Photosynthesis--The Photo Part

In this course, I make the assumption that you thoroughly understand the structure of a plant cell, the endosymbiont theory of organelle evolution, and the basic features of respiration and photosynthesis. Indeed your Cellular and Molecular Biology course should have covered these concepts in reasonable depth. In case this assumption is invalid for you, perhaps you should go over it again in your textbook or website for CMB.

The history of discovery and the photo-electronic-chemi-osmotic processes involved in the so-called "light reactions" is interesting, but I'll leave that to your CMB textbook as well. You should read it closely if your gleanings from prerequisite courses were weak on photosynthesis.

I think the parallelisms between respiration and photosynthesis are worthy of emphasis, hence this over-simplified diagram:

RESPIRATION:
sugar → glycolysis
cytosol
→ pyruvate → Krebs cycle
mito matrix
→ CO2
→ NADH →
→ ATP
ETS + Ox Phos
mito cristae
← O2
→ H2O
==> ATP!!
PHOTOSYNTHESIS:
sugar ← sucrose syn
cytosol
← triose ← Calvin cycle
chloro stroma
← CO2
← NADPH ←
← ATP ←
Light R + P Phos
chloro thylakoid
→ O2
← H2O
<== LIGHT!!

You should be able to reproduce these basic relationships between these two processes. This is a "forest" diagram...don't get too caught up in looking at the trees. You should be aware of the fundamental arrow-direction differences, and the three distinct phases shared by the two reaction pathways:

Here is an even simpler comparative diagram:

Respiration:
CH2O + O2 → CO2 + H2O + ATP
Photosynthesis:
CO2 + H2O + light → CH2O + O2

The two reaction systems are basically the reverse of each other. One undoes the other in plants. Since plants do not eat to supply their energy needs, the plant must do enough photosynthesis to meet its needs for maintenance, growth, and reproduction while compensating for losses due to respiration both night and day.

There are two important differences between the pathways outlined above...making them not quite the reverse of each other. One critical difference is the role of NADPH rather than NADH between the electronic and enzymatic portions. If these two very similar reducing agents were identical, it would be possible to use the Krebs Cycle to drive the Calvin Cycle. This would make the Calvin Cycle truly a "dark reaction," but NADH produced by the Krebs cycle has insufficient reducing power to drive the Calvin Cycle reactions. The phosphorylated NADPH does have this power, so the Calvin Cycle is dependent upon the light reactions of photosynthesis, and so occurs only in the light. The second critical difference between the pathways is an energy conversion solution. While the respiration pathway yields energy trapped in the form of a phosphate bond in ATP, photosynthesis requires the harnessing of energy in the form of light. The energy transduction step is a very important fundamental difference; this is where chlorophyll becomes a critical part of photosynthesis. It is this very special molecule that has the capacity to absorb light energy while hanging onto its electron pool long enough to drive an electron transport system. The heterocyclic ring system with conjugated double-bonds (an electron cloud) in association with a magnesium ion (another large electron cloud) provides a large volume in which electrons can be boosted for this energy-harnessing transduction to occur. Because of this critical feature of chlorophyll in photosynthesis, we really need to write our simplest form of photosynthesis as shown here:

Photosynthesis:

CO2 + H2O light
-------------->
chlorophyll
O2 + CH2O

The light reactions

In prerequisite courses you have studied the light reactions quite extensively. Here I would just like to hit the high spots. First, light has wave properties. the length of the wave determines the energy and color of the light. Human vision and photosynthesis cover the range of wavelengths from 400 to 700 nanometers. The high-energy end of this spectrum is the shorter wavelength purple light at 400 nm. The rest of the rainbow and decreasing energy follows as wavelengths increase to the lowest-energy red light at 700 nm. Light also has photon, particle, properties. Light intensity is measured as photon flux density: photons per square meter per second. Sunlight provides a plant with tremendous energy. Plants tap small amounts of energy in some very interesting biochemistry, releasing in each step some energy in the form of heat. So photosynthesis obeys the Second Law of Thermodynamics in every way.

