Quantum Coherence in Photosynthesis

Human beings are profoundly affected by quantum physics.  I have asserted this in my previous post and have used this as evidence for God who is an intelligent power behind the Quantum Veil.  So, what is the actual scientific evidence that such quantum effects occur in living organisms?  The most striking evidence to date comes from analysis of plant photosynthesis.  The amazing thing about plant photosynthesis is that it converts sunlight into food with almost 100% efficiency.  The theoretical limit for currently designed solar panels is about 40% efficiency. The solar panels that you can buy and put on your roof do no better than 15% efficiency.  In this case, learning how nature does this remarkable conversion of sunlight could help us solve our energy problem.  Go outside on a hot summer day and feel a leaf from a typical tree.  It will feel cool to the touch when other things in direct sunlight such as concrete, metal, and wood planks will feel hot.

The evidence for quantum effects during photosynthesis was published in 2007 in the Journal Nature (“Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems”, G. S. Engel, et. al., Vol. 446, page 782, 12 April, 2007).  This work was done by a team at the Lawrence Berkeley National Laboratory.  The reason that the efficiency is so high in living plants is that the entire process of transmitting sunlight energy inside the leaf takes place while that energy remains in a quantum state.  This is remarkable because there are many steps in the transmission of energy that had previously been thought to require an energy hopping scheme for transmission.  Quantum transmission is inherently more efficient than predicted by the energy hopping theory.

Photosynthesis takes place in two stages within Chloroplasts which are one part of some plant cells in the leaf.  Stage one is the light harvesting phase where sunlight energy is gathered by chlorophyll molecules.  Stage one takes place within Thylakoids inside the Chloroplasts and there are two main parts: the light harvesting antenna which gathers sunlight energy and a reaction center which initiates the chemical conversion of water and carbon dioxide into plant food.  Stage two is initiated by stage one and is the chemical production of plant sugars for food.  Stage two is fueled by energy in the form of free electrons from stage one and takes place in the body (Soma) of the Chloroplast.  Stage one is sometimes called the light phase and stage two, the dark phase.

Chlorophyll is the primary molecule used during stage one of photosynthesis.  This molecule has different types, each of which is sensitive to a broad spectrum of light color.  Overall, chlorophyll is more sensitive to the blue and red colors than to green and therefore green light is reflected, giving leaves their characteristic green color.  (An interesting side note is that when deciduous trees shed their leaves in the fall season, the color change is due to the absence of the green Chlorophyll which has been shielding the underlying colors during the growth season.  Leaves don’t really change color so much as reveal existing colors already present.)  Sunlight comes as a mixture of different colors, from low energy red to high energy violet; green is in the middle.

The light harvesting antenna inside the Thylakoids are densely packed Chlorophyll molecules embedded in protein scaffolding.  When a photon of sunlight energy is captured by the light harvesting antenna, it must be transported to the reaction center where it can initiate the conversion of water and carbon dioxide into plant food.  The main theory for the transport process prior to evidence for quantum coherence was based on Forster resonance energy transfer (FRET) theory.  The Forster-based theory proposed that the photon energy randomly hopped from chlorophyll molecule to chlorophyll molecule until it reached the reaction center.  This was a semi-classical description with few quantum effects because it had usually been supposed that quantum coherence could not last long enough in living systems for the complete transmission.

All that changed in 2006 when the Lawrence Berkeley team used advanced microscopy technology to capture data that unambiguously demonstrated long-lived quantum coherence in biological systems.  The actual experiments were performed on a popular target of research named the FMO (Fenna-Matthews-Olson) complex.  Although the initial experiments were done at cryogenic temperatures (77 degrees Kelvin), they were soon replicated, with little loss of quantum coherence, at room temperature (300 degrees Kelvin).  In these experiments, quantum coherence lasted up to 1 picosecond (one trillionth of a second), about 20 times longer than usually assumed for these biological systems.  (The original press release from the Lawrence-Berkeley Labs can be found here: http://www.lbl.gov/Science-Articles/Archive/PBD-quantum-secrets.html).

The demonstration of long-lived quantum coherence in plant photosynthesis proves it is possible for living system to make use of quantum effects.  It is still an active area of research how quantum effects lead to high efficiency of sunlight conversion.  But there is sufficient confidence in these findings that projects exploring artificial photosynthesis have been funded with the hope that one day we will have high efficiency solar power.

Plant life and animal life are the two main categories of life on earth.  Plants definitely use quantum effects to their evolutionary advantage when they convert sunlight into food.  It has been proposed that animal life also makes use of quantum effects through the ubiquitous microtubules.  While this has not been conclusively demonstrated, why wouldn’t evolution make use of every benefit at its disposal?

Quantum theory predicts some very remarkable characteristics such as entanglement, energy tunneling, and multiple state superposition.  These are all counter-intuitive properties.  Albert Einstein called quantum entanglement “spooky action at a distance”.  Energy tunneling allows particles to penetrate a so-called “impenetrable barrier.”   And quantum state superposition allows a particle to be in more than one place at a time. Superposition is the principle behind Schrodinger’s cat paradox, where Schrodinger’s pet cat is inside a box with a mechanism for dispensing poison that is triggered by a single particle.  Since it cannot be determined from outside the box whether the poison has been released, the cat is presumed to be in a superposition state of both alive and dead until the box is opened and the cat is actually observed.

There is a fascinating book on the strange and mysterious properties of quantum physics.  It is titled Quantum Enigma: Physics Encounters Consciousness, by Bruce Rosenblum and Fred Kuttner (Oxford University Press, 2006).  This book is written for the layperson and uses written narrative rather than mathematics to describe the strange quantum world.  Science cannot explain consciousness, but the study of quantum physics leads inexorably to an encounter with the nature of consciousness.  There are undoubtedly many surprises awaiting our discovery and we may find that the quantum world is the source of our remarkable experience of consciousness.