Examiner Analysis. Topics 2.1 - 2.4 (Biological Molecules) Analysis of Exam Series 2020-2025

"A significant proportion of candidates incorrectly thought that glycosidic bonds were involved in holding adjacent cellulose molecules together. Of those who knew that adjacent cellulose molecules are held together by hydrogen bonds, a significant minority incorrectly thought that the cellulose molecules in different layers of the cell wall are organised in a parallel arrangement."

"Many candidates knew the correct orientation of the –H and –OH on carbon one to distinguish an alpha-glucose molecule from a beta-glucose molecule, and most completed the rest of the structure correctly. A number of candidates did not fully complete the structure with the –H on carbon 5 being the most common omission."

"Candidates selecting option A had recalled that water is sometimes referred to as the universal solvent but had disregarded the fact that a number of biological molecules, including triglycerides and phospholipids, are insoluble in water. Those selecting option B had not realised that non-polar molecules are usually hydrophobic and therefore insoluble in water."

"The most common error was to indicate that glycogen is used for energy storage in plants."

"About one fifth of the cohort selected the correct option, D (four hydrogen bonds). This question required candidates to think about the three-dimensional structure of a water molecule in the liquid state, where one hydrogen bond can form between each of its hydrogen atoms and other water molecules, and two other water molecules can form hydrogen bonds with the two lone pairs associated with its oxygen atom."

"A common misconception identified in this question was that candidates believed that hydrogen bonds are strong bonds within the tertiary and quaternary structure of a protein, with a considerable number of candidates opting for A, over the correct option, B."

"A few candidates read this question carefully and correctly attributed the cause of the polarity of phosphate heads to them becoming ionised in water (option C). A considerable number of candidates incorrectly selected option A. Although this is a true statement, it does not answer the question."

"Many candidates knew that the quaternary structure of a protein depends on the primary structure of the polypeptides (option C). Options A and D were both popular incorrect answers."

"Most candidates stated that water is a good solvent and listed some of the substances that are transported in solution in the blood, such as glucose, albumen, and hydrogencarbonate ions. Some candidates only stated that water was a solvent and could gain credit if they explained that ions and polar molecules are dissolved in water. The other property that is relevant to blood is the high specific heat capacity of water. Credit was not given to those who omitted 'specific' and only stated that water has a 'high heat capacity'. It was not relevant in this question to refer to the latent heat of vaporisation."

"For Question 4(a), some candidates showed a lack of understanding of what constitutes a cellulose molecule. Many wrote about how cellulose molecules contribute to a cellulose cell wall by becoming part of cellulose microfibrils."

"Almost all who knew that the molecule was composed of beta-glucose monomers, either wrote out the term or gave the correct beta symbol rather than writing the letter B or b, which did not gain credit. A large proportion gave irrelevant details about cellulose microfibrils or cellulose fibres, even though the question had clearly asked about a molecule."

"The ability to make the distinction between a collagen molecule and a collagen fibre was critical in gaining credit in Question 2(a). There were correct statements about collagen molecules that appeared in the collagen fibre section and vice versa."

"Candidates needed to give an extended response for Question 4(a), which asked about two different polymers, lignin and suberin. The strongest answers addressed each polymer in separate sections, describing clearly their location and features and explaining how these were relevant to the transport of water. These contrasted with responses that either tried to mention both polymers in the same sentence or gave a number of statements where suberin or lignin appeared in no apparent order."

"Many knew that a collagen molecule was composed of three polypeptides that together formed the collagen triple helix. Some noted that the polypeptides were tightly wound and had a repeating pattern involving glycine. It was not correct to state that a polypeptide was an alpha helix or to state that glycine formed covalent bonds with other amino acids in the other polypeptides."

"The most common examples chosen were skin, blood vessels and tendons. Responses not credited simply stated a structure, without any qualification to show how the structure was affected, or they just wrote about the effect on 'the tissue' and did not give an example."

"Two thirds of weaker candidates incorrectly suggested that covalent bonding was found in the secondary structure of proteins."

