characters during the baking process. The proteins are coagulated (gel formation) and the degree of dispersion of the starch is increased. Adsorbed gases are set free and the bread "rises." Further alterations take place in the loaf after it is removed from the oven.

The physical nature of flesh is profoundly altered by subjection to cooking. In roasting, grilling, boiling, or frying, the meat is exposed directly to heat. The proteins in the outer layers are immediately coagulated, thus forming a more or less impermeable covering which prevents the escape of the meat juices, leaving the centre portion of the flesh only slightly altered chemically, but with all sols converted into hydrogels. On the other hand, if the meat is immersed in cold water and boiled much of the protein-sol and practically all the salts and extractives are dissolved out and form soup. In this soup the protein-sol is coagulated as the temperature rises, and on cooling it is adsorbed to the surface and often is removed with the fats as a scum. The remaining meat undergoes coagulation, but is flavourless. Stewing is a modification of boiling, but the extractives, salts and soluble proteins, are served as gravy.

CHAPTER IX

ENZYMES

THE TOOLS OF THE CELL

"Instances of Magic;

By which I mean those wherein the material or efficient cause is scanty and small as compared with the work or effect produced; so that even when they are common, they seem like miracles, some at first sight, others even after attentive consideration."

BACON.

THE living cell is a factory where, without any great display of energy, work is carried on which, outside the body, could only be done by the use of strenuous processes. In the cell are prepared secretions which act on insoluble raw material, rendering it soluble and so fit for transit to the cell and passage into it. Within the cell, these prepared materials undergo further change; some are used as sources of energy; from others, the cell builds up complex tissue; others again are altered somewhat and stored for future use. The cell manufactures from the material supplied, various substances such as are required, it may be by distant cells which are so occupied by some special process that they are unable to perform the particular synthesis. The by-products of manufacture are rendered harmless by processes possible, as yet, only in the cell. Some cells, as indicated above, have a specialised function. To a certain extent, all the cells of a multicellular organism are specialised. They are divided into communities, each engaged on some special work and requiring special raw material. Some of these communities, however, engage to a certain extent in general manufacture. They are almost, though not quite, self-supporting. The white cells of blood, for instance, are really unicellular organisms. Other communities are almost entirely dependent on imports for their sustenance. Nerve cells, for example, form the means for intercommunication between. cell-communities. Their general metabolism is peculiar.

Contrast the quiet, economical, and neat living-factories with the places where things are made outside the body. Our manu

facturing cities are not spotless nor are our processes there economical. Smoke, sound, and slag-heaps are universal accompaniments of a manufacturing community. Most of the processes carried on in the cell have not been reproduced in the laboratory. Fischer, the finest physiological chemist of this or any century, has failed to synthesise the simplest protein. Fat and carbohydrates are interconvertible in vivo but not in vitro. True, steps have been taken towards the building up of a protein. Polypeptides compounds containing eighteen amino acidshave been the crown of Fischer's efforts, but at what a cost of material, time, and energy. It has been well said that laboratory processes are just a roundabout way to the sink.

How does nature accomplish her work? What tools does she use? How does she harness her power?

Nature employs catalytic methods. A catalyst is defined as a substance which, while not entering into the final product of the reaction, alters its rate and in some cases alters the point of equilibrium. A model may make this clearer. A sheet of glass may be inclined at such an angle that a body placed at its upper end just slips slowly to the foot. The momentum of the sliding body may be insufficient to carry it to the foot of the glass plate, and motion may thus stop midway down the plane. If a small quantity of oil be placed either on the glass or on the bottom of the weight, it will slide rapidly to the foot of the plane. The oil remains unchanged. No energy has passed from the oil to the weight, and yet the rate of falling and the point of equilibrium have been altered. The lubricant may be taken as representing a catalyst. Some one has said that a catalyst, like a tip to a waiter, accelerates a reaction that otherwise would proceed with infinite slowness. It takes no part in the main reaction, is adsorbed to the reacting body, and may be recovered intact at the end of the reaction by destruction of the substrate.

Catalysts are of very many kinds, and the mechanism of their action is so varied and so little understood that few, if any, general principles can be enunciated. They may be classified according to the means they adopt to influence a reaction.

1. Contact agents. Many reactions seem to be accelerated by the adsorption of the reacting substance on the surface of the catalyst, e.g. effect of colloidal catalysts.

