file:///biology Exploring Life/BiologyExploringLife04/

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1 Objectives Identify carbon skeletons and functional groups in organic molecules. Relate monomers and polymers. Describe the processes of building and breaking polymers. Key Terms organic molecule inorganic molecule hydrocarbon functional group hydrophilic monomer polymer Life without carbon would be as unlikely as life without water. Other than water, most molecules of a cell are carbon-based. These "biomolecules" are composed of a backbone of carbon atoms bonded to one another. Atoms of other elements may branch off this carbon backbone. This basic structure is the foundation of the wide range of life's molecules. Carbon Skeletons and Functional Groups Why are carbon atoms so common in living things? Remember from Chapter 4 that an atom's bonding ability is related to the number of electrons in its highest occupied energy level. Carbon has only 4 electrons in its highest occupied energy level. Because this energy level can hold 8 electrons, carbon can form up to 4 bonds with other atoms. Each carbon atom is a connecting point from which other atoms can branch off in up to four directions. 1 of 5 10/5/11 6:49 AM

2 Carbon can form bonds with one or more other carbon atoms, producing an endless variety of carbon skeletons (Figure 5-1). Most carbon-based molecules are classified as organic molecules. In contrast, non-carbon-based molecules such as water (H 2 O), oxygen (O 2 ), and ammonia (NH 3 ) are classified as inorganic molecules. Figure 5-1 The carbon backbones of organic molecules can take many shapes. These molecules may include single, double, and rarely, triple bonds. The only rule is that each carbon forms a total of four bonds. Besides bonding with other carbon atoms, carbon may also bond with atoms of other elements. For example, organic molecules that are composed of only carbon and hydrogen are known as hydrocarbons. Many hydrocarbons are important fuels. Methane (CH 4 ) is one of the most abundant hydrocarbons in natural gas, a fuel used to heat homes. In your body, energy-storing fat molecules contain long hydrocarbon chains. In addition to hydrogen, two other atoms frequently found in organic molecules are oxygen and nitrogen. A group of atoms within a molecule that interacts in predictable ways with other molecules is called a functional group. Figure 5-2 shows some of the functional groups important in the chemistry of life. Together, the carbon skeleton and the attached functional groups determine the properties of an organic molecule. For example, hydroxyl groups are hydrophilic, meaning they attract water molecules. Thus, most organic molecules that contain 2 of 5 10/5/11 6:49 AM

3 hydroxyl groups are hydrophilic, which literally means "water-loving." These molecules tend to become surrounded by water molecules in an aqueous environment. Figure 5-2 These four common functional groups give specific properties to the organic molecules that contain them. Monomers and Polymers Some biomolecules may be composed of hundreds or even millions of atoms. These large molecules are built from many similar, smaller molecular units called monomers. Your cells link monomers together into long chains called polymers. A polymer may be a straight chain of monomers, much as a train is a string of many individual cars. Other polymers have branching chains or chains that fold back on themselves. The diversity of life's polymers is vast. Every living cell has thousands of different kinds of polymers. The specific molecules vary from cell to cell within an organism. The variety of polymers differs among individuals of the same species, and even more among organisms of different species. And yet, all of these polymers are built from a collection of fewer than 50 kinds of monomers. Life's large molecules are classified into four main categories: carbohydrates, lipids, proteins, and nucleic acids. In the next sections of this chapter, you'll explore the properties of carbohydrates, lipids, and proteins. The structure and role of nucleic acids, such as DNA, will be explored in depth in Chapter 11. But first, find out how polymers are constructed and broken down. Building and Breaking Polymers Each time a monomer is added to a chain, a water molecule is released (Figure 5-4). This is called a dehydration reaction because it involves removing (de-) water (hydro-). This same type of reaction occurs regardless of the specific monomers and the type of polymer being produced. 3 of 5 10/5/11 6:49 AM

4 Figure 5-4 In the dehydration reaction, two monomers bond to each other, making a polymer chain longer. The hydroxyl group of one monomer reacts with a hydrogen atom from the other monomer. The reactions involved ultimately release a water molecule. Organisms not only build polymers; they also have to break them down. For example, many of the molecules in your food are polymers. You must break down these giant molecules to make their monomers available to your cells. Your cells can either further break down the monomers to obtain energy, or, alternatively, use them to build new polymers. Cells break bonds between monomers by adding water to them, the reverse of dehydration (Figure 5-5). This process is called a hydrolysis reaction because water (hydro-) is used to break down (-lysis) the polymer. To summarize, water is removed to build a polymer, and water is added to break it down. 4 of 5 10/5/11 6:49 AM

5 Figure 5-5 In the hydrolysis reaction, the addition of a water molecule breaks the polymer chain. Concept Check Draw a molecule that has a three-carbon skeleton and a hydroxyl group on the middle carbon. (Hint: The molecule's formula is C 3 H 8 O.) 2. Explain the connection between monomers and polymers. 3. What molecule is released during construction of a polymer? What is this reaction called? 4. Draw at least three ways in which five carbon atoms could be joined to make different carbon skeletons. Copyright 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved. 5 of 5 10/5/11 6:49 AM