"Over two fifths of the weaker candidates did not realise that beta-glucose is not present in amylopectin and incorrectly selected options A or D."

"Over two fifths of all candidates incorrectly selected options containing process 3. Allowing leaves to cool down quickly at night is not a process in which hydrogen bonding in water is important on hot sunny days."

"Three quarters of the weaker candidates answered this question incorrectly. Candidates needed to understand that the many hydrogen bonds linking the unbranched chains of beta-glucose result in the cellulose fibres having great strength."

"In Question 2(c), candidates could use the information provided and apply knowledge learned in Topic 2, Biological molecules, to suggest features of keratin structure that contribute to its stability. Some needed to read the question properly and described collagen instead."

"The most common point put forward was the existence of the covalent disulfide bonds. Hydrogen bonds were also frequently mentioned, with only a proportion correctly identifying that collectively many hydrogen bonds were providing the stability required. A common error here was to state that hydrogen bonds are strong bonds."

"Over a third of weaker candidates incorrectly selected options A or C. Cellulose contains beta-glucose, not alpha-glucose. Additionally, over a third of weaker candidates incorrectly suggested that cellulose was a branched chain."

"Almost two thirds of the weaker candidates incorrectly indicated that the glucose molecules in cellulose coil into a helix."

"Half of the weaker candidates incorrectly believed that triglycerides are polar molecules."

"Just over two fifths of all candidates answered correctly, realising that disulfide bonds are also a type of covalent bond."

"Some stated that glycogen is 'more branched' than cellulose in Question 2(a), implying that cellulose is branched."

"This question asked for a description of the breakdown of protein molecules. Many responses stated that this is carried out by hydrolytic enzymes, and a few used their knowledge of protein structure to state that peptide bonds are broken to release peptides and/or amino acids. Many candidates wrote about the mode of action of enzymes, including formation of enzyme-substrate complexes and the lock and key hypothesis, which was not required and so did not gain credit."

"Good answers described the structure of cellulose in detail, for example by referring to the many hydrogen bonds between cellulose molecules. Some candidates incorrectly thought that cellulose is a fibrous protein and some suggested it is branched. There was some confusion with collagen as candidates referred to a 'triple helix' of cellulose molecules. Careful reading of the question should have guided candidates to refer to cellulose molecules rather than cellulose fibres in cell walls."

"There were many different ways to gain full credit for an explanation of a polysaccharide. Common errors included stating two or more, rather than more than two monosaccharides, and to use the term 'monomer' without further qualification. Some gave a correct statement but then included reference to amino acids or gave an incorrect bond type."

"There were some very clear drawings of the general structure of an amino acid. Labelling of the different parts was not required, but if the R group was labelled and annotated correctly, credit was given for the explanation of how it is possible to have many different amino acids. It was not acceptable to have NH2 – written on the left and shown joined to the central carbon, but H2N was credited. One error was to show three, rather than two, hydrogen atoms attached to the nitrogen. Some gave H instead of R, which meant that they drew a glycine amino acid rather than the general structure."

"Candidates selecting this option had not noted that the secondary structure of proteins is based on hydrogen bonds and does not involve covalent bonds."

"Most candidates did not realise that triglycerides always contain at least three double bonds. When triglycerides are formed from fatty acids and glycerol, the C=O double bond in the carboxyl group of fatty acids is unaffected by the condensation reaction."

"Some candidates incorrectly made reference to hydrogen ions, oxygen ions and OH– ions, or suggested that hydrogen bonds are examples of covalent bonds."

"Others confused a high latent heat of vaporisation with a high specific heat capacity. Some wrote about other properties of water that were not relevant to the question."

"The majority of candidates described how the structure of collagen molecules, rather than fibres, provides the skin with strength. This did not answer the question. Few responses described features of collagen fibres that were relevant, such as cross-linking between molecules in fibres and a staggered alignment of adjacent fibres."

"More than a quarter of the weaker candidates could correctly identify beta-glucose."

"Many weaker candidates incorrectly suggested that fructose or glucose was a disaccharide."

