Light Reactions

Antenna pigments serve as an energy funnel

In addition to the reaction center pigments of P680 and P700 chlorophyll a molecules, the transfer of energy from antenna pigments explain the rather flat curve shown in the action spectrum for photosynthesis. Below are the structures for some of the antenna pigments.

The absorption spectra of these pigments are shown below demonstrating how these pigments add to the range of wavelengths of light that are useful in photosynthesis...assuming that the energy absorbed by these antenna pigments can be transferred to P680 and P700.

Because blue-light is of higher energy than red light, and these pigments absorb light energy at shorter wavelengths than 680 or 700 nm, it makes sense that these pigments can transfer energy to the P680 and P700 versions of chlorophyll a. This transfer can be diagrammed as shown below; a similar diagram could have been sketched for PSI.

As you can see the light source is of very high energy, which is absorbed first at high-energy, short-wavelenths by antenna pigments. Carotenoid excitation is at the higher energy blue-green wavelengths. The energy absorbed by the carotenoid is transferred to another antenna pigment; in this case chlorophyll b which has an absorption peak in the somewhat lower energy yellow wavelengths. The energy absorbed by chlorophyll b is then transferred to chlorophyll a antenna pigments, which have an absorption peak in the yet lower energy red wavelengths. Ultimately in the example here, the energy is transferred to the reaction center chlorophyll a molecule, P680, that absorbs at 680 nanometers...a deep red color. It may seem counter-intuitive that the energy would be lowest at the reaction center...but remember that all aspects of life are affected by the 2nd law of thermodynamics. So it is no surprise that evolution has resulted in reaction center pigments that absorb in the lowest-energy end of the visible spectrum!

It is also no surprise, then, that energy is lost in each of these transfers thanks to the 2nd law of thermodynamics. This is partially shown here, where the drops in energy in each transfer are lost as heat and only the energy actually transferred to the reaction center pigment can be used to drive photosynthesis.

The antenna pigments have to be physically close to each other and to the reaction center chlorophyll a in order to achieve these energy transfers. An exploded view of the light-harvesting complex of photosystem II is shown below:

As you can see here, there are membrane proteins that bind to chlorophylls and carotenoids to form a light-harvesting complex. The LHCII absorbs energy most efficiently at 680 nm, other wavelengths release heat through the antenna transfers...but contribute to that broad action spectrum of wavelengths that can drive photosynthesis.

The electron transfer system is a multi-step process

A fuller view of the light reactions can be made, again diagrammed as an energy reaction. Perhaps this is the more-traditional view of the Z-scheme.

This view shows that the electron transfer system (ETS) is a group of interacting electron transfer carrier proteins and associated prosthetic groups. The ETS between the two photosystems consist of pheophytin, plastoquinone, cytochromes b and f, and plastocyanin. The ETS after PSI consists of special quinones, iron-sulfur proteins, ferredoxin, and a flavoprotein that reduces NADP+. That path is often called non-cyclic electron flow. Also shown here is a dashed line indicating that electrons can also pass from the PSI ETS back to the cytochromes in the PSII ETS. This path is called cyclic electron flow, and permits PSI to operate without PSII. Because the PSII ETS drives photophosphorylation, cyclic flow can produce an almost unlimited supply of ATP without generating NADPH. This permits a cell to produce a variable ratio of ATP to NADPH which has obvious advantages.

Again, at the far left of this diagram you can see that the source of electrons is photolysis (the Hill reaction) that splits water and releases oxygen gas to the atmosphere.

The ETS carriers are membrane protein complexes

The energy diagram (Z-scheme) above does not represent how the ETS is positioned physically inside a cell. To observe where the light reactions occur, we need to examine the membrane proteins more closely...

Here you can see that the electron carriers are membrane proteins, some integral and some peripheral or associated. The membrane that hosts them is the thylakoid membrane of a chloroplast. This membrane has two faces, one side faces the stroma of the chloroplast, the other side faces the lumen, an intermembrane space (the inside of the thylakoid sac). The membrane proteins are complexes of polypeptides and prosthetic groups.

The PSII light harvesting complex includes all proteins involved in photolysis, the antenna and reaction center pigments we saw above, the pigment binding proteins, and the first electron acceptors of the PSII ETS. Photolysis occurs on the lumen side of the thylakoid, leaving protons in the lumen.

Plastoquinone is a membrane-associated molecule that moves from one face of the membrane to the other. It shuttles protons from the stroma to the lumen and electrons from the light-harvesting PSII complex to the cytochrome complex.

The cytochrome complex is similar to the ETS of respiration. These are protein-associated cytochromes with heme-chelated iron atoms. The cytochromes pass the electrons in turn to plastocyanin.

In the lumen is plastocyanin, a water-soluble element of the light reactions. This protein is able to shuttle electrons from the cytochrome system to the light-harvesting complex of PSI thanks to a chelated copper (Cu) ion.

The light harvesting complex of PSI includes the antenna and reaction center pigments, the pigment-binding proteins, and the first electron acceptors of the PSI ETS. Light at 700 nm optimally excites the electrons from plastocyanin to high enough energy to pass them on to ferredoxin. On the stroma side of the thylakoid, are the associated ferredoxin and flavoproteins that carry out the NADP+ reductase reactions.

