Principles of General Physiology

The chief function of carbohydrate food, as also of fat, is to afford energy; but, in the process of its oxidation, a number of intermediate products are produced, given in the form of a diagram in the text (page 273). These substances are of importance in that they give opportunity for the occurrence of reactions of importance to the organism in other ways. Pyruvic aldehyde, lactic, and pyruvic acids may be especially mentioned. All of these reactions, with the exception of the last stages of oxidation, have been shown to be reversible under conditions obtaining in the living' organism.

Fat is of additional importance as being readily stored in considerable quantity. It can be formed from the carbohydrates of the food, and the manner in which there is every reason to suppose that the process takes place is, in general terms, as follows. By condensation of an aldehyde with a ketonic acid, we obtain another aldehyde with two more carbon atoms than the original one and, by repetition of the process, with final reduction, fatty acids with straight chains of carbon atoms, increasing by two at a time, are produced.

The frequent occurrence of pyruvic acid in the processes of metabolism, of carbohydrate, fat and protein, is pointed out. This substance forms, as it were, a meeting place of the three different classes of food-stuffs. The value of perfusion experiments and experiments in vitro as extended to processes in the whole organism is discussed in the text. Carbohydrates and fats are readily stored in the tissues as glycogen and neutral fats, respectively. There does not seem to be any particular form in which protein is stored, except as tissue or protoplasmic substance, although amino-acids can be adsorbed by tissue colloids.

The effect of ammonia and of urea in diminishing the nitrogen loss is probably due to a diminution by mass action of the de^amination of amino-acids and of the formation of urea from ammonia. The question of optical activity is discussed in the text and the way in which compounds of this kind may have first arisen is described. The preferential use of one optical isomer, at all events for energy purposes, is shown to be merely one of degree, although the cell constituents are finally composed of one set of isomers.

Results obtained by the growth of tissues in vitro show that proteins can be utilised, or dealt with in some way, by cells themselves, a fact also evident from the using up of cell substance in starvation. The process is to be explained by the presence of autolytic enzymes. Under normal conditions, the proteins of the blood do not serve as nitrogen food to the cells of the tissues. The fact that an organ of one animal has not been satisfactorily transplanted into another one, apart from exceptional cases, argues an extraordinary complexity of some kind or other on the part of the protoplasmic systems of the cell.

There is no evidence that the processes of nutrition in cells are directly influenced by the nervous system ; although the existence of nerve fibres supplying cells forbids a categorical denial of the possibility of such influence. Certain processes of growth and metabolism obey definite known mathematical laws. It is pointed out that the investigation of functions of the lower organisms is less likely to lead to valuable knowledge than that of the higher organisms. The methods of comparative physiology are of value in enabling us to exclude unessential factors and, in certain cases, allow experiments to be made under conditions in which it would be impossible to preserve the organs of warm-blooded

animals in a normal state. The fundamental phenomena of general physiology cannot be discovered by confining our attention to the lower organisms. The essential fact in the physiology of sexual reproduction is the advantage gained by the union of the capacities and qualities of two cells from different individuals. Special cells are set apart for this purpose, each being incomplete and incapable of development without the concurrence of the cell of the opposite sex. In the case of the female cell, this incapacity of development is to a certain degree only a relative one. The eggs of some invertebrates can be made to develop by chemical agency and a few rare cases are known where the unfertilised eggs develop into adult animals.

A short account is given of the facts of heredity as treated on the principles laid down by Mendel. In certain cases, plant and animal cells live side by side in the same organism (symbiosis). The plant cells contain chlorophyll and afford carbohydrate material for the animal ; while the cells of the latter appear to provide nitrogenous food for the growth of the plant. SINCE the work of Berthelot and Pean de St Gilles (1862) it has been a familiar fact that, if ethyl acetate and water be mixed in molecular proportions, and allowed to remain for some weeks, the ester is hydrolysed with formation of ethyl alcohol and water; but, however long a time be allowed to elapse, only a certain part of the ester undergoes conversion, although there is sufficient water to hydrplyse the whole. The rate of change becomes slower and slower until it ceases, and at this time it is found that the four components of the system are present in the proportion of one-third of a gram-molecule each of alcohol and acid, two-thirds of a gram-molecule each of ester and water.

Further, suppose that we have commenced with acetic acid and alcohol, also in molecular proportion, and have allowed the reaction to proceed until it stops, we find that we obtain the same proportion of the four components. Clearly we have to deal with a case of equilibrium, or balance of opposite reactions. Now these reactions, which for the present purpose we may regard as being spontaneous, are extremely slow. We can, however, increase their rate enormously by adding some mineral acid. In this case, the attainment of equilibrium, which, left to itself, takes weeks, can be brought about in a few hours. There are three important facts to be noticed here. FifStt^^he composition of the system in equilibrium is the same under the action of acidal^vhen reached spontaneously. Secondly, the acid added is found, after its work is done, still present in the same state as it was originally. Lastly, whether we start from ester and water, or acid and alcohol, we find that the rate of the reaction is accelerated by the addition of acid. This latter fact follows, as we shall see later, from the other fact that the equilibrium position is not changed by the presence of the acid.

