Nearly 100- efficiency of photosynthesis Quantum experiments reveal why!
All biological systems are extremely "chaotic". Yet somehow this chaos allows plants to photosynthesize with almost 100% efficiency.
In physics, a system is 100% efficient if it utilizes 100% of the input energy to perform some energy-intensive task. In plants, almost 100% of the incident photon energy from the sun is converted into electron energy, which ultimately powers the production of sugar - this is the process of photosynthesis.
From the moment sunlight is absorbed by chlorophyll molecules until the energy is transferred to the photosynthetic reaction center, the efficiency of energy transfer is close to 100%. Thanks to a groundbreaking experiment involving quantum physics, chemistry and biology, we may finally understand how and why this process occurs.
The research results, titled "Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria," were published on July 3 in the Journal of the American Physical Society. Journal of the American Physical Society.
The "holy grail" of any physical system in terms of energy is 100% efficiency. In most cases, this is an almost impossible goal to achieve, because from the moment any form of energy is first transferred into a system, it is inevitably lost to a variety of factors: heat, collisions, chemical reactions, and so on, before it finally does its job.
The only way physicists have managed to create systems with near-perfect efficiency is to push nature to its "extremes": at temperatures close to absolute zero, by emitting monochromatic (laser) photons into (crystalline) systems with an absorbing lattice; or in extreme cases, such as superconductivity and superfluidity.
But nature has provided us with a very surprising exception: plants. The humble plant, as well as other more primitive photosynthetic organisms (such as certain species of bacteria and protozoa), absorbs a portion of sunlight of specific (blue and red) wavelengths through the complex process of photosynthesis, converting that light (photon) energy into sugar. Somehow, however, almost 100% of that absorbed energy is converted into electron energy, which then creates sugars through photosynthesis. This has been an unsolved problem, but thanks to the combination of quantum physics, chemistry and biology, we may finally have the answer - "biological disorder" is the key.
This photo shows chloroplasts inside a plant cell of the organism Plagiomnium affine. This energy transport is almost 100% efficient in transferring absorbed sunlight energy to the photosynthetic reaction centers that produce sugar: an anomaly in almost all biological processes.
It is very important to recognize that whenever scientists talk about "efficiency", there are two different definitions, depending on which scientist is talking about it.
Efficiency can mean examining the total amount of energy produced from a reaction as a fraction of the total energy input to a system; this is the definition usually used when considering the overall efficiency of a complete, end-to-end system.
Alternatively, efficiency can mean examining an isolated part of a system: the part of the input energy that is involved in the reaction being considered, and then the part of that energy that is used or released from that reaction; this is more often used when considering a single component of an end-to-end interaction.
The difference between the first and second definitions is "How is it that two different physicists can look at the huge fusion energy breakthrough made last year at the National Ignition Facility and come up with seemingly inconsistent statements?" --We have simultaneously surpassed the break-even point for fusion energy, and the fact that fusion still uses 130 times more energy than it produces.
The first statement is true if one considers the energy incident on the hydrogen bomb compared to the energy released from the reaction, and the second statement is true if one considers the entire complete apparatus, including the inefficient charging of the capacitor banks that produce the incident energy.
It is true that, taken as a whole, plants are even less efficient than solar panels: the latter can convert about 15-20% of the total incident solar energy into electricity. The chlorophyll found in plants (especially the chlorophyll a molecule) is only able to absorb and utilize sunlight in two specific narrow wavelength ranges: blue light, which peaks at around 430 nanometers, and red light, which peaks at around 662 nanometers.
When all the sunlight incident on a plant is taken into account, taken together, the amount of radiation that can be converted into useful energy for the plant is only a few percent of the total amount of energy that hits the plant from the sun; photosynthesis is not particularly efficient in this strict sense. However, if we limit ourselves to looking only at the individual photons that can excite the chlorophyll a molecule-photons at or near the two absorption peaks of chlorophyll a-photons at red wavelengths are about 80 percent efficient, while those at blue wavelengths are more than 95 percent efficient: close to that perfect 100 percent efficiency.
This graph shows the absorption efficiency of the chlorophyll a molecule, which peaks mainly around a particular set of blue (430 nanometers) and a particular set of red (662 nanometers) wavelengths.
This is where the conundrum arises.
The light absorbed by the chlorophyll molecule is not monochromatic, but consists of individual photons with a fairly broad range of energies. These photons excite electrons within the chlorophyll molecule, which then emit photons when the electrons weaken: again, over a range of energies.
These photons are then absorbed by a series of proteins; where they excite electrons within the proteins, which then spontaneously de-excite and re-emit photons - until these photons are successfully transported to the photosynthetic reaction center.
Then, when the photons hit the photosynthetic reaction center, the cell converts the photon energy into electron energy, and then these high-energy electrons are used in the photosynthesis process, which ultimately leads to the production of sugar molecules.
This is a broad overview of the photosynthetic pathway, from the associated incident photons to the high-energy electrons that ultimately produce sugars.
The puzzle in all of this is why is it that for every photon that is absorbed in the first step, at the end of the final step, approximately 100% of the photons produce excited electrons? In terms of efficiency, there really is no known naturally occurring physical system that behaves in this way. But somehow photosynthesis does.
Various energy levels and selection rules for electron transitions in iron atoms. While many quantum systems can be controlled to lead to extremely efficient energy transfer, no biological system works in the same way.
In most laboratory situations, a quantum system must be specially prepared in a very specific way if the energy transfer is to be 100% efficient. It must be ensured that the incident energy is homogeneous: each photon has the same energy and wavelength, as well as the same direction and momentum.
