CIE A Level Biology
CIE A Level Biology Topic 2: Biological Molecules Exam Questions
Free CIE A Level Biology exam questions for Topic 2: Biological Molecules, with a mark scheme for every pack. Covers biochemical tests, carbohydrates, glycosidic bonds, lipids, proteins and their structure, haemoglobin, collagen and the properties of water.
Topic 2: Biological Molecules exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Biology Topic 2: Biological Molecules
Biological molecules introduces the carbohydrates, lipids and proteins that build cells, along with the properties of water. In the Cambridge 9700 syllabus you need to know how monomers join by condensation and separate by hydrolysis, how the structure of each molecule suits its role, and how to carry out and interpret the standard food tests, including a semi-quantitative Benedict's test. Protein structure, from primary to quaternary, links directly to enzymes in Topic 3 and to membranes in Topic 4. Examiners regularly ask you to compare starch, glycogen and cellulose, describe haemoglobin and collagen, and explain how hydrogen bonding gives water its useful properties.
What you need to know
- Biochemical tests Benedict's test for reducing sugars (heat, blue to green, yellow, orange or brick red); non-reducing sugars give a positive result only after acid hydrolysis and neutralisation; iodine for starch; emulsion test for lipids; biuret for proteins (lilac).
- Carbohydrates Alpha and beta glucose differ in the position of the OH on carbon 1. Amylose (1,4 bonds, helical), amylopectin and glycogen (1,4 and 1,6 bonds, branched) store energy; cellulose is made of beta glucose with alternate molecules inverted, forming straight chains held by hydrogen bonds into microfibrils.
- Lipids A triglyceride is glycerol joined to three fatty acids by ester bonds; it is an efficient energy store and insulator. Phospholipids have a hydrophilic phosphate head and two hydrophobic tails and form bilayers.
- Proteins Amino acids join by peptide bonds. Primary structure is the sequence; secondary is the alpha helix or beta pleated sheet held by hydrogen bonds; tertiary involves hydrogen, ionic and disulfide bonds and hydrophobic interactions; quaternary involves more than one polypeptide.
- Haemoglobin and collagen Haemoglobin is a globular, soluble protein of four polypeptides each with a haem group containing Fe2+. Collagen is fibrous: three polypeptides in a triple helix, with glycine every third amino acid, cross-linked into fibrils for tensile strength.
- Water Water is polar and forms hydrogen bonds, giving it a high specific heat capacity, high latent heat of vaporisation and its ability to act as a solvent.
Common mistakes in Topic 2: Biological Molecules
- Writing that hydrolysis joins molecules; condensation joins monomers and releases water, hydrolysis breaks bonds using water.
- Saying cellulose is made of alpha glucose; it is beta glucose, and the straight chains allow hydrogen bonding between chains.
- Calling the Benedict's test quantitative; it is semi-quantitative unless a colorimeter or standard colour series is used.
- Describing peptide bonds as being broken when a protein is denatured; denaturation breaks the bonds holding the tertiary structure, not the primary sequence.
- Confusing hydrogen bonds within a water molecule with those between molecules; hydrogen bonds form between separate water molecules.
Try three Topic 2: Biological Molecules exam-style questions
1. A polypeptide contains 120 amino acids. State how many water molecules are released when it is made and explain your answer. (2 marks)
Show the mark scheme answer
119; one water molecule is released by condensation for each peptide bond and there is one fewer bond than amino acids.
2. Compare the structures of amylopectin and glycogen. (3 marks)
Show the mark scheme answer
Both are polymers of alpha glucose; both have 1,4 and 1,6 glycosidic bonds; glycogen is more branched with shorter chains; so glycogen is more compact and can be hydrolysed faster.
3. Explain how the structure of collagen makes it suited to its function. (4 marks)
Show the mark scheme answer
Three polypeptide chains wound in a triple helix; glycine every third amino acid allows close packing; hydrogen bonds hold chains together; molecules are cross-linked into fibrils and fibres; giving high tensile strength; it is insoluble.
Topic 2: Biological Molecules FAQs
What does Cambridge International A Level Biology Topic 2 Biological molecules cover?
It covers food tests, the structure and roles of carbohydrates, lipids and proteins, the structure of haemoglobin and collagen, and the properties of water.
Which paper is Biological molecules examined in?
It is AS Level content, so it is assessed in Paper 1 (multiple choice) and Paper 2 (AS Level structured questions), with related practical skills tested in Paper 3. A Level candidates can also meet it again in Paper 4 and Paper 5, which assume AS knowledge.
How do I revise the biochemical tests?
Learn each test as method, positive result and negative result, including the extra hydrolysis steps for non-reducing sugars and how to make the Benedict's test semi-quantitative with a colour standard or colorimeter.