Let us take now, instead of acid, an extract of the pancreas and, in place of ethyl acetate, another similar ester, amyl butyrate. The experiment was made by Dietz (1907) and the higher ester was used for convenience in calculating the results, since the spontaneous reaction is so slow as to be undetectable during the time of the experiment ; in other respects, the system may be regarded as precisely similar to the previous one. The effect of the pancreatic extract is even more powerful than that of acid in accelerating the reaction. Otherwise, the three facts to which attention was called in that case are also to be noticed in this, with one slight exception, namely, that the position of equilibrium is not quite the same as that under acid or the spontaneous one. Fig. 4 (p. 86) in my monograph on " Enzyme Action " shows that the position of equilibrium is the same when attained from either direction, a fact also obvious from Fig. 80 of the present work.

One more case will be instructive, before we proceed to discuss the meaning of the facts before us. This is the one to which Fig. 81 refers. The system here is one of glucose, glycerol, glycerol-glucoside, and water. The equilibrium is brought about under the agency of a substance obtained from almonds, and known as emulsin. The curves, taken from experiments of my own (1913, 1), show that the equilibrium position is the same, whether we start from glucose and glycerol or from glucoside and water. The additional fact is that we are dealing with

FIG. 80. SERIFS OF CURVES SHOWING THE DIFFERENT EQUILIBRIUM POSITIONS OF THE »\ .KH ACID-GLYCEROL- FAT- WATER SYSTEM, AS ATTAINED UNDER THE ACTION OF LIPASE WITH Note that the greater the concentration of water, the nearer is the equilibrium point to that of compli-h- li\iln.|\-i- The presence of exce«j of glycerol (lowest pair of curves) leads to increase of synthesis, by removal of water a- w .•!! as by mass action. Ordinates — percentage of free acid. Abscissae — time in hours.

FIG. 81. EQUILIBRIUM ATTAINED I-SDF.R THE ACTION OF KMI I.SIN, <.\ CI.Y< F.I:OI. AND »;i,rn»sK (UPPER CURVE), ON GLYCEROL-GHTCOSIDE (LOWER ITRVE). — Tinposition is the same. Ordinates— optical rotation of diluted samples. Abscissa — time in days. optically active substances. Glucose is dextro-rotatory and we find that, as glucoside is formed, the rotation of the mixture diminishes, finally passing to the laevo-side of zero. Now there are two possible optical isomers of the glucoside, according to the position of the glyceryl group in relation to the terminal hydrogen atom of the glucose. Thus, diagrammatically, putting G for glyceryl : —

The one on the left, called for convenience the a-glucoside, has a higher dextrorotation than glucose, while the /2-glucoside is laevorotatory. The preparation from almonds, which was used in the experiments, is found to cause hydrolysis of the series of /3-glucosides only, of which there are a great number. The experiment quoted shows that it also brings about synthesis of the ftform of the glycerol-glucoside, since that one formed is Isevo-rotatory. Similar results were obtained by Bourquelot and Bridel (1913) in the case of numerous glucosides of alcohols. Note that the two glucosides are not mirror-images (see page 284 above). An important fact, which will be found to be of much significance in later pages, is that the reaction takes place in alcohol of such a strength that the agent, emulsin, is completely insoluble in it and can be filtered off, leaving no trace in solution. The same statement applies to the experiment of Dietz with extract of pancreas, or lipase, as the active constituent has been called.

To begin with, it will have been plain from the facts given in the chapter on Nutrition that the chemical changes which take place in the living organism are of a kind such as, in the laboratory, can only be brought about by powerful reagents and high temperatures. Take the hydrolysis of protein to amino-acids. This is effected in the laboratory by boiling concentrated hydrochloric acid, but in the organism it takes place, at an equal rate, at ordinary temperatures and in a medium which is only just faintly alkaline or neutral. This fact especially attracted the notice of Schonbein (1863).

