BIOCHEMISTRY. Biochemistry may be defined as the study of the chemical or physico-chemical processes which play a part in the life phenomena of plants and animals. The sub division of the science of chemistry into a number of special branches, physical, inorganic, organic, biological, etc., has been the natural consequence of the rapid advances in knowledge which have been made since about 1870. Although the study of the chemical aspect of the physiology of living organisms has only been generally termed biochemistry for some 20 years, it is actually one of the oldest branches of chemical science.
Disregarding events which occurred before the birth of modern chemistry at the end of the 18th century, we find that Lavoisier, the creator of the new science, himself spent no small portion of his time in prosecuting biochemical research. He showed clearly for the first time that the life-processes of the animal body can be investigated by chemical means, for he proved by quantitative methods that the body temperature is maintained by the combustion of foodstuffs just as the heat generated in the burning of wood or a candle is produced by the oxidation of organic matter. The first half of the next century produced three outstanding figures, whose united labour extended widely the foundations on which the modern study of biochem istry is based. In the first place Liebig, carrying back with him from Paris to Germany the inspiration gained by contact with those who had been students or colleagues of the great Lavoisier, not only established the study of organic chemistry, on which biochemistry is dependent, but himself opened up wide avenues of research on problems of plant and animal chemistry.
In 1837, at the request of the British Association for the Advancement of Science, he undertook to prepare a report on the state of knowledge in organic chemistry. This led to the publication in 1840 of his memorable work Die Chemie in ihrer Anwendung auf Agrikultur and Physiologic, followed in 1842 by Die Thierchemie, oder die organische Chemie in ihrer An wendung auf Physiologic and Pathologic. These two classics mark clearly the point at which the study of plant and agricultural chemistry on the one hand, and that of animal chemistry on the other, first attracted wide interest amongst scientists in gen eral and chemists in particular. Hofmann, speaking of this great chemist said, "In the study of biology, vegetal and animal, Liebig was the first to disentangle intricacies that had before seemed problems beyond the grasp of human intellect to solve ; and it was one of the grandest results of his philosophical and experi mental investigations that he traced amidst the multitudinous and apparently ever-varying manifestations of life, in its count less modifications of kind and degree, the operation of a few simple laws, physical and chemical, affording, by their determinate combination, the precise and proved conditions of vital develop ment, nutrition, growth, and perpetuation from generation to generation, in unaltered individuality." Claude Bernard and Pasteur.—At this time progress in this field of thought was still hampered by the persistence of ideas favouring the existence of a vital force. The first blow to shatter these theories was dealt by Wohler in 1828 when he announced the synthesis from inorganic materials of urea, a typical animal product for the formation of which some influence exerted by the vitality of the organism had, until then, been considered essen tial. The belief in a vital force or vital principle would have died slowly had it not been for the brilliant series of researches in ex perimental physiology that have made the work of Claude Bernard immortal. The third figure is the illustrious Pasteur, whose re searches on fermentation, putrefaction and disease revealed not only the organisms that gave rise to these conditions, but also, in many cases, the chemical or physico-chemical changes that occurred.
During the second half of the 19th century researches in plant and animal chemistry were prosecuted with increasing vigour. Unfortunately, however, there was a tendency for the studies in each field to be kept apart from the other, and there were com paratively few investigators of that period who were sufficiently broad in their outlook to attempt to view the problems from the standpoint of the cell rather than from that of the animal or plant.
Much progress was made in both animal chemistry or, as it was usually termed, physiological chemistry, and in plant and agri cultural chemistry, but there were few attempts at correlation.
The more general appreciation of the need of a wider outlook seems to have become apparent at the opening of the 2oth century, when the term biological chemistry or biochemistry began to displace the older terms. In Great Britain the Biochemical Club, now the Biochemical Society, was founded in 1912, six years after the appearance of the first volume of The Biochemical Journal. The corresponding scientific journals in the United States and Germany, The Journal of Biological Chemistry and Biochemische Zeitschri f t, also appeared for the first time in that year. The rapid advance of the new branch of chemistry can be illustrated by the growth of The Biochemical Journal, of which the first volume (1906) contained 32 papers, amounting in all to 495 pages of printed matter, whereas the volume for 1926 con tained 171 papers and a total of 1,376 pages.
