1. 1
   A student wearing a heart-rate monitor jogs steadily for 20 minutes. Throughout the run, her breathing rate and depth increase noticeably, and her muscle cells continue to release energy from glucose using the oxygen delivered by her blood.

   (a)

   Which equation correctly represents aerobic respiration?

   [1 mark]

   • Aglucose + oxygen → carbon dioxide + water
   • Bglucose → lactic acid
   • Ccarbon dioxide + water → glucose + oxygen
   • Dglucose + carbon dioxide → oxygen + water

   (b)

   Which statement best explains why respiration is described as an exothermic reaction?

   [1 mark]

   • AIt absorbs energy from the surroundings to build glucose
   • BIt releases energy that is used by the cell and lost as heat to the surroundings
   • CIt only takes place in plant cells during the day
   • DIt requires light energy in order to proceed

   (c)

   Using the scenario, explain why the student's muscle cells need a continuous supply of energy from respiration during her run, and state where in the cell most of this aerobic respiration takes place.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A 100 m sprinter accelerates explosively out of the blocks. During the race, her leg muscles work so hard that oxygen cannot be delivered fast enough by the blood to meet demand, so her muscle cells switch to producing energy without oxygen. After the race, she breathes heavily for several minutes before her breathing returns to normal.

   (a)

   What substance builds up in the sprinter's muscle cells as a result of anaerobic respiration?

   [1 mark]

   • ACarbon dioxide and water only
   • BEthanol and carbon dioxide
   • CLactic acid
   • DOxygen and glucose

   (b)

   Compared with aerobic respiration, anaerobic respiration in the sprinter's muscle cells releases:

   [1 mark]

   • ANo energy at all
   • BExactly the same amount of energy per glucose molecule
   • CMore energy per glucose molecule
   • DMuch less energy per glucose molecule

   (c)

   Explain why the sprinter continues to breathe heavily for several minutes after finishing the race.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the rate of respiration in germinating pea seeds. She places 20 germinating peas in a boiling tube with a layer of soda lime, which absorbs carbon dioxide, below a gauze platform so the peas do not touch it. The tube is sealed with a bung holding a capillary tube connected to a scale, with a drop of coloured liquid inside the capillary tube. As the peas respire, the drop moves along the scale over a fixed time. She repeats the experiment using 20 dried, non-germinating peas of the same total mass as a comparison, keeping both tubes in a water bath at the same constant temperature throughout.

   (a)

   Explain how the movement of the coloured liquid drop indicates the rate of respiration of the peas, and describe the role of the soda lime in this experiment.

   [3 marks]

   (b)

   The student found that the drop moved much further per minute in the tube with germinating peas than in the tube with non-germinating peas. Explain this difference, and describe two variables she should control to make the comparison valid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A sports scientist compares two athletes: a marathon runner who sustains a steady pace for over three hours, and a weightlifter who performs single explosive lifts lasting only a few seconds. Blood and muscle samples show that the marathon runner's muscle cells rely almost entirely on aerobic respiration throughout the race, while the weightlifter's muscle cells produce a burst of lactic acid during each lift.

   (a)

   Compare the process of aerobic respiration in the marathon runner's muscle cells with the process of anaerobic respiration in the weightlifter's muscle cells, referring to the reactants, products and relative amount of energy released in each case.

   [6 marks]

   (b)

   Evaluate why the marathon runner is unlikely to build up significant lactic acid during the race, whereas the weightlifter builds up lactic acid rapidly during each lift, and explain how each athlete's body deals with the demands of their type of exercise.

   [6 marks]

   Total for question 4: 12 marks

End of questions