Dear ScarletReinder, I appreciate your thoughtful comments. Defining life is complex; that's why I consider a bacterium to be the smallest living system, without controversy. I'll revisit this topic later.
Obviously, as I mentioned and also as you noted, the mechanics that describe microscopic systems is quantum mechanics (QM). On the other hand, as I stated in the essay, if QM is more fundamental than classical mechanics (CM) and contains it, then, ultimately, CM, which describes the macro world, manifests itself as an average of QM, which we have historically called precisely CM. In other words, these two extremes, micro and macro, are related to QM. On the other hand, we can also consider, for example, living beings on an intermediate (mesoscopic) scale, such as unicellular organisms that live in a world where there is Brownian motion or molecular agitation due to the presence of matter quanta. We could say that, in this unicellular world, there is something quantum. However, for conscious beings capable of asking these questions, the size may need to be larger, so that the result of molecular collisions cancels out. Perhaps we are too large to experience QM in life directly. However, a macroscopic effect on life can be observed, for example, with the replacement of ordinary water with heavy water. Effects governed by classical thermodynamics, such as altered diffusion, are seen, but reaction rates slow or even stop due to the inhibition of quantum tunneling in the exchange of hydrogen for deuterium, which has twice the mass but whose effect on tunneling is exponential.
Another issue concerns the application of smooth, continuous laws of CM. The continuous, differentiable mathematics we use in CM is a limit as N, the particle number, approaches infinity or continuum matter — that is, an idealization. In other words, CM lacks the precision to describe very fine details. We use CM in the macro limit because it is easier to describe this limit than with QM. If this limit is sufficiently accurate to describe an event, we adhere to CM; otherwise, we require QM.
Biochemical effects, such as molecular machines, could be simulated by quasi-classical artifacts that follow specific rules, which seem classical but are fundamentally quantum molecular in nature. For example, ATP synthase involves the mechanical rotation of subunits; the ribosome moves linearly along the mRNA; helicase exhibits helical and linear movement along the strand; DNA polymerase performs linear polymerization, "walking" along the template strand; kinesin takes step-by-step 'walks' along microtubules; dynein moves linearly with coordinated bending; bacterial flagellum involves reversible rotation of the basal motor; and so on. At larger scales, quantum effects tend to exhibit more classical characteristics.
Now, the question of the open system arises. Even without precisely defining what life is, a basic assumption would be that it must be described as an open system, far from thermodynamic equilibrium. Thermodynamic death would occur when a system reaches equilibrium and becomes isolated from its environment. The concept of open systems is crucial to understanding life. An embryo and a seed are only viable in open systems—specifically, in the systems where they evolved. As RoseWren put it. Probably, for a virus, the system needs to be more open than for a bacterium, since its metabolism may depend more on its environment. A host carrying a virus could be seen as a more complex form of life. The endosymbiotic theory of mitochondrial and chloroplast formation could be a successful example of this. We can also view the concept of a living individual as an approximation, since it is not possible to precisely separate living systems from their environments. Every living organism has limitations in space and time, from the most sensitive to the most resilient, such as extremophiles.
I have more questions than answers. Still, I believe we need to combine our knowledge to better understand life and the universe. Thank you, ScarletReindeer and RoseWren, for your comments.
