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2.3 Carbon-Based Molecules VOCABULARY Key Concept Carbon-based molecules are the foundation monomer of life. polymer MAIN...

2.3 Carbon-Based Molecules VOCABULARY Key Concept Carbon-based molecules are the foundation monomer of life. polymer MAIN IDEAS carbohydrate Carbon atoms have unique bonding properties. lipid Four main types of carbon-based molecules are found in living things. fatty acid protein amino acid Connect to Your World nucleic acid Car manufacturers often build several types of cars from the same internal frame. The size and style of the cars might differ on the outside, but they have the same structure underneath. Carbon-based molecules are similar, but they are much more varied. There are millions of different carbon-based molecules, but they form around only a few simple frames composed of carbon atoms. MAIN IDEA Carbon atoms have unique bonding properties. Carbon is often called the building block of life because carbon atoms are the basis of most molecules that make up living things. These molecules form the structure of living things and carry out most of the processes that keep organ- isms alive. Carbon is so important because its atomic structure gives it bond- ing properties that are unique among elements. Each carbon atom has four unpaired electrons in its outer energy level. Therefore, carbon atoms can form covalent bonds with up to four other atoms, including other carbon atoms. As FIGURE 3.1 shows, carbon-based molecules have three fundamental structures—straight chains, branched chains, and rings. All three types of molecules are the result of carbon’s ability to form four covalent bonds. Carbon chains can bond with carbon rings to form very large, very complex molecules. These large molecules can be made of many small molecules that are bonded together. In a sense, the way these molecules form is similar to the way in which individual links of metal come together to make a bicycle chain. FIGURE 3.1 Carbon Chains and Rings Straight chain Branched chain Ring O H H H H H CH H C C C C C CH3 C H H H H CH2 CH CH A simplified structure can also be shown as: CH3 CH CH2 CH3 CH C C O CH3 CH2 CH2 CH CH2 CH3 OH Pentene Hexane Vanillin 42 Unit 1: Introducing Biology  In many carbon-based molecules, VISUAL VOCAB small molecules are subunits of an Each smaller molecule is a subunit entire molecule, like links in a chain. called a monomer. mono- = one Each subunit in the complete molecule poly- = many is called a monomer. When monomers are linked, they form molecules called monomer polymers. A polymer is a large mol- ecule, or macromolecule, made of many monomers bonded together. All of the polymer monomers in a polymer may be the A polymer is a molecule that contains same, as they are in starches, or they many monomers bonded together. may be different, as they are in proteins. Synthesize Write your own analogy for the formation of a polymer from monomers. MAIN IDEA R E A D I NG T O O L B o x Four main types of carbon-based molecules are TAKING NOTES found in living things. Use a content frame to help you understand monomers and All organisms are made of four types of carbon-based molecules: carbohydrates, polymers in carbon-based lipids, proteins, and nucleic acids. These molecules have different structures and molecules. functions, but all are formed around carbon chains and rings. Monomer Polymer Example Function Carbohydrates Fruits and grains are in different food groups, but they both contain large amounts of carbohydrates. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen, and they include sugars and starches. Carbohydrates can be broken down to provide a source of usable chemical energy for cells. Carbohydrates are also a major part of plant cell structure. The most basic carbohydrates are simple sugars, or monosaccharides (mahn-uh-SAK-uh-rydz). Many simple sugars have either five or six carbon atoms. CH2OH Fruits contain a six-carbon sugar called C O fructose. Glucose, one of the sugars made by H H H plant cells during photosynthesis, is another C OH H C six-carbon sugar. Simple sugars can be HO OH C C bonded to make larger carbohydrates. For example, two sugars bonded together make H OH the disaccharide you know as table sugar, Glucose (C6H12O6) can be ring shaped and is shown in figure 3.2. Many glucose molecules often shown as a simplified hexagon. can be linked to make polysaccharides (pahl-ee-SAK-uh-rydz), which are polymers of monosaccharides. Starches, glycogen, and cellulose are polysaccharides. Starches and glycogen ©Mike Powell/Getty Images are similar, but they differ from cellulose because their glucose monomers are bonded together differently. Most starches are branched chains of glucose molecules. Starches are made