Another observation you should make here is that 4 protons are accumulated on the lumen side of the membrane for every oxygen molecule that is produced. Some of the light energy is used to split the water and shuttle the stroma protons. Thus the accumulation of protons in the lumen represents a conservation of energy.

These protons are leaked back through an ATP synthase membrane protein complex down the electrochemical gradient for protons. This downhill leakage releases energy, and that energy is trapped into a phosphate bond in the formation of ATP on the stroma side of the thylakoid membrane. This process is called photophosphorylation. You might notice how the structure of this ATP synthase resembles that of the H+-ATPase membrane protein that works in a reverse direction.

The light reactions occur in the chloroplast thylakoids

Again, the location of this electron transfer system is the thylakoid membrane of the chloroplast. Here is an electron micrograph of a chloroplast.

The boundary is a double membrane (two phosophlipid bilayers) that contains the stroma. From an endosymbiotic origin perspective, the outer membrane is the host vesicle, the inner membrane is the endosymbiont cell membrane, and the stroma is the endosymbiont cytosol. Within this cytosol is the histone-free circular DNA nucleoid, the endosymbiont genome, and 70S ribosomes for typical prokaryotic-style transcription and translation. The mesosomes of the endosymbiont are found here as thylakoid membranes. These thylakoids are the site of the chlorophyll, the electron transport system, and photophosphorylation.

Each of these membrane proteins is really a complex of proteins and associated molecules. What would be appropriate to do now, is to get a closer look at each of the steps in the light reactions from photolysis to NADP+ reductase.

Here is a diagrammatic view of the PSII light harvesting complex.

This view shows some of the many proteins in the PSII LHC. P680 marks the reaction center protein-chlorophyll complex, Pheo indicates pheophytin, the initial electron acceptor. The D1 and D2 proteins are indicated, as is an antenna complex. On the lumen face of the membrane you can see the manganese-chelating areas of the D1 protein which catalyze the photolysis reaction.

The photolysis reaction requires the energy of four photons

Four managanese atoms are held in chelation by the proteins of the PSII complex:

Because manganese has multiple valence states, one can imagine that its oxidizing potential is increased by increasing its charge. The relationship of these four manganese atoms, their chelation with lots of oxygen (remember those unshared electron pairs?), and the dissection of water molecule is shown above. The loss of electrons by the manganese ions ultimately pulls electrons from water to produce oxygen gas and protons.

Quinones serve as proton and electron shuttles

The quinones that accept the electrons from PSII are shown below. You can see how the molecule can receive electrons and protons. These reactions are not unidirectional so the quinone can pick up electrons and protons to become the hydroquinone on the stroma side, migrate to the thylakoid lumen side of the thylakoid and release the proton to the lumen and pass the electron to the cytochrome complex.

Heme in Cytochrome transfers electrons

The quinones pass the electrons to the cytochrome b6f complex and of course dump the protons on the lumen side of the thylakoid. The cytochromes are able to hold an electron briefly as a result of their structure:

What this diagram shows is a porphyrin ring system with conjugated double bonds, allowing for their blueish color as a pigment, and it shows them attached to membrane proteins. But of functional significance is the chelated iron (Fe) atom. As you probably know from chemistry, Fe ions can be either Fe+2 or Fe+3. Because an atom can be stable in either cationic state, the Fe atom allows the heme of cytochromes to briefly hold an electron and pass it on to another element of the ETS.

Plastocyanin shuttles electrons from cytochromes to PSI

Plastocyanin is a soluble protein found in the lumen of the thylakoid. This globular protein chelates a copper ion. Copper, just like iron, has two valence states, allowing it to accept and donate electrons. Plastocyanin accepts electrons from the cytochrome complex and transfers them to the PSI reaction center.

The PSI reaction center reduces NADP+

The PSI reaction center is somewhat similar in organization to the PSII reaction center and LHC. However, its reaction center pigment is a molecule of P700 chlorophyll a. Its peripheral proteins on the stroma side of the membrane are iron-sulfur types, flavoproteins, and include association with the soluble protein Ferredoxin.

The peripheral and soluble proteins of PSI shuttle electrons to NADP+, thereby reducing it to NADPH. The proton that joins this molecule comes from the supply leaking through the ATP synthase of photophosphorylation.

ATP synthase is driven by proton movement

The ATP synthase complex transports protons across the thylakoid from lumen to stroma. These protons flow down an electrochemical gradient and the energy released by that flow is trapped on the stroma side of the membrane by ATP synthesis.

ATP synthase and cyclic electron flow

Let's not forget that non-cyclic flow will be able to generate only a fixed ratio of NADPH to ATP as product. It is no surprise that this ratio is not always ideal, so evolution must have provided a way to work around this inflexibility. The workaround is cyclic electron flow. Electrons can flow from PS1 back to the cytochrome complex. This allows more quinones to pick up electrons and stroma protons, shuttling them to the thylakoid lumen, thus adding to the pool of protons available to drive photophosphorylation at the ATP synthase complex.

Two very interesting chemicals help us separate and study the photosystems, and cyclic and non-cyclic flow. These chemicals block the path of electrons in the ETS systems following each of the two photosystems.

Obviously in addition to helping us isolate and study the separate photosystems and electron flows, these chemicals have practical use as herbicides! With either one of them blocking one of the ETS pathways, the plant cannot survive.