Metabolism


Isn’t it interesting that so many cultures have historically honored or revered the sun as the source of life — and then science found that the sun is indeed the source of the energy for our individual lives and for the life of the entire earth ecosystem. And we know it is accomplished by metabolism, and we know how metabolism functions to convert the energy from sunlight into organic energy that the ecosystem then uses for every little bit of work in every cell that it requires to stay alive.

According to Webster’s Handy College Dictionary, metabolism is: “The chemical process of absorbing food.” The computer says: “the ongoing interrelated series of chemical interactions taking place in living organisms that provide the energy and nutrients (that is food) needed to sustain life.” I don’t like either of those definitions, and it is easy to find books, videos, all sorts of information about human physiology that tell us how we eat and absorb our food and get rid of the waste products. So I also won’t discuss those in detail, for the same reason. All these resources tend to be so anthropocentric (human centered, as though the whole process is all about us) that many people imagine, as did the primitive peoples, that food is magically provided for us on this earth and the only thing we need to do is find it and eat it. This is so far from the reality that we are threatening the health of the ecosystem by our voracious finding and eating.

The reality is that our food is only a subunit, a natural part of the whole system of energy flow through the ecosystem that is and must be maintained in a delicate balance if the entire ecosystem is to be properly nourished. Because we can not survive without the ecosystem, it is important that we know as much as we can about the entire system if we are to help it maintain that balance so we can continue within it.

The whole scheme of energy flow in the ecosystem is built upon the first and second laws of thermodynamics. How was the whole system created? We have no primary data. But we do have measurable facts that describe how the flow of energy NOW maintains the life of the ecosystem minute by minute and day by day in our physical universe, and that will be my definition of metabolism.

The incredible beauty of the living process is the way in which organisms convert the light energy into organic form to circulate the energy throughout the ecosystem so that it can be used to do all the tiny and huge kinds of work that all of life requires. What I am calling organic energy consists of several sorts of energy relationships — energy bonds that join atoms together to make molecules.

A molecule is a group of atoms that are joined together by energy bonds. Big molecules have more energy bonds than small molecules. Organic molecules are big molecules. They are composed mostly of atoms of carbon, hydrogen, oxygen and nitrogen because of the ways in which they are capable of joining with each other. We will remember again what you probably already know, that the major organic molecules are proteins, carbohydrates, lipids and nucleic acids.

We will not try to describe all the chemistry of these reactions for two reasons:

1. That sort of information is rather easy to find in books, text books and on the web and it is usually presented more or less accurately — though out of context.

2. The context is the most important part if we want to survive within the living earth ecosystem. So that’s what we will talk about. However, we do need to know a couple things about molecules and energy. So for now, we will make one point of chemistry that matches the point we made recently about the second law of thermodynamics.

The major organic molecules are actually macromolecules. Macromolecules are big molecules that are made of smaller molecules that are joined together by energy bonds. Proteins are made of long strings of amino acids; large carbohydrates are composed of smaller carbohydrates; lipids are composed of smaller lipids; nucleic acids are made of nucleotides. Just to give you an idea of comparative sizes of these molecules, the smaller subunits might consist of 12 to 25 or 30 atoms joined together, while the larger sizes can contain hundreds or thousands of the smaller molecules. All held together with energy bonds. So there is more energy in big molecules than there is in little molecules.

When you remove or break the energy bonds of a large molecule, it comes apart. For example, when a protein is “digested” in your gut it is broken down to the amino acids of which it is composed. In organisms, both the joining together and the taking apart is controlled by other molecules in the cell. Every cell is a hotbed of molecules doing this kind of work and other kinds of work that keep the cell alive.

If that is confusing, please let me know so I can make it more clear because from now we will refer back to this idea. Living cells are able to make large organic molecules from their subunits by adding energy to the system. This requires work because it is an “uphill” process relative to the second law of thermodynamics. Living cells are also able to break down the large molecules. This is a “downhill” process according to the second law, and so of course it could happen spontaneously with the release of energy. However, these are strong bonds so it might take as long as a mountain falling down if we wait for it to fall apart by itself, and the energy would be of no use to us in that case. Living cells, therefore, also have systems to break apart large molecules into smaller molecules, in a controlled way, at the right time when the energy is needed, and save the energy and circulate it around in the cell to do whatever work is needed for the cell to stay alive.

VERY IMPORTANT POINT. At every step of using the energy that is released, about 10% of the energy is lost from the system as a byproduct in the form of heat energy that can not be used to run the metabolism of life. That’s another manifestation of the second law. The WHOLE SYSTEM — the whole ecosystem — takes energy from the sun and converts it to organic energy that it shunts around in the ecosystem to do the work of staying alive. However energy does not RECYCLE. When it is used up it goes away as heat energy and it is gone from the system forever. Light energy comes in, it gets used and converted to heat energy and it goes away. As always, this balance must be maintained if life is to be.