Because short-wavelength light has higher energy, you might expect plants to use that part of the solar spectrum for photosynthesis. However, evolution would not favor that approach...much of the light energy from the sun would be lost to photosynthesis because most photons would lack sufficient energy to drive the reactions. So it turns out the the reaction center pigments, P680 and P700, are chlorophyll pigments absorbing light maximally at 680 and 700 nanometers...in the low-energy red-end of the visible spectrum. Of course if plants could only absorb 680 and 700 nanometer light, they would be missing a lot of light. So evolution has provided chlorophyll a with a range of other pigments, antenna pigments, that absorb higher-energy wavelengths and pass the absorbed energy on to chlorophyll a. Thus plants can utilize light from every wavelength in the visible spectrum from 400 to 700 nm. This is best observed by examining the action spectrum for a particular plant:

Here you can see that wavelength does change the rate of photosynthesis. You can see that all the wavelengths from 400-700 nm do drive photosynthesis. However, you do notice a drop in effectiveness at 540 nm, corresponding to green light. Obviously plants will be using less of this light and reflecting more of this color to our eyes. Plants look green. But green still drives photosynthesis! So obviously then, a plant must have more than green pigments that assist in photosynthesis. Other pigments must trap light...for example green light...and pass the energy on to chlorophyll. Also you might notice that wavelengths outside the range of the graph shorter than 400 nm might be able to drive photosynthesis! Indeed the near-ultraviolet wavelengths can be absorbed and the energy is sufficient to survive the transfer to chlorophyll P680 and P700. However, wavelengths longer than 700 nm appear to be unable to drive photosynthesis. The wavelength is too long, the energy content of the photons too low, to be able to excite the electrons of P680 or P700. Between 680 and 700 nm, the light can probably drive just one of the two photosystems and so efficiency drops rapidly in this range of wavelengths.

Now let us examine the rate of photosynthesis as a function of light intensity (photon flux density):

Here you can see that respiration rates (red line) do not change with photon flux density...it is not a light-driven pathway. You can also see that photosynthesis (green lines) cannot be driven in the dark (0 PFD) and increases with increasing light intensity. Because Respiration and photosynthesis basically undo each other, there is a place on this graph where the respiration and photosynthesis lines cross. The photon flux density at this particular point is called the compensation point because the two processes compensate for each other. At densities below this point, the plant is dying. At densities above this point, the plant can be growing and reproducing. At the compensation point, the plant is "breaking even" in terms of producing carbohydrate and using it.

You also observed there are two green lines representing photosynthesis in two different plants...a fern and corn. Obviously ferns are shade-tolerant and can grow and thive in darker conditions (compensation point is lower for ferns). They are also "scorched" by intense sunlight probably by oxygen free-radicals produced by excessive photolysis. But this brings up the interesting question of why our government spends research dollars on crop photosynthesis but does not fund much research on the really efficient photosynthesis of ferns or other shade-tolerant species. Clearly as we must farm more intensely, finding ways to crop under orchard trees would be one way to maximize food production. Getting the fern photosynthesis adaptations into corn might be worth a bit of research.

Obviously light energy drives photosynthesis. We have learned above that photosynthesis is not a single step, but in fact consists of two major groups of reactions. One set of reactions is largely electronic and light drives the electrons through an electron transport system coupled to phosphorylation of ATP. This has been called the "light reactions." The other half of photosynthesis has been called the "dark reactions" even though they never have and do NOT occur at night. I often call them light-independent reactions because light is not directly reacting with the elements. But that is misleading too; light only directly participates in two of the many electronic exchanges in the light reactions. Some people want to call the dark reactions the "carbon fixation reactions" even though carbon fixation is just one chemical reaction of many in that pathway. So we have a hard time choosing names for this second group of reactions. Suffice it to say that these are largely enzymatic reactions rather than electronic, and they occur in the stroma rather than in the thylakoid.

Here is a diagram of the light reactions...this is the famous Z-scheme. Not much of a Z here, since I have put energy on the vertical axis. But if you put energy on the horizontal axis (rotate the screen 90° right) you will end up with a Z. This energy diagram has compounds of low energy at lower positions and higher-energy compounds at higher positions.

