Biochemistry questions regarding organic molecules, carbohydrate and protein functions, fatty acid structures and properties, and DNA replication enzymes.

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SECTION A

1. • Enantiomers: Stereoisomers that are non-superimposable mirror images of each other. Example: D- and L-glyceraldehyde.
   • Tautomers: Isomers that readily interconvert, often involving the migration of a hydrogen atom or proton and a double bond. Example: Keto-enol tautomers.
   • Zwitterions: A molecule that contains both positive and negative electrical charges, though it is net neutral. Example: Amino acids at physiological pH.
   • Anomers: specific type of epimer found in cyclic forms of sugars, differing only in the configuration at the anomeric carbon. Example: α-D-glucose and β-D-glucose.
2. • a) A naturally occurring aldohexose that may account for the sweet taste in Mbuh is glucose. You would need to draw the linear (Fischer projection) and ring (Haworth projection) forms of glucose.
   • b) A reducing disaccharide derived from glucose is maltose. Maltose is formed from two glucose molecules linked by an α(1→4) glycosidic bond. An epimer of glucose is galactose. A reducing disaccharide formed from galactose is lactose which consists of a β-D-galactose and either a α- or β-D-glucose molecule joined by a β(1→4) glycosidic linkage.
   • c) Two polysaccharides that can be obtained from trees are cellulose and starch. Compare their structures, noting that cellulose is a linear polymer of glucose linked by β(1→4) glycosidic bonds, while starch is a mixture of amylose (linear α(1→4) glucose polymer) and amylopectin (branched α(1→4) glucose polymer with α(1→6) branches).
   • d) Two roles of carbohydrates in humans:
     • Energy source: Glucose is a primary energy source for cells.
     • Structural component: Carbohydrates are part of cell membranes and extracellular matrix.
3. • a) The amino acids in the pentapeptide YANKE belong to the following classes:
     • Y (Tyrosine): Polar, aromatic
     • A (Alanine): Nonpolar, aliphatic
     • N (Asparagine): Polar
     • K (Lysine): Basic, positively charged
     • E (Glutamic acid): Acidic, negatively charged
   • b) i. The C-terminal amino acid (E) serves as a buffer by donating or accepting protons at its carboxyl group.ii. A dipeptide between A and K would involve a peptide bond between the carboxyl group of alanine and the amino group of lysine. You would need to illustrate the formation of the dipeptide.
   • c) Two functions of proteins in plants:
     • Enzymes: Catalyze biochemical reactions (e.g., RuBisCO in photosynthesis).
     • Structural support: Provide structure to cells and tissues (e.g., collagen).

SECTION B

1. • a) You would need to draw the chemical structures of the fatty acids, showing the 18-carbon chain and the double bonds (where applicable) at the specified positions.
   • b) The fatty acids in increasing order of melting point: 18:3Δ9,12,15 < 18:2Δ9,12 < 18:1Δ9 < 18:0
   • c) Explanation: The more unsaturated a fatty acid (i.e., the more double bonds it has), the lower its melting point. This is because the presence of double bonds introduces kinks in the fatty acid chain, which reduces the efficiency of packing and lowers the Van der Waals forces between the chains.
   • d) Two functions of lipids in living organisms, apart from energy storage:
     • Structural component of cell membranes (phospholipids)
     • Hormone signaling (steroid hormones)
   • e) Migratory birds prefer to store metabolic energy as lipids because lipids provide more energy per unit weight compared to carbohydrates. Lipids are also hydrophobic and do not require water for storage, unlike carbohydrates.

2. Three enzymes involved in the initiation phase of DNA replication:

   • Helicase: Unwinds the DNA double helix.
   • Primase: Synthesizes RNA primers to provide a starting point for DNA polymerase.
   • DNA polymerase: Adds nucleotides to the 3' end of the RNA primer to start DNA synthesis.

3. Two similarities between DNA replication and DNA transcription:

   • Both involve the use of a DNA template.
   • Both involve the synthesis of a new strand of nucleic acid based on base-pairing rules.