Hydroponic Science: Light as an Energy Source
This is a topic that I’ve been meaning to write about for a while. The dawn of LED horticultural lighting has brought about it waves of confusion over wavelengths (or wavenumbers/frequencies/electron volts—pick your unit!). If anything, the one important realization that people have come across is that not all white light is the same! What appears white to the human eye is, in fact, a combination of colors across the visible spectrum, with perhaps some infrared and ultraviolet wavelengths mixed in. Look around the net, and you’ll see articles about why you absolutely need 5:1 red/blue, why far-red light is the bee’s knees, why this spectrum is more efficient than that spectrum, etc.
A full discussion of the effects of all wavelengths of light on a plant is most certainly beyond the scope of this article. Light can do a number of things to a plant, but for the most part it boils down to two factors: light as a signal and light as an energy source. These are not mutually exclusive terms, and in many cases the same wavelength of light can act as both. For example, red light is the principle energy source for a chlorophyll-containing plant, however red photons can also interact with plant phytochromes, which are proteins involved in light sensing and signaling.
I want to focus this article on the energy source side of things. We all know plants are absorbing light for energy, but what, exactly, is doing the absorbing? Where is all this energy going? How is absorption of light different for different colors of light?
Chlorophyll: Our Main Protagonist
In order for light to be used for energy, it must bump into a chlorophyll molecule in a plant. What does a chlorophyll look like? Let’s pull up the molecular structure up from Wikipedia:
Figure 1. Chemical structure of plant chlorophyll a. Source: Wikipedia
Figure 1 depicts the chemical structure of a chlorophyll a molecule from a plant. Ok, so how do you interpret that figure above? For those that haven’t taken organic chem, each of the bends or nodes in the lines in the figure above represents a carbon atom. Sometimes carbon atoms are explicitly shown (like the CH3 methyl groups in Figure 1), but more often than not, they’re just the bends on the lines. It's too much effort to write a C down a multitude of times, so scientists figured out a shortcut. When two bends are connected with a single line, you’re looking at a single C-C bond. Two lines between two bends represents a double bond, and alternating single-double bond represents something called delocalization whereby the electrons on the carbons are free to move around the bond network like a little electron highway. If you look closely at the “square” portion of the chlorophyll molecule, you’ll notice there is LOTS of delocalization going on. This gives the molecule the ability to absorb light energy (check out the structure of lycopene, the red pigment from tomatoes, and try to note the delocalization in the chemical structure).
The delocalized electrons reside in a molecular orbital called the HOMO (highest occupied molecular orbital). Given a little boost of energy (as governed by the laws of quantum mechanics), the electrons can get bumped into another molecular orbital that’s currently unoccupied—the LUMO (lowest unoccupied molecular orbital). With even more energy, you can bump the electrons from the HOMO into other unoccupied orbitals (LUMO+1, LUMO+2, etc). In the case of a chlorophyll, light provides this energy to “bump” up the electrons to a LUMO or LUMO+1. Red photons (~660 nm) provide enough energy to make the HOMO -> LUMO transition. Blue light (~430 nm) is more energetic, and can provide enough energy to promote a HOMO -> LUMO+1 transition.
Figure 2. Optical absorption spectra of chlorophylls a and b. Spectra are normalized to the Soret band in the blue region of the spectrum.
This is why there are two main absorption bands in the spectrum of chlorophyll: one in the blue region of the visible spectrum (called the B, or Soret band) and one in the red region (called the Q band). Energy that doesn’t match the energy gap between HOMO and LUMO/LUMO+1 is not absorbed by the chlorophyll (see Blankenship, R.E. Molecular Mechanisms of Photosynthesis, ISBN: 978-1-405-18976-7).
Chlorophyll comes in two flavors in a plant: a and b (some algae and cyanobacteria also contain chlorophylls c, d, and f, but I’m not getting into those). Chlorophyll b is different from chlorophyll a that the C7 methyl group (on the top right of the structure in Figure 1) is replaced with another chemical group called a formyl group. This causes the chlorophyll b to absorb light at ~644 nm and ~450 nm instead. I’ve overlaid the spectra of the two chlorophylls in Figure 2.
