CHAPTER EIGHT—THE TRAPPING OF THE SUN

  The first and greatest discovery by man was the use of fire. That discovery, more than anything else, was the point at which he was raised from beast to man.

  The Greeks recognized the importance of the discovery and viewed it as a gift of the demi-god, Prometheus, who stole fire from the sun and brought it to naked and shivering man. To the Greeks, fire was a piece of the sun, trapped and made tame, bent to the use of man.

  If for ‘sun’, you say ‘energy’, the Greeks were right.

  When man learned to start a fire by rubbing two sticks together, he put at his own disposal, for the first time, a source of energy other than that contained in his own body. It was because man, with fire, had more energy at his disposal than had any other animal in creation that he became something more than animal.

  But man’s discovery some thousands of years ago was only an echo of a similar and even greater discovery made by a primitive bit of life perhaps a billion years ago.

  In the previous chapter, we left life a nucleoprotein molecule adrift in the primordial ocean. It was alive, but it had no source of energy but what happened to come its way. (It was like a man who had to wait for lightning to hit a tree before he could count on a bit of fire.)

  In this chapter we consider the way in which a microscopic organism anticipated Prometheus by a billion years and, to raise itself to higher estate, stole the fire of the sun.

  Let’s begin the story with ourselves here and now. Our body makes use of energy constantly. Our muscles contract. Our nerves carry electrical impulses. Our kidneys filter our blood stream. Our cells manufacture complicated molecules out of simple ones. All these things take energy. Where does it come from?

  We can be specific and take a chemical reaction such as the union of two amino-acids to form what is called a dipeptide. The dipeptide can join up with a third amino-acid to form a tripeptide; that with still another to form a tetrapeptide; then a pentapeptide; a hexapeptide; a heptapeptide; and so on indefinitely (or at least as far as your knowledge of Greek numeral prefixes will allow you to.)

  When enough amino-acids have combined with one another, a protein molecule is formed, so this type of reaction is the very basis of life. Without it, a nucleoprotein molecule could not duplicate itself out of the raw materials about it and without that, there could be no life.

  Yet there is a catch. Two amino-acids, if brought together, will not combine of their own accord. A dipeptide contains more energy than two amino-acids separately. Every time another amino-acid is pushed into line and bound to the peptide chain, the energy of the peptide is increased. That energy must come from somewhere.

  The amount of energy that has to be put into the assembling of each amino-acid varies from 0.5 kilocalories per mole to 4.0 kilocalories per mole, depending on the particular amino-acid involved. (If you happen to know what a ‘kilocalorie per mole’ is, I am happy for you. If not, it doesn’t matter. Just keep your eye on the numerals.)

  The body gets the energy it needs for this and almost all other similar jobs from ‘high-energy phosphate bonds’ present in its tissues (and in all living tissue).

  There are certain compounds, you see, the molecules of which contain a phosphate group (made up of a phosphorus atom, two hydrogen atoms and four oxygen atoms, -OPO3H2) that hangs on rather precariously to the rest of the molecule. The chemical bond between the phosphate group and the rest of the molecule is taut, in a manner of speaking, ready to give with a bang. When the phosphate group does break off, nearly 5 kilocalories per mole of energy are turned loose. That is more than enough energy to tie any two amino-acids together.

  The high-energy compound most used by the body for such jobs is called adenosine triphosphate. This compound carries no less than three phosphate groups in a line and we can write it A-P-P-P for short. Sometimes one phosphate group is knocked off, sometimes two.

  When the A-P-P-P breaks up, part of it sticks to an amino-acid in the vicinity and forms a high-energy amino-acid complex. The complex now contains enough energy to be able to attach itself to another amino-acid without trouble and while it is doing that, it lets go of the piece of the phosphate it was holding. That leaves a dipeptide. Repeat the process over and over and a protein can be built up.

  If all this wordage has you frowning just about now, try Figure 15, which says the same thing more schematically.

  The only trouble with all this is that someone is bound to ask: and where does the body get its high-energy phosphates from? After all, for every amino-acid stuck on to a peptide chain, one high-energy phosphate goes down the drain, and the body’s supply of such phosphates is exceedingly limited.

  Obviously, the body has to make high-energy phosphates as fast as they are used up—but how? To stick a phosphate group back on to the molecule from which it was broken requires just as much energy as was released by the original break; that means nearly 5 kilocalories per mole. (In matters of energetics, remember this above all: you can’t get something for nothing. That’s called the First Law of Thermodynamics.)

  Well, if the body has trouble putting amino-acids together at 4 or less kilocalories per mole a throw, how will it manage when faced with finding 5 kilocalories per mole?

  It seems there is another type of chemical, which biochemists have only grown to appreciate quite recently, called an acyl mercaptan, in which the key group of atoms is made up of a carbon, an oxygen, and a sulfur—(CO)-S. The acyl mercaptan is even more energetic than the high-energy phosphate. When the (CO)-S combination is broken, a little over 8 kilocalories per mole are let loose.

  That’s enough to form a high-energy phosphate bond.

  Only—and you’re ahead of me, I know—where do the acyl mercaptans come from? The body makes them, but how? Now it has to find 8 kilocalories per mole to put an acyl mercaptan back together again. (It’s like the question that used to plague me when I was young. You need tools of a particularly hard steel alloy to shape ordinary steel objects. Then you need tools of a harder steel to shape the hard-steel tools. Then you need tools of a still harder steel to shape—You get the idea.).

  To see where the acyl mercaptans come from, we have to consider the food we eat.

  Our food consists of a number of kinds of compounds but, as far as energetics is concerned, the two important classes are the carbohydrates and the fats. Both carbohydrates and fats are made up of carbon atoms, hydrogen atoms, and oxygen atoms, but not in the same proportions.

  Both carbohydrates and fats are slowly combined with oxygen (i.e. ‘oxidized’) in the body, through dozens of steps, until nothing is left but carbon and hydrogen atoms combined with all the oxygens they can hold. The final products are carbon dioxide (CO2) and water (H2O).

  We can summarize by writing the following:

  Carbohydrates (or fats) plus oxygen give rise to carbon dioxide and water.

  But carbohydrates and fats contain more energy than do the carbon dioxide and water molecules to which they give rise. The energy left over in the conversion is turned loose so that we should really write the following:

  Carbohydrates (or fats) plus oxygen give rise to carbon dioxide and water and energy.

  This last bit is obvious if carbohydrates or fats are strongly heated. Fats will begin burning. Carbohydrates will char first and then glow and burn slowly. Both will be converted to carbon dioxide and water and the energy released will be given off in the form of heat and light.

  The same quantity of energy, not an iota more nor an iota less, is given off when the carbohydrates and fats are combined with oxygen in the body. The chemical pathway of change in the slow oxidation in the body is radically different from that of the rapid burning in a flame, but the energy developed in either case is the same. (It’s the First Law of Thermodynamics again.)

  The big difference is that oxidation in the body, being slow, is under control. The energy given off is not in the form of a dancing flame pouring heat and light uselessly into space. Instead, the energy is given off in little spurts that are captured in neat packets in the form of high-energy compounds.

  The crucial step in oxidation within the body is the combination of hydrogen and oxygen. The hydrogens that occur in a molecule of fat or carbohydrate (or which are stuck on in the course of the chemical changes they undergo) are combined with oxygen—two hydrogen atoms for each oxygen atom. Every time two hydrogens are removed from a molecule and combined (via a number of steps) with an oxygen, 45 to 65 kilocalories per mole are released. This is more energy than even an acyl-mercaptan bond represents; 6 to 8 times as much.

  However, the energy of such a hydrogen-oxygen combination within the body is put into the formation of only two to four high-energy phosphates.

