Classes of Chemical Reactions

A chemical reaction is a process in which a covalent or ionic bond is formed or broken. The course of a chemical reaction is symbolized by a chemical equation that typically shows the reactants on the left, the products on the right, and an arrow pointing from the reactants to the products. For example, consider this common occurrence: If you open a bottle of wine and let it stand for several days, it turns sour. Wine "turns to vinegar" because oxygen gets into the bottle and reacts with ethanol to produce acetic acid and water. Acetic acid gives the tart flavor to vinegar and spoiled wine. The equation for this reaction is

Decomposition reaction

Synthesis reaction


Oxygen ch3cooh +

Acetic acid


Ethanol and oxygen are the reactants, and acetic acid and water are the products of this reaction. Not all reactions are shown with the arrow pointing from left to right. In complex biochemical equations, reaction chains are often written vertically or even in circles.

Chemical reactions can be classified as decomposition, synthesis, or exchange reactions. In decomposition reactions, a large molecule breaks down into two or more smaller ones (fig. 2.13a); symbolically, AB ^ A + B. When

Decomposition reaction

Starch molecule

Amino acids

Starch molecule

Glucose molecules

Starch Molecules
Protein molecule

Exchange reaction

Exchange reaction

Chemical Decomposition Illustration

Figure 2.13 Decomposition, Synthesis, and Exchange Reactions. (a) In a decomposition reaction, large molecules are broken down into simpler ones. (b) In a synthesis reaction, smaller molecules are joined to form larger ones. (c) In an exchange reaction, two molecules exchange atoms.

To which of these categories does the digestion of food belong?


Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

70 Part One Organization of the Body you eat a potato, for example, digestive enzymes decompose its starch into thousands of glucose molecules, and most cells further decompose glucose to water and carbon dioxide. Starch, a very large molecule, ultimately yields about 36,000 molecules of H2O and CO2.

Synthesis reactions are just the opposite—two or more small molecules combine to form a larger one; symbolically, A + B ^ AB (fig. 2.13b). When the body synthesizes proteins, for example, it combines several hundred amino acids into one protein molecule.

In exchange reactions, two molecules exchange atoms or groups of atoms; AB + CD ^ AC + BD (fig. 2.13c). For example, when stomach acid (HCl) enters the small intestine, the pancreas secretes sodium bicarbonate (NaHCO3) to neutralize it. The reaction between the two is NaHCO3 + HCl ^ NaCl + H2CO3. We could say the sodium atom has exchanged its bicarbonate group (—HCO3) for a chlorine atom.

Reversible reactions can go in either direction under different circumstances and are represented with double-headed arrows. For example, carbon dioxide combines with water to produce carbonic acid, which in turn decomposes into bicarbonate ions and hydrogen ions:

CO2 Carbon dioxide


Carbonic acid

Bicarbonate ion

Hydrogen ion

This reaction appears in this book more often than any other, especially as we discuss respiratory, urinary, and digestive physiology.

The direction in which a reversible reaction goes is determined by the relative abundance of substances on each side of the equation. If there is a surplus of CO2, this reaction proceeds to the right and produces bicarbonate and hydrogen ions. If bicarbonate and hydrogen ions are present in excess, the reaction proceeds to the left and generates CO2 and H2O. Reversible reactions follow the law of mass action: they proceed from the side with the greater quantity of reactants to the side with the lesser quantity. This law will help to explain processes discussed in later chapters, such as why hemoglobin binds oxygen in the lungs yet releases it to muscle tissue.

In the absence of upsetting influences, reversible reactions exist in a state of equilibrium, in which the ratio of products to reactants is stable. The carbonic acid reaction, for example, normally maintains a 20:1 ratio of bicarbonate ions to carbonic acid molecules. This equilibrium can be upset, however, by a surplus of hydrogen ions, which drive the reaction to the left, or adding carbon dioxide and driving it to the right.

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Essentials of Human Physiology

Essentials of Human Physiology

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