Biology

1. 1.

   Explain negative feedback using a rise in blood glucose after a meal.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The response opposes the original deviation. As the regulated variable returns towards its range, the initiating signal weakens, preventing an indefinitely escalating response.

   Marking points

   • Pancreatic beta cells respond to increased glucose by releasing insulin.
   • Insulin promotes uptake in responsive tissues and storage such as glycogen synthesis.
   • Falling glucose reduces the stimulus for insulin release.

   Examiner tip: Name the regulated variable and show how the response reduces its deviation.

2. 2.

   Explain why an action potential is described as all-or-nothing rather than becoming larger for every stronger stimulus.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Below threshold, local depolarisation need not propagate. Above threshold, channel activation follows the same regenerative sequence, so stimulus intensity is encoded by firing patterns rather than proportional spike height.

   Marking points

   • A threshold depolarisation is needed to trigger it.
   • Voltage-gated channel feedback generates a stereotyped action potential.
   • Stronger stimuli can increase frequency/recruitment, not the amplitude of each action potential in the same axon.

   Examiner tip: Do not confuse a graded receptor potential with an axonal action potential.

3. 3.

   Explain how increased ADH affects collecting-duct permeability and urine concentration during dehydration.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The medullary gradient provides the driving force, and aquaporins alter access to it. ADH changes water permeability rather than directly pumping water or adding solute to urine.

   Marking points

   • ADH promotes aquaporin insertion into collecting-duct membranes.
   • More water is reabsorbed osmotically into the surrounding tissue/blood.
   • Urine volume falls and urine becomes more concentrated.

   Examiner tip: Water reabsorption is osmosis, not active transport of water.

4. 4.

   Explain how myelination increases conduction speed while allowing action potentials to be regenerated.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The signal does not physically teleport. Passive electrical spread bridges insulated regions, while active channel events at nodes restore a full-amplitude signal.

   Marking points

   • Myelin insulates internodal membrane and reduces ion leakage.
   • Local current spreads rapidly to the next node.
   • Voltage-gated channels at nodes regenerate action potentials, producing saltatory conduction.

   Examiner tip: Regeneration happens at nodes of Ranvier, not uniformly under the myelin.

5. 5.

   Glucose is normally absent from urine, but appears when plasma glucose becomes very high. Explain using filtration and selective reabsorption, and why this observation alone does not locate a kidney lesion.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Urinary glucose can reflect overload rather than failed filtration selectivity. Measure plasma glucose and assess tubular function before attributing the finding to a damaged kidney structure.

   Marking points

   • Glucose is small enough to enter the glomerular filtrate.
   • Proximal-tubule transport normally reabsorbs it.
   • High filtered load can exceed carrier capacity, leaving glucose in urine.
   • This may arise from high plasma glucose without structural kidney damage; additional measurements are required.

   Examiner tip: Normal absence from urine does not mean glucose was never filtered.

6. 6.

   An inhibitor blocks acetylcholinesterase at a neuromuscular junction. Predict an initial effect and a reason prolonged exposure need not produce indefinitely stronger muscle contraction.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Stopping transmitter breakdown initially extends signalling, but excitable membranes need repolarisation to reset. Persistent signalling can therefore become failure of repeated activation rather than sustained useful contraction.

   Marking points

   • Acetylcholine is removed more slowly from the cleft.
   • Receptors are stimulated for longer, initially prolonging depolarisation.
   • Persistent depolarisation can prevent sodium-channel recovery and further action potentials.
   • Desensitisation/depolarisation block can impair contraction rather than increase it indefinitely.

   Examiner tip: Distinguish an initial response from effects of persistent receptor activation.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.