Asimov's New Guide to Science

by Isaac Asimov

  Thus, the skeleton of the process of photosynthesis was established. In sunlight, a plant takes up carbon dioxide and combines it with water to form its tissues, giving off “left-over” oxygen in the process. Hence, it became plain that green plants not only provide food but also renew the earth’s oxygen supply. Were it not for this renewal, within a matter of centuries the oxygen would fall to a low level, and the atmosphere would be loaded with enough carbon dioxide to asphyxiate animal life.

  The scale on which the earth’s green plants manufacture organic matter and release oxygen is enormous. The Russian-American biochemist Eugene I. Rabinowitch, a leading investigator of photosynthesis, estimates that each year the green plants of the earth combine a total of 150 billion tons of carbon (from carbon dioxide) with 25 billion tons of hydrogen (from water) and liberate 400 billion tons of oxygen. Of this gigantic performance, the plants of the forests and fields on land account for only 10 percent; for 90 percent we have to thank the one-celled plants and seaweed of the oceans.

  CHLOROPHYLL

  We still have only the skeleton of the process. What about the details? Well, in 1817, Pierre Joseph Pelletier and Joseph Bienaime Caventou of France—who were later to discover quinine, strychnine, caffeine, and several other specialized plant products—isolated the most important plant product of all—the one that gives the green color to green plants. They called the compound chlorophyll, from Greek words meaning “green leaf.” Then, in 1865, the German botanist Julius von Sachs showed. that chlorophyll is not distributed generally through plant cells (though leaves appear uniformly green), but is localized in small subcellular bodies, later called chloroplasts.

  It became clear that photosynthesis takes place within the chloroplasts and that chlorophyll is essential to the process. Chlorophyll was not enough, however. Chlorophyll by itself, however carefully extracted, could not catalyze the photosynthetic reaction in a test tube.

  Chloroplasts generally are considerably larger than mitochondria. Some one-celled plants possess only one large chloroplast per cell. Most plant cells, however, contain as many as 40 smaller chloroplasts, each from two to three times as long and as thick as the typical mitochondrion.

  The structure of the chloroplast seems to be even more complex than that of the mitochondrion. The interior of the chloroplast is made up of many thin membranes stretching across from wall to wall. These are the lamellae. In most types of chloroplasts, these lamellae thicken and darken in places to produce grana, and it is within the grana that the chlorophyll molecules are found.

  If the lamellae within the grana are studied under the electron microscope, they in turn seem to be made up of tiny units, just barely visible, that look like the neatly laid tiles of a bathroom floor. Each of these objects may be a photosynthesizing unit containing 250 to 300 chlorophyll molecules.

  The chloroplasts are more difficult than mitochondria to isolate intact. It was not until 1954 that the Polish-American biochemist Daniel Israel Arnon, working with disrupted spinach-leaf cells, could obtain chloroplasts completely intact and was able to carry through the complete photosynthetic reaction.

  The chloroplast contains not only chlorophyll but a full complement of enzymes and associated substances, all properly and intricately arranged. It even contains cytochromes by which the energy of sunlight, trapped by chlorophyll, can be converted into ATP through oxidative phosphorylation.

  Meanwhile, though, what about the structure of chlorophyll, the most characteristic substance of the chloroplasts? For decades, chemists had tackled this key substance with every tool at their command, but it yielded only slowly. Finally, in 1906, Richard Willstätter of Germany (who was later to rediscover chromatography and to insist, incorrectly, that enzymes are not proteins) identified a central component of the chlorophyll molecule: the metal magnesium. (Willstätter received the Nobel Prize in chemistry in 1915 for this discovery and other work on plant pigments.) Willstätter and Hans Fischer went on to work on the structure of the molecule—a task that took a full generation to complete. By the 1930s, it had been determined that chlorophyll has a porphyrin ring structure basically like that of heme (a molecule that Fischer had deciphered). Where heme has an iron atom at the center of the porphyrin ring, chlorophyll has a magnesium atom.

