Wild Science

As I was on my morning birdwatching tour in Costa Rica (Monte Verde, “The Cloud Forest”), I was searching hard for the big birds: toucans, quetzals, trogons. Then, the guide pointed out a tiny bird, barely visible in the dense forest: the rufous-tailed hummingbird. Little did I know that this little cutie constantly uses quantum shortcuts to cheat death. Hummingbirds flap their wings up to 80 times per second and require so much energy that if they were humans-sized, they would need to drink a can of soda every minute just to survive! They are always 10 minutes away from starvation. They must have some crazy metabolism! Enter, quantum tunneling…

Hummingbirds rely on a foundational process called quantum tunneling within cellular respiration that occurs in all animals but is pushed to its absolute physical limits to support the hummingbird’s extreme metabolism. In the time it takes you to read this sentence, a typical hummingbird would flap its wings 150-320 times! Because hummingbirds have the highest metabolic rate of any vertebrate–with hearts beating up to 1,200 times per minutes–classical physics alone cannot account for the speed at which they generate energy. They exploit quantum mechanics to shortcut energy barriers and keep their cells alive!

To understand this incredible process fully, we dive into the hummingbird’s mitochondria; inside the mitochondria, cellular respiration (the process through which living organisms produce energy/ATP from glucose) relies on the Electron Transport Chain (ETC). This chain moves electrons through a series of protein complexes. This flow of electrons powers the pumping of protons, which ultimately drives the synthesis of ATP. Now that we understand the basics, let’s analyze what would occur if this process were explained using classical chemistry. In classical chemistry, for an electron to move from one protein complex to the next, it must acquire enough thermal kinetic energy to “climb over” an activation energy barrier. Also, the molecular gaps between their proteins are relatively wide. If electrons had to rely solely on classical “hopping”, the energy generation process would be far too slow to sustain complex life, let alone a hummingbird.

So what really occurs? To overcome the gaps, the hummingbird’s mitochondria exploit a quantum property known as wave-particle duality. Because electrons are infinitesimally small, they behave not just like solid pool balls but like spread-out waves of probability. When an electron encounters an energy barrier in the ETC, its probability wave naturally extends through the energy barriers. If the next complex is close enough, there is a non-zero chance that the electron will instantly appear there. This is quantum tunneling.

While this electron tunneling occurs in humans and other mammals, evolution has optimized the hummingbird’s cellular architecture to make it incredible robust. First, the proteins inside the hummingbird’s mitochondria are precisely aligned to hold the exact “sweet spot” distance required for maximum tunneling efficiency. Second, their mitochondrial protein complexes act as highly specialized shields, minimizing molecular noise so that quantum tunneling can repeat without interruption. Lastly, the high-speed enzymes responsible for breaking down the sugars from nectar also rely on proton tunneling to accelerate metabolic chemical reactions.

Did you know that without quantum tunneling, life would fail for three interconnected reasons:

1. The Sun would go dark–Quantum tunneling allows protons in the Sun to bypass electrical repulsion and fuse at temperatures lower than classical physics requries.
2. Enzymes would fail–As we discussed earlier, vital biological enzymes inside our cells use tunneling to rapidly move electrons and protons across molecular barriers, accelerating metabolic reactions.
3. DNA Mutations (driver of evolution)–Protons can spontaneously tunnel across the hydrogen bons of DNA strands, causing structural changes during replication that drive the genetic variation necessary for evolution.

Thank goodness for quantum tunneling!