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AQA A Level Biology 7402/3 Paper 3 Actual
Exam June 2026 | Complete Exam-Style
Questions | 100% Verified – Detailed
Rationales – Pass Guaranteed – A+ Graded
SECTION 1: BIOLOGICAL MOLECULES AND CELLS ═
Question 1 of 50
Researchers analyzing a novel protein isolated from a thermophilic bacterium discovered that the
protein maintains its tertiary structure at extremely high temperatures. They determined that the
stability is primarily due to extensive covalent cross-linking between specific amino acid side
chains. Which type of bond is most likely responsible for this increased thermal stability?
A. Hydrogen bonds forming between polar R-groups B. Ionic interactions between positively
and negatively charged R-groups C. Disulfide bridges formed between cysteine residues ✓
CORRECT D. Hydrophobic interactions between non-polar R-groups
Correct Answer: C Rationale: Disulfide bridges are strong covalent bonds formed between the
sulfur atoms of cysteine residues, providing significant structural rigidity that helps proteins
maintain their tertiary shape under thermal stress. Hydrogen bonds and ionic interactions are
much weaker non-covalent forces that break easily at high temperatures, while hydrophobic
interactions are a result of the exclusion of water rather than direct bonding. In exam questions
involving extreme environmental conditions, looking for covalent cross-links like disulfide
bridges is often the key to identifying the stabilizing factor.
Question 2 of 50
A student investigates the effect of different organic solvents on the permeability of the beetroot
cell membrane, which contains the pigment betalain. The student places equal-sized discs of
beetroot in test tubes containing water at 70°C, ethanol at 20°C, and detergent solution at 20°C
for exactly 5 minutes. Assuming the ethanol and detergent disrupt the membrane structure
similarly to heat, which prediction regarding the absorbance of the solutions is correct?
A. The water at 70°C will show the lowest absorbance as high temperatures denature the pigment
B. The ethanol and detergent solutions will show high absorbance due to the disruption of the
phospholipid bilayer ✓ CORRECT C. All solutions will show equal absorbance because
temperature does not affect membrane integrity D. The detergent solution will show lower
absorbance than ethanol because detergents repair the membrane
,2
Correct Answer: B Rationale: Both ethanol and detergent disrupt the hydrophobic interactions
within the phospholipid bilayer, increasing membrane permeability and allowing the betalain
pigment to leak out into the surrounding solution, thereby increasing absorbance. High
temperature in water also increases permeability, so option A is incorrect as it suggests the
opposite effect, and option D is incorrect because detergents disrupt rather than repair
membranes. When analyzing membrane permeability experiments, remember that any factor
damaging the lipid bilayer—heat, solvents, or pH changes—will result in increased leakage of
cell contents.
Question 3 of 50
During a laboratory practical, students are asked to calculate the size of a mitochondrion
observed under a transmission electron microscope. The image of the mitochondrion measures
30 mm in length, and the scale bar on the micrograph indicates that 1 µm represents 5 mm on the
image. What is the actual length of the mitochondrion?
A. 6 µm ✓ CORRECT B. 15 µm C. 150 µm D. 0.6 µm
Correct Answer: A Rationale: The actual size is calculated by dividing the image size by the
magnification factor; since 5 mm represents 1 µm, the magnification is 5000x (5 mm / 0.001
mm), making the actual size 30 mm / 5000 = 0.006 mm or 6 µm. Option B results from
multiplying instead of dividing, a common error in magnification calculations, while option C
incorrectly assumes the scale is 1:1. Always ensure units are consistent (converting mm to µm or
vice versa) before performing magnification calculations.
Question 4 of 50
Scientists studying the properties of water in biological systems noted that water has a high
specific heat capacity. This property is vital for living organisms because it allows water to act as
a temperature buffer. Which molecular feature of water is directly responsible for this high
specific heat capacity?
A. The covalent bonds between hydrogen and oxygen within the molecule B. The small size of
the water molecule allowing rapid movement C. The non-polar nature of the oxygen atom D. The
hydrogen bonds between water molecules that require energy to break ✓ CORRECT
Correct Answer: D Rationale: A significant amount of heat energy is required to break the
numerous hydrogen bonds between water molecules before the molecules themselves can move
faster and increase in temperature. Covalent bonds within the molecule are not broken during
heating, and the small size of the molecule does not inherently buffer temperature changes.
Understanding the link between hydrogen bonding and thermal properties like specific heat
capacity and latent heat of vaporization is essential for explaining homeostasis.
Question 5 of 50
, 3
In an experiment to determine the water potential of potato tissue, cores were placed in a range
of sucrose solutions ranging from 0.0 mol dm⁻³ to 1.0 mol dm⁻³. After 24 hours, the percentage
change in mass was calculated. A student observed that the potato cores in the 0.4 mol dm⁻³
solution showed no change in mass. What does this indicate about the potato tissue and the
solution?