Therefore, it must be ensured that there is an absorption system that does not dissipate the incident energy: like a lattice, all the internal components are regularly spaced and ordered. And it is necessary to impose conditions as close as possible to "lossless", in which no energy is lost due to internal vibrations or rotation of the particles.
But in the process of photosynthesis, these conditions are absolutely non-existent. The incoming light is ordinary white sunlight: it consists of various wavelengths, and no two photons have exactly the same energy and momentum. There is no order in the absorption system, because the distances between the various molecules are not fixed in a lattice, but vary enormously: even neighboring molecules have scales of several nanometers between them. And all these molecules are free to vibrate and rotate; there are no special conditions that prevent these movements from occurring.
That's what's so exciting about this new research: what the scientists did was to start with one of the simplest examples of photosynthesis known in all of nature: a photosynthetic bacterium known as a purple bacterium (as opposed to a blue-green cyanobacterium), which is one of the oldest, simplest, and yet most potent examples of organisms known to carry out photosynthesis (the lack of chlorophyll b helps to give the bacterium its purple color) .
The key steps the researchers are trying to isolate and study are after the initial absorption of the photon, but before the last re-emitted photon reaches the photosynthetic reaction center, because these early and final steps are well understood. But in order to accurately understand why the process is so lossless in terms of energy, those intermediate steps need to be quantified and identified.
This is what makes this problem so difficult, and why it makes so much sense to choose a simple, ancient and efficient bacterial system to study.
The way the researchers approached this problem was to try to quantify and understand how energy is transferred between these series of proteins (antennae proteins) to reach the photosynthetic reaction centers.
The major antenna protein in purple bacteria is known as LH2: for light-harvesting complex 2. Whereas in purple bacteria the protein known as LH1 (light-harvesting complex 1) is tightly bound to the photosynthetic reaction centers, LH2 is distributed elsewhere, and its biological function is to harvest and transport energy to the reaction centers. To perform direct experiments on these LH2 antennae proteins, two independent variants of the protein (the conventional LH2 and the low-light variant known as LH3) were embedded in a small-scale disk that resembles, but is slightly different from, the native membranes where these light-harvesting proteins naturally occur. These disks close to the native membrane are called nanodiscs, and by varying the size of the nanodiscs used in these experiments, the researchers were able to replicate the behavior of energy transfer between the proteins at various distances.
Molecular structure of LH2
Surface charge density (left) and structural organization (right) of LH2 and LH3.
The researchers found that as they varied the size of the nanodiscs, they found that the time scale of energy transfer between proteins increased rapidly: from a minimum of 5.7 picoseconds (a picosecond is a trillionth of a second) to a maximum of 14 picoseconds. When they combined these experimental results with simulations that better represent the actual physical environment inside purple bacteria, they were able to show that the presence of these steps of rapid energy transfer between neighboring antennae proteins can dramatically increase the efficiency and distance of energy transfer.
In other words, it is the pairwise interactions between these closely spaced LH2 (and LH3) proteins that are likely to be the key mediators of energy transfer: from the moment the first incident sunlight photon is absorbed, all the way through to the point where that energy is finally delivered to the photosynthetic reaction center.
A key finding of this study is that these light-harvesting proteins can only transport this energy very efficiently over long distances, because the spacing of the proteins in the purple bacteria themselves is irregular and disordered. If this arrangement were regular, periodic, or organized in a conventional way, this long-distance, highly efficient energy transfer would not be possible.
And this is exactly what the researchers actually found in their study. If the proteins are arranged in a periodic lattice structure, energy transfer is less efficient than if the proteins are arranged in a "random organization" pattern - which is more representative of the way proteins are normally arranged in living cells.
The time for a photon to be transferred from one antenna protein (LH2 or LH3) to another as a function of the distance between them. Experiments performed at the three key distances agree well with the predictions of the underlying (quantum) theory.
According to the senior author of this latest study, MIT professor Gabriela Schlau-Cohen, "When a photon is absorbed, they only have so long before the energy is lost through unwanted processes such as non-radiative decay, so the faster it can be converted, the more efficient it is ...... Ordered organization is actually less efficient than the disordered organization of living things, which we think is very interesting because living things tend to be disordered."
"This finding tells us that (disorder in systems) may not just be an unavoidable drawback of biology, but that organisms may have evolved to take advantage of it."
In other words, what we usually think of as a "mistake" in biology - namely, that biological systems are inherently disordered - may actually hold the key to how photosynthesis occurs in nature.
This key insight comes from a combination of experiments, theories and simulations that ultimately point the way for this ultra-fast, ultra-efficient energy transfer of sunlight energy directly to the photosynthetic reaction centers.
We usually think of quantum physics as being relevant only to the simplest of systems: to the interaction of individual quantum particles or electrons and photons. In fact, however, it is the fundamental explanation behind every non-gravitational phenomenon in our macroscopic world: from how particles bond together to form atoms, to how atoms combine to form molecules, to the chemical reactions that take place between atoms and molecules, and how photons are absorbed and emitted by these atoms and molecules.
In photosynthesis, by bringing together humanity's combined knowledge of biology, chemistry and quantum physics, we have finally solved the mystery of exactly how one of the most energy-efficient processes in all of the life sciences occurs.
Reference link:
[1]https://bigthink.com/starts-with-a-bang/photosynthesis-100-efficient-quantum-physics/
[2]https://phys.org/news/2023-07-chemists-photosynthetic-light-harvesting-efficient.html
[3]https://www.pnas.org/doi/10.1073/pnas.2220477120