The study of the chemical processes occurring in living cells or brought about in the ex ternal medium by their action has all along been dependent to a very large extent on the rate of advance in our knowledge of organic and physical chemistry. The contents of the cell, known generally as protoplasm, represent to the chemist a mixture of such complexity that despair of ever gaining any clear concep tion of its actual composition might well be pardoned. Not only is the task one of great difficulty by reason of the wide variety of substances which are to be found present in cells, but also because a large proportion of those compounds exist in the cell in the condition known as the colloidal state. (See COLLOIDS.) Conditions of Progress.—Progress in biochemistry has, there fore, been dependent to a large extent on the rate of advance of knowledge in the neighbouring fields of organic and physical chem istry. One example may make this clear. A large proportion of the solid material of cellular contents is composed of complex nitro genous substances termed collectively the proteins. No satisfac tory theory regarding their significance in living cells could be ad vanced until information concerning both the molecular structure of the proteins and their physical properties as colloids had been obtained. The former was provided by the classic researches of the great German chemist Emil Fischer, who revealed, by his analytical studies, that the proteins are large, complex molecules formed by the intercombination of a number of amino-acids by virtue of the basic and acidic groupings. Although it is still un certain whether the only form of linkage between the constituent amino-acids in the protein molecule is that described by Fischer, it is apparent that it is by far the most common and a very impor tant one. His work has given us a general picture of the internal structure of the protein molecule that is in agreement with the vast majority of known facts concerning its chemical properties.
Parallel with these researches in the field of organic chemistry on the constitution of the proteins, extensive investigations were being made in many quarters on the colloidal properties of these remarkable substances. As a result of this work it has at last been possible to attack with reasonable chances of success the major question concerning the role of the proteins in the living cell.
In general, it can be seen that advance in biochemistry has fol lowed much the same path that has been traced by the growth of other branches of experimental science. Broadly speaking, the de velopment can be divided into two phases, the descriptive phase and the quantitative phase. In the former period the efforts of investigators were devoted mainly to the isolation of the sub stances present in living tissues and to the investigation of their nature and properties, whilst in the later phase attention has been more and more given to elucidating their significance in the or ganism and to the quantitative examination of the dynamics of the reactions in which they are concerned.
The study of biochemistry may be regarded as having very definitely entered the second phase about 191o, for in almost any journal devoted to this branch of science the great proportion of the papers published at the present time are devoted to the study of the dynamics of cellular reactions. The importance of pursuing this line of study must be apparent when it is borne in mind that it is particularly by reason of the extraordinary complexity and variety of the energy exchanges that occur in even the simplest cell that living matter is best differentiated from non-living.
From the earliest time philosophers have attempted to define life, without one wholly satisfactory definition having been advanced. To-day the difficulties of their task can be appreciated because it becomes increasingly evident that there is no clear line of demarcation between the living and the non-living. Nevertheless, as Claude Bernard indicated in his striking essay on the "Phenomena of Life," there are a number of properties of living matter which, taken collectively, serve as a rough and ready means of differentiating it from systems relatively inert. These are: (I) Assimilation and respiration; (2) reproduction; (3 ) growth and development ; (4) movement ; (5) secretion and excretion.
Considered singly, it is obvious that no one of these is in itself characteristic of living matter alone, but as yet no system that could be considered in the light of present opinion a non-living one has been found to exhibit all together. It may serve to show the scope of modern biochemistry if examples be given of the ap plication of the new branch of science to the study of the chemical aspect of these phenomena of life.
The biochemical study of assimila tion has somewhat naturally fallen into two sections, depending upon whether plants or animals are being considered. The power possessed by green plants to synthesize complex organic substances from carbon dioxide, water and simple inorganic salts, such as nitrates, sulphates and phosphates, places them in a class apart from other beings and calls for particular study. (See PHOTO SYNTHESIS.) The essential difference is, of course, that for the synthesis of organic matter from carbon dioxide a supply of energy is necessary—the reaction being an endothermic one. Com paratively long ago it was recognized that the source of this supply of energy is the sun, but only recently has the knowledge of the nature of the pigments in the leaf, of the absorption of energy in the form of light by these pigments, of the mechanism of absorp tion of carbon dioxide by the leaf and of the nature of the sub stances formed during assimilation, enabled biochemists to con struct reasonable theories as to the process of photosynthesis in green plants. In this field much remains to be done. In spite of many strong indications that the first step in carbon assimilation is the photochemical reduction of carbon dioxide to formaldehyde, precise confirmation is lacking. The efficiency of the process is also undetermined. Early investigators believed it to be of the order of 3-5%, but Warburg has recently recorded that the value may be much higher under optimum conditions.
