1. 1
   An enzyme extracted from a thermophilic bacterium is tested at a range of temperatures using excess substrate. Reaction rate is low at 20 °C, rises steadily to a maximum at 70 °C, and then falls sharply at 80 °C, with almost no activity remaining at 90 °C. In a separate trial at 70 °C, adding a molecule that binds reversibly to a site on the enzyme distinct from the active site reduces the reaction rate, and this reduction cannot be reversed by adding more substrate.

   (a)

   What causes the rise in reaction rate between 20 °C and 70 °C?

   [1 mark]

   • AIncreased kinetic energy leads to more frequent successful collisions between substrate and active site
   • BMore of the substrate is broken down without enzyme catalysis
   • CThe enzyme's active site changes shape to fit a wider range of substrates
   • DThe enzyme denatures, exposing more active sites

   (b)

   What causes the sharp fall in reaction rate above 70 °C?

   [1 mark]

   • ASubstrate concentration becomes limiting
   • BThe enzyme denatures: heat disrupts the bonds holding its tertiary structure, changing the shape of the active site so substrate no longer binds
   • CThe reaction becomes exothermic
   • DThe enzyme's optimum pH is exceeded

   (c)

   Explain, using the description, why the molecule added in the second trial is a non-competitive inhibitor rather than a competitive inhibitor.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates respiration in yeast cell suspensions. In flask A, provided with glucose and free access to oxygen, carbon dioxide is produced at 0.8 cm³ per minute and ethanol is not detected. In flask B, provided with the same glucose solution but sealed to exclude oxygen, carbon dioxide is produced at 0.3 cm³ per minute and ethanol accumulates in the flask.

   (a)

   Why is the rate of carbon dioxide production more than twice as high in flask A as in flask B?

   [1 mark]

   • AAnaerobic respiration in flask A produces more carbon dioxide per glucose molecule
   • BOxygen inhibits glycolysis in flask B
   • CAerobic respiration in flask A yields far more ATP per glucose molecule via the Krebs cycle and electron transport chain, so more glucose is processed per minute
   • DEthanol production in flask B consumes carbon dioxide

   (b)

   Which molecule accepts electrons at the end of the electron transport chain in flask A's aerobic yeast cells?

   [1 mark]

   • AGlucose
   • BPyruvate
   • CNAD
   • DOxygen

   (c)

   Explain why ethanol accumulates in flask B but not in flask A.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Photosynthetic pigments extracted from spinach leaves are separated by paper chromatography using a non-polar solvent. Four bands are seen: a blue-green band that travels 8.4 cm, a yellow-green band that travels 8.0 cm, a yellow band that travels 10.5 cm, and an orange band that travels 11.0 cm, with the solvent front measured 12.0 cm from the origin. In a separate experiment, illuminated chloroplasts are given carbon dioxide but light is then switched off; the concentration of glycerate 3-phosphate (GP) rises and the concentration of ribulose bisphosphate (RuBP) falls over the next few minutes.

   (a)

   Calculate the Rf value of the orange band, and identify which type of pigment it most likely represents.

   [3 marks]

   (b)

   Explain why GP concentration rises and RuBP concentration falls in the few minutes after light is switched off, and explain why this pattern cannot continue indefinitely.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A person accidentally touches a hot pan. Free nerve endings in the skin of the hand respond to the high temperature, generating action potentials that travel along a sensory neuron to the spinal cord. In the grey matter of the spinal cord, this sensory neuron synapses with an interneuron, which in turn synapses with a motor neuron; the motor neuron carries impulses to the biceps muscle of the arm, causing it to contract and withdraw the hand within a fraction of a second, before the sensation of pain is consciously perceived in the brain.

   (a)

   Explain, in terms of ion movement across the neuron membrane, how an action potential is generated and propagated from the free nerve ending along the sensory neuron's axon to the spinal cord.