"Most candidates were able to explain that globular proteins are spherical in shape. Fewer were able to develop their answer further to give other properties of globular proteins such as solubility in water. The strongest responses linked solubility in water to the tertiary structure of the enzyme, carefully describing the position of the amino acids with hydrophilic R groups and those with hydrophobic R groups."

"In Question 5(a), it was important for candidates to know the difference between a collagen polypeptide, a collagen molecule and a collagen fibre. In this question, candidates were only required to give a response relating a collagen polypeptide to a collagen molecule. Some gave details extending to a collagen fibre."

"Strong answers gave clear statements about the solvent properties of water related to xylem sap and phloem sap and went on to explain how water contributed to this transport. Fewer explained how water was important in the hydrostatic pressure gradient that was created for mass flow in phloem. Weaker answers confused the two different transport mechanisms."

"Peptide was the most common answer. Some candidates gave other bonds that are found in other biochemical polymers, such as glycosidic, ester and phosphodiester. 'Polypeptide' bond was not accepted."

"The secondary structures alpha helix and beta pleated sheet were given by many candidates. Some candidates needed to make it clear that they were writing the letter beta (beta) as they often wrote 'B pleated sheet'. These answers did not gain credit."

"Strong answers stated that the protein is composed of a single polypeptide and not composed of two or more polypeptides. These went on to explain that in tertiary structure there are bonds between R groups of amino acids in the polypeptide chain to form the specific structure, whereas in quaternary structure there are intermolecular bonds between the polypeptides."

"Whilst almost all candidates knew that water has properties dependent on hydrogen bonds, approximately half of all respondents knew that this was also true for cellulose and haemoglobin."

"The majority of candidates were able to state that ester bonds are formed between glycerol and fatty acids. The most common incorrect answer was glycosidic bonds."

"Some candidates confused the term saturated with unsaturated and there were often references to the number of kinks in the hydrocarbon tails or the fluidity of each fatty acid, which were not credited."

"References to these molecules as those that hate or repel water did not gain credit."

"The terms disaccharide and monosaccharide were required and it was not sufficient to state that sucrose was composed of two monomers and fructose was a single monomer. Some attempted to name the monomers but did not get the detail correct, usually stating that sucrose was composed of two glucose monomers."

"To gain full credit, the response needed to address the structure of a cellulose molecule as well as a cellulose microfibril. Those who wrote correctly about microfibrils knew that the cellulose molecules were parallel and/or that hydrogen bonds were present, and a few answers went on to describe the staggered arrangement of molecules."

"Candidates were confident that 1,4 glycosidic bonds occur in cellulose but there was some confusion as to the nature of the monomers from which it is constructed."

"Hydrolysis uses water to split molecules and cannot therefore result in the release of metabolic water."

"Candidates had not recognised that peptide bonds are limited to the primary structure of proteins and had not learnt that many fibrous proteins, including the syllabus example of collagen, have a quaternary structure."

"Less than half of all candidates answered this correctly. A positive Benedict's test only shows the presence of reducing sugars, not a specific named reducing sugar."

"Less than a half of all candidates answered this correctly with almost a third incorrectly selecting option D. Not all protein molecules have a metal ion in the quaternary structure."

"Almost half of the weaker candidates incorrectly selected option B. β-pleated sheets are not formed by the chain of amino acids coiling."

"The most common incorrect answer was Benedict's solution, which is used to test for reducing sugars. Some answered the question and gave a correct colour change; others stated only blue-black and needed to reference the original orange-brown colour of the iodine solution."

"Reference to a-glucose was not accepted. Fewer went on to describe the bonding in the molecules and their different shapes. Some candidates confused the two molecules, describing amylose as branched or containing 1,6 glycosidic bonds."

"To gain full credit, it was necessary to state the initial colour of biuret solution as well as give the colour of a positive result for protein. A common error was to state that biuret solution is colourless, rather than pale blue or blue."

"Some candidates did not make this change clear. Some wrote about haemoglobin having an R group, instead of individual amino acid residues having R groups. They stated that there are different bonds or interactions between R groups but were vague as to the effect that a change in R group would have. Many candidates were confused between molecules of haemoglobin and red blood cells."

