Representations such as these show how atoms bond and arrange in various molecules. Single lines represent single bonds, and double lines represent double bonds. Illustrations of this type are called structural formulas (fig. 2.7).

Chemical Reactions

Chemical reactions form or break bonds between atoms, ions, or molecules. Those being changed by the chemical reaction are called reactants. Those formed at the reaction's conclusion are called products. When two or more atoms, ions, or molecules bond to form a more complex structure, as when hydrogen and oxygen atoms bond to form molecules of water, the reaction is called synthesis (sin'the-sis). Such a reaction can be symbolized this way:

If the bonds of a reactant molecule break to form simpler molecules, atoms, or ions, the reaction is called decomposition (de"kom-po-zish'un). For example, molecules of water can decompose to yield the products hydrogen and oxygen. Decomposition is symbolized as follows:

Synthetic reactions, which build larger molecules from smaller ones, are particularly important in growth of body parts and repair of worn or damaged tissues. Decomposition reactions occur when food substances are digested and they release energy.

A third type of chemical reaction is an exchange reaction (replacement reaction). In this reaction, parts of two different kinds of molecules trade positions. The reaction is symbolized as follows:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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