Quantum Biology: When Nature Plays by Planck's Rules

For a long time, the scientific community treated quantum mechanics and biology as entirely separate domains. Quantum mechanics was reserved for subatomic particles, describing phenomena happening in ultra-cold vacuums. Biology, on the other hand, was the science of warm, messy, and complex living organisms. The idea that fragile quantum states could survive in the chaotic environment of a living cell seemed impossible. Yet, over the last two decades, researchers have uncovered evidence suggesting nature may harness the strange rules of the quantum realm. Welcome to the emerging field of quantum biology.

The Mystery of Photosynthesis

One of the most profound examples of quantum biology occurs in almost every leaf on Earth. Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight into chemical energy. The energy-transfer step of this process is staggering: sunlight energy absorbed by photosynthetic pigments reaches the reaction centers with nearly 100-percent efficiency. How is energy transferred so efficiently? When a photon hits a light-harvesting complex in a plant cell, it creates a package of energy called an exciton. This exciton needs to navigate through a dense forest of chlorophyll molecules to reach the reaction center where it can be utilized. If it moves randomly, much of the energy would be lost as heat before reaching its destination.

Early experiments suggested that instead of taking a single, random path, the exciton behaves like a quantum wave, using a phenomenon called quantum superposition to explore multiple paths to the reaction center at once. But later reviews of that evidence argue the long-lived signals were vibrations rather than electronic coherence, and that inter-exciton coherence is too short-lived to matter functionally — so whether quantum coherence actually boosts efficiency remains debated.

The Avian Compass

Another fascinating application of quantum biology takes place in the skies. Every year, millions of birds migrate thousands of miles across the globe, navigating with pinpoint accuracy. The European Robin, for instance, relies on a built-in compass that senses the Earth's magnetic field. For years, biologists struggled to find this compass. The breakthrough came when scientists discovered a light-sensitive protein called cryptochrome in the retinas of these birds. When light strikes a cryptochrome molecule, it displaces an electron, creating a pair of molecules known as a "radical pair." These two electrons form a radical pair with linked spin states. Because of this linkage, the pair is sensitive to external magnetic fields. The inclination of the Earth's magnetic field alters the spin states of these electrons, creating a chemical signal that helps the bird sense the field's direction. The bird may effectively "see" the magnetic field thanks to a quantum reaction happening in its eye.

Scent and Quantum Tunneling

Our own sense of smell may also rely on quantum mechanics. The traditional "lock and key" model of olfaction suggests that odor molecules fit into receptors in our nose based strictly on their shape. However, this model struggles to explain why some molecules with identical shapes but different isotopes (and therefore different vibrational frequencies) smell different to us — though tests of that prediction in humans have given mixed results. A competing theory proposes that our olfactory receptors act like quantum spectroscopists. Through a process called quantum tunneling, an electron within the receptor can jump across a physical gap if the odor molecule has the exact right vibrational frequency to absorb the energy difference. If the theory is right, we would not just be smelling the shape of a molecule, but the quantum vibrations of its atomic bonds — though the theory remains controversial.

Key Takeaways

• Efficiency of Life: Photosynthetic energy transfer is highly efficient; whether quantum superposition helps find the fastest path remains debated.
• Navigation: Migratory birds like the European Robin may use radical-pair spin dynamics in retinal cryptochrome to sense the Earth's magnetic field.
• Sensory Perception: Emerging theories suggest human olfaction might rely on quantum tunneling to differentiate between the vibrational frequencies of distinct molecules.

References

• Al-Khalili, J., & McFadden, J. (2014). Life on the Edge: The Coming of Age of Quantum Biology. Crown.
• Lambert, N., et al. (2013). "Quantum biology". Nature Physics.
• Engel, G. S., et al. (2007). "Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems". Nature.