A light-harvesting complex has a complex of subunit proteins that may be part of a larger supercomplex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. Light-harvesting complexes are found in a wide variety among the different photosynthetic species. The complexes consist of proteins and photosynthetic pigments and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment, toward the reaction center using Förster resonance energy transfer. 14
Blankenship, Photosynthesis, page 62:
All chlorophyll-based photosynthetic organisms contain light-gathering antenna systems. These systems function to absorb light and transfer the energy in the light to a trap, which quenches or deactivates the excited state. In most cases, the trap is the reaction center itself, and the excited state is quenched by photochemistry with energy storage. In some cases, however, the quenching is by some other process, such as fluorescence or internal conversion. The antenna pigments are arranged in well-defined, three-dimensional structures, so that only a few energy transfer steps are required to connect any two pigments in the array.
If chlorophylls would be arranged so that the energy had to diffuse along a linear, or one-dimensional, array of chlorophyll pigments, then the concept of energy transfer in photosynthesis would not be feasible. One-dimensional diffusion is very inefficient because many, many transfers are required to move the excitation from one point in the array to another.
Question: How could the right arrangement have emerged in a gradual, stepwise, evolutionary fashion, if only the arrangement in well-defined, three-dimensional structures, where only a few energy transfer steps are required to connect any two pigments in the array is feasible?
One-dimensional and three-dimensional antenna organization models.
In the one-dimensional model, excitation must be transferred by many steps before encountering a trap where photochemistry takes place. In the three-dimensional model, the trap is always no more than a few energy transfer steps from any of the pigments in the antenna complex.
A chlorophyll molecule, which is roughly square with dimensions of ∼10 Angstroms per side, will be illuminated by approximately 1100 photons per second, although not all of these photons will be absorbed. Calculating the target size that a chlorophyll molecule presents to this incoming rain of photons determines how many photons will actually be absorbed.
The target concept is schematically illustrated in Figure below:
A photon is a strange combination of particle and wave, with a physical size that is roughly similar to the wavelength of the photon, which in the visible region is much larger than a molecule. The photon interacts with a molecule and has its energy deposited in the molecule, whereupon the photon ceases to exist and the molecule is left in an excited state. Even with full sunlight, there is approximately a tenth of a second between photons being absorbed by any given chlorophyll molecule, and it can easily be several orders of magnitude longer under most conditions. A tenth of a second is an eternity on the molecular scale.
Even in full sunlight, a chlorophyll molecule only absorbs a few photons each second. If a reaction centre had no antenna pigments then it would be active only a small percentage of the time. However, if there is a large antenna
complex, energy absorbed by a large number of pigments can be channelled to the reaction centre increasing the efficiency of the photochemistry.
If every chlorophyll had associated with it the entire electron transfer chain and enzymatic complement needed to finish the job of photosynthesis, then these expensive components would sit idle most of the time, only occasionally springing into action when a photon is absorbed. This would obviously be wasteful, and ultimately such an arrangement would be unworkable. It is as if a factory were to have a number of expensive manufacturing machines sitting idle most of the time while a key raw material is being brought in at a slow pace. It makes more sense to buy only a few expensive machines and somehow to improve the delivery system of raw materials. This is what antennas do for photosynthetic organisms.
My comment: Did you observe the language employed? " It makes more sense". This is teleology. Goal orientation. Why would unguided evolutionary mechanisms produce chlorophylls wherein an inadequate arrangement and chaotic position would be of no use at all? - but only, in a workable arrangement? This is once again a formidable example where only intelligent setup explains rationally the setup of the biological system in question.
In addition, in all cases, photochemistry produces unstable initial products that will be lost if a second photochemical event does not take place in a relatively short time. If all pigments were functionally independent, most of these unstable products would be lost. All these factors demand that photons be collected and their energy delivered to and used in a central place. Because of these considerations, every
known photosynthetic organism has an antenna of some sort associated with it.
However, it appears that in many cases the size of the antenna system is larger than what might be expected to be the most efficient in terms of conversion of solar energy. Many organisms, especially those that live in full sunlight, must dissipate much of the energy that the antenna system collects or risk photodamage.
