The metabolism of a multicellular organism is simply the sum of its cellular metabolism to a great degree. Again, if ATP is being generated in the complete catabolism of glucose within one cell of a complex animal body, a similar involvement on the part of other cells only adds to total ATP production.
Nevertheless, the metabolism of a complex organism is more than the sum total of its cellular activities. We can best conceive of metabolism at this level as an emergent characteristic which reflects the high degree of coordination among tissues, organs, and organ systems.
In an attempt to exemplify metabolism at the organism level, we shall consider separate cases of a complex animal and a complex plant.
Since we can assume some familiarity with the structure and function of the human body, and because we have a natural interest in our own metabolic processes, we shall devote some attention to human metabolism. After this, we shall consider the metabolism of a generalized seed plant.
Nutrition as one of the animals that possess a tube- within-a-tube body plan, man exhibits a gastro-intestinal tract beginning with a mouth and ending with an anus. During embryonic development, the tract grows in length at a more rapid rate than does the body wall, which causes it to be thrown into folds. In a mature human being, it averages about thirty feet in length. Certain accessory organs of digestion, the salivary glands, liver, gall bladder, and pancreas, develop as outpushings from the embryonic tract, and, when fully developed, they lie in close proximity to it. The digestive tube itself consists of the cavity just posterior to the mouth, the pharynx, esophagus, stomach, small intestine, and the large intestine.
Food enters the mouth, and any solid portions are retained within the oral cavity for a time, where the teeth render them more susceptible to the chemical action which follows by dividing them into smaller pieces.
Simultaneously, the secretion of the salivary glands moistens the food mass, making passage along the esophagus possible, and an enzyme, salivary amylase, begins a process of hydrolysis whereby the polysaccharide carbohydrates glycogen and starch, if present, are broken down to the disaccharide maltose. Fats and proteins are not chemically affected in the oral cavity.
By the act of swallowing, the moistened food passes through the pharynx and esophagus, entering the stomach through a muscle-enclosed opening. Here it encounters an acid environment, due to the presence of hydrochloric acid in the gastric juice secreted by tiny glands in the stomach wall.
The salivary amylase, which is inactivated by acid, continues to hydrolyze starch or glycogen for a time, however, because penetration of the food mass by the acid is not instantaneous. Although the gastric juice contains no enzymes capable of acting on carbohydrates, a degree of hydrolysis occurs through the action of hydrochloric acid on some of the bonds that link monosaccharide units together.
It appears that very little digestion of fats occurs in the stomach, although they are softened. Protein digestion begins in the stomach through the action of pepsin, an enzyme present in gastric juice which acts specifically on certain bonds within protein molecules, thus hydrolyzing many of them to shorter chains.
Whenever a quantity of food is present in the stomach, liquefied portions are released at intervaJs into the duodenum through an opening similar to the one between the esophagus and the stomach. There is usually sufficient alkalinity in the secretions that collect in the small intestine to counteract the acid nature of the liquefied food as it comes from the stomach, thus rendering it near the point of neutrality in reaction.
It is in the small intestine that the major portion of digestion occurs, the stomach having served chiefly as a site of storage. Secretions from the pancreas and the liver enter the duodenum. Small glands located within the intestinal wall secrete enzymes also.
As a result of pancreatic and intestinal secretion, the food mass is exposed to a large number of enzymes in the small intestine, each of which is rather specific for certain types of chemical bonds.
Molecules of starch or glycogen which escaped the action of salivary amylase are hydrolyzed to maltose by an amylase secreted by the pancreas. Maltose and other undigested disaccharides are split into their component monosaccharides by enzymes which form a part of the intestinal secretion. Carbohydrate digestion is completed with the production of monosaccharide’s, which can be absorbed.
It will be recalled that virtually no digestion of fats occurs prior to the time that these enter the small intestine. At this point, however, lipases of the pancreatic and intestinal secretions hydrolyze some of the fat molecules to glycerol and fatty acids, both of which can be absorbed.
Perhaps more than half of the fat is not hydrolyzed but is finely emulsified to molecular aggregates called chylomicrons, which eventually pass through the membranes of the cells lining the inner lumen of the intestine.
It appears that this emulsification is made possible by the combined action of bile salts, found in the secretion of the liver which empties to the duodenum, and the products of fat digestion resulting from the action of lipases. Thus, the first step in the breakdown of fats is the bydrolytic production of glycerol and fatty acids, which then proceed to emulsify the remaining fat with the help of the bile salts.
