OCR A-Level Biology: Exchange and Transport
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.
A bony fish ventilates its gills by drawing water in through the mouth and passing it out over the gill filaments and lamellae. Within each lamella, blood flows in the opposite direction to the water flowing over it.
Describe and explain how the structure of a fish gill, together with countercurrent flow, is adapted for efficient gas exchange. In your answer you should refer to the features that affect surface area, diffusion distance and the concentration gradient.
(6 marks)
A student modelled organisms of different sizes using solid cubes of agar jelly. For each cube they recorded the side length and calculated the surface area and volume. Two of the cubes are shown in the table.
| Cube | Side length / cm | Surface area / cm² | Volume / cm³ |
|---|---|---|---|
| P | 1.0 | 6.0 | 1.0 |
| Q | 3.0 | 54.0 | 27.0 |
(a) Calculate the surface area to volume ratio of cube P and of cube Q, each expressed in the form number : 1. Show your working. (2 marks)
(b) The student stated that cube Q is a useful model for why large, active organisms need a specialised gas exchange system. Using your answers to part (a), explain what cube Q models about a large organism, and why such an organism cannot rely on diffusion across its body surface alone. (4 marks)
The table gives the percentage saturation of two haemoglobins at different partial pressures of oxygen (pO₂). Haemoglobin A is the normal adult form. Haemoglobin F is the foetal form found in an unborn mammal.
| pO₂ / kPa | Saturation of haemoglobin A / % | Saturation of haemoglobin F / % |
|---|---|---|
| 1.0 | 8 | 20 |
| 2.0 | 22 | 42 |
| 4.0 | 55 | 75 |
| 6.0 | 78 | 90 |
| 8.0 | 90 | 96 |
| 12.0 | 97 | 99 |
(a) Using the data, compare the affinity of haemoglobin F for oxygen with that of haemoglobin A. (2 marks)
(b) In the placenta, the maternal blood reaching the exchange surface has a pO₂ of about 4.0 kPa. Explain how the difference shown in the table allows the foetus to obtain oxygen from its mother's blood. (3 marks)
The table shows the pressure in the left atrium, left ventricle and aorta of a mammal at five points in time during one cardiac cycle.
| Time / s | Left atrial pressure / kPa | Left ventricular pressure / kPa | Aortic pressure / kPa |
|---|---|---|---|
| 0.10 | 1.2 | 0.6 | 10.6 |
| 0.20 | 0.5 | 8.0 | 10.4 |
| 0.30 | 0.4 | 15.8 | 14.5 |
| 0.45 | 0.7 | 6.0 | 12.0 |
| 0.55 | 1.0 | 0.8 | 11.0 |
The atrioventricular (AV) valve and the semilunar (aortic) valve open and close because of pressure differences.
(a) Identify the time interval during which the semilunar (aortic) valve is open, and explain how the data show this. (3 marks)
(b) Using the data, explain what is happening to the atrioventricular valve between 0.20 s and 0.30 s. (2 marks)
A leafy shoot was set up so that the rate at which it took up water could be measured. The shoot was first left in still, humid air. A gentle fan was then switched on to blow moving, drier air across the leaves. The rate of water uptake by the shoot increased.
Water travels up the xylem of the shoot to the leaves, where it evaporates and is lost through the stomata.
Explain why blowing moving, drier air across the leaves increased the rate of water uptake by the shoot. (4 marks)
During ventricular systole the pressure inside the ventricles rises sharply. The atrioventricular (AV) valves are prevented from turning inside out (everting) into the atria by tough cords attached to their cusps.
(a) Name the cords that attach to the cusps of the AV valves and prevent them from everting. (1 mark)
(b) Explain why it is important that the AV valves do not evert into the atria during ventricular systole. (2 marks)