Examining the diagram above we notice that light is funneled to two photosystems with the red-absorbing reaction center pigments P680 and P700. These two reaction center pigments are chlorophyll a molecules that are different in absorption maxima due to the way they are held by the chlorophyll binding proteins. You isolated those in cellular and molecular biology classes by doing electrophoresis. You will notice that the light funnel consists of antenna pigments that absorb different colors of light. Because this is an energy diagram, the funnel shows purple at the top and red at the bottom. By having P680 and P700 absorbing such long-wavelength/low-energy light, evolution has provided a pathway for photosynthesis to harvest virtually the entire visible spectrum!

Next you notice that light excites P680 and shoots electrons up to Phaeophytin. The huge jump in energy is not achieved by magic...we did not violate the second law of thermodynamics. Clearly this diagram is not showing the light energy with enough magnitude! Huge amounts are coming downhill to the reaction center pigment, and only some of that will boost the electrons the amount you see. The rest will be lost as heat.

The loss of electrons allows P680 to split a water molecule and replace its electrons. This action releases protons and oxygen gas. The protons participate in the electron transport chain and the coupled photophosphorylation steps. Thus water is the source of the electrons used in photosynthesis. Now, where do these electrons go? What is the amazing part about how chlorophyll works?

Chlorophyll is an amphipathic molecule. It has a polar head and a non-polar tail. This allows it to bind to chlorophyll binding proteins in the thylakoid membranes and to interface with the similarly amphipathic phospholipid bilayer. The "head" portion of chlorophyll is a porphyrin ring system. It is polar because of unshared electron pairs on oxygen and nitrogen atoms that comprise the porphyrin system. The bonding pattern in the ring system is resonant...notice the notation with alternating single and double bonds. This provides a large cloud of resonating electrons that are shared collectively over the large surface of the head of the chlorophyll molecule. Adding a nice big cloud of electrons and holding them in place is a chelated Magnesium atom. Metals like magnesium have wonderful ability to lose electrons or gain electrons; they have multiple valence states. The surrounding tetrapyrrole ring system just enhances this ability to gain and lose electrons.

So the large vertical arrows in the "z" scheme of the light reactions indicate the movement of electrons into the large electron cloud at the head of chlorophyll. From here, Phaeophytin picks the electrons up and passes them in turn to Q, cytochrome b, plastoquinone, cytochrome f, and plastocyanin. These electron transport molecules are embedded in the thylakoid membrane. Protons from photolysis are shuttled by plastoquinone from the stroma into the thylakoid lumen. This accumulation of protons represents a conservation of energy. The leakage of protons through an ATP synthase from lumen to stroma releases that energy which is trapped in the phosphate bond in ATP. So the electron transport system of the light reactions is coupled to phosphorylation; this is termed photophosphorylation.

The electrons then move to P700 to replace electrons boosted by light and passed to X, ferredoxin, and then either NADP or cytochrome b6. Again, the boosing light can be just about any wavelength in the visible spectrum because the energy is sufficent to transfer from antenna pigments to P700 and leave some energy to satisfy the second law of thermodynamics.

If ferredoxin passes the electron to NADP, the NADP is joined by some of the protons that have returned to the stroma. This produces a supply of NADPH for the carbon fixation reactions. Because these electrons do not stay in the light reactions, we call this non-cyclic flow. If ferredoxin passes the electron to cytochrome b6, it is then passed back through the electron transport system between the photosystems as shown above. Because this electron returns to P700, we call this cyclic electron flow. Since an electron can cycle through this pathway indefinitely, producing ATP from the energy lost along the ETS, a plant cell can produce any ratio that is needed of NADPH (reducing equivalents from non-cyclic flow) and ATP (energy from cyclic flow). Evolution will favor this kind of flexibility, so there is little surprise that we find it!

Our final graphic is to examine how these light reactions are arranged in the cell.

In this diagram you can see that the electron transport carriers and photosystems are embedded in the thylakoid membrane of the chloroplast. While photolysis, phosphorylation, and NADP reduction are stroma-side events, most of the light reactions occur within the thickness of the thylakoid membrane. The plastoquinone shuttling protons from stroma to thylakoid lumen and the return of protons through the two coupling factors of ATP synthase represent photophosphorylation.

In summary, the light reactions split water and drive the electrons and protons to NADPH, trap energy in the form of ATP, and releases oxygen gas to the atmosphere.