Biomolecules

All living organisms are made of the same chemical elements as non-living matter, but in different proportions; for example, carbon makes up about 18.5% of the human body but only about 0.03% of the earth's crust. This chapter explains how the chemical composition of a tissue is analysed by grinding it in trichloroacetic acid. The acid-soluble pool contains small molecules such as amino acids, sugars, nucleotides and lipids. The acid-insoluble fraction contains macromolecules: proteins, polysaccharides and nucleic acids. You will study amino acids and their zwitterionic form, fatty acids, glycerol, phospholipids like lecithin, nitrogen bases, nucleosides, nucleotides, and secondary metabolites such as alkaloids, terpenoids and toxins. You will learn about the primary, secondary, tertiary and quaternary structure of proteins, and polysaccharides such as starch, cellulose, glycogen, inulin and chitin. Bonds such as peptide, glycosidic and phosphodiester bonds link these molecules. The chapter ends with enzymes: how they lower activation energy, how they bind substrates at the active site, how temperature, pH and substrate concentration affect them, and the roles of competitive inhibitors and cofactors.

Detailed Worksheet: Biomolecules Section A - Definitions (10 marks) 1. What are biomacromolecules? Name three of them. (2 marks) 2. What is a zwitterion? (2 marks) 3. Distinguish between a nucleoside and a nucleotide with one example each. (2 marks) 4. What is a prosthetic group? Give an example. (2 marks) 5. Distinguish between primary and secondary metabolites. (2 marks) Section B - Calculations and Applications (15 marks) 6. A polypeptide chain contains 150 amino acids. How many peptide bonds are present, and how many water molecules are released during its formation? If the average mass of an amino acid residue is 110 Da, estimate the mass of the polypeptide. (3 marks) 7. A double-stranded DNA molecule contains 20% adenine. Calculate the percentages of thymine, guanine and cytosine. State the rule you used. (3 marks) 8. A B-DNA molecule has 5,000 base pairs. Given that the distance between two base pairs is 0.34 nm and one turn has 10 base pairs, calculate the length of the molecule in nm and the number of turns. (3 marks) 9. One molecule of carbonic anhydrase converts about 6,00,000 molecules of substrate per second. Calculate the number of molecules it converts in one minute. Why can enzymes speed up reactions so much? (3 marks) 10. A starch molecule contains 1,000 glucose units. How many glycosidic bonds join them in a linear chain, and how many water molecules are released? Compare the iodine test results for starch and cellulose with reasons. (3 marks) Section C - Diagrams (10 marks) 11. Draw the structures of glycine, alanine and serine, showing their R groups. (4 marks) 12. Draw a diagram showing the formation of a triglyceride from glycerol and fatty acids. (3 marks) 13. Draw a graph showing the effect of temperature on enzyme activity, and label the optimum temperature. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the four levels of protein structure. How does a change in tertiary structure affect the function of an enzyme? (5 marks) 15. Explain the mechanism of enzyme action using the concept of the enzyme-substrate complex and activation energy. Draw an energy diagram for a catalysed and uncatalysed reaction. (5 marks) 16. Explain how temperature, pH, substrate concentration and inhibitors affect enzyme activity. Give the example of malonate and succinic dehydrogenase. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw structures and graphs neatly with labels. Show the steps of calculations.