A. The potato tissue had a higher water potential than the solution B. The sucrose solution was
hypertonic, causing water to leave the cells C. The water potential of the potato tissue was equal
to that of the sucrose solution ✓ CORRECT D. The potato cells were fully plasmolysed at this
concentration
Correct Answer: C Rationale: No net change in mass occurs when there is no net movement of
water into or out of the cells, meaning the water potential of the tissue and the external solution
are in equilibrium. If the tissue had a higher water potential, mass would increase (hypotonic),
and if it were lower, mass would decrease (hypertonic), making options A and B incorrect for
this specific observation. Identifying the isotonic point is the fundamental goal of osmosis
practicals.
Question 6 of 50
A biochemist is analyzing two DNA samples. Sample X contains 28% adenine, while Sample Y
contains 22% cytosine. According to Chargaff's rules regarding base pairing in double-stranded
DNA, what are the percentages of thymine in Sample X and guanine in Sample Y, respectively?
A. 28% and 22% B. 22% and 28% ✓ CORRECT C. 28% and 28% D. 22% and 22%
Correct Answer: B Rationale: Chargaff's rules state that adenine always pairs with thymine and
guanine always pairs with cytosine in equal amounts due to complementary base pairing.
Therefore, if Sample X has 28% adenine, it must have 28% thymine, and if Sample Y has 22%
cytosine, it must have 22% guanine, resulting in the pair (28%, 22%). The question asks for
thymine in X (28%) and guanine in Y (22%), which matches option B. Students often confuse
the pairing partners, so memorizing A=T and C≡G is crucial for quick calculations.
Question 7 of 50
A research team is investigating the structure of a new enzyme. They treat the enzyme with a
strong acid, which breaks all the peptide bonds but leaves the individual amino acids intact.
Which level of protein structure is primarily destroyed by this acid hydrolysis?
A. Primary structure ✓ CORRECT B. Secondary structure C. Tertiary structure D. Quaternary
structure
Correct Answer: A Rationale: The primary structure of a protein is defined by the sequence of
amino acids linked by peptide bonds, so breaking these bonds destroys the primary structure.
While the acid also destroys higher structures (secondary, tertiary, quaternary), the fundamental
AQA A Level Biology 7402/3 Paper 3 Actual
Exam June 2026 | Complete Exam-Style
Questions | 100% Verified – Detailed
Rationales – Pass Guaranteed – A+ Graded
SECTION 1: BIOLOGICAL MOLECULES AND CELLS ═
Question 1 of 50
Researchers analyzing a novel protein isolated from a thermophilic bacterium discovered that the
protein maintains its tertiary structure at extremely high temperatures. They determined that the
stability is primarily due to extensive covalent cross-linking between specific amino acid side
chains. Which type of bond is most likely responsible for this increased thermal stability?
A. Hydrogen bonds forming between polar R-groups B. Ionic interactions between positively
and negatively charged R-groups C. Disulfide bridges formed between cysteine residues ✓
CORRECT D. Hydrophobic interactions between non-polar R-groups
Correct Answer: C Rationale: Disulfide bridges are strong covalent bonds formed between the
sulfur atoms of cysteine residues, providing significant structural rigidity that helps proteins
maintain their tertiary shape under thermal stress. Hydrogen bonds and ionic interactions are
much weaker non-covalent forces that break easily at high temperatures, while hydrophobic
interactions are a result of the exclusion of water rather than direct bonding. In exam questions
involving extreme environmental conditions, looking for covalent cross-links like disulfide
bridges is often the key to identifying the stabilizing factor.
Question 2 of 50
A student investigates the effect of different organic solvents on the permeability of the beetroot
cell membrane, which contains the pigment betalain. The student places equal-sized discs of
beetroot in test tubes containing water at 70°C, ethanol at 20°C, and detergent solution at 20°C
for exactly 5 minutes. Assuming the ethanol and detergent disrupt the membrane structure
similarly to heat, which prediction regarding the absorbance of the solutions is correct?
A. The water at 70°C will show the lowest absorbance as high temperatures denature the pigment
B. The ethanol and detergent solutions will show high absorbance due to the disruption of the
phospholipid bilayer ✓ CORRECT C. All solutions will show equal absorbance because
temperature does not affect membrane integrity D. The detergent solution will show lower
absorbance than ethanol because detergents repair the membrane
,2
Correct Answer: B Rationale: Both ethanol and detergent disrupt the hydrophobic interactions
within the phospholipid bilayer, increasing membrane permeability and allowing the betalain
pigment to leak out into the surrounding solution, thereby increasing absorbance. High
temperature in water also increases permeability, so option A is incorrect as it suggests the
opposite effect, and option D is incorrect because detergents disrupt rather than repair
membranes. When analyzing membrane permeability experiments, remember that any factor
damaging the lipid bilayer—heat, solvents, or pH changes—will result in increased leakage of
cell contents.
