The Chemical Building Blocks of Life

Chapter 2 The Chemical Building Blocks of Life 2.1 The Elements of Life • Matter is Composed of Atoms • An atom is the smallest unit of an element having the properties of that element • Atoms are made of a nucleus containing protons and neutrons, surrounded by a negatively charged cloud of electrons • Atoms Can Vary in the Number of Neutrons or Electrons • Isotopes are atoms of the same element that have different numbers of neutrons • An ion is an atom with a charge • Electron Placement Determines Chemical Reactivity • An atom with an unfilled outer electron shell is unstable. • It can become stable by interacting with another unstable atom 2.2 Chemical Bonding • Ionic Bonds Form between Oppositely Charged Ions • In forming an ionic bond, one atom gives up its outermost electrons to another atom • Covalent Bonds Share Electrons • Covalent bonds are formed when atoms share outer shell electron pairs • Carbon frequently forms covalent bonds with other atoms • Covalent bonds can be nonpolar (no electrical charges) or polar (has electrical charges) • Hydrogen Bonds Form between Polar Groups or Molecules • Hydrogen bonding is the electrostatic attraction between a partially negative atom and a partially positive atom • Chemical Reactions Change Bonding Partners • Chemical reactions can • break larger compounds into smaller ones (e.g., hydrolysis), or • add smaller reactants together to form a larger product (e.g., dehydration synthesis) 2.3 Water, pH, and Buffers • Water Has Several Unique Properties • All cellular chemical reactions occur in water • Water is an excellent solvent for other polar molecules • Acids and Bases Must Be Balanced in Cells • An acid is a chemical substance that donates a H+ to a solution; a base accepts the H+ • The pH scale indicates the acidity or alkalinity of a solution • Cell chemistry is very sensitive to changes in pH • Buffers Are a Combination of a Weak Acid and Base • Buffers prevent pH shifts 2.4 Major Organic Compounds of Living Organisms • Functional Groups Define Molecular Behavior • Functional groups are groups of atoms projecting from biological molecules • Chemical reactions can occur at functional groups if facilitated by an enzyme • Carbohydrates Consist of Sugars and Sugar Polymers • Monosaccharides are simple sugars like glucose and fructose • Disaccharides are composed of two monosaccharides • Carbohydrates Consist of Sugars and Sugar Polymers (Cont.) • Polysaccharides are complex carbohydrates made of many thousands of monosaccharides • For example, starch • Carbohydrates provide structure and energy • Lipids Are Water-Insoluble Compounds • Lipids are hydrophobic; they do not dissolve in water • Nucleic Acids Are Large, Information-Containing Polymers • Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are both composed of nucleotides • Nucleotides are composed of a sugar molecule, a phosphate group, and a nitrogenous base • In DNA, complementary base pairs hold the double-stranded molecule together • The double strand twists to form a double helix • Nucleic Acids Are Large, Information-Containing Polymers (Cont.) • DNA is the genetic material in all organisms • The genetic information is contained in thousands of genes • Genes are located in chromosomes • RNA is a single-stranded molecule that copies gene information for use in protein synthesis • Many viruses carry their genetic information as RNA instead of DNA • Damage to nucleic acids inevitable injures or kills the organism • Proteins Are the Workhorse Polymers in Cells • They are built from amino acids joined together by a (covalent) peptide bond • Each of the 20 amino acids has a unique R group (side chain) • Amino acids are joined through dehydration synthesis • Proteins have several structural levels • Primary structure: the sequence of amino acids in the polypeptide • Secondary structure: regions form an alpha helix, pleated sheet, or random coil • Tertiary structure: part of the polypeptide folds back on itself and sulfur atoms join through disulfide bridges • Quaternary structure: occurs when two or more polypeptides join to form the protein • Protein shape is held using ionic and hydrogen bonds • When these relatively weak bonds are disrupted, the protein is denatured