AQA A-Level Biology: Energy Transfers in and Between Organisms
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 light-independent reaction (Calvin cycle) takes place in the stroma of the chloroplast and uses products of the light-dependent reaction to synthesise organic molecules.
Describe and explain the events of the Calvin cycle, including the fixation of carbon dioxide, the reduction of glycerate-3-phosphate, the use made of the triose phosphate produced, and the regeneration of ribulose bisphosphate.
A student used a respirometer to measure the rate of oxygen uptake by 2.0 g of germinating pea seeds at 20 °C. The seeds were sealed in a tube containing a chemical that absorbs carbon dioxide, connected to a manometer containing coloured fluid. As the seeds respired, the fluid moved towards the seeds. For this manometer, every 1 mm of fluid movement corresponds to 0.01 cm³ of oxygen taken up.
The student recorded the distance moved by the fluid over 10 minutes:
| Time / min | Distance moved by fluid / mm |
|---|---|
| 0 | 0 |
| 2 | 10 |
| 4 | 20 |
| 6 | 30 |
| 8 | 40 |
| 10 | 50 |
(a) Calculate the rate of oxygen uptake per gram of seeds, in cm³ g⁻¹ min⁻¹. Show your working. (4 marks)
(b) Explain the purpose of the carbon-dioxide-absorbing chemical and why this causes the manometer fluid to move towards the seeds. (2 marks)
A researcher measured the rate of photosynthesis of a pondweed at increasing light intensities, at two temperatures (20 °C and 30 °C). Carbon dioxide concentration was kept constant and in excess throughout. The results are shown below.
| Light intensity / arbitrary units | Rate at 20 °C / arbitrary units | Rate at 30 °C / arbitrary units |
|---|---|---|
| 10 | 20 | 20 |
| 20 | 40 | 40 |
| 30 | 60 | 60 |
| 40 | 70 | 80 |
| 50 | 70 | 100 |
| 60 | 70 | 100 |
(a) State the limiting factor of photosynthesis in the steep initial region of the curves, and in the region where the rate at 30 °C has plateaued. (2 marks)
(b) Explain why the rate at 30 °C eventually reaches a plateau, and why the plateau at 30 °C is higher than the plateau at 20 °C. (3 marks)
A toxic substance called compound X makes the inner mitochondrial membrane freely permeable to hydrogen ions (protons). When isolated mitochondria are treated with compound X, the synthesis of ATP by oxidative phosphorylation stops, but the rate at which the mitochondria consume oxygen actually increases.
Using your knowledge of chemiosmosis, suggest and explain:
(a) why compound X stops ATP synthesis; (3 marks) (b) why oxygen consumption increases rather than stopping. (2 marks)
When a human muscle cell is well supplied with oxygen it respires aerobically, but during very vigorous exercise the same cell may respire anaerobically.
Compare and explain the approximate yield of ATP from the complete aerobic respiration of one molecule of glucose with the yield from anaerobic respiration of one molecule of glucose, and explain why anaerobic respiration releases so much less usable energy.
The electron transport chain is located on the inner mitochondrial membrane and plays a central role in oxidative phosphorylation.
Describe the role of the electron transport chain in setting up the conditions needed for ATP synthesis.