422 9.5 Extending Length and Time Scales to Quantum and Ecological Biophysics
9.5 EXTENDING LENGTH AND TIME SCALES TO QUANTUM AND
ECOLOGICAL BIOPHYSICS
Established biophysical tools have focused mainly on the broad length scale between the
single molecule and the single cell. However, emerging techniques are being developed to
expand this length scale at both extremes. At the high end, this includes methods of investi
gating multiple cells, tissues, and even large whole organisms of animals and plants. Beyond
this organismal macroscopic length scale is a larger length scale concerning populations of
organisms, or ecosystems, which can be probed using emerging experimental and theoret
ical biophysical methods. In addition, there are also biophysical investigations that are being
attempted at the other end of the length scale, for far smaller lengths and times, in terms of
quantum biology effects.
9.5.1 QUANTUM BIOLOGY
Quantum mechanical effects are ubiquitous in biology. For example, all molecular orbitals
have an origin in quantum mechanics. The debate about quantum biology, however, turns on
this simple question:
Does it matter?
Many scientists view “everyday” quantum effects as largely trivial. For example, although
the probability distribution functions for an electron’s positions in time and space operate
using quantum mechanical principles, one does not necessarily need to use these to pre
dict the behavior of interacting atoms and molecules in a cellular process. Also, many
quantum mechanical effects that have been clearly observed experimentally have been at low
temperatures, a few Kelvin at most above absolute zero, suggesting that the relatively enor
mous temperatures of living matter on an absolute Kelvin scale lies in the domain of classical
physics, hence the argument that quantum effects are largely not relevant to understanding
the ways that living matter operates.
However, there are some valuable nontrivial counterexamples in biology that are diffi
cult to explain without recourse to the physics of quantum mechanics. The clearest example
of these is quantum tunneling in enzymes. The action of many enzymes is hard to explain
using classical physics. Enzymes operate through a lowering of the overall free energy barrier
between biochemical products and reactants by deconstructing the overall chemical reaction
into a series of small intermediate reactions, the sum of whose free energy barriers is much
smaller than the overall free energy barrier. It is possible in many examples to experimen
tally measure the reaction kinetics of these intermediate states by locking the enzyme using
genetic and chemicals methods. Analysis of the reaction kinetics, however, using classical
physics in general fails to explain the rapid overall reactions rates, predicting a much lower
overall rate by several orders of magnitude.
The most striking example is the electron transport carrier mechanism of oxidative phos
phorylation (OXPHOS) (see Chapter 2). During the operation of OXPHOS, a high-energy
electron is conveyed between several different electron transport carrier enzymes, losing
free energy at each step that is in effect siphoned off to be utilized in generating the biomol
ecule ATP that is the universal cellular energy currency. From a knowledge of the molecular
structures of these electron transport carriers, we know that the shortest distance between
the primary electron donor and acceptor sites is around ~0.2–0.3 nm, which condensed
matter theory from classical physics is equivalent to a Fermi energy level gap that is equiva
lent to ~1 eV. However, the half reaction 2H+ + 2e−/H2 has a reduction potential of ca. −0.4 eV.
That is, to transfer an electron from a donor to a proton acceptor, which is the case here that
creates atomic hydrogen on the electron acceptor protein, results in a gain in electron energy
of ~0.4 eV, but a cost in jumping across the chasm of 0.2–0.3 nm of ~1 eV. In other words,
the classical prediction would be that electron transfer in this case would not occur, since it