A key point here is that the absorption spectra shown above are of chlorophylls in isolation, i.e. they’ve been removed from their natural source, purified, and their spectra captured in an organic solvent. In nature, most of the chlorophylls in photosynthesis are embedded in protein molecules. This shifts their absorption spectra (either to the red or blue) because i) the chlorophylls are interacting with the nearby amino acids of the protein matrix, and ii) the chlorophylls are interacting with each other. What do I mean by “interacting”? Primarily I mean electrostatic effects; an electron-rich oxygen from a chlorophyll hydrogen bonding to a nearby amino acid amine group, or the interaction between the delocalized electrons on a chlorophyll with those of a nearby cation (positive charge), for example. Thankfully (for the sake of explaining in this blog), plant chlorophylls don’t shift their absorption spectra too much as a result of this. For example, the chlorophylls in plant antenna complexes absorb light maximally at 675 nm (with a smaller 650 nm peak) in the context of the antenna complex, but this becomes a single 663 nm band once you extract the chlorophylls with solvents. Many photosynthetically active proteins also contain carotenoids, pigments that absorb complementary wavelengths to the chlorophylls and can donate their excitation energy to them, though not at 100% efficiency. Overall, the complementary pigments and the protein matrix makes the absorption spectrum of a protein like LHCII (light harvesting complex II, a main light gathering protein in plants) broader than that of chlorophylls in isolation.
The Fate of Absorbed Light – A Game of Energetic Hot Potato
Once a chlorophyll absorbs a photon of light, that energy needs to go somewhere useful, and fast—think 5-7 nanoseconds. This timescale is beyond the realm of traditional protein kinetics, where proteins undergo structural changes or movements to do what they need to do. In the case of photosynthetic processes, much of what’s happening is governed by the geometry of the chlorophylls in an otherwise static protein matrix. Evolution has essentially gamed the laws of physics through pigment arrangements to make useful ultrafast reactions far more probable.
Anyhow, back to excited states. Here’s a breakdown of the “choices” for an excited state chlorophyll:
The chlorophyll can dissipate the energy as heat and become “unexcited” again. This is known as returning to the ground state.
The chlorophyll can do the same as above, however it releases the energy as a photon of light. In other words, it fluoresces (a second photon can assist with this process in a phenomenon called stimulated emission).
The chlorophyll can transfer the energy or the excited electron itself to another nearby molecule.
The chlorophyll can decay to a lower energy state that’s still at a higher energy than the ground state, and then do one of the three options listed above from this state.
The first two cases above are not useful for the plant. Energy was absorbed and then lost. Whoop dee do! The third case is interesting as it is the only beneficial fate as far as the plant is concerned (it can also be very dangerous if the energy gets passed to a nearby oxygen molecule). I’m not really going to talk about the 4th option, but read up on triplet state chlorophylls if you’re ever curious—it’s too unnecessarily complicated for the sake of this article.
Let’s go back to option #3. Chances are that the chlorophyll that absorbed the energy is not capable of itself doing something useful with it; it needs to move the energy somewhere useful. Thus begins the game of energetic hot potato. Why is this the case? There are two main types of proteins involved in this part of photosynthesis: antenna complexes and reaction centers. These two proteins are purpose-built, and are very different in structure.
Antenna complexes need to absorb as much light as possible, so they arrange their chlorophylls to achieve a high absorption cross section. Furthermore, antennas need to efficiently transfer energy to other antennas. This is made easier by having many chlorophylls on the periphery of the protein, which reduces the distance between inter-antenna chlorophyll molecules. Reaction centers, on the other hand, are built to convert excitation energy into electrical charge, thus performing electron transfer. This ability limits the arrangements that reaction centers can have with respect to their chlorophylls, and in turn their absorption cross section is much lower. Reaction centers also need to make room for other molecules, such as pheophytins, quinones, and iron-sulfur clusters, that are important for the electron transfer reactions that convert excitation energy to electrical charge. To more clearly illustrate this distinction, let’s take a look at Figure 3 below.
