The analogy between chemical and biological systems is not always a fruitful one. When I was pondering the question of the most efficient chemical rocket fuel, I noted that energy was always wasted in heating the exhaust. A hot exhaust jet does not deliver more thrust than the same mass expelled cold. Greater efficiency would therefore be obtained if the exhaust could somehow be at room temperature.
That sounds impossible, but we and all other animals have in our bodies large numbers of specialized proteins known as enzymes. The purpose of an enzyme is to control chemical reactions, making them proceed at much lower temperatures than usual, or faster or slower.
Suppose we build an "enzymatic engine" in which the chemical fuels are combined to release as much energy as usual, but the temperature remains low? We might then have a better rocket for launches to space.
I soon realized that the idea would not work, because as we point out in Chapter 8, the whole idea in using rockets for a launch is to burn the fuel as fast as possible. There is no way to achieve a fast burn, yet avoid a temperature rise in the fuel's combustion products. Forget that, then. But what about an enzymatic engine for other purposes? Say, to power a vehicle that moves on the ground. For such a use, slow and steady fuel consumption is preferable to a single, giant, near-explosion. There would be other advantages, too. The intense heat generated when we burn fuels such as gasoline is a big factor in a vehicle's wear and tear. The heat also generates nitrogen oxides, which are a serious form of air pollution.
Let us imagine, then, a method of ground transportation, powered by some kind of slow-burning enzymatic engine that can operate at close to room temperature. Such a device would have numerous uses, and be free of environmental problems.
I was expounding on this idea with some fervor when a more hardheaded friend of mine pointed out that I seemed to be designing a horse.
5.6 Fullerenes: a chemical surprise. Textbooks on inorganic chemistry for the past couple of centuries have stated, without a hint of doubt, that carbon occurs in two and only two elementary forms: diamond, and graphite. In diamond, the carbon atoms form tetrahedra, triangular pyramids with one carbon atom at each vertex and one in the center. This is a strong and stable configuration, so diamond is famously hard. In graphite, the carbon atoms form hexagons with an atom at each vertex, and the hexagons line up as layers of flat sheets. Since the sheets are not strongly coupled with each other, graphite is famously slippery and a well-known lubricant.
The discovery in 1985 of a third elementary form of carbon was a shock in two different ways. First, the existence of the third form could have been predicted, or at least conjectured, since the middle of the eighteenth century. In fact, its existence was suggested in 1966, as a piece of near-whimsical speculation, by a columnist in the New Scientist magazine. No one took any notice. Second, and almost a disgrace to a self-respecting chemist, the third form is not at all hard to make. In fact, it had been around, waiting to be discovered, in every layer of soot produced by a hot carbon fire. Every time you light a candle, at least some of the soot will be this new and previously unknown form of carbon.
I said, this new form of carbon, but actually there is a family of them. The simplest form, C60, is sixty carbon atoms arranged in a round hollow shape involving 12 pentagons and 20 hexagons. Technically, this form is called a truncated icosahedron, but the name is neither suggestive nor catchy. However, the structure looks exactly like a tiny soccer ball.
Leonhard Euler, the great Swiss mathematician, studied the possible geometry of closed spheroidal structures more than two hundred years ago, and proved that while they must have exactly 12 pentagons, the number of hexagons may vary. And vary they do. Continuing the sporting motif, the next simplest form, C70, is an oblong spheroid of 12 pentagons and 25 hexagons that closely resembles a rugby ball. And after that there are carbon molecules with 76, 84, 90, and 94 atoms, and still bigger versions that form hollow closed tubes. All of these are known by the generic name of "fullerenes," or if they are round, "buckyballs." The form with 60 atoms, C60, is the simplest, most stable, and most abundant form, with C70 in second place. Not surprisingly, C60 was the first form to be discovered.