An antenna complex is composed of numerous molecules of chlorophylls and carotenoids connected by proteins. PSII antenna complexes can be divided into
1. Internal antennae composed of protein and chlorophyll complexes, such as CP43 (43 kDa) and CP47 (47 kDa) proteins, which contain chlorophyll a and β - carotene molecules
2. Trimeric external antennae composed of LHCII, which contain chlorophyll a , chlorophyll b , and xanthophylls
3. Monomeric proteins CP29 (29 KDa), CP26 (26 kDa), and CP24 (24 kDa) binding chlorophyll a , chlorophyll b , and xanthophylls
The Light harvesting antenna is located peripherally with respect to reaction centers and is specialized in harvesting light and in transferring excitation energy to the reaction center, thus increasing the amount of photons absorbed per photosystem. 12
Despite the existence of a variety of models and theoretical methods that complement the experimental results, to our knowledge, there is currently no method that can describe the photophysics of individual photosynthetic pigments as well as the dynamical evolution of their coupled excitations when embedded within a proteic environment from an atomistic, time dependent perspective. The main problem lies in that due to the large size of the antenna complex, i.e. a few thousand atoms, this entire system cannot be simulated with the present computational methods found in literature 13
Most of the photons that are converted to biochemical energy and biomass through photosynthesis are harvested by the major light-harvesting chlorophyll a/b binding antenna complex light-harvesting complex II (LHCII), which is one of the most abundant proteins on earth. 10 Light harvesting is important for the function of photosynthesis8 — for a variety of reasons — so exploring the operation of natural light-harvesting complexes provides an excellent platform for understanding how photo-excitation can be directed and amplified using assemblies of light-absorbing molecules [url=https://sci hub.bz/http://www.nature.com/nchem/journal/v3/n10/full/nchem.1145.html?wt..]11[/url] This extraordinary chain of events is started and regulated by light harvesting. Energy from sunlight is captured by complexes that subsequently funnel it to reaction centers on a 10–100 picosecond timescale ( A picosecond is one trillionth (10 -12 ) of a second ). More specifically, light harvesting relies on the process of electronic energy transfer moving electronic excitation energy — which is stored fleetingly (nanoseconds - a nanosecond (ns) is equal to one billionth of a second (10−9 or 1/1,000,000,000 s) by molecules in excited states — within networks of light-absorbing molecules (chromophores) to a target chromophore or trap.
Natural solar-energy conversion in photosynthesis is one of the most important biological processes in which electronic energy-transfer plays a decisive role. 7 The initial capture of light energy is a kinetic challenge. Chlorophylls (Chls) and bacteriochlorophylls (BChls), the workhorse pigments of photosynthesis, will dissipate their absorbed light energy as fluorescence in 3–6 ns! Even the fastest enzymes, such as carbonic anhydrase, cannot keep up, they operate in kinetic regimes that are at least an order of magnitude slower . In this respect, the light-harvesting (LH) antennas of photosynthetic organisms are some of the most impressive proteins
A photon – a particle of light – after a journey of billions of kilometres hurtling through space, collides with an electron in a leaf outside your window. The electron, given a serious kick by this energy boost, starts to bounce around, a little like a pinball. It makes its way through a tiny part of the leaf’s cell, and passes on its extra energy to a molecule that can act as an energy currency to fuel the plant. 4 The trouble is, this tiny pinball machine works suspiciously well. Classical physics suggests the excited electron should take a certain amount of time to career around inside the photosynthetic machinery in the cell before emerging on the other side. In reality, the electron makes the journey far more quickly. What’s more, the excited electron barely loses any energy at all in the process. Classical physics would predict some wastage of energy in the noisy business of being batted around the molecular pinball machine. The process is too fast, too smooth and too efficient. It just seems too good to be true.
Then, in 2007, photosynthesis researchers began to see the light. Scientists spotted signs of quantum effects in the molecular centres for photosynthesis. Tell-tale signs in the way the electrons were behaving opened the door to the idea that quantum effects could even be playing an important biological role. This could be part of the answer to how the excited electrons pass through the photosynthetic pinball machine so quickly and efficiently. One quantum effect is the ability to exist in many places at the same time – a property known as quantum superposition. Using this property, the electron could potentially explore many routes around the biological pinball machine at once. In this way it could almost instantly select the shortest, most efficient route, involving the least amount of bouncing about. Quantum physics had the potential to explain why photosynthesis was suspiciously efficient – a shocking revelation for biologists.