and stored by plants, and they can be broken Figure 3.2 Household sugar (sucrose) is a disaccharide, or down as a source of energy by plant and animal cells. Glycogen, which is made two-sugar molecule, of glucose and stored in animals, is more highly branched than plant starches. (inset) and fructose. Chapter 2: Chemistry of Life 43  FIGURE 3.3 Carbohydrate structure Polymer (starch) HMDScience.com Starch is a polymer of glucose Premium Content monomers that often has a branched structure. Macromolecules of Life Polymer (cellulose) monomer Cellulose is a polymer of glucose monomers that has a straight, rigid structure. CONNECT TO Cellulose is somewhat different from starch and glycogen. Its straight, rigid Cell Structure structure, shown in figure 3.3, makes the cellulose molecule a major building A cell wall made of cellulose block in plant cell structure. Cellulose makes up the cell wall that is the tough surrounds the membrane of outer covering of plant cells. You have eaten cellulose in the stringy fibers of plant cells. You will learn more vegetables such as celery, so you know that it is tough to chew and break up. about cell walls in Cell Structure and Function. Lipids Lipids are nonpolar molecules that include fats, oils, and cholesterol. Like carbohydrates, most lipids contain chains of carbon atoms bonded to oxygen and hydrogen atoms. Some lipids are broken down as a source of usable energy for cells. Other lipids are parts of a cell’s structure. Fats and oils are two familiar types of lipids. They store large amounts of chemical energy in organisms. Animal fats are found in foods such as meat and butter. You know plant fats as oils, such as olive oil and peanut oil. The structures of fats and oils are similar. They both consist of a molecule called glycerol (glihs-uh-rawl) bonded to molecules called fatty acids. Fatty acids are chains of carbon atoms bonded to hydrogen atoms. Two different types of fatty acids are shown in figure 3.4. Many lipids, both fats and oils, contain three fatty acids bonded to glycerol. They are called triglycerides. Most animal fats are saturated fats, which means they have the maximum number of hydrogen atoms possible. That is, every place that a hydrogen atom can bond to a carbon atom is filled with a hydro- gen atom, and all carbon–carbon bonds are single bonds. You can think of the fatty acid as being “saturated” with hydrogen atoms. In contrast, fatty acids in Figure 3.4 Fatty acids can be oils have fewer hydrogen atoms because there is at least one double bond either saturated or unsaturated. between carbon atoms. These lipids are called unsaturated fats because the fatty acids are not saturated Saturated fatty acid with hydrogen atoms. Fats and Saturated fats contain O CH2 CH2 CH2 CH2 CH2 CH2 CH2 fatty acids in which all oils are very similar, but why C are animal fats solid and plant HO CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 carbon–carbon bonds are single bonds. oils liquid? The double bonds Unsaturated fatty acid in unsaturated fats make kinks Unsaturated fats have in the fatty acids. As a result, O CH CH CH CH CH2 CH2 CH2 fatty acids with at least the molecules cannot pack C HO CH2 C CH2 C CH2 CH2 CH2 CH3 one carbon–carbon together tightly enough to double bond. form a solid. 44 Unit 1: Introducing Biology  All cell membranes are made mostly of another type of lipid, called a phospholipid (fahs-foh-lihp-ihd). A phospholipid consists of glycerol, two fatty acids, and a phosphate group (PO4–) that is part of the polar “head” of the molecule. The fatty acids are the nonpolar “tails” of a phospholipid. Compare the structure of a phospholipid to the structure of a triglyceride in figure 3.5. Figure 3.5 Lipid Structure Phospholipid A phospholipid Triglyceride A triglyceride has has nonpolar fatty three fatty acids PO4– acid “tails” and a and a molecule polar “head” that of glycerol, but contains a phos- no phosphate head tails phate group. group. Cholesterol (kuh-lehs-tuh-rawl) is a lipid that has a ring structure. You may hear about dangers of eating foods that contain a lot of cholesterol, such as eggs, but your body needs a certain amount of it to function. For example, cholesterol is a part of cell membranes, and your body uses it to make chemi- cals called steroid hormones. Cholesterol-based steroids have many functions. Some regulate your body’s response to stress. Others, such as testosterone and estrogen, control sexual development and the reproductive system. Proteins Proteins are the most varied of the carbon-based molecules in organisms. In Figure 3.6 Serine is one of 20 movement, eyesight, or digestion, proteins are at work. A protein is a polymer amino acids that make up proteins in organisms. made of monomers called amino acids. Amino acids are