Life on the Edge: The Coming of Age of Quantum Biology

Overview

The Coming of Age of Quantum Biology by Johnjoe McFadden and Jim Al-Khalili isn’t about pushing yourself to the limit, but rather exploring the surprising role of quantum mechanics in biological processes.

Key Concepts

The Quantum World Meets Biology

• Why quantum biology is surprising: Classical biochemistry treats molecules as tiny billiard balls governed by thermodynamics and Brownian motion; quantum effects (superposition, tunnelling, entanglement) were thought to be washed out by the warm, wet, noisy conditions inside living cells. McFadden and Al-Khalili argue that life has instead evolved to exploit quantum phenomena precisely because they confer functional advantages
  • The measurement problem in biology: Quantum coherence typically collapses on femtosecond timescales in warm environments; the central question is how biological systems might protect or harness coherent states long enough for them to matter functionally

Photosynthesis and Quantum Coherence

• Exciton energy transfer: In photosynthetic complexes (e.g., the FMO complex in green sulphur bacteria), absorbed photon energy must travel from antenna pigments to the reaction centre with near-perfect efficiency (~95%). Experiments using 2D electronic spectroscopy (Fleming lab, 2007) revealed long-lived quantum coherences in these complexes at physiological temperatures
  • Quantum walk: Rather than hopping randomly between pigment molecules (classical random walk), the exciton may simultaneously sample multiple pathways (quantum walk), arriving at the reaction centre faster and more efficiently — though the interpretation and biological relevance of these coherences remains debated
• Implications: If quantum coherence genuinely aids photosynthetic efficiency, it suggests that ~3.5 billion years of natural selection has optimised molecular architectures to exploit quantum mechanics — a profound intersection of physics and evolutionary biology

Quantum Tunnelling in Enzymes

• Proton and hydrogen tunnelling: Enzymes catalyse reactions at rates far exceeding what classical transition-state theory predicts; one proposed explanation is that protons and hydrogen atoms tunnel through energy barriers rather than climbing over them. The particle’s wave-function has a non-zero probability of appearing on the other side of a barrier that it classically could not surmount
  • Kinetic isotope effects: When hydrogen is replaced with the heavier isotope deuterium, reaction rates drop more than classical theory predicts — consistent with tunnelling, since the heavier particle has a shorter de Broglie wavelength and tunnels less efficiently
• Electron tunnelling in respiration: In the mitochondrial electron transport chain, electrons tunnel between iron-sulphur clusters and cytochrome complexes across distances of ~14 Å through protein medium — this is well-established quantum mechanics applied to bioenergetics

Magnetoreception — The Quantum Compass

• Radical pair mechanism: Some migratory birds (e.g., European robins) navigate using Earth’s magnetic field; the leading quantum hypothesis is the radical pair mechanism in cryptochrome proteins in the retina. Blue light excites a flavin-tryptophan radical pair whose singlet-triplet interconversion rate is sensitive to the orientation of the external magnetic field
  • Evidence: Behavioural experiments show that bird navigation is light-dependent (works under blue/green light, not red), disrupted by weak oscillating magnetic fields at radical-pair resonance frequencies, and localised to the right eye — all consistent with the cryptochrome hypothesis
  • Outstanding questions: Whether the quantum coherence time of the radical pair is long enough at physiological temperatures, and how the signal is transduced into a neural percept, remain open research problems

Speculative Frontiers

• Quantum olfaction: Luca Turin’s vibrational theory proposes that olfactory receptors detect molecular vibrations (via inelastic electron tunnelling) rather than just molecular shapes — explaining why molecules with similar shapes but different vibrational spectra can smell different. The hypothesis is provocative but lacks conclusive experimental support
• Quantum mutations: McFadden explores whether quantum tunnelling of protons along hydrogen bonds in DNA base pairs could cause tautomeric shifts that lead to point mutations — a quantum-mechanical origin for some genetic variation
• Consciousness and quantum biology: The book briefly touches on (but does not endorse) more speculative proposals linking quantum processes to consciousness (e.g., Penrose-Hameroff orchestrated objective reduction); the authors maintain a more cautious position focused on well-characterised quantum effects in specific molecular systems

Personal Reflection

[To be added]

Related Books

• Transformer - Lane explores deep chemistry; McFadden adds the quantum dimension — both argue physics shapes biology
• An Immense World - Yong describes bird magnetoreception; McFadden explains it via the radical-pair quantum mechanism
• What is Life? - Nurse asks what defines life; McFadden suggests quantum effects may be part of the answer

---

Parent: Books