Chlorophyll.—There is also the question of the origin of the chlorophyll pigments in relation to their role as energy absorbers and transformers. Obviously so complex a molecule as Will statter's researches revealed chlorophyll to be must have an ex ceedingly long evolutionary history. From what type of substance is it descended? At a very early period in the history of living organisms it is probable that supplies of energy for assimilation of carbon dioxide and formation of organic substances were de rived not from the absorption of solar radiation by suitable pig ments, but from simple exothermic chemical reactions of the type exhibited by the existing species of autotrophic bacteria. (See BACTERIOLOGY.) These remarkable organisms, amongst which are grouped certain of the sulphur bacteria, the nitrite bacteria and the hydrogen bacteria, possess the power to carry out simple re actions, such as the oxidation of sulphur to sulphates, ammonia to nitrites, and hydrogen to water, and to use the energy liberated by these reactions to effect the conversion of carbon dioxide into organic matter. It is also interesting to note that certain of the sulphur bacteria contain a pigment, bacterio-purpurin, which appears to function somewhat in the manner of chlorophyll in higher plants, when the bacteria are exposed to the light, whereas in the dark these organisms satisfy their energy requirements by the oxidations referred to above. Possibly these species represent the transitional forms that ultimately led to the evolution of the assimilatory system of the green plant. Little as is our knowledge of the synthesis of carbon compounds in green plants, it is pro found when compared with that concerning the formation of sub stances containing other elements, in particular nitrogen. We are, to all intents and purposes, entirely ignorant of the mode of formation in the plant of the great groups of the proteins, alkaloids and plant bases.
The outstanding fact that the animal organism is essentially analytic and not synthetic as is the green plant, has, of course, been recognized for many years. The study of assimilation by the animal becomes, therefore, to a large extent, the study of the breakdown, or metabolism, as it is termed, of the foodstuffs that are ingested by the organism to supply, on the one hand, the energy for heat production or work and, on the other, the molecular units required to construct or maintain its tissues.
Biochemical investigations of assimilation in the animal world have made rather more rapid progress than corresponding efforts in the field of plant chemistry. To a large degree this is due to the fact that, in the higher animals at least, it is to some extent pos sible to follow the fate of substances by examining the body fluids, the individual organs, or, more particularly, the excretions.
The study of the metab olism of foodstuffs in the animal body represents a large and im portant branch of biological chemistry. It entails in the first place an examination of the mode of action of the remarkable catalytic agents possessed by the living cell and termed enzymes (see ENZYMES), by means of which the complex molecules of the pro teins, polysaccharides, and fats are broken down so that the simpler molecules of amino-acids and sugars can pass through the absorb ing membranes of the alimentary canal.
The actual mechanism of ab sorption of substances into the tissue fluids must then be studied, after which we must enquire into the fate of the molecules that have been assimilated. Some of these go to form tissues that are be ing constructed, especially during the period of growth; of the others, the great majority are in due course oxidized so that the en ergy liberated by their oxidative degradation may be available for maintaining body temperature, or for the performance of work. A whole field of biochemistry is concerned with the mechanisms by which organic molecules are oxidized in the living cell to car bon dioxide and water, for a large proportion of the substances that are rapidly and fully oxidized in the tissues at temperatures below 4o° C are oxidized only by drastic treatment with chemical reagents and by the employment of high temperatures when sub jected to experiment in the test tube.
It was discovered by the researches of Mayow, Priestley and Lavoisier that living creatures support life by the process of respiration, in which oxygen is taken into the system, and the product of oxidation of organic matter, car bon dioxide, is given off. To-day it is recognized that respiration is in no way peculiar to living tissues, for many non-living sys tems can be constructed that will absorb oxygen and eliminate carbon dioxide under conditions more or less comparable with those under which the living cells respire. The very striking ex periment described in recent years by the distinguished German chemist Otto Warburg may be taken as an illustration. The oxi dation of certain substances which are oxidized in the body, e.g., certain amino-acids, will take place along apparently similar paths, at any rate leading to the formation of carbon dioxide and ammonia, when their aqueous solutions are shaken with carefully prepared charcoal in a fine state of division. A measure of the oxygen absorbed and of the carbon dioxide evolved in such cases is as truly a measure of the respiration of the charcoal as a de termination of the respiratory quotient is an indication of the oxidative activity of a living tissue. The parallel becomes even more remarkable when it is learned that the respiration of the charcoal particles can be depressed by the addition of narcotics or poisons in a manner entirely comparable with the influence of these substances on the respiration of living cells.
