Link reaction and Krebs cycleEdexcel International A Level Biology: Subtopic test

1. 1
   In a muscle cell, glycolysis in the cytoplasm produces pyruvate. This pyruvate is transported into the mitochondria, where it is converted to a two-carbon compound that can enter the Krebs cycle.

   (a)

   Where in the cell do the link reaction and the Krebs cycle take place?

   [1 mark]

   • AIn the matrix of the mitochondrion
   • BOn the inner membrane of the mitochondrion
   • CIn the space between the two mitochondrial membranes
   • DIn the cytoplasm

   (b)

   Which compound is formed in the link reaction and carries the two-carbon group into the Krebs cycle?

   [1 mark]

   • APyruvate
   • BLactate
   • CAcetyl coenzyme A
   • DGlucose phosphate

   (c)

   Explain why the conversion of pyruvate to the two-carbon compound is described as oxidative decarboxylation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher studies the Krebs cycle in isolated liver mitochondria supplied with the product of the link reaction. For one molecule of glucose, the Krebs cycle turns twice, once for each pyruvate formed.

   (a)

   How many molecules of reduced NAD are made by the Krebs cycle itself (not the link reaction) from one molecule of glucose?

   [1 mark]

   • A2
   • B6
   • C8
   • D10

   (b)

   By which process does the Krebs cycle make ATP?

   [1 mark]

   • AOxidative phosphorylation
   • BChemiosmosis through ATP synthase
   • CThe electron transport chain
   • DSubstrate-level phosphorylation

   (c)

   The Krebs cycle does not use oxygen directly, but it stops when oxygen is absent. Explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A suspension of isolated mitochondria is supplied with 0.0020 mol of pyruvate and plenty of oxygen. Assume that all of the pyruvate is completely oxidised by the link reaction and the Krebs cycle, and that one mole of any gas occupies 24 000 cm³ at room temperature and pressure.

   (a)

   Calculate the volume of carbon dioxide released from the 0.0020 mol of pyruvate.

   [3 marks]

   (b)

   Calculate the amounts, in mol, of reduced NAD, reduced FAD and ATP produced from the 0.0020 mol of pyruvate. State how these amounts, produced by the link reaction and the Krebs cycle, would change if 0.0020 mol of glucose were supplied instead, giving a reason.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the complete oxidation of one molecule of glucose, four molecules of ATP are made directly by substrate-level phosphorylation (two in glycolysis and two in the Krebs cycle). Aerobic respiration in a typical cell makes about 30 molecules of ATP from each glucose overall, and most of the rest is made by oxidative phosphorylation using the reduced coenzymes.

   (a)

   Describe how the link reaction and the Krebs cycle contribute to the complete oxidation of glucose.

   [6 marks]

   (b)

   The Krebs cycle makes only two ATP per glucose directly. Evaluate the importance of the Krebs cycle for the production of ATP in aerobic respiration.

   [6 marks]

   Total for question 4: 12 marks

End of questions