The link reaction and Krebs cycleEdexcel A-Level Biology B: Flashcards

Exam questions on The link reaction and Krebs cycle

1. A researcher supplies isolated liver mitochondria with pyruvate in which all three carbon atoms are labelled with radioactive carbon-14. The mitochondria are in an aerated medium that contains NAD, ADP and phosphate. She collects the gas released from the medium and finds that it is radioactive.
   Explain why radioactive carbon dioxide is released from the mitochondria.2 marks
2. A student incubates isolated mitochondria with 3.0 mmol of pyruvate and excess oxygen, NAD, ADP and phosphate. All of the pyruvate is completely oxidised by the link reaction and the Krebs cycle.
   Calculate the amount of ATP formed directly in the Krebs cycle. Show your working.2 marks
3. Mitochondria are isolated and supplied with pyruvate, NAD, ADP, phosphate and oxygen. A researcher measures the rate of carbon dioxide release and the amount of NADH formed. She then repeats the experiment after adding a poison that blocks the Krebs cycle but has no effect on the link reaction. She records measurements during the first few minutes after the poison is added.
   Explain why carbon dioxide is still released after the poison is added, but at a lower rate.3 marks