So how was it discovered? Not, as one might think, by direct observation. The C60 molecule is less than a millionth of a millimeter across (about 7x10-10 meters), but it is big enough to be seen using a scanning tunneling microscope. It wasn't, though. It was found by a very curious and apparently improbable route. A British chemist, Harold Kroto, was studying how carbon-rich stars might lead to the production of long chains of carbon molecules in open space. In the United States, at the Houston campus of Rice University, American chemists Robert Curl and Richard Smalley had suitable lab equipment to simulate the carbon-rich star environment and see what might be happening.
The team did indeed find evidence of a variety of carbon clusters, but as the carbon vapor was allowed to condense, everything else seemed to fade away except for a 60-atom cluster, and, much less abundant, a 70-atom cluster. It seemed that there must be a very stable form of carbon with just 60 atoms, and another, rather less stable, with 70 atoms.
At this point, the team faced a problem. Carbon is highly reactive. If the cluster had the form of a flat sheet, like graphite, it ought to have free edges which would latch on to other carbon atoms, and so grow rapidly in size. The only way around that would be if the structure could somehow close in on itself, and tie up all the loose ends.
The research team was guided at that point not by the eighteenth-century mathematical researches of Euler, but by the geodesic dome idea of Buckminster Fuller. That, too, is a closed structure of pentagons and hexagons. With faith that a closed 60-atom sphere like a geodesic dome was the only plausible structure for the cluster, the researchers went ahead and named it "buckminsterfullerene." They did have the grace to apologize for such a mouthful of a name, and it was quickly shortened to "fullerenes" when it was realized that there was not one but a multitude of molecules.
The first fullerenes were produced in minute quantities. Research on them was therefore difficult. Then in 1990 a German team discovered a shockingly simple production method. By burning a graphite rod electrically, the resulting soot contained a substantial percentage of C60. Combining this with the suitable use of a benzene solvent, an almost-pure mixture of fullerenes was formed. Now anyone who wants fullerenes for research can easily buy them. And they are doing so, in ever-increasing numbers. The buckyball was named "Molecule of the Year" by Science magazine in 1991, and today the most frequently cited chemistry papers all seem to be on the subject of fullerenes. The 1996 Nobel Prize in chemistry went to Robert Curl, Richard Smalley, and Harold (now Sir Harold) Kroto.
One natural question is, all right, so fullerenes exist, and they are of scientific interest. But what are they good for, apart from winning Nobel Prizes? Potentially, many things. Because they are hollow, buckyballs can be used to trap other atoms inside them and to provide miniature "chemical test sites." They are phenomenally robust and stable, and could be the basis for materials stronger than anything we have today. They have been proposed as nanotechnology building blocks. They are already being used to improve the growth of diamond films. And they have interesting properties and potential as superconductors.
The best answer to the question, though, is that it is too soon to say. Like lasers in 1965, five years after the first one was built, fullerenes seem to be a solution waiting for a problem. And like lasers, fullerenes will almost certainly become enormously valuable technological tools in the next thirty years.
5.7 A burning home: the oxygen planet. Why is combustion normally referred to as combination with oxygen? Why did we discuss aliens breathing oxygen, and exhaling carbon dioxide?
Only because we live on a planet in which free oxygen is a major component of the atmosphere. We do not think of this as unusual, but we ought to. As pointed out earlier, oxygen combines readily—even fiercely—with other elements. A planet with an oxygen atmosphere is
unstable. If a world starts out with an atmosphere of pure oxygen, before long the normal processes of combustion will combine the oxygen to other, more stable compounds.
Clearly, that has not happened to the Earth. The presence of life, and in particular of plant life that performs photosynthesis, makes all the difference. Using the energy from sunlight, a plant reverses the process of combustion. It takes carbon dioxide and water, producing from them hydrocarbons, and releasing pure oxygen into the atmosphere. This is a dynamic, self-adjusting process. If there is more carbon dioxide in the air, plant activity will increase, serving to remove carbon dioxide and increase oxygen. Too little carbon dioxide, and plant growth decreases.
