Chapter 3 Notes

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Biological Sciences
Mark Fitzpatrick

Chapter 3 Macromolecules include carbohydrates, proteins, lipids, and nucleic acids. They are larger, with molecular weights ranging from hundreds of Daltons (sucrose) to billions (some nucleic acids). These molecules all contain carbon atoms, and so belong to a group of what are known as organic chemicals. Third, they are held together largely by covalent bonds, which gives them important structural stability and forms the basis of some of their functions. And finally, carbohydrates, proteins, lipids, and nucleic acids are all unique to the living world. These molecular classes do not occur in inanimate nature. You wont find proteins in rocksand if you do, you can be sure they came from some living organism. Most of these biological molecules are large polymers (poly, many; mer, unit) constructed by the covalent bonding of smaller molecules called monomers (Table 3.1). The monomers that make up each kind of biological molecule have similar chemical structures. Thus chains of chemically similar sugar monomers (saccharides) form the different carbohydrates; the thousands of different proteins are formed from combinations of a mere 20 amino acids, all of which share chemical similarities. The Building Blocks of Organisms MONOMER COMPLEX POLYMER (MACROMOLECULE) Amino acid Polypeptide (protein) Monosaccharide (sugar) Polysaccharide (carbohydrate) Nucleotide Nucleic acid Each functional group has specific chemical properties, and when it is attached to a larger molecule, it confers those properties on the larger molecule. Isomers are molecules that have the same chemical formulathe same kinds and numbers of atomsbut the atoms are arranged differently. (The prefix iso-, meaning same, is encountered in many biological terms.) Of the different kinds of isomers, we will consider two: structural isomers and optical isomers. Structural isomers differ in how their atoms are joined together. Consider two simple molecules, each composed of four carbon and ten hydrogen atoms bonded covalently, both with the formula C4H10. These atoms can be linked in two different ways, resulting in two different forms of the molecule: www.notesolution.comThe different bonding relationships in butane and isobutane are distinguished by their structural formulas, and the two molecules have different chemical properties. Optical isomers occur when a carbon atom has four different atoms or groups of atoms attached to it. This pattern allows two different ways of making the attachments, each the mirror image of the other (Figure 3.2). Such a carbon atom is called an asymmetrical carbon, and the two resulting molecules are optical isomers of each other. You can envision your right and left hands as optical isomers. Just as a glove is specific for a particular hand, some biochemical molecules that can interact with one optical isomer of a carbon compound are unable to fit the other. _____________________________________________________________________________________ Both carbohydrates and proteins can play structural roles, supporting and protecting tissues and organs. However, only the nucleic acids specialize in information storage. These macromolecules function as hereditary material, carrying the traits of both species and individuals from generation to generation. The functions of macromolecules are directly related to their three-dimensional shapes and to the sequences and chemical properties of their monomers. Polymers are constructed from monomers by a series of reactions called condensation reactions (sometimes called dehydration reactions; both terms refer to the loss of water). Condensation reactions result in covalently bonded monomers. These reactions release a molecule of water for each covalent bond formed (Figure 3.4A). The condensation reactions that produce the different kinds of polymers differ in detail, but in all cases, polymers form only if energy is added to the system. In living systems, specific energy-rich molecules supply this energy. The reverse of a condensation reaction is a hydrolysis reaction (hydro, water; lysis, break). Hydrolysis reactions digest polymers and produce monomers. Water reacts with the covalent bonds that link the polymer together, and the products are free monomers. The
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