In the body, chemical energy of foods is trapped in the bonds of a high-energy chemical called ATP (adenosine triphosphate), and ATP's energy may ultimately be transformed into the electrical energy of a nerve impulse or mechanical energy of shortening muscles. ATP transfers its terminal phosphate group to glucose to form glucose- phosphate. The synthesis of adenosine triphosphate, or ATP, is all-important be- cause it provides a form of chemical energy that all body cells can use. Without ATP, molecules cannot be made or broken down, cells cannot maintain their boundaries, and all life processes grind to a halt. Structurally, ATP is a modified nucleotide; it consists of an adenine base, ribose sugar, and three phosphate groups. ATP can be compared to a tightly coiled spring that is ready to uncoil with tremendous energy when the "catch" is released. Like salts, acids and bases are electrolytes. That is, they ionize and then dissociate in water and can then conduct an electrical current. The bases come in five varieties: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). (A) and (G) are large, two-ring bases, whereas the others are smaller, single-ring stmctures. The nucleotides are named according to the base they contain: A-containing bases are adenine nu- cleotides, C-containing bases are cytosine nu- cleotides, and so on. Carbohydrates provide a ready, easily used source of food energy for cells, and glucose is at the top of the "cellular menu." When glucose is oxidized (combined with oxygen) in a complex set of chemical reactions, it is broken down into carbon dioxide and water. Small amounts of carbohydrates are used for structural purposes and represent 1 to 2 percent of cell mass. Like carbohydrates, all lipids contain carbon, hydrogen, and oxygen atoms, but in lipids, carbon and hydrogen atoms far outnumber oxygen atoms, as illustrated by tl1e formula for a typical fat named tristearin: C57H1100 6. Carbohydrates, which include sugars and starches, contain carbon, hydrogen, and oxygen. With slight va1iations, the hydrogen and oxygen atoms appear in the same ratio as in water; that is, 2 hydrogen atoms to 1 oxygen atom. Exchange reactions involve both synthesis and decomposition reactions; bonds are both made and broken. During exchange reactions, a switch is made between molecule parts (changing partners, so to speak), and different molecules are made. Thus, an exchange reaction can be generally indicated as AB+C->AC+B and AB+CD->AD+CB. An exchange reaction occurs, for example, when ATP reacts with glucose and transfers its end phosphate group to glucose, forming glucose- phosphate (Figure 2.10c). At the same time, the ATP becomes ADP. This important reaction, which occurs whenever glucose enters a body cell, effectively traps the glucose fuel molecule inside the cell. form when electrons are completely transferred from one atom to another. Atoms are electrically neutral, but when they gain or lose electrons during bonding, their positive and negative charges are no longer balanced, and charged particles, called ions, result. When an atom gains an electron, it acquires a net negative charge because it now has more electrons than protons. Negatively charged ions are more specifically called anions. When an atom loses an electron, it becomes a positively charged ion, a cation, because it now possesses more pro- tons than electrons. (It may help you to remember that a cation is a positively charged ion by thinking of its "t" as a plus [+ ) sign.) Both anions and cations result when an ionic bond is formed. Because opposite charges attract, the newly created ions tend to stay close together. Lipids are a large and diverse group of organic compounders. They enter the body in the form of fat-marbled meats, egg yolks, milk products, and oils. The most abundant lipids in the body a re triglycerides, phospholipids, and steroids. Like carbohydrates, all lipids contain car- bon, hydrogen, and oxygen atoms, but in lipids, carbon and hydrogen atoms far outnumber oxy- gen atoms, as illustrated by tl1e formula for a typical fat named tristearin: C57H1100 6. Most lipids are insoluble in water but readily dissolve in other lipids and in organic solvents such as alcohol and acetone. Nucleic acids, composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus atoms, are the largest biological molecules in the body. The building blocks, the nucleotides. The two major kinds of nucleic acid are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA differ in many respects. DNA is the genetic material found within the cell nucleus (the control center of the cell). It has two fundamental roles: It replicates itself exactly before a cell divides, thus ensuring that the genetic information in every body cell is identical. And it provides the instructions for building eve1y protein in the body. For the most part, RNA is located outside the nucleus and can be considered the "molecular slave" of DNA; that is, RNA carries out the orders for protein synthesis issued by 0 1A. account for over 50 percent of the organic matter in the body, and they have the most varied functions of the organic molecules. Some are consumption materials; others play vital roles in cell function. Like carbohydrates and lipids, all proteins contain carbon, oxygen, and hydrogen. In addition, they contain nitrogen and sometimes sulfur atoms as well. The building blocks of proteins are small molecules called amino acids. About 20 common varieties of amino acids are found in proteins. All amino acids have an amine group (NH2) , which gives them basic properties, and an acid group (COOH), which allows them to act as acids. In fact, all amino acids are identical except for a single group of atoms called their R-group. Differences in the R-groups make each amino acid chemically unique. For example, an extra acid group (COOH) in the R-group makes the amino acid more acidic. For the most part, RNA is located outside the nucleus and can be considered the "molecular slave" of D A; that is, R A carries out the orders for protein synthesis issued by 0 1A. Whereas 0 A is double-stranded, RNA molecules are single nucleotide strands. The RNA bases are A, G, C, and U (U replaces the T found in 0 A), and its sugar is ribose instead of deoxyribose. Three major varieties of RNA exist- messenger, ribosomal, and transfer RNA-and each has a specific role to play in carrying out DNA's instructions for building proteins. RNA carries the information for building the protein from the 0 A genes to the ribosomes, the protein-synthesizing sites. Transfer RNA ferries amino acids to the ribosomes. Ribosomal RNA forms part of the ribosomes, where it oversees the translation of the message and the binding together of amino acids to form the proteins. A salt is an ionic compound containing cations other than H+ and anions other than the hydroxyl ion (OH- ). Salts of many metal elements are commonly found in the body, but the most plentiful salts are those containing calcium and phosphorus, found chiefly in bones and teeth. When dis- solved in body fluids, salts easily separate into their ions. This process, called dissociation, occurs rather easily because the ions have already been formed. All that remains is to pull the ions apart. This is accomplished by the polar water molecules, which orient themselves with their slightly negative ends toward the cations and their slightly positive ends toward d1e anions and thereby over come the attraction between them. Salts, both in their ionic forms and in combination with other elements, are vital to body functioning. For example, sodium and potassium ions are essential for nerve impulses, and iron forms part of the hemoglobin molecule that transports oxygen within reel blood cells. Because ions are charged particles, all salts are electrolytes-substances that conduct an electrical current in solution. When ionic (or electrolyte) balance is severely disturbed, virtually nothing in the body works. are basically flat molecules formed of four interlocking rings; thus, their structure differs quite a bit from that of fats. However, like fats, steroids are made largely of hydrogen and carbon atoms and are fat-soluble. The single most important steroid molecule is cholesterol. We ingest cholesterol in animal products such as meat, eggs, and cheese, and some is made by the liver, regardless of dietary intake. Cholesterol has earned bad press because of its role in arteriosclerosis, but it is essential for human life. Cholesterol is found in cell membranes and is the raw material of vitamin D, steroid hormones, and bile salts. Although steroid hormones are present in the body in only small quantities, they are vital to homeostasis. Without sex hormones, reproduction would be impossible, and a total lack of the corticosteroids produced by the adrenal glands is fatal. is the most abundant inorganic compound in the body. It accounts for about two-thirds of body weight. Body tissues are 60 to 80 percent water. Among the properties that make water so vital are the following:
1. High heat capacity. Water has a high heat capacity; that is, it absorbs and releases large amounts of heat before its temperature changes appreciably. Thus, it prevents the sudden changes in body temperature that might otherwise result from intense sun expo- sure, chilling winter winds, or internal events (such as vigorous muscle activity) that liberate large amounts of heat.
2. Polarity/ solvent properties. Because of its polarity, water is an excellent solvent; indeed, it is often called the "universal solvent." A solvent is a liquid or gas in which smaller amounts of other substances, called solutes (which may be gases, liquids, or solids), can be dissolved or suspended. The resulting mixture is called a solution when the solute particles are exceedingly tiny, and a suspension when the solute particles are fairly large. Translucent mixtures with solute particles of intermediate size are called colloids.
3. Chemical reactivity. Water is an important reactant in some types of chemical reactions. For example, to digest foods or break down biological molecules, water molecules are added to the bonds of the larger molecules. Such reactions are called hydrolysis reactions, a term that specifically recognizes this role of water (hydro= water; lys = splitting).
4. Cushioning. Water also serves a protective function. In the form of cerebrospinal fluid, water forms a cushion around the brain that helps to protect it from physical trauma. Amniotic fluid, which surrounds a developing fetus within the mother's body, plays a similar role in protecting the fetus.