Quantum phenomena such as superposition had previously been observed mostly under highly controlled conditions. Typical experiments to observe quantum phenomena involve cooling down materials to bitingly cold temperatures in order to dampen down other atomic activity that might drown out quantum behaviour. Even at those temperatures, materials must be isolated in a vacuum – and the quantum behaviours are so subtle that scientists need exquisitely sensitive instruments to see what’s going on.
The first step in photosynthesis is the capture of a tiny packet of energy from sunlight that then has to hop through a forest of chlorophyll molecules to makes its way to a structure called the reaction center where its energy is stored. The problem is understanding how the packet of energy appears to so unerringly find the quickest route through the forest. An ingenious experiment first carried out in 2007 in Berkley, California, probed what was going on by firing short bursts of laser light at photosynthetic complexes. The research revealed that the energy packet was not hopping haphazardly about, but performing a neat quantum trick. Instead of behaving like a localized particle traveling along a single route, it behaves quantum mechanically, like a spread-out wave, and samples all possible routes at once to find the quickest way. 5
Quantum biology 6
One of the simplest and most well-studied examples is the light-harvesting apparatus of green-sulphur bacteria (Fig. 1)
These have a very large chlorosome antenna that allows them to thrive in low-light conditions. The energy collected by these chlorosomes is transferred to the reaction centre through a specialized structure called the FennaMatthewsOlson (FMO) complex. Owing to its relatively small size and solubility in water, the FMO complex has attracted much research attention and as a result has been well characterized. What is remarkable is the observed efficiency of this and other photosynthetic units. Almost every photon (nearly 100%) that is absorbed is successfully transferred to the reaction centre, even though the intermediate electronic excitations are very short-lived (1 ns). In 2007, Fleming and co-workers demonstrated evidence for quantum coherent energy transfer in the FMO complex12, and since then the FMO protein has been one of the main subjects of research in quantum biology. The FMO complex itself normally exists in a trimer of three complexes, of which each complex consists of eight bacteriochlorophyll a (BChl-a) molecules. These molecules are bound to a protein scaffold, which is the primary source of decoherence and noise, but which also may assist in protecting the coherent excitations in the complex and play a role in promoting high transport efficiency. The complex is connected to the chlorosome antenna through what is called a baseplate. Excitations enter the complex from this baseplate, exciting one of the BChl molecules into its first singlet excited state. The molecules are in close proximity to one another (roughly 1.5 nm), enabling the excitation energy to transfer from one BChl molecule to another, until it reaches the reaction centre.
A light-harvesting complex is a complex of subunit proteins that may be part of a larger supercomplex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. Light-harvesting complexes are found in a wide variety among the different photosynthetic species. The complexes consist of proteins and photosynthetic pigments and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment, toward the reaction center using Förster resonance energy transfer.
LIGHT HARVESTING COMPLEX II
The complex is formed from nine protomers each consisting of an alpha and beta polypeptide, three bacteriochlorophyll a (Bchl a) molecules, one rhodopin glucoside and one beta-octylglucoside molecule. The structure has precise 9 fold symmetry and comprises two concentric rings of trans-membrane helices. A continuous ring of 18 Bchl a molecules are situated between these helices. A further 9 Bchl a molecules are found between beta peptide helices at a distance of 18.0Å from the first ring. The two rings of molecules are linked through the intertwining of their phytol chains and the contacts of rhodopin glucoside molecules.
The structure explains many of observed spectral transfer processes between the various chromaphores within the complex. The peripheral antenna complexes aggregate into two dimensional arrays, incorporating the core complex comprising LH1 and the Reaction Centre. The homology of the LH1 subunit and that of LH2 suggests that the 850nm bacteriochlorin molecules from LH2 and the 870nm absorbers in LH1 are at the same point in the membrane. Thus efficient energy transfer within these arrays occurs without special relative orientations of the light harvesting molecules.
One of its remarkable features is the efficiency with which energy is transferred within the light-harvesting complexes comprising the photosynthetic apparatus. Suspicions that quantum trickery might be involved in the energy transfer processes at the core of photosynthesis are now confirmed by a new spectroscopic study.This wavelike characteristic of the energy transfer process can explain the extreme efficiency of photosynthesis 13 This wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path.