Berzelius (1837, pp. 19-25), however, directed the attention of chemists to what he called a " force which differs from those hitherto known." On account of the importance of the question, I will give, in an abbreviated form, the description given by Berzelius, whose portrait is reproduced in Fig. 82. The difficulty to which attention has just been called, is pointed out by this chemist. Blood is supplied to an organ and, without the assistance of any other liquid, we obtain saliva, milk, urine, arid so on. A discovery was made by Kirchhof (1812) which gave the first clue to an understanding of the vital processes, but which, as it is scarcely necessary to remark, are still far from complete explanation. Kirchhof found that starch could be converted into glucose by the action of dilute sulphuric acid, which was itself unchanged in the process, since it could be recovered at the end. The next step was, according to Berzelius, the discovery of hydrogen peroxide by Thenard. This was noticed to be decomposed, not only by soluble alkalies, but also by many various kinds of solid insoluble substances, such as manganese peroxide, silver, platinum, and the fibrin of blood. These do not take part themselves in the new

compounds formed, but remain unaltered ; they are stated to act by an " indwelling force, whose nature is still unknown." Shortly before Thenard's discovery, Humphrey Davy had found that platinum, under certain conditions, had the power of causing the oxidation of alcohol vapour in air. Edmund Davy, his cousin, made a more active preparation, which was actually platinum in a very finely divided state, and Dtibereiner made a spongy platinum which could even cause the union of oxygen and hydrogen gases. As we shall see later, this greater activity is due to the greater extent of surface. Berzelius points out

that this property is not confined to platinum, but, in a less degree, is possessed by other substances. Thus, while platinum is active even below 0° C., gold requires a higher temperature, silver still higher, and glass at least 300°. He next refers to the phenomena of alcoholic fermentation, known since the earliest times, before written history. But, until Cagniard de Latour (1838), the fact that it was produced by a living organism was unknown. The words used by Berzelius are worth quoting: "We had made acquaintance with the fact that, for example, the change of sugar into carbonic acid and alcohol takes place in fermentation under the influence of an insoluble body, which we call 'ferment,' and also with the fact that this ferment could be replaced, although less effectively, by animal fibrin, coagulated plant albumin, cheese and similar substances, as well as with the experience that the process could not be explained by a chemical action between the sugar and the ferment analogous to double decomposition. Comparing it with known relations in the inorganic world, it was seen to be most like the decomposition of hydrogen peroxide under the influence of platinum, silver, or fibrin ; it was, therefore, natural to suppose that the action of the ferment was an analogous one." The investigations of Mitscherlich on the formation of ether from alcohol by sulphuric acid are next brought into connection with those of Kirchhof on sugar and with the action of alkalies in decomposing hydrogen peroxide. What is common to all is the manifestation of a "new force," different from chemical affinity in the ordinary meaning of the words,

in that a substance may effect chemical changes without itself taking part in them. Berzelius is careful, however, to guard himself from the supposition that this force is other than a special manifestation of known properties of matter. We shall see later that, in certain cases, explanation on the lines of known chemical and physical laws is actually possible. To return to our author, we find a definition of the process given as follows : " I will call it the catalytic power of substances and the decomposition effected thereby, catalysis ; just as we understand by analysis the separation of the constituents of substances by means of ordinary chemical affinity. Catalytic power appears to consist essentially in the fact that substances are able to set into activity affinities which are dormant at this particular temperature, and this, not by their own affinity, but by their presence alone."

Turning to living nature, it is pointed out that " we have justifiable reasons to suppose that, in living plants and animals, thousands of catalytic processes take place between the tissues and the liquids and result in the formation of the great number of dissimilar chemical compounds, for whose formation out of the common raw material, plant juice or blood, no probable cause could be assigned. The cause will perhaps in the future be discovered in the catalytic power of the organic tissues of which the organs of the living body consist."

With respect to the name itself, it must be admitted that " catalysis " suggests an opposite kind of process to that of " analysis " ; so that, since this latter implies the separation of a process or compound into its constituents, catalysis might be taken to mean a synthetic process. The word has come into general use, however, to denote such processes as those referred to by Berzelius. It is also convenient to have a word for the agent itself : " catalyst " is most frequently used, sometimes "catalyser." Both have the same meaning, but the former seems to me to be more euphonious and to correspond better to the Greek form of the word, although it may have the suggestion of human personality.

If now we turn back to the examples given at the beginning of this chapter, we see at once that they belong to those called catalytic, in that the agent concerned, acid or tissue extract, does not itself form a part of the final chemical system in equilibrium. Again, considering the first of these, the ester system, we note that the catalyst does not actually set into action a new process, but merely hastens one that was already in progress. Ostwald ([1903, II., 1., p. 515 ; 2nd edition) therefore defines a catalyst a's a substance that increases the rate at which equilibrium is reached, but at the same time he points out that the reaction, without the catalyst, may be so slow that it appears not to take place at all.