"Candidates were often unclear about the arrangement of cellulose molecules in microfibrils. Some described only the structure of a cellulose molecule. Others confused cellulose with collagen and wrote about the structure and arrangement of triple helices. The strongest responses explained that the straight, unbranched molecules of cellulose are aligned in parallel and are held together by hydrogen bonds."

"Most of the stronger candidates correctly multiplied the number of molecules by 2, since each molecule has 2 α-globin chains."

"The role of hydrogen bonding in the secondary structure of proteins was understood by many who made it clear that hydrogen bonds are involved in stabilising alpha-helices and beta-pleated sheets."

"Good answers went on to give the detail that these form between -NH and -CO or -OH groups. Many stated incorrectly that the hydrogen bonds in tertiary structure are strong bonds."

"This question was about the solvent action of water and the strongest responses focused only on this, rather than describe the importance of other properties of water. Many answers explained that ions and polar molecules dissolve in water and are transported through plants... Some stated incorrectly that glucose is the main sugar transported in phloem rather than sucrose."

"Some omitted to state that there were two fatty acid tails in a phospholipid and many did not mention glycerol at all."

"It was quite common for candidates to think that the peptide bond was weaker than one or more of the tertiary structure bonds. Where no or only partial credit was given, this was often because many simply listed the different types of bonds used to form the tertiary structure of proteins, without identifying them as R group interactions, and making no attempt at comparison with peptide bonds."

"Some explained that disaccharides were composed of two sugars or two sugar molecules. This did not confirm the understanding that they were composed of monosaccharides... Some candidates mistakenly described a polysaccharide as a sugar."

"Candidates had not noticed that condensation reactions release water, rather than using water."

"Most candidates knew that collagen is an example of a protein. A number of candidates incorrectly stated polypeptides or amino acids."

"Candidates needed to consider that if the enzyme was boiled it would be denatured so no reducing sugar would be produced."

"The key knowledge that candidates did not realise was that hydrogen bonding in the tertiary structure of proteins occurs between the R groups."

"Nearly half of candidates incorrectly chose option A with candidates thinking that both reducing sugars and non-reducing sugars must be present."

"Option B was the most common incorrect response chosen as candidates did not recall that starch contains 1,6 glycosidic bonds."

"Those that did not gain full credit confused condensation reactions with hydrolysis reactions. Some candidates did not appreciate that three water molecules would be produced, one for each ester bond formed."

"A significant number of candidates thought that lipase enzymes were fibrous proteins. Candidates should be aware that enzymes are globular proteins, having an approximately spherical shape."

"Most of the responses that gained credit described a collagen molecule made up of three polypeptides forming a triple helix. Some went on to describe the hydrogen bonds that hold the three polypeptides in this structure. A few candidates mentioned glycine in their descriptions. Weaker answers simply described collagen as a fibrous protein."

"Candidates often did not make it clear that each collagen molecule is composed of three polypeptides that are closely packed together in a triple helix. Many explained that the polypeptides are found in a 'tight coil' which does not convey the right idea. Many knew that glycine is the smallest amino acid but did not always relate this to the close packing of the three polypeptides."

"Few candidates stated clearly that there are many hydrogen bonds between the polypeptides in each collagen molecule and covalent bonds between collagen molecules... Many also gained a mark for stating one place in the mammalian body where collagen is found... Some candidates thought individual hydrogen bonds were strong, rather than stating that it is the presence of many bonds that provides strength."

"Stronger explanations were based on consideration of energy density, which is the amount of energy stored per unit volume. These explanations typically focused on the compact nature of the packing of triglyceride molecules and the presence of many carbon–hydrogen bonds in the hydrocarbon chains of the fatty acids."

"Many candidates did not know the type of polypeptide in a haemoglobin molecule that is affected by the mutation causing sickle cell anaemia. Examples of incorrect responses included polypeptide, β-polypeptide, α-polypeptide, α-globin, primary structure, globular protein and valine."