Förster resonance energy transfer 1
Förster resonance energy transfer (FRET), Fluorescence resonance energy transfer (FRET), resonance energy transfer (RET) or electronic energy transfer (EET), is a mechanism describing energy transfer between two chromophores. A donor chromophore, initially in its electronic excited state, may transfer energy to an acceptor chromophore through nonradiative dipole–dipole coupling.The efficiency of this energy transfer is inversely proportional to the sixth power of the distance between donor and acceptor making FRET extremely sensitive to small distances.
Measurements of FRET efficiency can be used to determine if two fluorophores are within a certain distance of each other. Such measurements are used as a research tool in fields including biology and chemistry.
FRET is analogous to near field communication, in that the radius of interaction is much smaller than the wavelength of light emitted. In the near field region, the excited chromophore emits a virtual photon that is instantly absorbed by a receiving chromophore. These virtual photons are undetectable, since their existence violates the conservation of energy and momentum, and hence FRET is known as a radiationless mechanism. Quantum electrodynamical calculations have been used to determine that radiationless (FRET) and radiative energy transfer are the short- and long-range asymptotes of a single unified mechanism.
Ancient Bacteria Use Quantum Mechanics
The light-harvesting antenna complex of purple bacteria 2
The quantum world is a strange one. In a process called “quantum tunneling,” particles can pass through barriers as if they aren’t there at all. As a result of a process called “perturbation,” empty space can give rise to virtual particles that “blip” into and out of existence. Because of a phenomenon known as “quantum coherence,” a particle can be in several different places at once. These ideas defy common sense, but they have been experimentally verified in many different ways.
It turns out that photosynthesis (the process by which some organisms convert the energy in sunlight into energy that they can use) exploits quantum coherence in an incredible way. When light strikes a photosynthetic organism, its energy must be captured so that it can be used in an amazingly complex process that will convert it from radiant energy into chemical energy.
It has long been known that photosynthesis is about 95% efficient when it comes to the first step of capturing light’s energy.1 Until now, however, scientists have not understood how photosynthesis could be that efficient. The energy transfer efficiency between chlorophyll molecules is close to 100%, and it is lower when the transfer occurs from carotenoids to chlorophyll (Hall and Rao 1999).
After all, harvesting light in a biological environment is difficult. Even though photosynthetic organisms have a well-designed “antenna” system for capturing that light (an example is given above), a living organism is usually in motion. Its environment is also constantly stimulating it in different ways. As a result, even though the antenna system is well designed, it will be distorted and deformed as the organism moves and responds to its environment. This means there should be times when the antenna system is well-aligned, producing very efficient transfer of energy, but there should also be times where it is misaligned, reducing its efficiency. Nevertheless, photosynthesis stays very efficient, regardless of how the antenna complex is distorted.
How does the antenna complex stay efficient? The answer is incredible.
Richard Hildner and his colleagues studied the antenna complex of purple bacteria, single-celled organisms that perform photosynthesis. 3 Unlike plants and many other photosynthetic organisms, they do not produce oxygen as a byproduct of their photosynthesis, and they are generally considered to have very “simple” photosynthetic machinery. As Chandler, Hsin, and Gumbart of the University of Illinois write:
Among all photosynthetic organisms, purple bacteria are considered to have the oldest and simplest photosynthetic apparatus, making them ideal candidates for photosynthetic studies.
What Dr. Hildner and his colleagues found, however, was anything but “simple.” They found that the light-collecting antennae of purple bacteria exploit quantum coherence when they absorb a particle of light (which is called a photon). Because of this, the photon can essentially be everywhere in the antennae at once. What does this mean? It means that regardless of the current state of the antenna, the photon can explore all possible pathways in the absorption process. The most efficient pathway can then be chosen, regardless of how distorted or deformed the antenna might be! As the authors state:
…long-lived coherences contribute to the necessary robustness against external perturbations and disorder that are ubiquitous in biological systems at physiological temperatures. In this respect, the biological function of these complexes, light absorption and energy funneling toward the reaction center, is optimized for each individual aggregate, and long-lived quantum coherences herein play an important role.
Without exploiting quantum coherence, then, the photosynthesis of purple bacteria would not be as robust. It would vary depending on the specific external perturbations and disorder that happen to be occurring at the time.
