered first. In the light reactions PS II works first.
• Mixing up P680 and P700. PS II is P680. PS I is P700.
• Saying that every pigment is a reaction centre. Only one chlorophyll a molecule in each photosystem is.
• Forgetting where the electron for PS II comes from. It comes from water.
• Giving the oxygen to carbon dioxide. The oxygen released comes from the splitting of water.

Remember these

• Two photosystems, each with an antenna of hundreds of pigments and one chlorophyll a reaction centre.
• PS II is P680. PS I is P700.
• Water is split at PS II: $2\,\text{H}_2\text{O} \to 4\,\text{H}^+ + \text{O}_2 + 4\,e^-$.
• Stroma lamellae have PS I only, with no PS II and no NADP reductase.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the antenna of a photosystem, and what does it do?

2. Name the reaction centres of the two photosystems.

3. Where does the electron that photosystem II loses come from?

4. Write what happens to two water molecules in the light reactions.

5. Which photosystem can still be excited by light of wavelength $690\ \text{nm}$?