Biomolecules

Biomolecules are the complex organic molecules, such as carbohydrates, proteins, nucleic acids, lipids and vitamins, that build living systems and keep them running. In this chapter you will study carbohydrates as optically active polyhydroxy aldehydes or ketones, their classification into monosaccharides, oligosaccharides and polysaccharides, and into reducing and non-reducing sugars. You will learn how glucose is prepared from sucrose and starch, how its open-chain structure was deduced from reactions with HI, hydroxylamine, HCN, bromine water, acetic anhydride and nitric acid, why it is assigned the D-configuration, and why reactions such as the absence of a 2,4-DNP test require a cyclic pyranose structure with alpha and beta anomers. You will also study fructose, the disaccharides sucrose, maltose and lactose, inversion of cane sugar, and the polysaccharides starch, cellulose and glycogen. The chapter then covers amino acids, zwitterions, peptide bonds and the primary, secondary, tertiary and quaternary structure of proteins and their denaturation, the deficiency diseases of vitamins, and the structure and functions of nucleic acids, DNA and RNA.

Detailed Worksheet: Biomolecules Section A - Definitions (10 marks) 1. Define carbohydrates. Classify them on the basis of hydrolysis with one example of each class. (2 marks) 2. What are reducing sugars? Why is sucrose a non-reducing sugar? (2 marks) 3. What are anomers? Draw or name the two anomers of glucose. (2 marks) 4. What is a peptide bond? Write the structure of a dipeptide formed from glycine and alanine. (2 marks) 5. Distinguish between a nucleoside and a nucleotide. (2 marks) Section B - Calculations and Applications (15 marks) 6. Sucrose is dextrorotatory, but on hydrolysis it gives an equimolar mixture of D-(+)-glucose (specific rotation +52.5 degrees) and D-(-)-fructose (specific rotation -92.4 degrees). Calculate the approximate specific rotation of the mixture and explain why the product is called invert sugar. (3 marks) 7. A tripeptide is formed from glycine (molar mass 75), alanine (89) and valine (117). Calculate the molar mass of the tripeptide and the number of peptide bonds. How many different tripeptides can be formed if each amino acid is used once? (3 marks) 8. Glucose (C6H12O6 = 180) reacts with acetic anhydride to form glucose pentaacetate. Calculate the number of moles of acetic anhydride needed for 9 g of glucose, and the molar mass of the pentaacetate (C16H22O11). What does this reaction prove about glucose? (3 marks) 9. A DNA sample contains 18% adenine. Calculate the percentages of thymine, guanine and cytosine. Why does this calculation not apply to a single-stranded RNA molecule? (3 marks) 10. Match each vitamin with its deficiency disease and classify it as fat soluble or water soluble: vitamin A, vitamin B1, vitamin B12, vitamin C, vitamin D, vitamin K. (3 marks) Section C - Diagrams (10 marks) 11. Draw the Fischer projection of D-glucose and the Haworth structures of alpha-D-glucopyranose and beta-D-glucopyranose, labelling C1 and the anomeric hydroxyl group. (4 marks) 12. Draw the Haworth structure of sucrose showing the glycosidic linkage between C1 of alpha-D-glucose and C2 of beta-D-fructose. (3 marks) 13. Draw labelled diagrams of the alpha-helix and the beta-pleated sheet secondary structures of proteins, showing the hydrogen bonds. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the evidence for the open-chain structure of glucose from its reactions with HI, NH2OH, HCN, bromine water, acetic anhydride and nitric acid. Then explain the reactions that the open-chain structure cannot explain and how the cyclic structure accounts for them. (5 marks) 15. Compare starch, cellulose and glycogen on the basis of monomer units, type of glycosidic linkage, branching and function. Analyse why humans can digest starch but not cellulose. (5 marks) 16. Explain the four levels of protein structure. Analyse what happens to these levels when an egg is boiled or milk curdles, and why the biological activity of the protein is lost while the primary structure remains intact. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all structures neatly and label key atoms. Use the given molar masses for calculations.