Anaerobic respiration and respiratory substratesAQA A-Level Biology: Subtopic test

1. 1
   A brewery grows yeast in a sealed fermentation vessel containing glucose solution. Once the dissolved oxygen has been used up, the yeast respires only anaerobically: the liquid slowly gains ethanol and carbon dioxide bubbles out through an airlock.

   (a)

   Which pair of compounds is produced when the yeast converts pyruvate during anaerobic respiration?

   [1 mark]

   • ALactate and carbon dioxide
   • BEthanol and carbon dioxide
   • CEthanol and water
   • DLactate and water

   (b)

   What is the main reason why the yeast converts ethanal to ethanol?

   [1 mark]

   • ATo make extra ATP by substrate-level phosphorylation
   • BTo reduce NAD so that it can enter the Krebs cycle
   • CTo supply carbon dioxide for the airlock
   • DTo oxidise reduced NAD so that NAD is available for glycolysis

   (c)

   Explain why the yeast would run out of ATP if it could not convert ethanal to ethanol.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During a 400 m sprint, the leg muscle cells of an athlete use oxygen faster than the blood can deliver it. The concentration of lactate in her blood rises sharply during the race.

   (a)

   Which compound is reduced to form lactate in the muscle cells?

   [1 mark]

   • APyruvate
   • BEthanal
   • CGlucose
   • DAcetyl coenzyme A

   (b)

   What is the net yield of ATP per glucose molecule when the muscle cells respire only anaerobically?

   [1 mark]

   • A0
   • B4
   • C2
   • DAbout 32

   (c)

   Explain how the formation of lactate allows the muscle cells to go on making some ATP.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the effect of temperature on the rate of anaerobic respiration of a yeast culture. For each temperature she mixes 10 cm³ of yeast suspension with 10 cm³ of glucose solution, covers it with a layer of oil to exclude oxygen, and collects the carbon dioxide released in an inverted measuring cylinder over water for 5.0 minutes. The tubes are kept in water baths at 20, 30, 40 and 50 °C. At 40 °C the volume collected was greatest; at 50 °C it was less than at 40 °C.

   (a)

   At 30 °C the student collected 7.5 cm³ of carbon dioxide in 5.0 minutes from yeast of dry mass 0.50 g. Calculate the rate of carbon dioxide production in cm³ min⁻¹ g⁻¹ of yeast.

   [3 marks]

   (b)

   Explain why less carbon dioxide was collected at 50 °C than at 40 °C, and describe two variables the student should control to make the comparison valid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A runner takes part in a 100 km ultra-marathon. Over many hours her muscle cells use their stored carbohydrate first, then increasingly respire lipids, and some protein is broken down as her stores run low. Near the finish she sprints, and her muscle cells become short of oxygen.

   (a)

   Explain how lipids and amino acids can act as respiratory substrates during the race, and why this allows the runner to keep making ATP when her carbohydrate stores are low.

   [6 marks]

   (b)

   Explain how her muscle cells continue to make ATP during the final sprint when oxygen is short, and suggest why this cannot be relied on for the whole race.

   [6 marks]

   Total for question 4: 12 marks

End of questions