A simple experiment will assist us in understanding the essential properties of a catalyst and avoid confusion with some other processes, which have a superficial resemblance to those of catalysis. Take a piece of carefully-cleaned, polished plate glass about a metre long and some 20 c. broad. Rest one end on the table and raise the other end on an adjustable support. Now take a brass weight of about one kilogram, polish the bottom and place it on the top of the glass plate, which forms an inclined plane. By delicate adjustment of the angle of the plane, it will be found possible to find such a position that the weight slides down very slowly. This is the most difficult part of the experiment, since a speck of gritty dust will stop the descent, so that it is well to polish the surface with a little talc and a chamois leather immediately before the weight is placed thereon. This part of the experiment represents a reaction taking place of itself very slowly. Apply, next, a little oil to the bottom of the weight and again place it at the top of the plane. It will slide down with great rapidity. The oil represents the catalyst.

There are several instructive points about this scheme. Notice first that the energy available in the " reaction " is simply that due to the fall of the weight from the vertical height of the top of the plane to that of the lower end, and that this is unaffected by the addition of the catalyst, which therefore takes no part in the final state. A point of importance in relation to the catalytic reactions in the living organism is, however, that the form of the energy may be different in the two cases. Without the oil, the weight arrives at the bottom with very little kinetic energy, most of its potential energy having been lost as heat, due to friction along the glass. With oil, very little energy is lost as heat and the

weight arrives at the bottom with considerable kinetic energy. This teaches us that the actual products of a catalysed reaction are not necessarily identical with those obtained in the absence of a catalyst. The next point is that, within limits, we can vary the rate of fall by the application of much or little oil. Although the catalyst does not affect the position of the equilibrium point, the rate at which this is reached is directly proportional to the amount of the catalyst present. Moreover, comparing the relative efficiency of different amounts of oil, we note that small amounts produce at first a much greater effect than the same amounts added after there is already a considerable amount present. This is characteristic of adsorption and applies to enzymes, the catalysts of living organisms, particularly.

We may next note the difference between what is sometimes called " trigger action " and catalysis. Suppose that the plane is, for convenience, raised to a rather steeper position than before, and that the weight is prevented from sliding down by the support of a catch of some kind. When the catch is removed, the weight falls, but the amount of work done in moving the catch has no effect whatever on the subsequent process ; whether the trigger moves very stiffly or easily, the weight descends at the same rate. The true catalyst, oil, exerts its action throughout the whole of the descent, whereas the action of the trigger is completed before the fall begins. Supersaturated solutions are cases of "trigger action." They remain indefinitely as such until infected with a crystal, and then the rate of crystallisation is independent of the amount of crystals added. The same fact is exhibited in the case of supercooled acetic acid, as shown by B. Moore (1893) in his experiments, in which a long tube was used.

One more fact, the meaning of which will be appreciated later, is that in our model the oil partially disappears by sticking to the glass, so that the whole of it is not present on the weight at the bottom. In a certain sense we may say that it has " combined " with some other constituent of the system. In some catalytic reactions we meet with phenomena of this nature ; for example, in the chamber process of sulphuric acid manufacture, the nitric acid, which acts as a catalyst, slowly disappears, being used up in subsidiary reactions.

It is held by some that a catalyst may actually start a reaction which was not in progio> on account of chemical " friction." Our model, again, shows this phenomenon. The friction between the weight and the glass may be so great that no movement appears to take place until oil is applied. The question is rather of theoretical interest and may almost be said to be one of words. The use of the word "friction" implies the possibility of movement, and it may be said that the weight really does move, but is arrested again. There are also all degrees of friction, with corresponding rates of movement, and the rate of a reaction may be so slow that it is, in practice, impossible to say whether it is actually proceeding or not.

The remark may be made here, that the number of reactions known to be capable of catalytic acceleration is very large and increasing every day. Discussion of the mechanism of catalysis will best be deferred. It is probably of a different nature in different cases. Before passing on to the subject of enzymes proper, a few words are necessary with respect to reversible reactions in relation to catalysis. In the example chosen to begin with, namely that of the action of acid on the ester system, we saw that the position of equilibrium is unaltered by the presence of the catalyst. Now this position of equilibrium is due to the simultaneous existence of the two opposite reactions of hydrolysis and synthesis, which are proceeding at an equal rate at this moment.

In order to see how the actual position of this equilibrium depends on the relative rate of two opposite reactions, we may take a rough illustration, which must not be followed in too great detail. Suppose that two people start to walk towards one another from two distant places. Where they meet will clearly depend on the relative rates at which they \valk. Supposing that their rates are the same, they will meet half way between the places from which they start. Imagine that one of them is excited, " catalysed," so that, instead of walking, lie runs. He will meet the other man before he has taken many steps from home. It is also obvious that, if one ran, the only way by which the two could meet at the same place ("equilibrium position") is that the other man runs also, and at the same rate. He must be equally " catalysed " in fact.