ENZYME CATALYSIS: A TUNNEL THROUGH THE BARRIER

Traditional theories of enzyme catalysis hold that the proteins speed up reactions by lowering the activation energy. But some researchers argue that a quantum trick known as tunneling also plays a role, and that the structure of enzymes’ active sites might have evolved to take advantage of this phenomenon.

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A Many chemical reactions are prevented from happening spontaneously by an energy barrier, known as the activation energy. B Enzymes lower this barrier by stabilizing an intermediate, or “transition,” state that allows the reaction (such as the movement of a hydrogen atom within a molecule) to take place. C The intermediate state can be bypassed if particles within the molecule are transferred via quantum tunneling, where a particle instantaneously traverses the barrier with a certain probability.

PHOTOSYNTHESIS: ALL PATHS TRAVELED

During the light-harvesting reaction of photosynthesis in plants and some microbes, a photon excites an electron in a chlorophyll molecule to create a structure called an exciton—an entity containing both the excited electron and the positively charged hole it leaves behind. This exciton is then transferred via other chlorophyll molecules until it reaches a protein complex called the reaction center.

Traditional Model

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According to the traditional, or “incoherent,” model of this process, the exciton’s route to the reaction center is more or less random. Because energy can be lost during the transfer process, such a path can end up being wasteful.

Quantum Model

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By contrast, if the energy transfer process is “quantum coherent” such that the exciton travels like a wave, it can explore all possible paths simultaneously and only take the most efficient route.

MAGNETORECEPTION: SPINNING SENSORS

According to the radical-pair model of avian magnetoreception, cryptochrome, a protein found in the retinas of birds and other animals, may be the magnetosensor, detecting the direction of magnetic ?elds via changes to the spin states of some of its electrons.

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Reactions within the cryptochrome protein generate a pair of molecules that each have a lone electron. These electrons, which can be entangled with each other, occupy one of two states: a “singlet” state, meaning the spinning direction of one is related to the spinning direction of the other such that the spins are antiparallel; or a “triplet” state, in which the two electrons tend to have spins that are close to parallel. The radical pair oscillates between these two states, and the probability of finding it in one state or the other is influenced by the direction of magnetic fields. If the singlet and triplet states of the radical pair are associated with different biochemical reactions, then the yields of products from those reactions can provide information about the direction of a magnetic field. If those products go on to influence neural signaling from the bird’s retina, then this mechanism could provide the basis for magnetoreception.















Glossary: Quantum Terminology

The world at the scale of spinning atoms and subatomic particles is governed by the probabilistic rules of quantum mechanics, which often produce effects that seem counterintuitive to organisms living in a world usually described perfectly well by more-standard physics. These effects have been harnessed for multiple technological applications, and the possible role of quantum phenomena in several biological systems is now being explored.



Entanglement: Two particles are said to be quantumly entangled if their states are interdependent, regardless of the distance separating them. In the classic example of entanglement two entangled electrons, when measured, will have opposite spins.



Important for: Quantum computing, quantum cryptography

Studied in: Photosynthesis, magnetoreception, human consciousness

Qubits: These units of information are the quantum equivalent of standard binary digits or bits. While a bit can have a state of 0 or 1, qubits can have multiple states simultaneously, and may be entangled with other qubits to perform parallel computations. Qubits can be encoded in the spin states of electrons and other subatomic particles.



Important for: Quantum computing

Studied in: Human consciousness

Tunneling: Particles at the quantum scale have wave-like properties, and their exact location at any moment is described by a probabilistic wave function. As a result, particles such as electrons can, with certain probabilities, traverse—or tunnel through—apparently impermeable energy barriers.



Important for: Thermonuclear fusion, scanning tunneling microscopy

Studied in: Enzyme catalysis, photosynthesis, olfaction, DNA mutation

Coherence: Because quantum objects can behave like waves, they can exhibit a property of waves called coherence when they are in a particular rhythm with one another. Quantum coherence underlies several effects observed by quantum physicists, including entanglement as well as interference patterns manifested as so-called quantum beating. Loss of coherence has traditionally been thought to happen very quickly in the molecular bustle of ambient- temperature environments.



Important for: Lasers, superconductors, quantum computing

Studied in: Photosynthesis, magnetoreception, vision, respiration

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