  Any doubt on this point was removed by R. B. Woodward. That master synthesist—who had put together quinine in 1945, strychnine in 1947, and cholesterol in 1951—now capped his previous efforts by putting together a molecule in 1960 that matched the formula worked out by Wills tatter and Fischer, and, behold, it had all the properties of chlorophyll isolated from green leaves. Woodward received the 1965 Nobel Prize for chemistry as a result.

  Exactly what reaction in a plant does chlorophyll catalyze? All that was known, up to the 1930s, was that carbon dioxide and water go in and oxygen comes out. Investigation was made more difficult by the fact that isolated chlorophyll cannot be made to bring about photosynthesis. Only intact plant cells or, at best, intact chloroplasts, would do; hence, the system under study was very complex.

  As a first guess, biochemists assumed that the plant cells synthesize glucose (C6H12O6) from the carbon dioxide and water and then go on to build from this the various plant substances, adding nitrogen, sulfur, phosphorus, and other inorganic elements from the soil.

  On paper, it seemed as if glucose might be formed by a series of steps which first combine the carbon atom of carbon dioxide with water (releasing the oxygen atoms of CO2), and then polymerize the combination, CH2 a (formaldehyde), into glucose. Six molecules of formaldehyde would make one molecule of glucose.

  This synthesis of glucose from formaldehyde could indeed be performed in the laboratory, in a tedious sort of way. Presumably, the plant might possess enzymes that speed the reactions. To be sure, formaldehyde is a very poisonous compound, but the chemists assumed the formaldehyde to be turned into glucose so quickly that at no time does a plant contain more than a very small amount of it. This formaldehyde theory, first proposed in 1870 by Baeyer (the synthesizer of indigo), lasted for two generations, simply because there was nothing better to take its place.

  A fresh attack on the problem began in 1938, when Ruben and Kamen undertook to probe the chemistry of the green leaf with tracers. By the use of oxygen 18, the uncommon stable isotope of oxygen, they made one clear-cut finding: it turned out that when the water given a plant is labeled with oxygen 18, the oxygen released by the plant carries this tag, but the oxygen does not carry the tag when only the carbon dioxide supplied to the plant is labeled. In short, the experiment showed that the oxygen given off by plants comes from the water molecule and not from the carbon dioxide molecule, as had been mistakenly assumed in the formaldehyde theory.

  Ruben and his associates tried to follow the fate of the carbon atoms in the plant by labeling the carbon dioxide with the radioactive isotope carbon 11 (the only radiocarbon known at the time). But this attempt failed. For one thing, carbon 11 has a half-life of only 20.5 minutes. For another, they had no available method at the time for separating individual compounds in the plant cell quickly and thoroughly enough.

  But, in the early 1940s, the necessary tools came to hand. Ruben and Kamen discovered carbon 14, the long-lived radioisotope, which made it possible to trace carbon through a series of leisurely reactions. And the development of paper chromatography provided a means of separating complex mixtures easily and cleanly. (In fact, radioactive isotopes allowed a neat refinement of paper chromatography: the radioactive spots on the paper, representing the presence of the tracer, would produce dark spots on a photographic film laid under it, so that the chromatogram would take its own picture—a technique called autoradiography.)

  After the Second World War, another group, headed by the American biochemist Melvin Calvin, picked up the ball. They exposed microscopic one-celled plants (chlorella) to carbon dioxide containing carbon 14 for short periods, in order to allow the photosynthesis to progress only through its earliest stages. Then they mash
ed the plant cells, separated their substances on a chromatogram, and made an autoradiograph.

  They found that even when the cells had been exposed to the tagged carbon dioxide for only 1½ minutes, the radioactive carbon atoms turned up in as many as fifteen different substances in the cell. By cutting down the exposure time, they reduced the number of substances in which radiocarbon was incorporated, and eventually they decided that the first, or almost the first, compound in which the cell incorporated the carbon-dioxide carbon was glyceryl phosphate. (At no time did they detect any formaldehyde, so the venerable formaldehyde theory passed quietly out of the picture.)