  The energy changes in the known steps from the food we eat to the protein built up in our tissues is shown schematically in Figure 16.

  Figure 16 should make one point clear that some people manage to scramble rather badly.

  It is the long experience of mankind that everything tends to run down. Clocks stop, iron rusts, water runs downhill, living creatures age and die, the hills weather and erode into sand, the earth’s rotation is slowing, the sun is using up its hydrogen.

  This is an important and universal rule—that everything is gradually running down—and scientists call it the Second Law of Thermodynamics.

  Some people have been impressed by the fact that life seems to have a contrary effect. A human being can wind a stopped clock, resmelt rusted iron, pump water uphill again, rejuvenate age by giving birth to young and so on. There is the feeling that there is something in life which is not subject to this running-down rule and therefore something which makes it superior to the laws of physics or chemistry.

  Not so.

  It is all very well to point out that man can take a lump of iron ore and a mess of bauxite and sand and clay and make steel beams and aluminum and glass and bricks out of them and put them altogether to make a beautiful skyscraper. This is ‘building up’ rather than ‘running down’,—it seems.

  But in order to bring this about, man has had to use a mess of energy in the form of burning coal to smelt the iron ore and fuse the sand and bake the clay and make the electricity that will separate the aluminum out of the bauxite. And human energy has had to be used, too. All this burning coal and sweating humanity represents a ‘running down’ that is much greater than the ‘building up’ involved in making the skyscraper.

  Our whole civilization depends on the running down (as fast as possible) of the energy content of the coal and oil reserves of the world. And the running down of these reserves and the energy they represent is much greater than the building up we manage to do as a result. It can’t be helped. The Second Law of Thermodynamics has never been broken yet.

  See how Figure 16, now, shows the way in which the human body runs down. You start with 45 to 60 kilocalories per mole when a pair of hydrogen atoms are united with oxygen. You end up with two to four amino-acid links which represent an investment of 1 to 16 kilocalories per mole. You’re building up your protein—at a 1 to 16 rate. You’re running down your food—at a 45 to 65 rate. Anywhere between 65 and 98 per cent of the energy of your food is just wasted. It is given off as heat and if you work hard, you will yourself note that one of your body’s chief concerns is to get rid of all the heat that is being produced at the same time that some work is being turned out.

  Since evaporating water will absorb heat, the body is designed to perspire. On humid days, when water will not evaporate very well, you feel completely miserable. It’s not the heat, you say, it’s the humidity. But it is the heat, just the same; the body heat you are developing and don’t want and can’t get rid of fast enough.

  Not only we, but all living creatures get by on the energy developed by converting carbohydrates and fats to carbon dioxide and water. All organisms use a small bit of the energy and throw the rest away. But then where does the supply of carbohydrates and fats come from? In a billion years or so, we haven’t run out.

  We, and other creatures as well, make our own, of course, but that scarcely counts since the energy required to make it come from energy developed by oxidizing carbohydrates and fats to begin with. And since you can’t beat the Second Law, the amount of carbohydrate and fat you must run down to get energy is greater than the amount you can build up with that energy.

  And it’s no use saying you get your fat or carbohydrate from milk, or beef, or eggs, or poultry or pork because cattle, chickens, and pigs are busy burning carbohydrates and fats much faster than they are storing them in their own tissues or in eggs and milk.

  No, if we are to have life continue for more than a short time, we must find a way of creating carbohydrates and fats by some method that doesn’t use up carbohydrates and fats. A new source of energy must be found.

  The green plant does the trick; it has trapped the sun. It has found a way of taking the energy of sunlight and using it to break the water molecule into hydrogen and oxygen. (The energy required to break the water molecule is about 65 kilocalories per mole, but to manage the trick, the plant has to use probably 100 kilocalories per mole of light energy; possibly up to 200 kilocalories per mole. The Second Law wins out again, but fortunately the supply of sunlight is virtually endless.)