The biochemical study of respiration is, however, a problem presenting many aspects. Primarily it is necessary to investigate the means by which the oxygen is brought to the cells, a task that widens on every hand and takes us into many fields. The cells of the simpler forms of life draw their supplies of oxygen direct from solution in the sur rounding fluid, but for more complex organisms this would not suffice, and methods of distributing the oxygen to tissues more remote from the external medium have evolved. Thus arose the circulatory system of animals. Simple air-breathing species de pendent on the diffusion into the tissue spaces of oxygen from a more or less complicated system of tracheal tubes, gave rise to more complex organisms requiring the evolution of the lung with its enormous diffusion surface. Even this would have been insufficient to supply the oxygen requirements of the majority of animals if oxygen carriers of the type of haemoglobin had not been evolved to enable the circulating fluid to carry round to the tissues ample oxygen for their needs.
field of bio chemical research that attracts wide attention at the present time concerns the respiratory pigments, of which the haemo globins are the best known examples. These substances may serve a dual role by acting as oxygen carriers and as catalysts of oxidation reactions. The latter seems particularly true of a re markable pigment that has been found present in every form of life yet examined. Discovered in 1884 by MacMunn and ex haustively studied by him, its significance, as a substance related to haemoglobin and as an important factor in the oxidation re actions in the cell, which he emphasized, escaped notice until attention was again drawn to it by the recent studies of Keilin. The occurrence of cytochrome, as Keilin has renamed this pig ment, in plant as well as animal cells indicates that its significance is probably fundamental; it seems likely that it occupies an im portant position in the evolutionary history of the haemoglobins. But the interest of the biochemist cannot stop with the study of the mechanisms by which oxygen is brought to the cells or held there ; he must enquire how the oxygen is made available in the cell for oxidations.
Oxygen.—Atmospheric oxygen, whether obtained direct from solution in the surrounding medium, or by the dis sociation of such a pigment as oxyhaemoglobin, is relatively inert as an oxidizing agent. Of the foodstuffs oxidized in living cells only very few are appreciably attacked by oxygen in the molec ular form 02; the unsaturated fatty acids present in such oils as cod liver oil and linseed oil are examples of compounds that take up molecular oxygen, but the extent to which they are oxidized is very small when compared with the complete degradation to carbon dioxide and water that occurs with ease in the animal body. Early in the i 9th century Schonbein, in his studies of ozone, pointed out that oxygen must in some manner be activated before it is able to effect the majority of oxidations, and this view, in one form or another, has formed the basis of the many theories of oxidation that have been advanced since his time. We are still without clear ideas regarding the mechanism of activation of molecular oxygen which the living cell possesses, but the studies of Warburg on tissues and on the charcoal model to which reference has been made suggest that minute traces of iron and possibly of other heavy metals such as copper play an important part in the process.
This brings us in a natural se quence to refer to the oxidative mechanisms which the living organism possesses. In the first place, there are the oxidizing enzymes, the oxidases, a group of catalysts, many of them of a highly specific character, capable of oxidizing with great rapidity under suitable conditions a wide variety of substances. The biochemical examination of the oxidases has been extensive, and has, particularly in recent years, given us considerable information how molecules such as those of tyrosine, xanthine and uric acid are oxidized. According to the recent work of Thunberg, Battelli and Stern the cell possesses, in addition to the recognized type of oxidase, other types of oxidative catalysts of a thermolabile na ture. It would appear, however, from the results of the researches of Sir Gowland Hopkins and of Meyerhof that there are also present in the cells of plants and animals oxidative reductive systems which are thermostable. The remarkable substance glutathione, discovered by Hopkins, probably plays an important part in these last-mentioned systems.
Turning now to the second characteristic of life in our list, namely, reproduction, we find we are considering a phenomenon which might well, at first sight, appear to be out side the scope of experimental attack by biochemists. Surely in the processes underlying reproduction, if anywhere in the realm of biology, might be traced the "vital force" or "entelechy" that would at almost every turn frustrate experimental attack. The clear demonstration that such is not the case, and that the prob lems of fertilization and reproduction are no less open to experi mental biochemical investigation than those of digestion or respiration, we owe to the remarkable work of the great experi mental biologist Jacques Loeb. His investigations on the in fluence of the composition of the surrounding medium on the reproductive cells of certain marine animals dispel any doubt that many of the problems of fertilization, of specificity and of inheritance will in time be solved, and to a large extent by bio chemical methods. (See CYTOLOGY; EXPERIMENTAL EMBRYOLOGY.) Can this be doubted when we recall that the migration of the sperm to the egg has been shown to be directed by the physico chemical action of certain substances derived from the latter ; that the penetration of the ovum by the sperm can be controlled by altering the composition of the surrounding fluid, even to the extent of permitting the admission of a sperm cell foreign to the egg; that the mechanical act of penetration performed by the sperm, and resulting in the almost explosive outburst of oxidative activity that marks the initiation of fertilization, can be repro duced by perforation with needles so successfully that numbers of male frogs have been reared to adult size from eggs so ferti lized? Considering these striking facts, and at the same time bearing in mind the mass of evidence that is accumulating on every side to show how the development of the reproductive cells is under chemical control in the body and how these tissues themselves exert by chemical means a far-reaching control on the development and functions of other tissues, no reasonable doubt can be entertained that the phenomena associated with repro duction are open to biochemical investigation.