  Glyceryl phosphate is a three-carbon compound. Evidently it must be formed by a roundabout route, for no one-carbon or two-carbon precursor could be found. Two other phosphate-containing compounds were located that took up tagged carbon within a very short time. Both were varieties of sugars: ribulose diphosphate (a five-carbon compound) and sedoheptulose phosphate (a seven-carbon compound). The investigators identified enzymes that catalyze reactions involving such sugars, studied those reactions, and worked out the travels of the carbon-dioxide molecule. The scheme that best fits all their data is the following.

  First, carbon dioxide is’ added to the five-carbon ribulose diphosphate, making a six-carbon compound. This quickly splits in two, creating the three-carbon glyceryl phosphate. A series of reactions involving sedoheptulose phosphate and other compounds then puts two glyceryl phosphates together to form the six-carbon glucose phosphate. Meanwhile, ribulose diphosphate is regenerated and is ready to take on another carbon-dioxide molecule. You can imagine six such cycles turning. At each turn, each cycle supplies one carbon atom (from the carbon dioxide), and out of these a molecule of glucose phosphate is built. Another turn of the six cycles produces another molecule of glucose phosphate, and so on.

  This is the reverse of the citric-acid cycle, from an energy standpoint. Whereas the citric-acid cycle converts the fragments of carbohydrate breakdown to carbon dioxide, the ribulose-diphosphate cycle builds up carbohydrates from carbon dioxide. The citric-acid cycle delivers energy to the organism; the ribulose-diphosphate cycle, conversely, has to consume energy,

  Here the earlier results of Ruben and Kamen fit in. The energy of sunlight is used, thanks to the catalytic action of chlorophyll, to split a molecule of water into hydrogen and oxygen, a process called photolysis (from Greek words meaning “loosening by light”). This is the way that the radiant energy of sunlight is converted into chemical energy, for the hydrogen and oxygen molecules contain more chemical energy than did the water molecule from which they came.

  In other circumstances, it takes a great deal of energy to break up water molecules into hydrogen-for instance, heating the water to something like 2,000° C or sending a strong electric current through it. But chlorophyll does the trick easily at ordinary temperatures. All it needs is the relatively weak energy of visible light. The plant uses the light-energy that it absorbs with an efficiency of at least 30 percent; some investigators believe its efficiency may approach 100 percent under ideal conditions, If we humans could harness energy as efficiently as the plants do, we would have much less to worry about with regard to our supplies of food and energy.

  After the water molecules have been split, half of the hydrogen atoms find their way into the ribulose-diphosphate cycle, and half of the oxygen atoms are liberated into the air. The rest of the hydrogens and oxygens recombine into water. In doing so, they release the excess of energy that was given to them when sunlight split the water molecules, and this energy is transferred to high-energy phosphate compounds such as ATP. The energy stored in these compounds is then used to power the ribulose-diphosphate cycle. For his work in deciphering the reactions involved in photosynthesis, Calvin received the Nobel Prize in chemistry in 1961.

  To be sure, some forms of life gain energy without chlorophyll. About 1880, chemosynthetic bacteria were discovered: bacteria that trap carbon dioxide in the dark and do not liberate oxygen. Some oxidized sulfur compounds to gain energy; some oxidized iron compounds; and some indulged in still other chemical vagaries.

  Then, too, some bacteria have chlorophyll-like compounds (bacteriochlorophyll), which enable them to convert carbon dioxide to organic compounds at the expense of light-energy—even, in some cases, in the near infrared, where ordinary chlorophyll will not work. However, only chlorophyll itself can bring about the splitting of water and the conservation of the large energy store so gained; bacteriochlorophyll must make do with less energetic devices.