Figure 3. Chlorophyll arrangement in a plant antenna complex (LHCII from spinach) vs that of a reaction center (Photosystem II from cyanobacteria, right(REF: 10.1038/nature21400)). Chlorophylls a are shown in green, chlorophylls b are shown in gray, and the transparent red and blue “stuff” is the protein matrix that’s holding the pigments together. Image produced with VMD.
On the left side of Figure 3 you have an antenna complex. Lots of chlorophylls arranged for maximum absorption cross-section, optimized for energy transfer laterally between copies of itself and eventually to a reaction center. It is often the case that energy takes many “hops” across a multitude of antenna complexes before landing on a reaction center. On the right is an illustration of a reaction center. More protein with less chlorophyll. The reaction center is the end of the line for excitation energy. Here it gets trapped and converted to electrical charge. Within a reaction center, energy moves more in a “vertical” direction in the form of electrons (i.e. in the direction from your eyes towards the page in the example in Figure 3, right).
One important distinction here is that antenna complexes are for the most part engaged in excitation energy transfer, whereas reaction centers are mostly engaged in electron transfer. The physics that govern these processes are different, and so the arrangement of the pigments within these specialized proteins are also different. One obvious example is that chlorophylls in reaction centers are more “buried” into the protein. This plays into the rules of electron transfer, where well-insulated pathways are key to moving the electron to a useful location.
The two reaction centers in plants (photosystems I and II) use energy primarily transferred from an antenna complex to perform their functions. What this means is that, regardless of the initial energy (wavelength) of the photon, the two photosystems utilize the energy from a ~675 nm excitation. In the case of photosystem II, this becomes a 680 nm excitation on the chlorophylls of the reaction center, and in photosystem I this is a 700 nm excitation. Notice how these absorption maxima are slightly lower in energy than the antenna (675 nm). This contributes to the “trapping” effect of reaction centers, where having the energy hop back to an antenna is energetically unfavorable. Photosystem II uses its excitation energy to split water and create the oxygen we breathe, in the process funneling electrons to a molecule called plastoquinone. This plastoquinone provides the electrons required to both pump protons for ATP synthesis as well as to eventually “recharge” photosystem I following illumination (this requires several important intermediate steps that I’m skipping here). Photosystem I uses the 700 nm chlorophyll excited state energy to funnel electrons to a protein called ferredoxin, that is involved in making NADPH. NADPH is an important player in the process of taking CO2 out of the atmosphere and incorporating it in the form of stored sugars.
As far as energy utilization is concerned the excess energy provided by your blue, green, orange LEDs is lost as heat. Does this make those wavelengths useless? Absolutely not! Plants are 3D organisms. Even in the context of light as an energy source, sub-optimal wavelengths, such as green light, have the benefit of better penetrating leaf tissue. Despite the fact that plant antenna complexes don’t absorb well between the blue and red regions of the spectrum, the absorption is not zero. The surprisingly high utilization of green light for CO2 fixation was elegantly demonstrated by K.J. McCree for many field-grown and chamber-grown crops in the 1970s. When you toss in differences in leaf morphologies, plant growth behavior, and light-responsive signaling, the picture gets far more complicated. One thing that I would like to see from future research in the field (and perhaps it exists, but I haven’t found it yet—this happens a lot because there’s tens of thousands of research articles out there) is a repeat of the McCree curve with better resolution. McCree used 25 nm steps for his data, which is coarse by today’s standards. Also, we know that plants are very dynamic organisms, changing their biology in response to light quality, temperature, stress, etc. It may be interesting to see if any of these factors can affect the shape of the curve. What we do know, however, is that nature has developed clever tricks to use sub-optimal light despite the biochemical limitations of the proteins doing the actual work. Personally, I feel that once we hit the efficiency cap of LED lighting, much of the efforts in horticultural lighting development will be geared towards understanding these clever tricks of nature, and building newer products that capitalize on them.