College Board AP® Biology

Section II: Free Response (Set A)

Saturday 26 July 2025

Afternoon (Time: 1 hour 30 minutes) Total marks

/ 33

Instructions
Try to complete this mock exam paper in one sitting, under exam conditions. Use all the time available and
check your answers to each question at the end before submitting.
Remember this is PRACTICE. Mistakes are fine and will help you improve in time for the real exam - just do
your best.
Questions 1-2 are long free-response questions requiring 25 minutes each to answer
Questions 3-6 are short free-response questions requiring 10 minutes each to answer
Show all your working
You are expected to use a scientific calculator where appropriate

Materials
a scientific calculator
a pencil
a black ink or black ball-point pen

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1 (a) Scientists investigated how different substances affect the cohesion of water molecules.
They designed an experiment where a droplet of water was placed on a glass slide, and
the contact angle (θ) of the droplet with the surface was measured. The higher the
contact angle, the higher the cohesion of water molecules. Figure 1. shows two water
droplets and their contact angles.

Figure 1. Water droplets and their contact angles.

The scientists repeated the experiment after adding different substances to water. The
data is recorded in Table 1.

Table 1: Contact angle of water droplets containing different additional

substances.

Solution / mixture Contact angle (θ) / °

Pure water 98.2

0.1 M NaCl 97.5

0.1 M Glucose 95.1

0.1 M Oil 70.5

Describe the effect of adding a substance on the contact angle of water droplets.

(1 mark)

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(b) Calculate the percentage change in contact angle when oil is added to pure water.

(1 mark)

(c) (i) Identify the solute that disrupts water cohesion the most.

(ii) Explain how the differences in contact angle observed in Table 1 may relate to the
chemical properties of the added substance.

(2 marks)

(d) A student proposes an additional experiment to test the effect of solute concentration
on cohesion in water droplets.

(i) State a suitable hypothesis for this investigation.

(ii) Identify the independent and dependent variables.

(iii) State the control condition that would need to be used in this experiment.

(iv) Justify the inclusion of a control condition in this experiment.

(4 marks)

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2 (a) The primary enzyme responsible for carbon fixation in the Calvin cycle is RuBisCO
(ribulose-1,5-bisphosphate carboxylase/oxygenase). As an enzyme, its efficiency is
affected by temperature, however, RuBisCO also exhibits a temperature sensitive dual
affinity for CO₂ and O₂. At high temperatures, RuBisCO favors binding to O₂ instead of
CO₂.

Researchers measured carbon fixation rates (CO₂ uptake in μmol/m²/s) in Elodea

canadensis, an aquatic plant, at different temperatures. The experiment was repeated in
two CO₂ conditions:

1. Ambient CO₂ (~400 ppm, normal air levels)

2. Elevated CO₂ (~800 ppm, enriched conditions)

The results are presented in Table 1.

Table 1: Effect of temperature on carbon fixation rate at two CO₂ concentrations

Temperature Carbon fixation rate (μmol Carbon fixation rate (μmol

(°C) CO₂/m²/s) at 400 ppm CO₂ (± CO₂/m²/s) at 800 ppm CO₂ (±
SE) SE)

15 10.5 ± 0.6 12.1 ± 0.5

20 15.3 ± 0.8 18.6 ± 0.6

25 18.9 ± 0.7 22.4 ± 0.8

30 14.8 ± 1.0 18.1 ± 0.9

35 9.5 ± 1.2 13.0 ± 1.1

40 5.2 ± 1.4 7.6 ± 1.3

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(i) Using the information given and your knowledge of the Calvin cycle, describe the role
of RuBisCO in the production of glucose

(ii) Explain why temperature influences the efficiency of RuBisCO in the Calvin cycle.

(2 marks)

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(b) Construct an appropriate graph representing the data in Table 1.

(4 marks)

(c) (i) Describe the trend in the data from your graph shown in part (b).

(ii) The optimum temperature for RuBisCo is 30 °C, explain the trend in carbon fixation
rate observed at temperatures above 25°C.

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 (2 marks)

(d) Rising atmospheric carbon dioxide (CO₂) levels trap heat and drive global warming, but
scientists hypothesize that carbon fixation may remain stable under these changing
climate conditions.

Use evidence from the data and your knowledge to justify this claim.

(1 mark)

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3 (a) Fireflies emit light when the enzyme luciferase catalyzes a reaction in which its substrate,
D-luciferin, reacts to form oxyluciferin and other products (Figure 1). In order to
determine the optimal temperature for this enzyme, scientists added ATP to a solution
containing D-luciferin, luciferase, and other substances needed for the reaction. They
then measured the amount of light emitted during the first three seconds of the reaction
when it was carried out at different temperatures.