Question 3 of 50
During a laboratory practical, students are asked to calculate the size of a mitochondrion
observed under a transmission electron microscope. The image of the mitochondrion measures
30 mm in length, and the scale bar on the micrograph indicates that 1 µm represents 5 mm on the
image. What is the actual length of the mitochondrion?
A. 6 µm ✓ CORRECT B. 15 µm C. 150 µm D. 0.6 µm
Correct Answer: A Rationale: The actual size is calculated by dividing the image size by the
magnification factor; since 5 mm represents 1 µm, the magnification is 5000x (5 mm / 0.001
mm), making the actual size 30 mm / 5000 = 0.006 mm or 6 µm. Option B results from
multiplying instead of dividing, a common error in magnification calculations, while option C
incorrectly assumes the scale is 1:1. Always ensure units are consistent (converting mm to µm or
vice versa) before performing magnification calculations.
Question 4 of 50
Scientists studying the properties of water in biological systems noted that water has a high
specific heat capacity. This property is vital for living organisms because it allows water to act as
a temperature buffer. Which molecular feature of water is directly responsible for this high
specific heat capacity?
A. The covalent bonds between hydrogen and oxygen within the molecule B. The small size of
the water molecule allowing rapid movement C. The non-polar nature of the oxygen atom D. The
hydrogen bonds between water molecules that require energy to break ✓ CORRECT
Correct Answer: D Rationale: A significant amount of heat energy is required to break the
numerous hydrogen bonds between water molecules before the molecules themselves can move
faster and increase in temperature. Covalent bonds within the molecule are not broken during
heating, and the small size of the molecule does not inherently buffer temperature changes.
Understanding the link between hydrogen bonding and thermal properties like specific heat
capacity and latent heat of vaporization is essential for explaining homeostasis.
Question 5 of 50
, 3
In an experiment to determine the water potential of potato tissue, cores were placed in a range
of sucrose solutions ranging from 0.0 mol dm⁻³ to 1.0 mol dm⁻³. After 24 hours, the percentage
change in mass was calculated. A student observed that the potato cores in the 0.4 mol dm⁻³
solution showed no change in mass. What does this indicate about the potato tissue and the
solution?
A. The potato tissue had a higher water potential than the solution B. The sucrose solution was
hypertonic, causing water to leave the cells C. The water potential of the potato tissue was equal
to that of the sucrose solution ✓ CORRECT D. The potato cells were fully plasmolysed at this
concentration
Correct Answer: C Rationale: No net change in mass occurs when there is no net movement of
water into or out of the cells, meaning the water potential of the tissue and the external solution
are in equilibrium. If the tissue had a higher water potential, mass would increase (hypotonic),
and if it were lower, mass would decrease (hypertonic), making options A and B incorrect for
this specific observation. Identifying the isotonic point is the fundamental goal of osmosis
practicals.
Question 6 of 50
A biochemist is analyzing two DNA samples. Sample X contains 28% adenine, while Sample Y
contains 22% cytosine. According to Chargaff's rules regarding base pairing in double-stranded
DNA, what are the percentages of thymine in Sample X and guanine in Sample Y, respectively?
A. 28% and 22% B. 22% and 28% ✓ CORRECT C. 28% and 28% D. 22% and 22%
Correct Answer: B Rationale: Chargaff's rules state that adenine always pairs with thymine and
guanine always pairs with cytosine in equal amounts due to complementary base pairing.
Therefore, if Sample X has 28% adenine, it must have 28% thymine, and if Sample Y has 22%
cytosine, it must have 22% guanine, resulting in the pair (28%, 22%). The question asks for
thymine in X (28%) and guanine in Y (22%), which matches option B. Students often confuse
the pairing partners, so memorizing A=T and C≡G is crucial for quick calculations.
Question 7 of 50
A research team is investigating the structure of a new enzyme. They treat the enzyme with a
strong acid, which breaks all the peptide bonds but leaves the individual amino acids intact.
Which level of protein structure is primarily destroyed by this acid hydrolysis?
A. Primary structure ✓ CORRECT B. Secondary structure C. Tertiary structure D. Quaternary
structure
Correct Answer: A Rationale: The primary structure of a protein is defined by the sequence of
amino acids linked by peptide bonds, so breaking these bonds destroys the primary structure.
While the acid also destroys higher structures (secondary, tertiary, quaternary), the fundamental