By the time proteins reach the small intestine, hydrolysis has occurred to an extent through the action of pepsin. The pancreatic ermine’s tyrosine and chymotrypsin render the protein chains still shorter by acting upon links for which they are specific, with the result that peptides consisting of relatively few amino acids are produced.
Various peptidases of pancreatic and intestinal origin then remove amino acids one at a time and digestion of proteins is completed since amino acids are able to penetrate the cell membranes of the intestinal lining.
Monosaccharides, fatty acids, glycerol, chylomicrons, and amino acids are absorbed into the cells that line the inner surface of the small intestine and pass on to still deeper cells. Eventually, they reach small blood capillaries and lymph vessels which transport them from the intestine.
Some of the absorption can be accounted for by simple diffusion, but it is known that under certain conditions active absorption against concentration gradients occurs. The small intestine is well adapted to the process; not only is there a great deal.
The chief function of digested carbohydrates is that o furnishing the body with a ready source of energy, and the blood stream maintains a fairly constant level of glucose under normal dietary conditions.
If this level has fallen during a period of fasting or starvation, glucose and other monosaccharides received from the small intestine are picked up directly’ from the liver without alteration and are carried to the tissues.
Under normal circumstances, however, they are converted in the liver to glycogen and are stored by the liver in this form. Liver glycogen may be built up to a point of saturation, and excess glucose is then converted to fat in certain tissues of the body. Although the liver is the central “bank” for glycogen in the body, this substance may be maintained to some extent in “branch banks” such as muscles.
One glycogen is formed in the liver; however, it must be hydrolyzed to glucose before it can be transported to other tissues, where a reverse process results in its re-formation into glycogen. .
Under normal conditions, it appears that glycerol, fatty acids, and chylomicrons travel directly from the intestine to sites of storage called/at depots, of which the mesenteries and the tissues just beneath the skin are most important
Glycerol and fatty acids are recombined into fats which, together with the chylomicrons, serve as raw materials for the synthesis of the particular type of fat characteristic of the human species.
Whenever the energy requirement of the body is such that carbohydrate intake is insufficient to meet it and to maintain a normal level of glycogen in the liver and other tissues, these fat stores are called upon, it appears that they are transported under these conditions to the liver, where they are split to glycerol and fatty acids, which then become available to the cells of the body.
Although carbohydrates and fats are involved to a certain extent in anabolic processes within cells, they function primarily as sources of energy for the body. In the case of proteins, the reverse is true.
The primary function of amino acids as they are absorbed from the intestine is that of protein synthesis, and it is through this process that cells build up their enzymes and structural proteins. After these requirements are met, excess amino acids are converted in the liver to another readily utilizable substance.
There is virtually no storage of proteins and amino acids in the body, and these substances must be taken into the body at fairly frequent intervals. If the supply of amino acids is inadequate, the body eventually meets its requirements by utilizing certain of its own tissues. Under conditions of a normal diet, however, the intake of protein is somewhat greater than that necessary to maintain the tissues. This results in an excess of amino acids in terms of their primary function, and it is these units which enter into transformations in the liver. The conversion of amino acids to other substances of their entrance into the citric acid cycle involves the loss of their NH2 groups, which are eventually excreted as urea.
In spite of the inability of the body to build up amino acids from fats and carbohydrates alone, since these substances possess no nitrogen, a certain amount of amino acid synthesis takes place. Some are formed through modification of others, and some are produced from no protein materials. Certain of the amino acids, however, cannot be made: these must be included in the diet; hence they are termed essential amino acids.
Eight of the twenty amino acids which make up the proteins of the human body are essential to man. Not only must these be included in the diet, but they must be present in amounts sufficient to meet the anabolic requirements of the body.
It is therefore possible that the body may receive amounts of protein which are more than adequate in a quantitative sense, but if there is a deficiency in even one essential amino acid, the result is a break in the nutritional chain.
Fortunately, most proteins are complete in their inclusion of essential amino acids, although not all are equally rich in them. In general, proteins of animal origin are richer than those of plants in this respect, which means that the daily intake of protein must be larger when plant products are chiefly or exclusively utilized as a source.
With the build-up of fat, glycogen, and blood glucose, the body is equipped with immediate and reserve fuels with which to stoke the catabolic fires. Respiration is made possible in the cells through the passage of hydrogen to gaseous oxygen, this final acceptor being transported to the cells by way of the blood stream in loose chemical combination with hemoglobin, a pigment present in red blood cells.
In addition to a catabolic function, glucose and certain types of fat molecules may also enter into anabolic reactions of various sorts. Growth, repair, and maintenance are made possible by the absorption of amino acids from the blood stream into cells, and any surplus of these is diverted to the function of energy production through loss of their nitrogen in the liver.