Because we grew up with this process, we tend not to realize how extraordinary it is. But the first life on Earth had to deal with an atmosphere containing no oxygen, but plenty of hydrogen. When the first photosynthetic organism (almost certainly, some form of cyanobacteria) developed, a huge but unchronicled battle took place. To hydrogen-tolerant life, free oxygen is a caustic and poisonous gas. To oxygen-tolerant life, free hydrogen is an explosive.
The oxygen-producers and oxygen-breathers won, to become oaks and marigolds and tigers and humans. The hydrogen lovers remain as single-celled organisms, the anaerobic bacteria.
Free oxygen is so much a hallmark of life, James Lovelock (Lovelock, 1979) insists that the detection of substantial amounts of oxygen in a planetary atmosphere would prove, beyond doubt, that life must be present there. The converse, as shown by the early history of Earth, is not true: absence of oxygen does not mean absence of life. The science fiction writer is free to suppose that life has developed on other worlds in an atmosphere of hydrogen, or oxygen, or methane, or nitrogen, or carbon dioxide, or many other gases. Combinations are permitted, as our own atmosphere shows. But if you take the route of an exotic atmosphere, the chemical consequences must be worked out in detail. No atmosphere, please, of mixed oxygen and hydrogen.
The master of the design of alien planets and biospheres is Hal Clement. If you want to see how carefully and lovingly it can be done, read his Mission of Gravity (1953—with hydrogen-breathing natives, no less); Cycle of Fire (1957); Close to Critical (1964); and Iceworld (1953). If you want to see fascinating and exotic worlds that won't stand up to such close scrutiny, consult Larry Niven's Ringworld (1970), or wander the wild variety of planets to be found in his multiple volumes known collectively as Tales of Known Space.
TABLE 5.1
Materials, potential strengths
Element pairs*
Mol wt
Bond
Strength to
(kcal/mole)
strength
weight ratio
Silicon/carbon
40
104
2.60
Carbon/carbon
24
145
6.04
Fluorine/hydrogen
20
136
6.80
Boron/hydrogen
11
81
7.36
Carbon/oxygen
28
257
9.18
Hydrogen/hydrogen
2
104
52.0
——————————
Muonium/muonium
2.22
1,528
9,679
Positronium/positronium
1/919
104
95,576
* Not all these elements exist as stable molecules.
CHAPTER 6
The Limits of Biology
6.1 The miracle molecule. You will read in many places that if the twentieth century was from the scientific point of view the century of physics, then the one after it will be the century of biology. That should make the frontiers of biology of special interest to a science fiction writer. The question then is, where do we begin?
Fifty years ago, a writer on the limits of biology might have had trouble deciding where to start. The biological world offers such a dazzling diversity of forms and creatures at every scale, everything from bacteria and viruses to mushrooms and elephants. Today, there is no such problem. We have to begin with a single molecule, an organic compound with a long name but a famous abbreviation.
Deoxyribonucleic acid, universally shortened to DNA, was discovered in 1869 by the German chemist Friedrich Miescher. It was (and is) found in the nuclei of the cells of most living things, but no one knew its structure, what it did, or how important it was.
DNA is one of a class of chemicals known as nucleic acids. By the beginning of the twentieth century, the components of the DNA molecule were known to be sugars, phosphates, and two types of two chemical bases known as purines and pyrimidines. The functions of the molecule were still obscure, though in 1884 a zoologist, Hertwig, had written that it was the way that hereditary characteristics were passed on from generation to generation.
He was right, but most people didn't accept what he said. So when, in 1943, Erwin Schrödinger gave lectures in Ireland on the mechanisms of heredity, he did not talk about DNA. He proposed, in his lectures and in a short and very readable book What Is Life? (Schrödinger, 1944), that the basis for heredity must be some kind of code, in which specific sequences of chemicals were written and interpreted; however, he assumed that the "code-script," as he called it, was contained in proteins, in the form of an aperiodic crystal.
Schrödinger was right, in that heredity, and all cell reproduction, depends on what we now term the genetic code. But it took another decade before the nature of the code and the structure of the code carrier were determined.
