The term metabolism, strangely enough, is related closely to devil, with which it shares the Greek root ballein, meaning "to throw." By adding dia ("through" or "across"), one arrives at devil and many related words, such as diabolical ; on the other hand, the replacement of that prefix with meta ("after" or "beyond") yields the word metabolism. The connection between the two words has been obscured over time, but it might be helpful to picture metabolism in terms of an image that goes with that of a devil: a furnace.
Metabolism is indeed like a furnace, in that it burns energy, and that is the aspect most commonly associated with this concept. But metabolism also involves a function that a furnace does not: building new material. All metabolic reactions can be divided into either catabolic or anabolic reactions. Catabolism is the process by which large molecules are broken down into smaller ones with the release of energy, whereas anabolism is the process by which energy is used to build up complex molecules needed by the body to maintain itself and develop new tissue.
One way to understand the metabolic process is to follow the path of a typical nutrient as it passes through the body. The digestive process is discussed in Digestion, while nutrients are examined in Nutrients and Nutrition as well as in Proteins, Amino Acids, Enzymes, Carbohydrates, and Vitamins. Here we touch on the process only in general terms, as it relates to metabolism.
The term digestion is not defined in the essay on that subject, because it is an everyday word whose meaning is widely known. For the present purposes, however, it is important to identify it as the process of breaking down food into simpler chemical compounds as a means of making nutrients absorbable by the body. This is a catabolic process, because the molecules of which foods are made are much too large to pass through the lining of the digestive system and directly into the bloodstream. Thanks to the digestive process, smaller molecules are formed and enter the bloodstream, from whence they are carried to individual cells throughout a person's body.
The smaller molecules into which nutrients are broken down make up the metabolic pool, which consists of simpler substances. The metabolic pool includes simple sugars, made by the breakdown of complex carbohydrates; glycerol and fatty acids, which come from the conversion of lipids, or fats; and amino acids, formed by the breakdown of proteins. Substances in the metabolic pool provide material from which new tissue is constructed—an anabolic process.
The chemical breakdown of substances in the cells is a complex and wondrous process. For instance, a cell converts a sugar molecule into carbon dioxide and water over the course of about two dozen separate chemical reactions. This is what cell biologists call a metabolic pathway: an orderly sequence of reactions, with particular enzymes (a type of protein that speeds up chemical reactions) acting at each step along the way. In this instance, each chemical reaction makes a relatively modest change in the sugar molecule—for example, the removal of a single oxygen atom or a single hydrogen atom—and each is accompanied by the release of energy, a result of the breaking of chemical bonds between atoms.
Cells capture and store the energy released in catabolic reactions through the use of chemical compounds known as energy carriers. The most significant example of an energy carrier is adenosine triphosphate, or ATP, which is formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group. (A phosphate is a chemical compound that contains oxygen bonded to phosphorus, and the term group in chemistry refers to a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways.)
ADP will combine with a phosphate group only if energy is added to it. In cells, that energy comes from the catabolism of compounds in the metabolic pool, including sugars, glycerol (related to fats), and fatty acids. The ATP molecule formed in this manner has taken up the energy previously stored in the sugar molecule, and thereafter, whenever a cell needs energy for some process, it can obtain it from an ATP molecule. The reverse of this process also takes place inside cells. That is, energy from an ATP molecule can be used to put simpler molecules together to make more complex molecules. For example, suppose that a cell needs to repair a rupture in its cell membrane. To do so, it will need to produce new protein molecules, which are made from hundreds or thousands of amino-acid molecules. These molecules can be obtained from the metabolic pool.
The reactions by which a compound is metabolized differ for various nutrients. Also, energy carriers other than ATP may play a part. For example, the compound known as nicotinamide adenine dinucleotide phosphate (NADPH) also has a role in the catabolism and anabolism of various substances. The general outline described here, however, applies to all metabolic reactions.
Energy released from organic nutrients (those containing carbon and hydrogen) during catabolism is stored within ATP, in the form of the high-energy chemical bonds between the second and third molecules of phosphate. The cell uses ATP for synthesizing cell components from simple precursors, for the mechanical work of contraction and motion, and for transport of substances across its membrane. ATP's energy is released when this bond is broken, turning ATP into ADP. The cell uses the energy derived from catabolism to fuel anabolic reactions that synthesize cell components. Although anabolism and catabolism occur simultaneously in the cell, their rates are controlled independently. Cells separate these pathways because catabolism is a "downhill" process, or one in which energy is released, while anabolism is an "uphill" process requiring the input of energy.
Catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle. Named after the German-born British biochemist Sir Hans Adolf Krebs (1900-1981), the Krebs cycle is a series of chemical reactions in which tissues use carbohydrates, fats, and proteins to produce energy; it is part of a larger series of enzymatic reactions known as oxidative phosphorylation. In the latter reaction, glucose is broken down to release energy, which is stored in the form of ATP—a catabolic sequence. At the same time, other molecules produced by the Krebs cycle are used as precursor molecules for reactions that build proteins, fats, and carbohydrates—an anabolic sequence. (A precursor is a substance, cellular component, or cell from which another substance, cellular component, or cell—different in kind from the precursor—is formed.)
As noted earlier, many practical aspects of metabolism are discussed elsewhere, particularly in the essays Digestion and Nutrients and Nutrition. Also, two types of chemical compound, proteins and carbohydrates, are so important to a variety of metabolic processes that they are examined in detail within entries of their own. In the present context, let us focus on the third major kind of nutrient, lipids or fats.
Lipids are soluble in nonpolar solvents, which is the reason why a gravy stain or other grease stain is difficult to remove from clothing without a powerful detergent or spot remover. Water molecules are polar, because the opposing electric charges tend to occupy opposite sides or ends of the molecule. In a molecule of oil, whether derived from petroleum or from animal or vegetable fat, electric charges are very small, and are distributed evenly throughout the molecule.
Whereas water molecules tend to bond relatively well, like a bunch of bar magnets attaching to one another at their opposing poles, oil and fat molecules tend not to bond. (The "bond" referred to here is the fairly weak one between molecules. Much stronger is the chemical bond within molecules—a bond that, when broken, brings about a release of energy, as noted earlier.) Their functions are as varied as their structures, but because they are all fat-soluble, lipids share in the ability to approach and even to enter cells. The latter have membranes that, while highly complex in structure, can be identified in simple terms as containing lipids or lipoproteins (lipids attached to proteins). The behavior of lipids and lipid-like molecules, therefore, becomes very important in understanding how a substance may or may not enter a cell. Such a substance may be toxic, as in the case of some pesticides, but if they are lipid-like, they are able to penetrate the cell's membrane. (See Food Webs for more about the biomagnification of DDT.)
In addition to lipoproteins, there are glycolipids, or lipids attached to sugars, as well as lipids attached to alcohols and some to phosphoric acids. The attachment with other compounds greatly alters the behavior of a lipid, often making them bipolar—that is, one end of the molecule is water-soluble. This is important, because it allows lipids to move out of the intestines and into the bloodstream. In the digestive process, lipids are made water-soluble either by being broken down into smaller parts or through association with another substance. The breaking down usually is done via two different processes: hydrolysis, or chemical reaction with water, and saponification. The latter, a reaction in which certain kinds of organic compounds are hydrolyzed to produce an alcohol and a salt, is used in making soap.
Comment about this article, ask questions, or add new information about this topic: