· Summarize the characteristics of synthesis, decomposition,and exchange reactions.
· Explain how reversible reactions produce chemical equilibrium.
· Distinguish between chemical reactions that release energy and those that take in energy.
· Describe the factors that can affect the rate of chemical reactions.
In a chemical reaction, atoms, ions, molecules, or compounds interact either to form or to break chemical bonds. The substances that enter into a chemical reaction are called the reactants, and the substances that result from the chemical reaction are called the products.
For our purposes, chemical reactions can be classified as synthesis, decomposition, or exchange reactions.
When two or more reactants combine to form a larger, more complex product, the process is called a synthesis reaction,representedsymbolically as
Examples of synthesis reactions include the synthesis of the complex molecules of the human body from the basic “building blocks” obtained in food and the synthesis of adenosine triphos-phate (ATP) (ă-den′\ō-sēn trı̄-fos′\fāt) molecules. In ATP,Astandsfor adenosine, T stands for tri- (or three), and P stands for a phos-phate group (PO43−). Thus, ATP consists of adenosine and three phosphate groups. ATP is synthesized when adenosine diphosphate (ADP), which has two (di-) phosphate groups, combines with a phosphate group to form the larger ATP molecule. The phosphate group that reacts with ADP is often denoted as Pi, where the iindicates that the phosphate group is associated with an inorganic substance (see “Inorganic Molecules ” later).
All of the synthesis reactions that occur in the body are collectively referred to as anabolism (ă-nab′\-ō-lizm). Growth, maintenance, and repair of the body could not take place without anabolic reactions.
In a decomposition reaction, reactants are broken down into smaller, less complex products. A decomposition reaction is the reverse of a synthesis reaction and can be represented in this way:
AB → A + B
Examples of decomposition reactions include the breakdown of food molecules into basic building blocks and the breakdown of ATP to ADP and a phosphate group.
The decomposition reactions that occur in the body are collec-tively called catabolism (kă-tab′\-ō-lizm). They include the digestion of food molecules in the intestine and within cells, the breakdown of fat stores, and the breakdown of foreign matter and microorganisms in certain blood cells that protect the body. All of the anabolic and cat-abolic reactions in the body are collectively defined as metabolism.
An exchange reaction is a combination of a decomposition reaction and a synthesis reaction. In decomposition, the reactants are broken down. In synthesis, the products of the decomposition reaction are combined. The symbolic representation of an exchange reaction is
AB + CD → AC + BD
An example of an exchange reaction is the reaction of hydro-chloric acid (HCl) with sodium hydroxide (NaOH) to form table salt (NaCl) and water (H2O):
HCl + NaOH → NaCl + H2O
A reversible reaction is a chemical reaction that can proceed from reactants to products and from products to reactants. When the rate of product formation is equal to the rate of reactant formation, the reaction is said to be at equilibrium. At equilibrium, the amount of the reactants relative to the amount of products remains constant.
The following analogy may help clarify the concept of revers-ible reactions and equilibrium. Imagine a trough containing water. The trough is divided into two compartments by a partition, but the partition contains holes that allow the water to move freely between the compartments. Because the water can move in either direction, this is like a reversible reaction. The amount of water in the left compartment represents the amount of reactant, and the amount of water in the right compartment represents the amount of product. At equilibrium, the amount of reactant relative to the amount of product in each compartment is always the same because the parti-tion allows water to pass between the two compartments until the level of water is the same in both compartments. If the amount of reactant is increased by adding water to the left compartment, water flows from the left compartment through the partition to the right compartment until the level of water is the same in both. Thus, the amounts of reactant and product are once again equal. Unlike this analogy, however, the amount of reactant relative to the amount of product in most reversible reactions is not one to one. Depending on the specific reversible reaction, there can be one part reactant to two parts product, two parts reactant to one part product, or many other possibilities.
An important reversible reaction in the human body occurs when carbon dioxide (CO2) and water (H2O) form hydrogen ions (H+) and bicarbonate ions (HCO3−). The reversibility of the reaction is indicated by two arrows pointing in opposite directions:
CO2 + H2O < - - >
If CO2 is added to H2O, the amount of CO2 relative to the amount of H+ increases. However, the reaction of CO2 with H2O produces more H+, and the amount of CO2 relative to the amount of H+ returns to equilibrium. Conversely, adding H+ results in the formation of more CO2, and the equilibrium is restored.
Maintaining a constant level of H+ in body fluids is neces-sary for the nervous system to function properly. This level can be maintained, in part, by controlling blood CO2 levels. For example, slowing the respiration rate causes blood CO2 levels to increase, which causes an increase in H+ concentration in the blood.
Energy is defined as the capacity to do work—that is, to movematter. Energy can be subdivided into potential energy and kinetic energy. Potential energy is stored energy that could do work but is not doing so. For example, a coiled spring has potential energy. It could push against an object and move the object, but as long as the spring does not uncoil, no work is accomplished. Kinetic (ki-net′ ik; of motion) energy is energy caused by the movement of an object and is the form of energy that actually does work. An uncoiling spring pushing an object and causing it to move is an example. When potential energy is released, it becomes kinetic energy, thus doing work.
Potential and kinetic energy exist in many different forms: chemical energy, mechanical energy, heat energy, electrical energy, and electromagnetic (radiant) energy. Here we examine how chem-ical energy and mechanical energy play important roles in the body.
The chemical energy of a substance is a form of potential energy stored in chemical bonds. Consider two balls attached by a relaxed spring. In order to push the balls together and compress the spring, energy must be put into this system. As the spring is compressed, potential energy increases. When the compressed spring expands, potential energy decreases. In the same way, similarly charged particles, such as two negatively charged elec-trons or two positively charged nuclei, repel each other. As simi-larly charged particles move closer together, their potential energy increases, much like compressing a spring, and as they move farther apart, their potential energy decreases. Chemical bonding is a form of potential energy because of the charges and positions of the subatomic particles bound together.
Chemical reactions are important because of the products they form and the energy changes that result as the relative positions of subatomic particles change. If the products of a chemical reaction contain less potential energy than the reactants, energy is released. For example, food molecules contain more potential energy than waste products. The difference in potential energy between food and waste products is used by the human body to drive activities such as growth, repair, movement, and heat production.
An example of a reaction that releases energy is the break-down of ATP to ADP and a phosphate group (figure 2.8a). The phosphate group is attached to the ADP molecule by a covalent bond, which has potential energy. After the breakdown of ATP, some of that energy is released as heat, and some is available to cells for activities such as synthesizing new molecules or contract-ing muscles:
ATP → ADP + Pi + Energy (used by cells)
In the two reactions shown here, the larger\ “sunburst” \represents greater potential energy and the smaller \“sunburst” \represents less potential energy. (a) Energy is released as a result of the breakdown of ATP.(b) The input of energy is required for the synthesis of ATP.
According to the law of conservation of energy, the total energy of the universe is constant. Therefore, energy is neither created nor destroyed. However, one type of energy can be changed into another. Potential energy is converted into kinetic energy. Since the conversion between energy states is not 100% efficient, heat energy is released. For example, as a spring is released, its potential energy is converted to mechanical energy and heat energy. Mechanicalenergy is energy resulting from the position or movement ofobjects. Many of the activities of the human body, such as moving a limb, breathing, or circulating blood, involve mechanical energy.
Why does body temperature increase during exercise?
If the products of a chemical reaction contain more energy than the reactants (figure 2.8b), energy must be added from another source. The energy released during the breakdown of food molecules is the source of energy for this kind of reaction in the body. The energy from food molecules is used to synthesize mol-ecules such as ATP, fats, and proteins:
ADP + Pi + Energy (from food molecules) → ATP
The rate at which a chemical reaction proceeds is influenced by several factors, including how easily the substances react with one another, their concentrations, the temperature, and the pres-ence of a catalyst.
Reactants differ from one another in their ability to undergo chemical reactions. For example, iron corrodes much more rapidly than does stainless steel. For this reason, during the refurbishment of the Statue of Liberty in the 1980s, the iron bars forming the statue’s skeleton were replaced with stainless steel bars.
Within limits, the greater the concentration of the reactants, the greater the rate at which a chemical reaction will occur because, as the concentration increases, the reacting molecules are more likely to come in contact with one another. For example, the normal concentration of oxygen inside cells enables it to come in contact with other molecules, producing the chemical reactions necessary for life. If the oxygen concentration decreases, the rate of chemical reactions decreases. A decrease in oxygen in cells can impair cell function and even result in cell death.
Because molecular motion changes as environmental temperature changes, the rate of chemical reactions is partially dependent on temperature. For example, reactions occur throughout the body at a faster rate when a person has a fever of only a few degrees. The result is increased activity in most organ systems, such as increased heart and respiratory rates. By contrast, the rate of reac-tions decreases when body temperature drops. The clumsy move-ment of very cold fingers results largely from the reduced rate of chemical reactions in cold muscle tissue.
At normal body temperatures, most chemical reactions would take place too slowly to sustain life if not for substances called catalysts. A catalyst (kat′\ă-list) increases the rate of a chemical reaction, without itself being permanently changed or depleted. An enzyme is a protein molecule that acts as a catalyst. Many of the chemical reactions that occur in the body require enzymes.