   [6 marks]

   (b)

   Explain why the hand is withdrawn from the hot pan by a reflex arc before pain is consciously perceived, and outline the roles of the interneuron and motor neuron in producing this response.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A person under sudden stress releases epinephrine (adrenaline) from the adrenal glands into the blood. In liver cells, epinephrine binds to a transmembrane receptor on the plasma membrane, activating a G protein, which in turn activates an enzyme that increases the concentration of cyclic AMP (cAMP) inside the cell. This rise in cAMP triggers a cascade of reactions that rapidly increases the breakdown of glycogen to glucose, raising blood glucose concentration within seconds.

   (a)

   Why does epinephrine bind to a receptor on the outside of the liver cell's plasma membrane, rather than passing into the cytoplasm to bind an intracellular receptor?

   [1 mark]

   • AEpinephrine is an amine, a hydrophilic signalling molecule that cannot readily cross the hydrophobic core of the membrane
   • BEpinephrine is a steroid hormone that always binds outside the cell
   • CLiver cells have no cytoplasm for epinephrine to enter
   • DEpinephrine is too large to fit through nuclear pores

   (b)

   What is the role of cAMP in this signalling pathway?

   [1 mark]

   • AIt is the ligand that binds directly to the transmembrane receptor
   • BIt acts as a second messenger, relaying the signal from the activated G protein to trigger the cellular response
   • CIt is the antigen recognised by the receptor
   • DIt directly activates transcription of new genes

   (c)

   Explain why the rise in blood glucose concentration occurs within seconds of epinephrine release, rather than over minutes to hours as with some hormones that act via intracellular receptors.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A person is infected with a novel respiratory virus for the first time. Over the first few days, phagocytes engulf and digest virus particles at the site of infection, but the infection continues to spread. By day 10, specific antibodies against the virus are detected in the blood, and by day 14 the infection has been cleared. Three months later, the same person is re-exposed to the same virus; this time, specific antibodies appear within two days and the person shows no symptoms of infection.

   (a)

   Which type of immune response is responsible for the phagocyte activity described in the first few days?

   [1 mark]

   • AAdaptive immune response
   • BAntibody-mediated response
   • CInnate immune response
   • DVaccination response

   (b)

   Why does the antibody response on re-exposure appear within two days rather than the ten days taken during the first infection?

   [1 mark]

   • AThe virus mutates to become less virulent
   • BPhagocytes become more active after a first infection
   • CAntibodies from the first infection are still circulating in the blood after three months
   • DMemory B-lymphocytes from the first infection, capable of making the specific antibody, survive long-term and respond rapidly on re-exposure

   (c)

   Explain the roles of helper T-lymphocytes and B-lymphocytes in producing the antibody response described during the first infection.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Ecologists estimate the population size of a motile beetle species in a meadow using capture-mark-release-recapture. On the first day, 80 beetles are caught, marked and released. Two days later, 100 beetles are caught, of which 20 are found to be marked. In the same meadow, ecologists measure primary production by the grass at 6000 kJ m⁻² yr⁻¹ and estimate that grasshoppers feeding on the grass have a secondary production of 480 kJ m⁻² yr⁻¹.

   (a)

   Using the Lincoln index, calculate the estimated population size of the beetle in the meadow, showing your working.

   [3 marks]

   (b)

   Calculate the percentage of the grass's primary production that is converted to secondary production by the grasshoppers, and explain why this percentage is much less than 100%.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A sealed, illuminated aquarium contains aquatic plants and fish, with no gas exchange with the outside air. Over a 24-hour period, the concentration of dissolved oxygen in the water rises during the day and falls at night, while dissolved carbon dioxide shows the opposite pattern, and both gases remain within a stable range from one day to the next.

   (a)

   Explain, in terms of gas production and consumption, why dissolved oxygen rises during the day and falls at night in the sealed aquarium.

   [6 marks]

   (b)

   Explain why the concentrations of dissolved oxygen and carbon dioxide remain within a stable range from one day to the next in this sealed aquarium, referring to the relationship between respiration and photosynthesis.

   [6 marks]

   Total for question 8: 12 marks

End of questions