Edexcel 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.
The mammalian lung is a highly efficient gas exchange organ. Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses in the opposite direction.
Describe and explain how the structure of the mammalian gas exchange system is adapted for efficient exchange of gases between the air and the blood. In your answer you should refer to the alveoli, the capillaries and the way air and blood are kept moving.
(6 marks)
A student modelled cells of different sizes using agar cubes of three side lengths. For each cube she recorded the surface area, the volume and the surface area to volume ratio. Two values have been left blank.
| Cube side length / mm | Surface area / mm² | Volume / mm³ | Surface area : volume ratio |
|---|---|---|---|
| 1 | 6 | 1 | 6.0 : 1 |
| 2 | 24 | 8 | 3.0 : 1 |
| 4 | 96 | 64 | to calculate |
| 6 | 216 | 216 | to calculate |
(a) Calculate the surface area to volume ratio for the 4 mm cube and for the 6 mm cube. Show your working. (2 marks)
(b) Using the completed table, explain what your results show about the relationship between the size of an organism and the way it exchanges substances with its environment, and why large multicellular animals need a specialised exchange surface and a transport system. (4 marks)
The table shows the percentage saturation of haemoglobin with oxygen at different partial pressures of oxygen for human haemoglobin. Values are given at a normal partial pressure of carbon dioxide and, in the final column, at a higher partial pressure of carbon dioxide.
| Partial pressure of oxygen / kPa | Percentage saturation at normal pCO₂ | Percentage saturation at higher pCO₂ |
|---|---|---|
| 1 | 8 | 4 |
| 2 | 20 | 11 |
| 4 | 55 | 38 |
| 6 | 80 | 66 |
| 8 | 90 | 82 |
| 10 | 96 | 92 |
| 12 | 98 | 96 |
In actively respiring muscle tissue the partial pressure of oxygen is about 4 kPa and the partial pressure of carbon dioxide is raised.
(a) Using the normal pCO₂ column, calculate the percentage of the haemoglobin's oxygen that is unloaded as blood moves from the lungs (partial pressure of oxygen 12 kPa) to resting tissue at a partial pressure of oxygen of 6 kPa. Show your working. (2 marks)
(b) Using the data, explain the effect of the raised partial pressure of carbon dioxide on oxygen unloading in actively respiring muscle. (3 marks)
A species of small burrowing rodent lives at high altitude, where the partial pressure of oxygen in the air is much lower than at sea level, and it spends long periods in poorly ventilated burrows where the air also has a raised carbon dioxide concentration.
Researchers found that this rodent has a form of haemoglobin with a higher affinity for oxygen than the haemoglobin of a closely related lowland species. They also found that its oxygen dissociation curve is positioned further to the left.
Suggest and explain how having haemoglobin with a higher affinity for oxygen helps this rodent to survive in its low-oxygen environment. (5 marks)
A grower measured the rate of water uptake of a leafy plant shoot using a potometer. When she switched on a fan to blow air across the leaves, the rate of water uptake increased sharply. When she instead enclosed the shoot in a clear plastic bag (still well lit), the rate of water uptake fell almost to zero, even though the stomata remained open.
Explain why moving air increased the rate of water uptake but enclosing the shoot in a bag decreased it. (4 marks)
The heartbeat is initiated and coordinated by specialised tissue in the heart, without the need for nervous stimulation to start each beat (the heart is myogenic).
Describe the role of the sinoatrial node (SAN) in initiating and coordinating a single heartbeat, up to the point where the ventricles contract. (3 marks)