  All methods of fundamental energy gain, other than that which uses sunlight by way of chlorophyll, are essentially dead-end, and exist only under rare and specialized conditions complicated than a bacterium has successfully made use of them. For almost all of life, chlorophyll and photosynthesis, directly or indirectly, are the basis of life.

  Chapter 13

  * * *

  The Cell

  Chromosomes

  It is an odd paradox that until recent times, we humans have known very little about our own bodies. In fact, it was only some three hundred years ago that we learned about the circulation of the blood, and only within the last fifty years or so we have discovered the functions of many of the organs.

  Prehistoric people, from cutting up animals for cooking and from embalming their own dead in preparation for afterlife, were aware of the existence of the large organs, such as the brain, liver, heart, lungs, stomach, intestines, and kidneys. This awareness was intensified through the frequent use of the appearance of the internal organs of a ritually sacrificed animal (particularly the appearance of its liver) in foretelling the future or estimating the extent of divine favor or disfavor. Egyptian papyri, dealing validly with surgical technique and presupposing some familiarity with body structure, can be dated earlier than 2000 B.C.

  The ancient Greeks went so far as to dissect animals and an occasional human cadaver with the deliberate purpose of learning something about anatomy (from Greek words meaning “to cut up”). Some delicate work was done. Alcmaeon of Croton, about 500 B.C., first described the optic nerve and the Eustachian tube. Two centuries later, in Alexandria, Egypt (then the world center of science), a school of Greek anatomy started brilliantly with Herophilus and his pupil Erasistratus. They investigated the parts of the brain, distinguishing the cerebrum and the cerebellum, and studied the nerves and blood vessels as well.

  Ancient anatomy reached its peak with Galen, a Greek physician who practiced in Rome in the latter half of the second century. Galen worked up theories of bodily functions which were accepted as gospel for fifteen hundred years afterward. But his notions about the human body were full of curious errors—understandably so, for the ancients obtained most of their information from dissecting animals. Inhibitions of one kind or another made people uneasy about dissecting the human body.

  In denouncing the pagan Greeks, early Christian writers accused them of having practiced heartless vivisections on human beings. But this comes under the heading of polemical literature: not only is it doubtful that the Greeks did human vivisections, but obviously they did not even dissect enough dead bodies to learn much about the human anatomy. In any case, the Church’s disapproval of dissection virtually put a stop to anatomical studies throughout the Middle Ages. As this period of history approached its end, anatomy began to revive in Italy. In 1316, an Italian anatomist, Mondino de Luzzi, wrote the first book to be devoted entirely to anatomy, and he is therefore known as the “restorer of anatomy.”

  The interest in naturalistic art during the Renaissance also fostered anatomical research. In the fifteenth century, Leonardo da Vinci performed some dissections by means of which he revealed new facts of anatomy, picturing them with the power of artistic genius. He showed the double curve of the spine and the sinuses that hollow the bones of the face and forehead. He used his studies to derive theories of physiology more advanced than Galen’s. But Leonardo, though a genius in science as well as in art, had little
influence on scientific thought in his time. Either from neurotic disinclination or from sober caution, he did not publish any of his scientific work but kept it hidden in coded notebooks. It was left for later generations to discover his scientific achievements when his notebooks were finally published.

  The French physician Jean Fernel was the first modern to take up dissection as an important part of a physician’s duties. He published a book on the subject in 1542. However, his work was almost completely overshadowed by a much greater work published in the following year. This was the famous De Humani Corporis Fabrica (“Concerning the Structure of the Human Body”) of Andreas Vesalius, a Belgian who did most of his work in Italy. On the theory that the proper study of mankind was man, Vesalius dissected the appropriate subject and corrected many of Galen’s errors. The drawings of the human anatomy in his book (which are reputed to have been made by Jan Stevenzoon van Calcar, a pupil of the artist Titian) are so beautiful and accurate that they are still republished today and will always stand as classics. Vesalius can be called the father of modern anatomy. His Fabrica was as revolutionary in its way as Copernicus’s De Revolutionibus Orbium Coelestium, published in the very same year.