Biological Molecules: Biology 9700 (Cambridge International AS & A Level)
Syllabus 2.1, 2.2, 2.3, 2.4 · Strand 1 Cells and biological molecules
- Questions
- 10
- Total marks
- 53
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 2.1 2 questions completed
- 2.2 6 questions completed
- 2.3 2 questions completed
- 2.4 2 questions completed
Living things are built from a small set of molecule families, and this topic (syllabus 2.1 to 2.4) explains how their structures fit their jobs. It opens with the practical tests you must be able to carry out and interpret: Benedict’s test for reducing and non-reducing sugars, the iodine test for starch, the biuret test for proteins and the emulsion test for lipids. These give a hands-on way into the chemistry that follows.
Carbohydrates run from single monosaccharides such as glucose, through disaccharides joined by glycosidic bonds, to polysaccharides. Here structure explains function: the coiled chains of starch and glycogen make compact energy stores, while the straight, hydrogen-bonded chains of cellulose build strong plant cell walls. Lipids are covered through triglycerides, which are non-polar energy stores made from glycerol and fatty acids by ester bonds, and phospholipids, whose hydrophilic heads and hydrophobic tails are the basis of every membrane. Proteins are examined at four levels of structure, from the amino acid sequence to the folded three-dimensional shape, using haemoglobin as a soluble globular protein and collagen as an insoluble fibrous one. The topic closes with water, whose hydrogen bonding gives it the solvent action, high specific heat capacity and latent heat that make life possible.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
Two molecules of α-glucose react together to form a molecule of the disaccharide maltose.
Which statement correctly describes this reaction and the bond formed between the two glucose units?
Question 2
Starch, glycogen and cellulose are all polysaccharides built from glucose monomers, but each has a different structure that suits its biological role.
(a) Explain what is meant by a condensation reaction, using the formation of a glycosidic bond between two α-glucose molecules as your example. [2]
(b) Amylopectin (a branched component of starch) and glycogen are both highly branched polysaccharides made of α-glucose. Explain how this branched, coiled structure makes amylopectin and glycogen well suited to their function as storage carbohydrates. [3]
(c) Cellulose is made of β-glucose monomers linked into long, straight, unbranched chains, with hydrogen bonds forming between adjacent parallel chains. Explain how this structure makes cellulose an effective structural material in plant cell walls. [3]
Question 3
Proteins are polymers of amino acids, and their function depends on the way the polypeptide chain folds and, in some cases, associates with other chains.
(a) State what is meant by the primary structure of a protein, and name the type of bond that links adjacent amino acids together. [2]
(b) Describe the two common types of secondary structure found in proteins, stating the type of bond responsible for maintaining each. [2]
(c) Explain, with reference to at least three different types of interaction between R-groups, how the tertiary structure of a protein is formed and maintained. [3]
(d) Haemoglobin is a globular protein with a quaternary structure, while collagen is an insoluble fibrous protein. Explain how the structure of haemoglobin, and separately the structure of collagen, is related to its function. [3]
Question 4
Lipids and water are both essential biological molecules, though very different in their structure and behaviour in the cell.
(a) Describe, using the terms glycerol, fatty acid and ester bond, how a triglyceride molecule is formed. [3]
(b) Explain how the molecular structure of a triglyceride makes it well suited to its function as an energy-storage molecule in adipose (fat-storage) tissue. [2]
(c) A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. Explain why phospholipid molecules arrange themselves into a bilayer, rather than a single continuous layer, when surrounded by water on both sides, as in a cell-surface membrane. [2]
(d) State one property of water that arises from hydrogen bonding between water molecules, and explain how this property benefits a named organism or biological process. [2]
Question 5
A student carries out four biochemical tests on an unknown food extract and records the following results.
- Iodine test: the solution remains yellow-brown.
- Benedict's test (after heating): the solution turns brick-red, with a precipitate.
- Biuret test: the solution remains blue.
- Emulsion test: the mixture remains a clear solution, with no cloudy layer.
Which statement correctly describes the food extract, based on these results?
Question 6
Which statement correctly describes the general structure shared by all amino acids?
Question 7
Sucrose is a disaccharide formed from a molecule of glucose and a molecule of fructose. Unlike maltose, sucrose is a non-reducing sugar.
(a) Describe how you would carry out Benedict's test on a food sample to test for the presence of a reducing sugar, including the appearance of a positive result. [3]
(b) A student carries out the test in (a) on a pure sucrose solution and obtains a negative result. Explain, in terms of the glycosidic bond in sucrose, why sucrose does not reduce Benedict's reagent directly. [2]
(c) Describe how the student could modify the procedure to test whether a sample contains a non-reducing sugar such as sucrose. [3]
Question 8
Triglyceride X is made from three saturated fatty acids. Triglyceride Y is made from three unsaturated fatty acids, each containing several cis carbon-carbon double bonds.
Which statement correctly compares the melting points of X and Y, and correctly explains the reason for the difference?
Question 9
Many of water's biological roles arise from hydrogen bonding between its polar molecules.
(a) Explain how hydrogen bonding between water molecules gives rise to cohesion, and describe how this property helps transport water through the xylem of a flowering plant. [3]
(b) Explain how hydrogen bonding between water molecules gives water a high latent heat of vaporisation, and describe how this property benefits a mammal that cools itself by sweating. [3]
(c) State one reason why water's properties as a solvent are important for metabolic reactions inside a cell. [2]
Question 10
Glucose, maltose and glycogen are all carbohydrates built from glucose units, but they belong to different classes of carbohydrate.
(a) State whether each of the following is a monosaccharide, a disaccharide or a polysaccharide, giving a reason for each answer: glucose; maltose; glycogen. [3]
(b) A single unbranched amylose molecule is made of 600 α-glucose monomers joined end-to-end by 1,4-glycosidic bonds. Calculate the number of glycosidic bonds formed, and the number of water molecules released, when this amylose molecule is assembled by condensation reactions from 600 separate α-glucose monomers. Show your working. [2]
(c) State the type of reaction, and name the general type of enzyme, that would break this amylose molecule back down into its individual glucose monomers. [1]