AQA A-level Biology: Lipids and Protein Structure (7402) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sections 3.1.3 (Lipids) and 3.1.4.1 (General properties of proteins) of the AQA AS and A-level Biology specification, for first teaching 2015, used for A-level Biology 7402. Use the study guide first and the revision notes for recall. Course pages: AQA A-level Biology hub and printable checklist. Where a question gives relative atomic masses, use them: C 12, H 1, N 14, O 16.

Questions

1. Three fatty acids are shown as condensed formulae. Acid P: CH3(CH2)12COOH. Acid Q: CH3(CH2)5CH=CH(CH2)7COOH. Acid R: CH3(CH2)4(CH=CHCH2)2(CH2)6COOH. Identify which acid is saturated and which has the most double bonds. Give a reason for each. [2]

2. Glycerol has Mr 92. Lauric acid, C12H24O2, has Mr 200. One molecule of glycerol reacts with three molecules of lauric acid. (a) Name the bond formed and state how many water molecules are released. [2] (b) Calculate the Mr of the triglyceride. [2]

3. A food label gives the energy content of fat as 37 kJ per gram and of carbohydrate as 17 kJ per gram. (a) Calculate the mass of carbohydrate that releases the same energy as 15 g of fat. [2] (b) Using the structure of a triglyceride, explain why fat releases more energy per gram. [2]

4. Compare the structures of a triglyceride and a phospholipid, then explain why phospholipids but not triglycerides form a bilayer in water. [4]

5. The melting points of three fatty acids with 18 carbon atoms are: stearic acid 69 °C, oleic acid 13 °C and linoleic acid minus 5 °C. Stearic acid has no double bonds, oleic acid has one and linoleic acid has two. Explain the trend. [3]

6. A student tests four samples. The results are in the table.

Sample	Emulsion test	Biuret test
W	Cloudy white	Blue
X	Clear	Purple
Y	Clear	Blue
Z	Cloudy white	Purple

(a) State what each sample contains, using only these two tests. [3] (b) Describe how the emulsion test is carried out. [2]

7. (a) Draw the general structure of an amino acid, labelling every group attached to the central carbon except hydrogen. [2] (b) State what makes one of the twenty common amino acids different from another. [1]

8. Serine has the formula C3H7NO3 and valine has the formula C5H11NO2. (a) Name the reaction that joins them and the bond that forms. [2] (b) Calculate the Mr of the dipeptide. [2] (c) A polypeptide has 133 amino acids. State how many peptide bonds it contains and how many water molecules are needed to hydrolyse it completely. [2]

9. Describe the primary, secondary and tertiary structure of a protein, naming the bonds that hold each level. [5]

10. In a study of a protein, researchers replaced one cysteine with alanine. Alanine cannot form a disulfide bridge. The temperature at which half the protein had lost its shape was 72 °C for the normal protein and 58 °C for the altered protein. Explain these results and suggest why the replacement changes only some bonds. [5]

11. A biuret test was used to estimate protein concentration. The absorbance of standard solutions was 0.00, 0.11, 0.22, 0.33 and 0.44 at 0, 0.5, 1.0, 1.5 and 2.0 mg per cm³. The absorbance is proportional to concentration. (a) A sample gave an absorbance of 0.30. Calculate its protein concentration. [2] (b) A second sample was diluted so that one part sample was mixed with three parts water. The diluted sample gave 0.176. Calculate the concentration of the original sample. [3] (c) Explain why the biuret reagent changes colour only when protein is present. [2]

Answers

1. Acid P is saturated because its chain has only single bonds [1]. Acid R has the most double bonds because its formula shows two C=C groups, against one in acid Q [1]. [2] Examiner insight: Quote evidence from the formula; “it is long” or “it is bent” does not identify saturation.

2. (a) Ester bond [1]; three water molecules [1]. [2] (b) Mass of reactants = 92 + (3 × 200) = 692 [1]. Subtract water: 692 - (3 × 18) = 638 [1]. [2] Examiner insight: Show the subtraction of three waters; forgetting to remove them is a common loss of a mark.

