Aerobic respiration: glycolysis and the Krebs cycleEdexcel A-Level Biology A: Subtopic test

1. 1
   A student is calculating the energy yield of the first stage of respiration in a yeast cell. This stage takes place in the cytoplasm. It begins with the phosphorylation of a hexose sugar and ends with the formation of pyruvate.

   (a)

   What is the net yield of ATP from glycolysis per molecule of glucose?

   [1 mark]

   • A2
   • B4
   • C0
   • D36

   (b)

   What is the main purpose of phosphorylating the hexose at the start of glycolysis?

   [1 mark]

   • AIt oxidises the hexose to pyruvate
   • BIt splits the hexose directly into two triose molecules
   • CIt makes the hexose more reactive so that it can be split into two triose phosphates
   • DIt produces reduced NAD

   (c)

   Explain why the conversion of pyruvate to lactate allows glycolysis to continue when no oxygen is available.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A biochemist incubates isolated, intact mitochondria in a solution containing pyruvate, coenzyme A, NAD, FAD and ADP, with plenty of oxygen. She collects the gas released and analyses the compounds that accumulate during the reactions that take place in the mitochondrial matrix.

   (a)

   Which row correctly shows the products of the link reaction for each molecule of pyruvate?

   [1 mark]

   • AAcetyl CoA, ATP and reduced FAD
   • BCitrate, carbon dioxide and reduced NAD
   • CLactate and NAD
   • DAcetate (as acetyl CoA), carbon dioxide and reduced NAD

   (b)

   Which row shows the products of one turn of the Krebs cycle?

   [1 mark]

   • A2 ATP, 3 reduced NAD, 1 reduced FAD, 2 CO₂
   • B1 ATP, 3 reduced NAD, 1 reduced FAD, 2 CO₂
   • C1 ATP, 2 reduced NAD, 2 reduced FAD, 3 CO₂
   • D1 ATP, 3 reduced NAD, 1 reduced FAD, 1 CO₂

   (c)

   Calculate the total number of molecules of reduced NAD and of carbon dioxide produced in the link reaction and Krebs cycle from one molecule of glucose. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glycolysis takes place in the cytoplasm of a cell, whereas the link reaction and the Krebs cycle take place in the mitochondria. A mitochondrion has a double membrane, and its inner membrane is folded into cristae that enclose a fluid matrix. A student claims that it makes no difference where in the cell these stages happen.

   (a)

   Explain why the link reaction and the Krebs cycle take place in the mitochondria and not in the cytoplasm.

   [3 marks]

   (b)

   Describe the events of one turn of the Krebs cycle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A 400 m runner's blood lactate concentration rises sharply during the race. After the race, while she rests, her breathing stays deep and fast for several minutes and her blood lactate concentration falls back to its resting value over the next half hour.

   (a)

   Explain why the runner's blood lactate concentration rises during the race, and what happens to the lactate afterwards.

   [6 marks]

   (b)

   Explain how the link reaction and the Krebs cycle result in the complete oxidation of glucose to carbon dioxide, and why oxygen is needed even though neither stage uses it directly.

   [6 marks]

   Total for question 4: 12 marks

End of questions