Many of the closely associated problems of genetics are also well within the biochemist's purview, as can readily be judged by giving one or two examples. The inheritance of colours, whether we are thinking of the blue, red or purple colours of flowers or the black and brown markings of animals, has been shown to depend on the inheritance of a physico-chem ical system capable, under ordinary conditions, of producing the colour. Some of these systems and the mechanism of their action are fairly well understood. The systems may be more or less complex in character, but unless they are complete the mechanism for the production of the colour cannot function normally. Thus, for example, two white flowers from different plants of the same species may lack colour because one com ponent of the colour-producing system is absent. If the missing factor is the same in both cases, crossing will not result in the production of coloured flowers, but if their deficiencies are comple mentary, the seeds produced on cross fertilization will yield plants with coloured flowers. The mechanisms concerned in the pro duction of both animal and plant colours have been extensively studied in vitro, and are to some extent understood.
The biochemical study of growth and development has been followed along many paths. The influence of the composition of the soil on plant growth has attracted wide attention ever since the classic researches of Liebig were published. To-day the agricultural biochemist not only studies the significance of the more obviously essential constitu ents of the soil, such as nitrates, phosphates, etc., but is con cerned to no little extent with the influence which apparently in significant amounts of other substances may have. The curious fact that the broad-bean plant will not grow to full maturity without a minute amount of the element boron being present in the soil is an example of what has already resulted.
The biochemical study of the growth and development of animals provides many examples of both these aspects of the chemical control of growth. On the one hand the energy requirements of animals during the period of growth have been investigated in great detail, whilst on the other, it has been ascertained that a normal development is dependent on the satisfaction of a number of clearly defined requirements.
A small but definite amount of the amino acid tryptophane, a constituent of some, but not all, proteins, is necessary for growth of young animals, and the provision of any amount of a protein deficient in that particular building stone will fail to induce growth unless the missing unit is provided from some other source. Many biochemists at the present time are engaged in investigating the remarkable influence which the substances known as the vitamins have on the growth and nutri tion of animals. The number of these substances that are gener ally accepted as being clearly differentiated is already five and probably more exist as yet undiscovered. Their chemical nature remains unknown, and we are also ignorant as to the actual part they play in the economy of the organism. (See VITAMINS.) Lastly, there is a chemical aspect of growth in the investigation of the influence of inorganic elements on animal development. Problems such as the rOle of traces of iodine in inducing normal development of the thyroid gland, and the manner in which lime salts are deposited in growing bone illustrate the type of question in this field that calls for an answer from biochemists.
Study of the chemical or physico-chemical fac tors inducing the movements of plants and animals covers a wide field of experimental research, ranging from control of the move ments of free-swimming unicellular organisms to the unravelling of the complex series of events that occur during a muscle twitch in higher animals.
Little is known, at the present time, of the factors determin ing the growth movements of plants beyond Loeb's having shown by his researches on regeneration in Bryophyllum that chemical factors probably play a part in the directional growth of shoots and roots. The movements of many forms of simple animals can be to a certain extent controlled by making alterations in the composition of the medium in which they exist, as, for example, when the water flea, Daphnia, which normally swims away from the light moves in the reverse direction when carbon dioxide is bubbled through the water. Another remarkable example also described by Loeb is that of the larvae of Porthesia, which in the starving condition are attracted toward light and climb high up the stems of the plant on which they customarily feed, but which, having fed on the leaves, become negatively heliotropic and at once descend again to the darker regions. By starving them or by feeding on the leaves of the plant they can in the laboratory be made at will to move toward or away from a source of light.