3. (a) Energy from fat = 15 × 37 = 555 kJ [1]. Carbohydrate = 555 / 17 = 32.6 g [1]. [2] (b) Triglycerides have long hydrocarbon chains with many C-H bonds [1], so more energy is released on oxidation per gram than from carbohydrate [1]. [2] Examiner insight: Name the structural feature (C-H bonds in chains) and link it to energy. “Fat has more energy” alone repeats the question.

4. A triglyceride has glycerol with three fatty acids [1]; a phospholipid has one fatty acid replaced by a phosphate-containing group [1]. The phosphate group is hydrophilic and the fatty acid tails are hydrophobic [1]. In water the heads face the water and the tails face inward, forming a bilayer; a triglyceride has no hydrophilic part so cannot do this [1]. [4] Examiner insight: A comparison needs both molecules mentioned in each point; describing only the phospholipid scores less.

5. More double bonds lower the melting point [1]. Double bonds put bends in the chain, so molecules pack less closely [1]. Weaker forces between molecules need less energy to separate them, so melting happens at a lower temperature [1]. [3] Examiner insight: Describe the trend first, then explain it; the mechanism (packing and forces) earns the marks.

6. (a) W contains lipid only [1]. X contains protein only [1]. Y contains neither lipid nor protein; Z contains both [1]. [3] (b) Shake the sample with ethanol so lipid dissolves [1]. Pour into water; a cloudy white emulsion shows lipid [1]. [2] Examiner insight: Say “contains” and also state the negative result; the result alone is not a conclusion.

7. (a) Central carbon with NH2, COOH, H and R attached [1]; NH2 labelled amine, COOH labelled carboxyl and R labelled as the side chain [1]. [2] (b) Only the R group [1]. [1] Examiner insight: Attaching the groups to the wrong atom in your drawing can lose the structure mark, even when the labels are correct.

8. (a) Condensation [1]; peptide bond [1]. [2] (b) Mr of serine = 36 + 7 + 14 + 48 = 105; valine = 60 + 11 + 14 + 32 = 117 [1]. Dipeptide = 105 + 117 - 18 = 204 [1]. [2] (c) 132 peptide bonds [1] and 132 water molecules [1]. [2] Examiner insight: Remember the single water lost; and for a chain, bonds are one fewer than amino acids.

9. Primary: the sequence of amino acids joined by peptide bonds [1]. Secondary: the chain coils into an alpha helix or folds into a beta pleated sheet [1], held by hydrogen bonds [1]. Tertiary: further folding into a specific three-dimensional shape [1], held by hydrogen bonds, ionic bonds and disulfide bridges between R groups [1]. [5] Examiner insight: Match each bond to its level; “hydrogen bonds” alone is too vague.

10. The disulfide bridge is a covalent bond that holds the tertiary structure [1]. Without it, the altered protein is held only by weaker hydrogen and ionic bonds [1]. Less heat is needed to break these, so the shape is lost at a lower temperature [1]. Only the cysteine R group is changed, so other R groups still form hydrogen and ionic bonds [1], and the sequence otherwise is unchanged [1]. [5] Examiner insight: Link each change in bonding to the observed temperature difference; a description of the numbers is not enough.

11. (a) Standards give 0.22 per mg cm-3 [1]. 0.30 / 0.22 = 1.36 mg per cm³ [1]. [2] (b) Diluted concentration = 0.176 / 0.22 = 0.80 mg per cm³ [1]. Dilution factor is 4 [1]. Original = 0.80 × 4 = 3.2 mg per cm³ [1]. [3] (c) The reagent contains copper(II) ions in alkali [1]; these react with peptide bonds, giving a purple colour [1]. [2] Examiner insight: Multiply back by the dilution factor and give units; the unscaled value is a common error.

Where marks are usually lost

• Naming a bond as “hydrogen” when the question asks for the one linking amino acids.
• Giving the formula of a triglyceride without subtracting three waters.
• Saying a phospholipid has “a hydrophilic tail”.
• Describing a trend in data without an explanation.
• Giving only the colour of a food test, without saying what it shows.
• Putting disulfide bridges in secondary structure.
• Writing one peptide bond per amino acid instead of one fewer.

Next steps

• Re-read the study guide and the revision notes.
• Tick off the printable checklist and visit the course hub.
• Take one of the free 10-minute diagnostics.
• Book a free trial class at /trial/.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, for first teaching 2015, published by AQA: sections 3.1.3 and 3.1.4.1.