molecules that contain OH carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Organisms use 20 H CH2 O different amino acids to build proteins. Your body can make 12 of the amino acids. The others come from foods you eat, such as meat, beans, and nuts. N C C H H OH Look at FIGURE 3.6 to see the amino acid serine. All amino acids have similar structures. As FIGURE 3.7 shows, each amino acid monomer has a carbon atom that is bonded to four other parts. Three of these parts are the same in every amino acid: a hydrogen atom, an amino group (NH2), and a carboxyl group (COOH). Amino acids differ only in their side group, or the R-group. Amino acids form covalent bonds, called peptide bonds, with each other. The bonds form between the amino group of one amino acid and the car- boxyl group of another amino acid. Through peptide bonds, amino acids are linked into chains called polypeptides. A protein is one or more polypeptides. Figure 3.7 amino acid and Protein Structure All amino acids have a carbon atom Monomer (amino acid) Polymer (protein) bonded to a hydrogen atom, an O R O amino group (NH2), and a carboxyl group (COOH). Different amino acids C N C C N have different side groups (R). H H H H R O peptide bonds peptide bonds N C C Peptide bonds form between the amino A polypeptide is a chain of precisely ordered H H OH group of one amino acid and the carboxyl amino acids linked by peptide bonds. A pro- group of another amino acid. tein is made of one or more polypeptides. Chapter 2: Chemistry of Life 45  Proteins differ in the number and order of amino acids. The specific sequence of amino acids deter- mines a protein’s structure and function. Two types of interactions between the side groups of some amino acids are especially important in protein structure. First, some side groups contain sulfur atoms. The hydrogen bond sulfur atoms can form covalent bonds that force the protein to bend into a certain shape. Second, hydrogen bonds can form between the side groups of some amino acids. These hydrogen bonds cause the protein to fold into a specific shape. For example, FIGURE 3.8 shows the structure of one of the four polypeptides that makes up hemoglobin, the protein in your red blood cells that transports oxygen. Each of the four polypeptides contains an iron atom that bonds to an oxygen molecule. The four polypeptides are folded in a way that puts the four Figure 3.8 Hemoglobin in red oxygen-carrying sites together in a pocketlike structure inside the molecule. blood cells transports oxygen. The If a protein has incorrect amino acids, the structure may change in a way that structure of hemoglobin depends on hydrogen bonds between spe- prevents the protein from working properly. Just one wrong amino acid of the cific amino acids. Just one amino 574 amino acids in hemoglobin causes the disorder sickle cell anemia. acid change causes red blood cells to have the curved shape charac- Nucleic Acids teristic of sickle cell anemia. Detailed instructions to build proteins are stored in extremely long carbon- (colored SEM; magnification 35003) based molecules called nucleic acids. Nucleic acids are polymers that are made Web up of monomers called nucleotides. A nucleotide is composed of a sugar, a phosphate group, and a nitrogen-containing molecule called a base. There are HMDScience.com two general types of nucleic acids: DNA and RNA. Premium Content Nucleic acids work together to make proteins. DNA stores the information Prions and Public Health for putting amino acids together to make proteins, and RNA helps to build ©Eye of Science/Photo Researchers, Inc. proteins. DNA is the basis of genes and heredity, but cannot do anything by itself. Instead, the structure of DNA—the order of nucleotides—provides the code for the proper assembly of proteins. Many different kinds of RNA mol- ecules assist in assembling proteins based on the DNA code. RNA may even catalyze reactions. You will learn more about nucleic acids and how they build proteins in the Genetics unit. Apply What is the relationship between proteins and nucleic acids? Self-check Online HMDScience.com 2.3 Formative Assessment Premium Content Reviewing Main Ideas Critical thinking CONNECT TO 1. What is the relationship between a 3. Compare and Contrast How are Biochemistry polymer and a monomer? carbohydrates and lipids similar? 5. Why might fatty acids, 2. Explain how both nucleic acids and How are they different? amino acids, and nucleic proteins are polymers. Be sure to 4. Infer Explain how the bonding acids increase the hydrogen describe the monomers that make up properties of carbon atoms result in ion (H+) concentration of a the polymers. the large variety of carbon-based solution? Explain your molecules in living things. answer. 46 Unit 1: Introducing Biology