DNA, not proteins, carries the genetic code, for humans and for everything remotely like us. Nature is prodigal with DNA. In most (but not all; mature red blood cells lack a nucleus) of the 1014 cells of our bodies, we have the DNA to provide a complete description of the whole organism. Your DNA is in all important respects the same as the DNA in any other animal or plant, everything from a wisteria to a walrus. The same, that is, in all important respects but one: your DNA defines the unique you, the walrus DNA defines the complete walrus. In principle, given one cell from my body a full copy of me could be grown. This idea of "clones" has been widely used in fiction (Varley, 1977, 1979, 1980), with some of the fiction posing as fact (Rorvik, 1978). Sheep and other mammals have been cloned, but no one has yet cloned a human. We can look for that in less than twenty years, regardless of laws passed by those who disapprove of the concept on religious or ethical grounds.
The structure of the DNA molecule was determined by Crick and Watson in 1953. The story of their discovery is told in frank detail by Watson in his book The Double Helix (Watson, 1968). The title is appropriate, because the molecule has the form of a double helical spiral. Strung out along the spiral, at regular intervals, are molecule after molecule of the four chemical bases. Their names are adenine, cytosine, thymine, and guanine, and they are exactly paired. Wherever on one strand of the double spiral you find a cytosine nucleotide base, paired with it on the other strand you will find guanine; if there is thymine on one strand, on the corresponding site of the other strand there will be adenine. If we were to read off the sequence of nucleotide bases along a single strand, we would find a long string of letters, A-G-T-G-C-T-A-A-C-C-G-T-A- (we are using the obvious abbreviations). The corresponding sites on the other strand would then, without a choice, read T-C-A-C-G-A-T-T-G-G-C-A-T-.
Long strands of DNA nucleotide bases, each base with an accompanying sugar and phosphate molecule, make up the chromosomes found in the nucleus of every cell. Individual genes, with which the science of genetics is mainly concerned, are subunits within the chromosomes. The division of the DNA into many separate chromosomes (humans have forty-six of them) seems to be mainly a matter of packing convenience. Efficient packing is necessary. There are about three billion separate nucleotide bases in human DNA, tucked into a cell nucleus only a few micrometers across. The c
hromosomes that define your body and brain (though not its contents—writers of cloning stories beware) are invisible to the naked eye.
As an interesting sidebar to the development of life, not all the DNA in a cell of your body will be found in the nucleus. Some is located in other small units, known as mitochondria, that control cell energy production. However, the DNA in mitochondria is not your DNA. It belongs to the mitochondria themselves, and it is used to control their own reproduction. It seems that the mitochondria were originally independent organisms, but long, long ago they abandoned that independence in favor of a symbiotic relationship with other creatures.
The means by which the DNA molecule reproduces itself is simple and elegant: the double helix unwinds. One branch of the spiral goes in one direction, the other in the opposite direction. As each site on the helix is left with an unpaired base, the correct pairing, cytosine/guanine or adenine/thymine, takes place automatically (the pairs of bases have a natural chemical affinity). The correct base is collected from a pool of materials within the cell. At the same time, the necessary sugar and phosphates are added to the spine of the helix. When the double helix has finished unwinding, two new double helices, each identical to the original one, have miraculously appeared. The DNA molecule has reproduced itself.
This copying procedure is incredibly accurate, assisted by a "proofreading" enzyme called DNA polymerase that can correct mistakes. Very rarely, however, there will be a glitch, perhaps an A where the copy ought to have a C, or a short sequence duplicated or left out completely. If this occurs in the reproductive cells of an organism, the copying error will pass on to the offspring. A change in DNA due to imperfect copying, or accidental damage, gives rise to what we term a mutation. Most mutations have no apparent effect, and of those that do, most lead to changes for the worse. The offspring, if it survives at all, will be unable to perform as well as the parent. Occasionally, however, there will be a favorable mutation. The new version will be an improvement over the original, and produce its own superior offspring. This is the driving mechanism of evolution.
Borderlands of Science Page 13
