OCR A-Level Biology: Communication, Homeostasis and Excretion
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
During a long training run on a hot day, a marathon runner loses about 2 dm³ of water as sweat. This lowers the water potential of their blood plasma. By the time they finish, their kidneys are producing a small volume of very concentrated urine.
Describe and explain how negative feedback involving the hormone ADH restores the water potential of the blood towards its set point after the runner has been sweating. In your answer you should refer to the receptors, the coordinator, the effector, and the molecular action of ADH on the collecting duct.
(6 marks)
A physiologist sampled fluid from three points in a healthy person's nephron system and measured the concentration of three substances in the blood plasma, in the glomerular filtrate (in the Bowman's capsule) and in the urine. The results are shown below.
| Substance | Plasma / mg cm⁻³ | Glomerular filtrate / mg cm⁻³ | Urine / mg cm⁻³ |
|---|---|---|---|
| Glucose | 1.00 | 1.00 | 0.00 |
| Urea | 0.30 | 0.30 | 21.0 |
| Protein | 75.0 | 0.00 | 0.00 |
(a) Calculate how many times more concentrated the urea is in the urine than in the glomerular filtrate. Show your working. (2 marks)
(b) Using the data for glucose and for protein, explain how the values in the table provide evidence for ultrafiltration at the glomerulus and selective reabsorption in the proximal convoluted tubule. (4 marks)
Four healthy adult volunteers were placed in different hydration states. For each volunteer, the concentration of ADH in the blood and the solute concentration of the urine they produced were measured. The results are shown below.
| Volunteer | Hydration state | Blood ADH / pg cm⁻³ | Urine solute concentration / mOsm kg⁻¹ |
|---|---|---|---|
| W | Drank 1.5 dm³ of water | 0.8 | 90 |
| X | Resting, normal hydration | 2.5 | 600 |
| Y | After a 2-hour hike, no drink | 4.6 | 980 |
| Z | After a 2-hour hike, sweating heavily | 6.1 | 1200 |
(a) Describe the relationship between blood ADH concentration and urine solute concentration shown by these data. (1 mark)
(b) Explain, in terms of osmoregulation, why volunteer Z produced urine with a much higher solute concentration than volunteer W. (4 marks)
A patient visits their doctor feeling persistently thirsty and tired. The doctor uses a reagent dip-stick to test a fresh urine sample. The colour change of each pad is interpreted against a chart, giving the results below. A urine sample from a healthy person is shown for comparison.
| Dip-stick pad | Healthy person | This patient |
|---|---|---|
| Glucose | Negative | Strongly positive |
| Protein | Negative | Trace |
| Ketones | Negative | Positive |
(a) Using your knowledge of selective reabsorption, explain why glucose is normally absent from urine but is strongly positive in this patient. (3 marks)
(b) Suggest why the patient also shows a trace of protein in the urine, and state what this might indicate about their kidneys. (2 marks)
A patient with kidney failure is treated by haemodialysis. Their blood is passed through a dialyser, where it flows on one side of a partially permeable membrane while dialysis fluid (dialysate) flows on the other side. The dialysate is freshly prepared so that it contains glucose and mineral ions at their normal blood concentrations but no urea.
(a) Explain why the dialysate contains glucose and mineral ions at normal blood concentrations, rather than none at all. (2 marks)
(b) The blood and the dialysate are made to flow in opposite directions through the dialyser. Suggest why this improves the removal of urea from the blood. (2 marks)
Mammals cannot store excess amino acids, so any taken in beyond the body's needs are broken down in the liver. The nitrogen-containing part is eventually excreted as urea.
Describe how the liver deals with excess amino acids, naming the process that removes the amino group, the toxic product formed, and the cycle that converts this product into urea. (3 marks)