Turning to what we may regard as the other corner of this field, namely, the bio chemical investigation of the mechanism of muscular movement in animals we find one of the most complete chapters of modern biochemistry. The long series of researches of outstanding merit by Hopkins and Fletcher, Meyerhof and A. V. Hill have taught us how the glycogen of muscle is broken down to sugar ; how the sugar, passing through the intermediate stage of being combined with phosphoric acid, gives rise to the lactic acid that initiates the contraction, and how these anaerobic changes are followed by an oxidative phase of recovery during which part of the lactic acid passes back into its precursor, whilst the remainder is oxidized to carbon dioxide and water. These changes have been followed with such precision by chemical and physical methods that the heat production of the muscle during the whole cycle has been accounted for with considerable exactitude by a knowledge of the heat exchanges of the chemical reactions which are believed to occur. In spite of these carefully compiled results, we are as yet ignorant of the actual processes involved when the muscle fibre shortens in contraction.
Hormones.—The processes of secretion and excretion have been the subject of extensive biochemical research, although mainly on animal tissues. The discovery by Starling and Bayliss of the agents known as hormones (q.v.) showed for the first time how important a part such chemical messengers play in the co ordinating mechanisms of the higher animals. Their discovery of secretin, the substance of, as yet, undetermined nature which is produced in the mucosa of the upper part of the small intestine under the stimulus of the entry of acid food-material from the stomach, and which, passing into the blood stream, invokes, in a very short space of time, the secretion of the digestive juice of the pancreas, has paved the way for the discovery by Banting and Best of the internal secretion of the pancreas itself, and more recently of that of the internal secretion of the parathyroid gland by Collip. Few more striking examples of the service of chemistry to the study of biological problems concerning human welfare could be given than the discovery of the nature of adrenalin and its synthesis by Takamine, and the isolation of a specific sub stance of the thyroid gland (thyroxin) by Kendall and the more recent demonstration of the chemical constitution of this remark able substance and its synthesis by Harington. Viewed as a whole, the regulatory action (see ENDOCRINOLOGY) exerted by the secretions of the various glands and tissues of the higher animals is seen to be one of the most delicately balanced nature, but it is one that is quite definitely open to chemical investigation.
Excretory Processes.—The processes of excretion by which living organisms rid their tissues of waste products have, as yet, been scarcely investigated at all in plants. In animals, more particularly in the mammals, they have been extensively exam ined from the biochemical standpoint. The secretion of urine, the work done by the kidney in this process, the chemical and physicochemical principles underlying the concentrations of waste products that appear in the excreted fluid, all these have been the subject of prolonged and fruitful investigation.
Secretory Processes.—Of no less interest are problems con cerning the secretion by living cells, such as the gastric mucosa of animals, of fluids containing appreciable concentrations of free mineral acids. These questions are intimately bound up with the very general one concerning the mechanisms by which the re action of the body fluids is maintained within narrow limits. The study of the system of amphoteric colloids and simple electro lytes that constitute the tissue fluids of animal or plant tissues has, from the standpoint of physical chemistry, been almost ex haustive, and the analysis, step by step, of the influence of the many factors playing a role in the cycle of changes that occurs in blood during its circulation in the bodies of animals stands as one of the most impressive tributes to the application of the rigid, quantitative methods of modern physical chemistry to problems of outstanding biological importance. Many other spheres of extensive biochemical work might be mentioned, but it will be sufficient if brief reference be made to the chemical investigations of fermentation and bacterial changes.
Fermentation Processes.—To-day, as a result of the re searches of Harden and Young in Great Britain, of Fernbach in Paris, and of C. Neuberg in Germany, we are in possession of a reasonably clear idea of the stages by which the sugar mole cule is broken down to yield alcohol and carbon dioxide when fermented by yeast. Most of the intermediate products have actually been isolated and their part in the process proven. Of particular interest is the fact that the most recent work on the fermentation of sugar by yeast and on its degradation in the cells of the animal body points to the essential steps in the breakdown of the carbohydrate molecule being the same in both cases.
Probably in no other field of research in biochemistry have so many striking examples been found as in that of fermentation chemistry, of the course of a reaction being changed by alter ations in the conditions of the experiment. Certain of these diversions of the normal course of events have proved of con siderable economic value, as, for example, when, during the World War, German scientists were able to prepare considerable quantities of glycerol from sugar by causing inhibition of the fermentation at a certain stage by the addition of sulphites.
Apart from the alcoholic fermentations there are numerous other processes, many of them of considerable technical impor tance, in which the action of the bacteria (see BACTERIOLOGY) or other micro-organisms is the subject of biochemical research.
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(J. C. D.)
