Biological molecules questions

Biological molecules syllabus

Questions on biological molecules can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 2.7 identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)
• 2.8 describe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units: starch and glycogen from simple sugars, protein from amino acids, and lipid from fatty acids and glycerol
• 2.9 practical: investigate food samples for the presence of glucose, starch, protein and fat
• 2.10 understand the role of enzymes as biological catalysts in metabolic reactions
• 2.11 understand how temperature changes can affect enzyme function, including changes to the shape of active site
• 2.12 practical: investigate how enzyme activity can be affected by changes in temperature
• 2.13 understand how enzyme function can be affected by changes in pH altering the active site 2.14B practical: investigate how enzyme activity can be affected by changes in pH

How to answer biological molecules questions

1. Name the structure or process precisely before describing its biological role.
2. When comparing cells, give paired differences and link each adaptation to its function.
3. For microscopy and transport calculations, show the equation, unit conversion and substitution.
4. In practical questions, identify the independent, dependent and controlled variables before interpreting results.

Biological molecules example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Graphing the effect of temperature on lipase

Adapted from June 2023, Paper 1B · Question 7(d)

Question 1

Lipase digests lipid into fatty acids and glycerol. The fatty acids change the pH of the solution.

A student adds lipase to milk, sodium carbonate solution and phenolphthalein indicator. The mixture is initially pink. She records the time until it loses its pink colour, repeating the test at six temperatures.

The table shows the student’s results.

Temperature in °C	Time taken for contents to lose the pink colour in seconds
20	385
25	340
30	300
35	250
40	275
45	360

Plot a line graph to show the effect of temperature on the time taken for the contents of the test tube to lose the pink colour.

Use a ruler to join your points with straight lines.

Final answer

The line graph shows temperature against the time taken to lose the pink colour, with consecutive points joined by straight lines.

Mark scheme points

1. M1 Use a linear scale, with the plotted data spanning at least 2.5 large squares vertically. The axes may start at zero or be truncated.
2. M2 Join all consecutive points with straight lines. Do not extend the lines beyond the first or last point.
3. M3 Put temperature on the horizontal x-axis and time on the vertical y-axis.
4. M4 Label temperature in °C and time in seconds or s.
5. M5 Plot all six points correctly, within half a small square: (20, 385), (25, 340), (30, 300), (35, 250), (40, 275) and (45, 360).

Explanation

Temperature is the variable the student changes, so it belongs on the x-axis. The time measured belongs on the y-axis. Use this checklist when drawing:

• Choose evenly spaced scales that use most of the grid. Here, the time axis starts at 200 s rather than zero, which is allowed.
• Use a ruler to connect neighbouring points, stopping at the two end points.
• Check that temperature is horizontal and time is vertical.
• Include both the quantity and its unit on each axis.
• Check each plotted value against the table. For example, 385 s lies between 375 s and 400 s, not on either grid line.

Common mistakes

• Most candidates gained 4 or 5 marks. Those missing full marks usually chose a scale that used too little of the grid or omitted axis units. Check the vertical spread and both units before finishing.
• Drawing a smooth curve, a bar chart or lines extending beyond the end points loses the straight-line joining mark. Join only the recorded points with a ruler.

Example 2: Predicting amylase activity in agar

Adapted from June 2015 (R), Paper 2B · Question 6(d)(ii)

Question 2

A student is given a Petri dish containing starch mixed with agar jelly.

The student makes five wells each of diameter 10 mm in the agar. She fills the wells with solutions of amylase, each with a different pH.

After 24 hours at 20 °C she pours iodine solution on to the agar jelly.

The iodine solution turns the starch in the agar jelly a dark blue colour.

The diagram shows the appearance of the Petri dish.

The student repeats the experiment, keeping the Petri dish at 37 °C on this occasion. On the diagram, draw the results you would expect to see.

Final answer

Show no non-blue zone around the pH 2 and pH 13 wells. Draw a wider non-blue zone around the pH 7 well than the one shown at 20 °C.

Mark scheme points

1. M1 No starch-free zone around either the pH 2 or pH 13 well.
2. M2 The starch-free zone around pH 7 is wider than at 20 °C.

Explanation

Iodine turns starch blue, so a non-blue zone shows where amylase has broken down the starch. At pH 2 and pH 13, amylase does not break down the starch, so there should be no such zone. At 37 °C, amylase at pH 7 breaks down more starch than at 20 °C, making its non-blue zone wider.

Common mistakes

• Drawing a non-blue zone around the pH 2 or pH 13 well.
• Forgetting to make the pH 7 zone wider than it was at 20 °C.

Practise Biological molecules questions

Build a focused practice set from this part of the Edexcel IGCSE Biology syllabus.