Colloids, as we have seen, are characterised by the development of surface. If we take a sphere of metal which just fits into a cubical box, and divide that sphere into smaller spheres of uniform

CATALYSTS

3

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2

size, the same mass of metal may be packed into the box regardless of the size of the spheres, provided they are uniform in size. Mass and total effective volume are not altered, but surface is increased. The surface of a sphere is 47r2. If the original sphere be divided into 100 small shot, then the new surface would be 100 x4πr,2 where r1=radius of small shot. Now r1 =√186=4·64, i.e. the surface would be increased over four and a half times. If the subdivision were carried still further till there were 1030 small shot, then the total adsorbing surface would be increased 10,000,000,000 times. The intensity of adsorption is chiefly dependent on the area of adsorbing surface (cf. Table, p. IX.). In other words, contact catalysis is indicated where the specific surface of the catalyst comes within the colloidal range. Charcoal is used as an adsorbent in the clarification of sugar. A cubic metre of charcoal consisting of particles 1 mm. in diameter has a surface of about 600 sq. metres. If the particles are reduced to colloidal dimensions, say to 0.1μ diameter, then the adsorbing surface becomes 60,000,000 square metres.

2. Carriers. In some cases the catalytic agent combines chemically with one of the reacting substances to form an unstable intermediate compound. This, in turn, breaks up, regenerates the catalyst, and liberates the reagent in the active atomic state-so called nascent. Many oxidations and reductions are brought about in this way.

3. Ionic Catalysts. Hydrogen and hydroxyl ions act as catalysts for many reactions which occur in aqueous solution. The velocity of such a reaction in dilute solution is proportional to the concentration of the ions in question, provided the thermodynamic environment remains constant. The ion probably acts as a carrier, forming an unstable perhydrate as intermediate product.

The great majority of vital catalytic reactions have, as catalyst, an enzyme. Enzymes themselves cannot be detected or estimated. Their presence is made apparent by their action. By estimating the amount of the products of enzyme activity an idea of the rate of reaction may be gained. Many attempts have been made to isolate and purify certain enzymes and, though complete success has not been granted to any investigator, much has been learned of their nature and of the conditions necessary for enzyme action. (a) Enzymes are colloidal. They can readily be separated from crystalloids by dialysis or ultrafiltration. Chemically, they resemble their substrate or are so closely associated with their

substrate that existence apart is impossible. It may be that the colloidal character of enzymes is the secret of their action. At any rate an artificial oxidising enzyme has been prepared by mixing a suspensoid-finely divided manganese, with an emulsoid -gum acacia. The adsorption complex so formed, if suitable crystalloids were present, reacted as an artificial "laccase."

(b) Enzymes retain their activity only over a very well-defined range of temperature. It is common knowledge that physiological processes take place most readily at body temperature. Every biological laboratory is equipped with devices for keeping incubators at a constant temperature-say, 37°-40° C. Before these appliances had been perfected, investigators in this realm had to keep their experimental material on their person. The Abbé Spallanzani (1729-1799), in his classical work on digestion, carried his digest-tubes in small pockets in his armpits for several days. During the Great War, when scientific work had to be carried out in all sorts of places, at least one physiologist, bereft of gas regulators, had to resort to this simple but efficient method of maintaining a fairly uniform temperature. In this way, reactions in which they play a part differ from those usually styled chemical. The rate of most chemical processes is doubled or trebled when the temperature is raised 10° C. The enzymes follow this rule only from 0° C. to a temperature called their optimum temperature, above which the rate decreases rapidly. The optimum temperature of most enzymes lies between 30° and 40° C. The decrease in rate of reaction when the temperature is allowed to go over 40° C. is probably due to coagulation of the enzyme. Increase in temperature causes alterations in the physical state of colloidal matter. These alterations, in viscosity, in colour, and in conductivity, all indicate an increase in the size of the colloidal particles, and consequently a decrease in their specific surface. The effective adsorbing surface is diminished. At the optimum temperature the increased chemical action due to temperature more than balances the decreased adsorbing surface. Beyond this temperature, the loss of surface becomes relatively important. If the temperature is raised till the specific surface is reduced, by coagulation, to a value below 10,000, adsorbing power is totally lost, chemical action is stopped, and the enzyme is said to be dead.

In the appended figure (Fig. 12) curve 1 (dotted line) shows how, as the temperature increases, a pure chemical action is accelerated. Curve 2 (dash line) represents the rate at which the