Now think about this for a moment. Purple bacteria are supposed to have “simple” photosynthetic machinery. However, even this “simple” machinery is sophisticated enough to exploit quantum mechanics – an esoteric aspect of nature that even most scientists don’t understand. In fact, from an evolutionary point of view, purple bacteria were the first to evolve the process. Nevertheless, they use quantum mechanics! Now, of course, it is always possible that earlier photosynthetic machinery in purple bacteria was simple and that evolution “tinkered” with the process for billions of years to come up with the ability to exploit quantum mechanics. However, there is no evidence for this. The fact is that the simplest, most “primitive” version of photosynthesis that currently exists in nature has already mastered quantum mechanics. As far as I’m concerned, this provides even more evidence that photosynthesis is the product of an Incredible Designer.
Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature
Light-gathering macromolecules in plant cells transfer energy by taking advantage of molecular vibrations whose physical descriptions have no equivalents in classical physics, according to the first unambiguous theoretical evidence of quantum effects in photosynthesis published today in the journal Nature Communications.
The majority of light-gathering macromolecules are composed of chromophores (responsible for the colour of molecules) attached to proteins, which carry out the first step of photosynthesis, capturing sunlight and transferring the associated energy highly efficiently.
Previous experiments suggest that energy is transferred in a wave-like manner, exploiting quantum phenomena, but crucially, a non-classical explanation could not be conclusively proved as the phenomena identified could equally be described using classical physics.
Often, to observe or exploit quantum mechanical phenomena systems need to be cooled to very low temperatures. This however does not seem to be the case in some biological systems, which display quantum properties even at ambient temperatures.
Now, a team at UCL have attempted to identify features in these biological systems which can only be predicted by quantum physics, and for which no classical analogues exist.
We found that the properties of some of the chromophore vibrations that assist energy transfer during photosynthesis can never be described with classical laws, and moreover, this non-classical behaviour enhances the efficiency of the energy transfer.
“Energy transfer in light-harvesting macromolecules is assisted by specific vibrational motions of the chromophores,” said Alexandra Olaya-Castro (UCL Physics & Astronomy), supervisor and co-author of the research. “We found that the properties of some of the chromophore vibrations that assist energy transfer during photosynthesis can never be described with classical laws, and moreover, this non-classical behaviour enhances the efficiency of the energy transfer.”
Molecular vibrations are periodic motions of the atoms in a molecule, like the motion of a mass attached to a spring. When the energy of a collective vibration of two chromphores matches the energy difference between the electronic transitions of these chromophores a resonance occurs and efficient energy exchange between electronic and vibrational degrees of freedom takes place.
Providing that the energy associated to the vibration is higher than the temperature scale, only a discrete unit or quantum of energy is exchanged. Consequently, as energy is transferred from one chromophore to the other, the collective vibration displays properties that have no classical counterpart.
The UCL team found the unambiguous signature of non-classicality is given by a negative joint probability of finding the chromophores with certain relative positions and momenta. In classical physics, probability distributions are always positive.
“The negative values in these probability distributions are a manifestation of a truly quantum feature, that is, the coherent exchange of a single quantum of energy,” explained Edward O’Reilly (UCL Physics & Astronomy), first author of the study. “When this happens electronic and vibrational degrees of freedom are jointly and transiently in a superposition of quantum states, a feature that can never be predicted with classical physics.”
Other biomolecular processes such as the transfer of electrons within macromolecules (like in reaction centres in photosynthetic systems), the structural change of a chromophore upon absorption of photons (like in vision processes) or the recognition of a molecule by another (as in olfaction processes), are influenced by specific vibrational motions. The results of this research therefore suggest that a closer examination of the vibrational dynamics involved in these processes could provide other biological prototypes exploiting truly non-classical phenomena.[/quote]
Spatial propagation of excitonic coherence enables ratcheted energy transfer
Experimental evidence shows that a variety of photosynthetic systems can preserve quantum beats in the process of electronic energy transfer, even at room temperature. However, whether this quantum coherence arises in vivo and whether it has any biological function have remained unclear. Here we present a theoretical model that suggests that the creation and recreation of coherence under natural conditions is ubiquitous. Our model allows us to theoretically demonstrate a mechanism for a ratchet eect enabled by quantum coherence, in a design inspired by an energy transfer pathway in the Fenna-Matthews-Olson complex of the green sulfur bacteria. This suggests a possible biological role for coherent oscillations in spatially directing energy transfer. Our results emphasize the importance of analyzing long-range energy transfer in terms of transfer between inter-complex coupling (ICC) states rather than between site or exciton states
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