Course 9.2Lesson 2 of 3

The Z scheme: non-cyclic and cyclic photophosphorylation

In non-cyclic photophosphorylation electrons go from water through PS II, the electron transport chain (plastoquinone, cytochrome b6-f, plastocyanin) and PS I to NADP+, in a Z shape. It gives ATP, NADPH and oxygen. In cyclic photophosphorylation only PS I works, and the electron returns to PS I. It gives ATP only.

about 25 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

The path of the electrons

In the last lesson the electrons came from water. Now let us follow them. Their journey is drawn as a graph of redox potential. The most negative levels, where an electron is easiest to give away, are at the top. The graph is called the Z scheme, because its zig-zag has the shape of a letter Z lying on its side.

A graph with no numbers, drawn as a zig-zag in the shape of the letter Z lying on its side. The vertical direction is redox potential, with the most negative (electron-rich) level at the top. At the left, a label says water, with a small note that water splits to give oxygen and four protons. An arrow carries electrons from water up to the reaction centre of photosystem II, P680. A yellow wavy arrow labelled light lifts the electron steeply up to a high level, where a box names the primary electron acceptor of photosystem II. From there the path runs downhill in a gentle slope through three boxes in order, plastoquinone (PQ), the cytochrome b6-f complex, and plastocyanin (PC), and arrives at the reaction centre of photosystem I, P700. A second yellow wavy arrow labelled light lifts the electron again to a higher level, where a box names the primary electron acceptor of photosystem I. The path then runs downhill through ferredoxin and ends at NADP+, which becomes NADPH. A dashed curved arrow runs from the photosystem I acceptor back to the cytochrome b6-f complex and on to P700, labelled cyclic flow. A note beside the downhill slope says that ATP is made here by chemiosmosis.

Non-cyclic photophosphorylation

Here is the whole run, one step at a time.

Light excites PS II. The reaction centre P680 absorbs red light of $680\ \text{nm}$ . Its electrons are lifted to a high energy level and handed to an electron acceptor.
The acceptor passes the electrons down a chain. The electron transport system has plastoquinone (PQ), the cytochrome b6-f complex and plastocyanin (PC). The electrons move downhill, and energy is set free.
The electrons arrive at PS I. They fill the gap in P700.
Light excites PS I. P700 absorbs light of $700\ \text{nm}$ . Electrons are lifted again, to a level with a greater redox potential than before. They are taken up by another acceptor.
The electrons go downhill again and reach NADP+. There they reduce it to NADPH, with a proton from the stroma.

Meanwhile, PS II is short of electrons. Water is split and supplies them. This is why the process gives off oxygen.

This path is called non-cyclic photophosphorylation, because the electrons do not come back. Phosphorylation means that ADP is turned into ATP. The ATP is made as the electrons go downhill, by the process that you will see in the next lesson. The run gives ATP, NADPH and oxygen.

The two uphill steps need light. The downhill steps do not. Every electron is lifted twice, once at each photosystem, so each electron needs two photons.

Cyclic photophosphorylation

Sometimes the plant has no use for more NADPH, or the light is only of long wavelength. Then a second path is used.

Only PS I works. PS II does not take part.
The electron lifted at PS I goes to its acceptor, but it does not go to NADP+. It is passed back down a chain, through the cytochrome b6-f complex, to PS I.
This is a cycle, so the process is called cyclic photophosphorylation.

The cycle gives ATP only. There is no NADPH, because the electron never reaches NADP+. There is no oxygen, because water is not split and PS II is idle.

Cyclic flow occurs in the stroma lamellae. They have PS I but they lack PS II and NADP reductase. It also takes place when only light of wavelength beyond 680 nm is available.

	Non-cyclic	Cyclic
Photosystems	PS II and PS I	PS I only
Electron source	Water	The electron returns to PS I
Products	ATP, NADPH, oxygen	ATP only
Place	Grana and stroma lamellae	Stroma lamellae

You should find that $12$ water molecules give $6$ oxygen molecules and $24$ electrons. The electrons make $12$ NADPH. Each electron needs two photons, so the run absorbs $48$ photons.

Try it in Python

The program counts the products of the light reactions in white light and in far-red light alone.

You should see that white light gives oxygen, NADPH, photons and ATP, while far-red light gives only ATP. With $20$ water molecules, you get $10$ oxygen molecules and $80$ photons.

Practise with new numbers

Common mistakes

Saying the cyclic path makes NADPH. It does not. The electron goes back to PS I.
Saying the cyclic path gives oxygen. It does not. PS II is not working.
Counting one photon for each electron in the non-cyclic path. Each electron is lifted twice, so it takes two.
Putting PS I first. The electrons leave water, pass PS II, then the chain, then PS I.
Thinking the Z scheme is about the shape of the thylakoid. It is the shape of the graph.

Remember these

Non-cyclic: water, PS II, acceptor, PQ, cytochrome b6-f, PC, PS I, acceptor, NADP+. It gives ATP, NADPH and oxygen.
Cyclic: PS I only, the electron returns to PS I. It gives ATP only.
Uphill steps need light. Downhill steps release energy.
Cyclic flow occurs in the stroma lamellae, and when only far-red light is available.