RIGIDITY OF TISSUES 87 The hydrophilic properties of sodium and calcium soaps have already been mentioned. Their behaviour in emulsion-making throws light on some peculiar problems in physiology. Loeb and his co-workers found that certain marine organisms died when put into fresh water. This will not appear surprising to the student who remembers the phenomena of endosmosis, e.g. plasmolysis, haemolysis, etc. That this explanation is not correct is shown by putting the organisms into solutions of sodium chloride or of calcium chloride having the same osmotic pressure as sea water. If, however, the organisms which would have been killed by immersion in these isotonic solutions were placed in a solution having a definite ratio between the amount of sodium and calcium present, life was maintained quite normally. All protoplasm may be considered as an emulsion of lipoid material in a colloidal-crystalloidal complex. The presence of the sodium soap formed by interaction with the lipoids causes the formation of a lipoid-in-water emulsion while the calcium soaps emulsify water-in-lipoid. The two types of emulsion thus formed are in equilibrium with an environment containing a definite Na/Ca ratio, that of sea water. Alteration in this ratio upsets the balance between the two types of emulsion and causes the cessation of growth and subsequently of life (see Nerve, Chap. XVII.). The rigidity of tissue is to a large extent due to their emulsion character. We have up till now considered protoplasm as a liquid, arguing that it is so because it shows the phenomena of surface tension, because it allows the ready diffusion of crystalloids into and through it, and because it reacts chemically as a liquid. On the other hand, tissues, as we handle them, are more or less rigid, having elasticity and definiteness of form. Do Pickering's solid emulsions and the Na/Ca ratio not suggest a fairly plausible explanation of this double nature of protoplasm? The "softening " of tissues observed in various pathological states may be due to the breaking of the protoplasm-emulsion from any cause (Part II.). Our food materials as well as our tissues are colloidal complexes. They are derived in part from the animal, in part from the vegetable kingdoms. A. Animal foods may be classified as : (1) Milk and its products-cream, butter, and cheese. (3) Eggs. (1) Milk is a fine emulsion of fat in a protein-colloidal solution. (a) The fat globules each seem to be enveloped by a covering of adsorbed protein. (b) The chief protein in milk is caseinogen, a phospho-protein which exists in milk as a soluble calcium compound. This compound is broken by the action of acid, and protein separates as a curd. (c) The carbohydrate of milk, lactose, is split by various microorganisms, forming lactic acid, thus souring the milk and causing curdling. Butter is simply the fat of the milk more or less completely separated from the other constituents and forming a water-in-oil emulsion. Whole, unchanged milk shows no tendency to form butter. To form butter the fat particles are concentrated at the surface by centrifugal action (or merely by allowing the cream to rise), and then by causing the cream to sour, the fat is freed from its emulsion with the colloidal matter. Since the hydrated colloids tend to collect in the surface layer between the fat globules and the dispersant aqueous phase of the cream, churning is performed to break these layers and hasten the coalescence of the fat. "The combined efforts therefore bring about a progressive increase in the concentration of the oil with a decrease in the concentration of the hydrated colloid until the instability of the oil in hydrated colloid becomes so great as to 'break' and yield the hydrated colloid-in-fat emulsion which we call butter " (Fischer and Hooker). That milk and cream are oil-in-water emulsions can be proved microscopically. They wet paper and are not greasy to the touch. Butter is a water-in-oil emulsion, feels greasy, oils paper, and microscopically appears as a finely divided aqueous colloid phase in a continuous oil phase. (2) Flesh. Under this head is included, not only the muscles of various animals, but such cellular organs as the liver, kidneys, thymus, etc. The colloidal nature of such tissues has already been dealt with (see effect of cooking, below). (3) Eggs. The white of eggs is practically an albumin hydrosol containing some crystalloids, while the yolk is an emulsion of lipins (lecithin, etc.), in a hydrosol of protein (ordinary proteins, and vitellin, a phospho-protein). B. Vegetable Foods. In the food of man, vegetable foods play as important a part as animal products. Generally, their make up is that of a mixed hydrogel of protein, higher carbohydrates (and in the case of COLLOIDAL NATURE OF FOODS 89 oatmeal, maize, nuts, certain legumes and vegetables), a fair proportion of fat. This gel is enclosed in a capsule of cellulose— a higher carbohydrate which is very resistant to the action of the human digestive juices. The capsule must be destroyed by previous treatment, e.g. milling, cooking, chewing, etc., before the contents can be utilised. Far and away the most important of our foodstuffs are derived from cereals. From 30 to 50 per cent. of the energy of an ordinary diet comes from them. They are generally used as flour, baked into bread, or as meal made into porridge. Wheat flour is a complex gel powder consisting of about 10 per cent. protein, about 75 per cent. carbohydrate (starch and cellulose), and about 2 per cent. fat in the colloidal state. The individual particles contain molecularly dispersed salts, sugar, water, and adsorbed gases such as air and carbondioxide. Of the 10 per cent. of protein, gliadin forms about 4 per cent. and glutelin about 4 per cent. There is less than 1 per cent. of globulin (0.6 per cent.) and albumin (0·3) present. The mixture of glutelin and gliadin is known as gluten. Gluten is insoluble in water or in dilute salt solutions, and therefore readily forms a disperse system with water called dough. Dough is a polydispersoid composed of the glutelin (and other proteins) carbohydrates and crystalloids mentioned above bound together by colloidal gliadin. It is a viscous semi-liquid mass which, however, may be cut like a solid, and when torn exhibits a fibrous surface. The elastic properties of dough depend upon the proportion of electrolytes present, especially on the phosphates. When it is dried it changes into a gel and later becomes brittle like glue. There is doubtless a close connection between the viscosity of flour-water mixtures, and the stickiness, rising property, power of absorbing CO2 of the dough, hydration of the starch and the porosity and volume of the resultant loaf. The viscosity is found to increase with the concentration of the flour and also to become greater for some time after mixing. This is doubtless due to the slow swelling of the starch and albumin. If concentrated solutions are suddenly diluted the viscosity is too great at first, but gradually approaches a normal value. This is probably caused by a slow increase in the dispersion, because when the larger particles are removed by means of filter paper normal results are obtained. Cooking. While many reactions occur in cooking, the changes that are of paramount importance are of a colloidal nature. Dough, for instance, undergoes a marked alteration in its physical 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 |