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The Chemical Building Blocks of Life

3.1 Molecules are the building blocks of life.
Carbon: The Framework of Biological Molecules
• Biological molecules consist predominantly of carbon atoms bonded to other carbon atoms or to atoms of oxygen, nitrogen, sulfur, or hydrogen. (p. 36)
• Hydrocarbons are molecules consisting only of carbon and hydrogen; thus, they store considerable energy. (p. 36)
• Functional groups have definite chemical properties that are retained no matter where they occur, and tend to act as units during chemical reactions, conferring specific chemical properties to molecules that possess them. (p. 36)
• Polymers are composed of long chains of similar subunits joined by dehydration synthesis. (pp. 36—37)

3.2 Proteins perform the chemistry of the cell.
The Many Functions of Proteins
• The major functions of proteins within living organisms include enzyme catalysis, defense, transport, support, motion, regulation, and storage. (pp. 39—40)
Amino Acids: The Building Blocks of Proteins
• Proteins are polymers of 20 different kinds of amino acids, arranged in specific orders. (p. 41)
• The 20 common amino acids are grouped into five chemical classes: nonpolar, polar uncharged, charged, aromatic, and special function. (p. 41)
• A peptide bond is a covalent bond that links two amino acids together. (p. 41)
Protein Structure
• Proteins consist of long amino acid chains folded into complex shapes, and the shape of a protein determines its function. (p. 43)
• The specific amino acid sequence of a protein forms its primary structure. (p. 43)
• The folding of the amino acid chain by hydrogen bonding into coils and pleats forms a protein's secondary structure. (p. 45)
• The elements of secondary structure can combine in proteins in characteristic ways (motifs). (p. 45)
• The final folded shape of a globular protein forms a protein's tertiary structure. (p. 45)
• Each exon-encoded section of a protein folds into a structurally independent functional unit (domain). (p. 45)
• Quaternary structure refers to individual polypeptide subunits associated to form functional proteins. (p. 45)
How Proteins Fold into Their Functional Shapes
• Chaperone proteins help new proteins fold correctly. (p. 46)
How Proteins Unfold
• Denaturation occurs when a protein changes shape, or unfolds, and can be caused by a change in environmental pH, temperature, or ionic concentration. (p. 47)
• Denaturation usually renders a protein biologically inactive. (p. 47)

3.3 Nucleic acids store and transfer genetic information.
Information Molecules
• Deoxyribonucleic acid (DNA) is often referred to as heredity material, and encodes the information used to assemble proteins. (p. 48)
• Cells use ribonucleic acid (RNA) to read the DNA-encoded information and direct protein synthesis. (p. 48)
• Nucleic acids are long polymers of repeating subunits (nucleotides) that consist of a five-carbon sugar, a phosphate, and a nitrogen-containing base. (p. 49)
• DNA exists in a double-stranded helix, while RNA is a single-stranded molecule. (pp. 50—51)

3.4 Lipids make membranes and store energy.
Phospholipids Form Membranes
• A phospholipid is a composite molecule made up of glycerol, fatty acids, and a phosphate group. It contains a polar head and two nonpolar tails, forming the core of all biological membranes. (p. 52)
Fats and Other Kinds of Lipids
• A triglyceride is a fat, consisting of a glycerol molecule and three fatty acids. Because triglyceride molecules lack a polar end, they are not soluble in water. (p. 53)
• Other kinds of lipids include terpenes, steroids, and prostaglandins. (p. 53)
Fats as Energy-Storage Molecules
• Fats have a high concentration of C—H bonds, and are thus efficient energy-storage molecules. (p. 54)

3.5 Carbohydrates store energy and provide building materials.
• Carbohydrates are a loosely defined group of molecules containing carbon, hydrogen, and oxygen in 1:2:1 ratio. (p. 55)
• Structural differences between sugar isomers confer functional differences between the molecules. (p. 56)
Transport and Storage Carbohydrates
• Starches are glucose polymers found in plants, while glycogen serves as the animal version of starch. (p. 57)
Structural Carbohydrates
• Structural carbohydrates such as cellulose (in plants) and chitin (in arthropods) are chains of sugars that resist digestion because most organisms lack the necessary enzymes. (p. 58)

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