D-Luciferin + O2 + ATP ⎯⎯⎯⎯⎯⎯⎯⎯

Lucifefase
Oxyluciferin + CO 2 + AMP + PPi + Light

Figure 1. Light is emitted as a result of the reaction catalyzed by luciferase.

Describe a characteristic of the luciferase enzyme that allows it to catalyze the reaction.

(1 mark)

(b) Identify the dependent variable in the experiment.

(1 mark)

(c) State the null hypothesis for the experiment.

(1 mark)

(d) A student claims that, as temperature increases, there will be an increase in the amount
of light given off by the reaction in the first three seconds. Support the student's claim.

(1 mark)

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4 (a) Ectothermic animals such as lizards rely on external temperatures to regulate body
processes. Higher temperatures trigger conformational shape change in temperature-
sensitive proteins, which initiates a cascade. This subsequently increases the expression
of heat shock proteins (HSPs), which assist in the proper folding of other proteins,
especially under stress conditions, without becoming part of the final protein structure.
The production of HSPs is controlled by a signal transduction pathway that activates
transcription of hsp genes when the temperature rises above a threshold.

A biologist investigates this pathway by exposing lizards to five temperatures (between

25°C and 45°C) and measuring HSP concentration and hsp70 gene expression after 4
hours.

Table 1: HSP concentration and hsp70 expression at different temperatures

Temperature (°C) HSP protein level (μg/mL) hsp70 expression (a.u.)

25 5.2 1.0

30 10.6 3.5

35 22.1 7.9

40 30.4 5.3

45 33.2 2.6

Identify the trigger of this signal transduction pathway.

(1 mark)

(b) Identify the independent and dependent variables in this experiment.

(1 mark)

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(c) The researcher claimed that once HSP levels are high enough, negative feedback inhibits
further hsp 70 transcription to conserve resources.

Use evidence from the data to justify this claim.

(1 mark)

(d) If a lizard has a mutation in the heat shock factor (HSF) protein, it may fail to activate hsp
gene transcription even when the temperature rises, leaving the cell without HSP
protection.

Predict the effect of this on the organism.

(1 mark)

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5 (a) Cells use the electron transport chain (ETC) to transfer energy from electrons and
produce ATP. Figure 1 shows the electron transport chain used in cellular respiration.

Figure 1. The electron transport chain in cellular respiration.

Identify the source of the electrons donated to the electron transport chain in Figure 1.

(1 mark)

(b) Describe how the formation of a proton gradient leads to the synthesis of ATP.

(1 mark)

(c) The electron transport chain is located in the inner mitochondrial membrane.

Draw an 'X' in Figure 2 to identify the location of glycolysis in cellular respiration.

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 Figure 2. An animal cell

(1 mark)

(d) Respiration is an enzyme-controlled metabolic process.

Explain how temperature changes may affect the production of ATP.

(1 mark)

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6 (a) Cells use three primary macromolecules (carbohydrates, lipids and proteins) as energy
sources, breaking them down through distinct metabolic pathways to generate ATP, the
cell’s primary energy currency. Carbohydrates are generally the first macromolecule to
be metabolised, followed by lipids, and finally proteins, which are processed only when
other sources are insufficient.

Scientists measured energy yield per macromolecule to compare their efficiency in

energy production, considering both their energy content per gram and specific dynamic
action (SDA). SDA is the percentage of energy gained from a molecule that is used up
during its digestion, absorption, and processing for metabolism. Table 1 summarizes the
energy values and SDA percentages for three key macronutrients.

Table 1: Energy values and SDA of macronutrients.

Macronutrient Energy value (kcal/g) SDA (%)

Carbohydrates 4.0 5–10

Lipids 9.0 0–3

Proteins 4.0 20–30

A student concluded from Table 1 that lipids provide the highest net energy yield.

Use data in Table 1 to justify the student's conclusion.

(1 mark)

(b) Figure 1 below shows deamination, a process that occurs in the liver after protein
digestion.

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 Figure 1. Deamination of amino acids in the liver.

Use Figure 1 to explain the higher SDA value for proteins shown in Table 1.

(1 mark)

(c) Using the data in Table 1, calculate the difference in net energy output between
proteins and carbohydrates. Assume median SDA values for each molecule.

(1 mark)

(d) Scientists carried out an investigation into the effect of macronutrient composition on fat
loss. Two groups of participants were placed on calorie-controlled diets: one high in
protein, and the other high in carbohydrates. Each participant’s body fat mass was
recorded weekly over an 8-week period. The results are shown in Figure 1.

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 Figure 1. Change in body fat mass over time in individuals on high-protein and high-
carbohydrate diets

Using all the data provided, explain why a high-protein diet leads to the fat loss trend
seen in Figure 1.

(1 mark)

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