Molecular Biology: Proteins [19 marks] 1. Which molecule could be hydrolysed into amino acids? C 2. How does the proteome of a species contain a larger number of proteins than genes that code for these proteins? A. Some proteins have more than one polypeptide chain. B. There are genes that code for several proteins. C. Not all proteins are coded for by the genome. D. Some proteins are coded for by other proteins. B
3. The most abundant structural protein in the human body is found in ligaments and skin. What is the name of this protein? A. Collagen B. Hemoglobin C. Myoglobin D. Immunoglobulin A 4. Draw a molecular diagram of an amino acid to show its general structure. [3 marks] a. COO or COOH group at one end b. NH 2 or NH 3 + at the other c. CH in middle with H or R group attached If shown expanded, then carbonyl oxygen must attach to C If shown non-expanded, N of amine group must attach to C 5. Which is an effect of protein denaturation? A. The order of amino acids is changed when the protein overheats. B. The bonds between amino acids are broken by condensation. C. Parts of the protein become linked together by hydrolysis. D. The three-dimensional structure of the protein is altered. D This question helped to discriminate well.
6. Outline the production of a dipeptide by a condensation reaction, showing the structure of a generalized dipeptide. [5 marks] a. condensation is joining together two amino acids to form a dipeptide; b. carboxyl/cooh group of one amino acid reacts with amine/nh 2 group of another / diagrams of two (generalized) amino acids correctly shown; c. water/h2o is eliminated; d. diagram of dipeptide correctly shown; e. peptide/covalent bond is produced / peptide bond correctly labelled; f. occurs at the ribosomes; Most gained the mark for stating that water is eliminated in a condensation reaction. Unfortunately they could not explain the process in sufficient detail to gain any more marks. Even although the stem was about dipeptides, weaker candidates wrote about carbohydrates. There were some G2 comments that asking SL candidates to draw a dipeptide was beyond expectations. It is indeed on the limit of what could be expected from 3.2.2 and 3.2.5. However the candidates did have a choice of Section B questions. 7. The image shows the structural formula of a molecule. What is this molecule? A. Amino acid B. Ribose C. Deoxyribose D. Lactose
A Although the question could have confused candidates by adding the charges in the amino and carboxyl termini, this was not an unfair question. The fact that the R group was fully shown should not have confused them. 8. How many molecules of water are required to completely hydrolyse a polypeptide made up of 23 amino acids? A. 11 B. 22 C. 23 D. 44 B This question turned out to be one of the most difficult for candidates, but tested candidate understanding of the process of hydrolysis and was a good discriminator. 9a. List three functions of proteins, giving a named example of each. [3 marks] catalysts/digestion amylase/protease/lipase/catalase; defense immunoglobin / fibrinogen; structure collagen; movement actin/myosin; transport hemoglobin; synthesis ligase/dna polymerase; hormonal communication insulin/luteinizing hormone; light detection rhodopsin / plant phytochromes; storage ferritin/gluten/casein; Accept any other valid responses. Here some candidates did not read the entire question and only listed the functions without a named example. Explain the significance of polar amino acids and non-polar amino acids in membranes. 9b. [2 marks]
non-polar amino acids for hydrophobic part of the bilipid layer; polar amino acids for hydrophilic environment; polar amino acids allow hydrophilic channels; integral proteins are held in place by polar amino acids; To award [2 max] both polar and non-polar should be addressed. Accept answers in the form of a diagram. C3 (b) seemed to be very difficult and very few got this question right. Some candidates did mention the polarity and hydrophobicity of the amino acids, but did not mention the significance of them to membranes. This is clearly an area that needs reinforcing. International Baccalaureate Organization 2018 International Baccalaureate - Baccalauréat International - Bachillerato Internacional Printed for Pritzker College Prep