  Some of the separated hydrogen and oxygen recombine to liberate enough energy to form three high-energy phosphates for every molecule of water reformed. These high-energy phosphates are used to supply the energy that will enable the rest of the hydrogen to combine with the carbon dioxide of the air to form carbohydrates and fats. Figure 17 presents the process (called ‘photosynthesis’) in schematic form.

  Notice that photosynthesis represents almost the exact reverse of the process that goes on in our body. In our body, it is:

  Carbohydrates (or fats) plus oxygen yield carbon dioxide plus water plus chemical energy.

  Carbon dioxide plus water plus solar energy yield carbohydrates (or fats) plus oxygen.

  The oxygen produced and the carbon dioxide used up in photosynthesis changed the atmosphere from its primordial composition of ammonia and carbon dioxide to the present composition of nitrogen and oxygen.

  To summarize then, green plants convert solar energy into chemical energy, and their cells then live upon the chemical energy stored in carbohydrates and fats.

  All animal life lives upon this chemical energy, too, either by eating plants or by eating animals that have eaten plants, or by eating animals that have eaten animals that have eaten plants, and so on. No matter how many animals can be forced into the one-eats-another chain, at the bottom is some green plant and that supports all the rest. This includes sea-life, where the one-celled plants, called algae, swarm in the surface layers of the ocean and form the foundation upon which rests all other marine life from worms to whales.

  How does all this apply to the lonely little nucleoprotein molecule adrift in the primordial ocean?

  The only chemical property we know it must have had is the ability to construct another molecule of itself out of simpler molecules such as amino-acids. But tying amino-acids together takes energy. Where did the nucleoprotein molecule get the necessary energy? From carbohydrates and fats?

  Probably! The ocean was swarming with organic molecules formed by the action of lightning on the primordial atmosphere and the action of ultra-violet rays from the sun upon the simple compounds in the ocean. The end result must have included the simpler carbohydrates and fats. But there was no oxygen in the primordial atmosphere. The first step in getting energy is to combine the carbohydrates and fats with oxygen. Well, then?

  The most common solution to this problem involves a process known as glycolysis. In glycolysis, a molecule of glucose (a simple sugar) which contains 6 carbon atoms, 12 hydrogens and 6 oxygens is split (via a number of steps) into two molecules of lactic acid, each made up of 3 carbons, 6 hydrogens and 3 oxygens. Enough energy is released by this split to form 2 high-energy phosphates.

  Glycolysis is inefficient in comparison with the complete oxidation of glucose to carbon dioxide and water. That complete oxidation would give rise to no less than 32 high-energy phosphates. But glycolysis has this advantage: it doesn’t require molecular oxygen. Even today, when there is plenty of oxygen in the air, tissues sometimes make use of glycolysis when the demands for energy are greater than the rate at which oxygen can be supplied. Muscles, when engaged in active work, make use of glycolysis. Embryonic tissue, which is chronically short of oxygen, makes use of glycolysis to a certain extent.

  Presumably, then, the primordial nucleoprotein molecules made use of glycolysis to make their high-energy phosphates and got along without molecular oxygen.

  But how does the nucleoprotein bring about all the necessary changes? How does it split glucose molecules and make high-energy phosphates and split those and combine amino-acids and so on? It is so easy to say ‘the nucleoprotein does this and the nucleoprotein does that’, but how does it do it?

  Which brings us to the question of catalysis.

  There are a great many reactions which take place readily when the conditions are right, which take place scarcely at all when the conditions are not right.

  For instance, suppose it was vitally necessary for you to make a certain notation and you had both a pencil and a piece of paper in your possession. Suppose, however, you were standing in the middle of a vast and featureless plain, built of undulating sand. You would have nothing to rest the paper on and you could make your notation only with great difficulty and probably not very legibly.

  Suppose, however, a flat board of some hard smooth material suddenly appeared. Using that to write on, you would have no problem. The job could be done quickly and well.