THE RANDOM AND THE STABLE IN CHEMISTRY-BASED LIFE

Temperature affects how fast you can think and feel as well as how fast you can move and how fast you metabolize food. This may sound trivial. In fact, few people would be surprised to learn that temperature affects his or her performance as a living organism. But after only a few minutes thought, this simple statement about temperature and performance lead us to some of the most fundamental questions of physics and philosophy. We are quickly led to the boundary of what we can know as human beings.

Life lives in a narrow range of temperatures

Terrestrial life lives in a narrow range of temperatures, compared with the large range of temperatures observed in the universe. It is easy to see why this is true. If life gets too cold, it freezes into a solid. At the coldest it can be, called Absolute Zero, matter is just a slightly shivering frozen mass, incapable of even the most boring activity. If the temperature is as high as it is on the Sun, then terrestrial life turns into a random gas of molecules that spreads over the Sun and into space. At high temperatures, life is no more interesting than hot air in a balloon.Life must exist as both a solid and a fluid. Life must have some parts that are solid so that life in the form of organisms can persist for long periods of time. This is needed so that the blueprint or biological organization chart for life can be preserved over extended periods. It must have other parts that behave like a fluid and can adapt to changing conditions around the organism on short time scales.

Life must exist as both a solid and a fluid simultaneously.

Life must exist as both a solid and a fluid simultaneously. How this comes about leads to one of the most fascinating, but little known, stories in science. A botanist discovered the first clues. In 1827, the botanist Robert Brown noticed that very small grains inside pollen cavities experienced jittery random motion. The motion came to be called Brownian Motion. The mechanism for the motion was not clear. Thoughts at the time were that it might be due to some effect of light on the particles, or perhaps some hydrodynamic forces. Understanding the mechanism had to wait for eighty years until Albert Einstein, in his miracle year of 1905, looked at the problem. At that time, it was not entirely clear to physicists and chemists that matter was composed of very small atoms and molecules. Einstein was able to show that, if atoms and molecules existed, then the random collisions would lead to the observed motion of the microscopic grains. This is not at all obvious. If atoms existed, then they were certainly very small, much smaller than the grains they were pushing around. A single collision of an atom with a grain would not be noticeable. Einstein showed, however, that if the grains were surrounded by millions of atoms, then occasionally enough atoms would strike the grain at one time in the same direction as to observably move the grain. Matter is composed of atoms and molecules. This insight was important not specifically for the understanding of the motion of pollen grains. The implications were much more far-reaching. It firmly established that matter was composed of atoms and molecules. Moreover, the world of the very small was much different than the world of the macroscopic. Small objects were constantly being knocked and tumbled. An air molecule, for instance, is struck by other air molecules one billion times each second.

The microscopic world is violent and chaotic

The microscopic world that Einstein described is violent and chaotic. Objects are under constant bombardment at rates that would be unimaginable to macroscopic objects like us. Not only are molecules being knocked around, but occasionally the molecules react leading to other different molecules. The more molecules present, the more often were collisions that lead to chemical reactions. Also, the greater the energy of the molecules, the more often were the reactions. Chemical reactions do not occur often if the number of molecules bombarding each other is small. For a molecule to change from one state to another requires that the molecule exist in a sea of other molecules.

If molecules are being bombarded constantly, then how can they lead to stable life?

There is a problem though. Our condition for life is that life must exist stably for some period of time. If molecules are being bombarded constantly, then how can they lead to stable life? We noted that chemical reactions occur only occasionally. This an empirical observation. We know this is true, because we can measure how fast molecules react. But why is this true? If collisions that lead to chemical reactions were as common as collisions in general, then the atomic makeup of the molecules would be under constant flux. They would not be stable. As Schroedinger pointed out in his 1944 lectures, the reason chemical reactions are slower than the rate of collisions traces back to quantum mechanics. Historically, Max Planck observed that light could only appear as packets of energy. Einstein, again in his miracle year, explained this in terms of something known as the photoelectric effect. This is the removal of electrons from metals by shining light on the metal. Einstein was able to show that indeed light was composed of quanta of energy. These quanta, in the case of light, are known as photons.

At biological temperatures, the number of collisions with electrons that can move the electron out of an orbital is very small

Quantum theory describes how electrons around nuclei exist in orbitals, convoluted clouds of electrons. Specifically, in atoms, each orbital represents a different discrete energy. A discrete and specific quantum of energy is required to move an electron from a lower energy state to a higher energy state. If a smaller bit of energy comes along, then the electron will not be moved out of its orbital. Chemical reactions require electrons to move between or completely out of their orbitals. If the energy bombarding the atom is not sufficient to move electrons out of an orbital, then no chemistry occurs. At biological temperatures, the number of collisions with electrons that can move the electron out of an orbital is very small. The atoms and molecules remain unchanged.

Our second requirement for life is that life must do something in its environment repeatedly

In order for a molecule to do something, it must sense its surroundings and be constantly affected by it. We can see how this affects life. Our second requirement for life is that life must do something in its environment repeatedly. In order for a molecule to do something, it must sense its surroundings and be constantly affected by it. The life must constantly interact with its surroundings in order to act on it. Collisions are the way that life samples its environment. On the other hand, the collisions cannot be so great that order in life is completely disrupted. The quantization of energy states in atoms solves this problem. The energy of the bombarding molecules is great enough to move electrons between energy states that are very close in energy, but not between energy states that are very different in energy. There is a very delicate balance between the energy of the bombarding molecules and the energies of the atomic orbitals.This balance allows chemistry-based life to simultaneously satisfy competing requirements of stability and metabolism

Balance between stability and metabolism

The energy of electrons in atomic orbitals is usually measured in electron-volts (eV) by physicists. Chemists use a different unit of energy. It is not important to know exactly what an electron-volt is; it is only necessary to know that it takes 13.6 eV to rip an electron from out of its lowest energy level in a hydrogen atom. This gives us a baseline. Typically it takes a few or a fraction of an electron-volt to move an electron between orbitals. For comparison, the energy at room temperature of the kinetic energy of the bombarding molecules is about 1/40 eV. This number is small enough such that many electrons in orbitals will only occasionally be affected by a collision, yet large enough such that the electrons will be affected sometimes. This balance allows chemistry-based life to simultaneously satisfy competing requirements of stability and metabolism, that is, actually doing something.

Hydrophilic and Hydrophobic bonds

Let’s talk about an important example in biology. A very important set of “forces” in biology are due to the hydrophilic and hydrophobic effect. These forces are much weaker than the quantum-based covalent forces that hold molecules together, yet they are strong enough to cause bonding of molecules on short time scales. Because these two types of bonds are weaker than covalent bonds, random collisions break them much faster than covalent bonds. They are thus responsible for biological activities on rapid time scales.

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