1 Chemistry of Life

Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.

• Living systems depend on the properties of water to sustain life.
  • i. Water has polarity, because of the formation of polar covalent bonds between hydrogen and oxygen within water molecules. This polarity contributes to hydrogen bonding between and within biological molecules.
  • ii. Water has a high specific heat capacity, which allows for the maintenance of homeostatic body temperature within living organisms.
  • iii. Water has a high heat of vaporization, which allows for the evaporative cooling of the surrounding environment. In living organisms, this property allows for body temperature to be maintained.
• The hydrogen bonds between adjacent polar water molecules result in cohesion, adhesion, and surface tension.

Describe the composition of macromolecules required by living organisms.

• Atoms and molecules from the environment are necessary to build new molecules. Carbon, hydrogen, and oxygen are the most prevalent elements used to build biological molecules such as carbohydrates, proteins, lipids, and nucleic acids. Additionally:
  • i. Sulfur is used in the building of proteins.
  • ii. Phosphorus is used in the building of phospholipids (a type of lipid) and nucleic acids.
  • iii. Nitrogen is used in the building of nucleic acids.

Describe the chemical reactions that build and break biological macromolecules.

• Hydrolysis is a chemical reaction involving the cleaving of covalent bonds. This type of reaction breaks down molecules into smaller molecules. When water is added to the bond between monomers in a polymer, the bond is broken. The hydrogen ion from a water molecule is added to one monomer and the hydroxyl group of the water molecule is added to the other monomer, completing the reaction.
• Dehydration synthesis occurs when two smaller molecules are joined together through covalent bonding. A hydrogen ion is removed from one monomer and a hydroxyl group is removed from the other. This causes the loss of the equivalent of a water molecule from the reactants and the connection of the two remaining monomers. The connection of many monomers is known as polymerization.

Describe the structure and function of carbohydrates.

• Monosaccharides (simple sugars) are the monomers for polysaccharides (complex carbohydrates). These monomers are connected by covalent bonds to form polymers such as complex carbohydrates, which may be linear or branched.
  • Exclusion: The molecular structure of specific carbohydrate polymers is beyond the scope of the AP Exam.

Describe the structure and function of lipids.

• Lipids are typically nonpolar, hydrophobic molecules whose structure and function are derived from the way their subcomponents are assembled. Fatty acids can be described as either saturated or unsaturated.
  • i. Saturated fatty acids contain only single bonds between carbon atoms.
  • ii. Unsaturated fatty acids contain at least one double bond between carbon atoms, which causes the carbon chain to kink.
  • iii. The more double bonds in a fatty acid tail, the more unsaturated the lipid becomes.
  • iv. The more unsaturated a lipid is, the more liquid it is at room temperature.
• Lipids provide a variety of functions for living organisms. Some examples of lipids are fats, steroids including cholesterol, and phospholipids.
  • i. Fats provide energy storage and support cell function. In some cases, they can also provide insulation to help keep mammals warm.
  • ii. Steroids are hormones that support physiological functions including growth and development, energy metabolism, and homeostasis.
  • iii. Cholesterol provides essential structural stability to animal cell membranes.
  • iv. Phospholipids group together to form the lipid bilayers found in plasma and cell membranes.
  • Exclusion: The molecular structure of specific lipids is beyond the scope of the AP Exam.

Describe the structure and function of DNA and RNA.

• In nucleic acids (DNA and RNA), biological information is encoded in sequences of nucleotide monomers. Each nucleotide has the following structural components: a five-carbon sugar (deoxyribose or ribose), a phosphate, and a nitrogenous base (adenine, thymine, guanine, cytosine, or uracil).
• Nucleic acids have a linear sequence of nucleotides that have ends, defined by the 3’ (three prime) hydroxyl and 5’ (five prime) phosphates of the sugar in the nucleotide. During nucleic acid synthesis, nucleotides are added to the 3’ end of the growing strand, resulting in the formation of covalent bonds between nucleotides.
  • Exclusion: The molecular structure of specific nucleotides is beyond the scope of the AP Exam.
• DNA is structured as an antiparallel double helix, with two strands of nucleotides running in opposite 5’ to 3’ orientation. In DNA, adenine nucleotides pair with thymine nucleotides via hydrogen bonds (A-T), and cytosine nucleotides pair with guanine nucleotides via hydrogen bonds (C-G). In RNA, adenine pairs with uracil (A-U).
• Structural differences between DNA and RNA include:
  • i. DNA contains the sugar deoxyribose, and RNA contains the sugar ribose.
  • ii. DNA contains the nitrogenous base thymine, and RNA contains the nitrogenous base uracil.
  • iii. DNA is typically double stranded, while RNA is typically single stranded.

Describe the structure and function of proteins.

• Proteins comprise linear chains of amino acids connected by the formation of covalent (peptide) bonds that form between a carboxyl group (−COOH) of one amino acid and an amine group (N−H)2 of the next amino acid, resulting in a growing peptide chain.
• Amino acids are composed of a central carbon atom with a hydrogen atom, a carboxyl group, an amine group, and a variable R group covalently bound to it. The R group of an amino acid can be categorized by three possible chemical properties: hydrophobic/nonpolar, hydrophilic/polar, or ionic. The interactions of these R groups determine the structure and function of that region of the protein.
• The specific sequence of amino acids in proteins determines the primary structure of a polypeptide as well as the overall shape of the protein.
  • Exclusion: The molecular structure of amino acids is beyond the scope of the AP Exam.
• Secondary structures of proteins are made through the local folding that forms from interactions between atoms of the polypeptide backbone of the amino acid chain. Hydrogen bonding forms shapes such as alpha-helices and beta-pleated sheets.
• The three-dimensional shape of the tertiary structure of a protein results from the formation of hydrogen bonds, hydrophobic interactions, ionic interactions, or disulfide bridges.
• The quaternary structure arises from interactions between multiple polypeptides. All four levels of a protein structure determine the function of a protein.