Although most of the vitamins which are required by the human body are a part of the normal diet, two vitamins, D and K, are synthesized in quantity within the human body. The action of sunlight on a substance in the skin called ergosterol changes it to Vitamin D, and bacteria of the intestinal tract manufacture vitamin K, which is absorbed when their cells disintegrate. Apparently, no other vitamins are manufactured by the body or obtained from bacterial synthesis in the intestine, at least in appreciable quantities.
In addition to the synthesis of carbohydrates, fats, proteins, and certain vitamins, a variety of other compounds are formed. Among these are nucleic acids, complex lipids, and hormones. The complexity of these synthetic reactions is such that we shall not attempt to enter into this aspect of metabolism, but suffice it to say that the production of new cells alone involves the synthesis of a myriad of compounds.
Furthermore, the functioning of the human body involves many complex substances, such as hormones, and the total synthesis and utilization of such compounds are delicately balanced within a homeostatic system of grand proportions. We have already seen how isolated parts of this system work as steady-state mechanisms, but as we have pointed out, homeostasis at the organismic level involves a coordination of such mechanisms. Nowhere is this more clearly emphasized than in the synthetic reactions of the human body.
There is one additional aspect of synthesis which deserves some attention, and that is the formation of compounds which eventually leave the body as waste material. Here again, these synthetic reactions are an important part of homeostasis. Since the formation and elimination of these compounds involve special organ systems, we shall consider this aspect of synthesis in some detail.
Excluding digestive residues from consideration, the two major classes of waste products of the human body are carbon dioxide and a wide variety of nitrogenous materials. The elimination of carbon dioxide involves a ratheT simple transformation, but it is so closely identified with respiration that we shall discuss its elimination in the next topic of this section.
With regard to the nitrogenous wastes, two muscles of the chest undergo contraction, the thoracic cavity is expanded. Air rushes into the lungs in response to the partial vacuum created by such action, and oxygen is thus made available to the cells of the lungs. Relaxation of the muscles causes the lungs to undergo partial collapse due to their own elasticity, and a quantity of air is expelled.
Functionally, gaseous exchange occurs in the lungs by virtue of the thinness of the lung tissues and ofthe walls of capillaries which supply them. This exchange is apparently to a great degree a matter of simple diffusion; there is a continual tendency for oxygen to move into the capillaries, and for carbon dioxide to move into the lungs.
Intimate contact of cells makes it possible for this exchange to occur. As would be expected, inhaled air contains more oxygen and less carbon dioxide than exhaled air. As a matter of fact, the air ofthe atmosphere, as it is inhaled, contains about 20 per cent of oxygen and 0.03 per cent of carbon dioxide, whereas exhaled air contains about 16 per cent of oxygen and 4 per cent of carbon dioxide.
Once in the bloodstream, oxygen combines with the hemoglobin of red blood cells and is transported throughout the body in the form of oxyhemoglobin. The bonds which hold oxygen to hemoglobin are very weak ones, and eventually, dissociation of oxyhemoglobin occurs.
At this point, hemoglobin may be returned to the lungs for another load, and oxygen makes its way to the individual cells of the body. As we have seen, the capillary network of the blood-circulatory system is widespread in the body, and no cell is very far removed from a capillary.
Hence, a constant oxygen supply from the blood stream is assured under normal conditions of body activity. Conversely, carbon dioxide passes from the cells into capillaries, and is thus transported to the lungs. In the case of carbon dioxide, however, hemoglobin is not the chief carrier.
The movement of oxygen from the blood stream to the tissues and the movement of carbon dioxide from the tissues to the blood stream are consistent with the so-called gas laws of physics and chemistry, and in essence, this means that these gases are responsive to differential pressures. In other words, if there is less oxygen per volume of space in the tissues than in the blood stream, dissociation of oxyhemoglobin occurs, and gaseous oxygen moves into the tissues.
Conversely, if there is less carbon dioxide per volume of space in the bloodstream than in the tissues, carbon dioxide moves into the bloodstream. Since a continuous oxygen deficit is normally present in the tissues, and a carbon dioxide deficit is the usual condition in the capillary blood, the exchange of gases at the site of the tissues is assured. Similarly, the reverse situation prevails between the capillary network in the lungs and the air within the lung sacs.
Conversely, the addition of hydroxyl ions is counteracted by other components of the general buffering system. In actual practice, however, this is not an isolated situation which is merely confined within the body fluids. Both the kidneys and the lungs influence pH regulation directly, and ultimately, almost every organ system becomes involved in some way. In the final analysis, then, homeostasis at this level of organization is characteristic of the organism as a whole.