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Energy and Respiration: Question 7

Syllabus 12.2

Structured A2 6 marks

Glycolysis can be divided into an earlier phosphorylation phase, in which ATP is used, and a later oxidation phase, in which ATP is generated by substrate-level phosphorylation.

(a) Describe what happens to a glucose molecule during the phosphorylation phase of glycolysis, naming the products formed, and state why two molecules of ATP are used at this stage. [3] (b) Describe what happens during the oxidation phase of glycolysis, and use your answer to explain how glycolysis has a net gain of only 2 ATP molecules per glucose molecule despite 4 molecules of ATP being generated by substrate-level phosphorylation. [3]

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Worked solution

Part (a): The phosphorylation phase

Glycolysis begins in the cytoplasm with the phosphorylation phase, in which the 6-carbon sugar glucose is made more reactive so that it can later be split.

  1. Glucose is phosphorylated, using a phosphate group transferred from a molecule of ATP, forming glucose 6-phosphate.
  2. Glucose 6-phosphate is phosphorylated again, using a second molecule of ATP, forming fructose 1,6-bisphosphate (still 6C).
  3. This unstable 6-carbon, doubly-phosphorylated sugar is then split into two molecules of triose phosphate (3C each).

Two molecules of ATP are used in this phase for two reasons: adding phosphate groups destabilises the sugar, priming it so that the enzyme-catalysed splitting reaction can occur, and phosphorylating glucose as soon as it enters the cell also prevents it from diffusing back out across the plasma membrane, effectively trapping it inside the cell for use in respiration.

Part (b): The oxidation phase and the net ATP yield

In the second phase of glycolysis, the oxidation phase, each of the two molecules of triose phosphate formed above is oxidised in a series of enzyme-catalysed steps and converted into pyruvate (3C).

  • As each triose phosphate is oxidised, hydrogen is removed and used to reduce a molecule of NAD to reduced NAD.
  • ATP is generated directly during this phase by substrate-level phosphorylation, in which a phosphate group is transferred directly from an intermediate compound onto ADP.

Because there are two molecules of triose phosphate formed from every glucose molecule, and each one generates ATP by substrate-level phosphorylation as it is oxidised to pyruvate, a total of 4 molecules of ATP are produced during the oxidation phase.

Combining both phases:

4−2=24 - 2 = 2

Four ATP molecules are produced during the oxidation phase, but 2 ATP molecules were used earlier during the phosphorylation phase, so the net gain across the whole of glycolysis is only 2 ATP molecules per glucose molecule. Two molecules of reduced NAD are also produced overall, one from the oxidation of each triose phosphate molecule.

Final answers

  • (a) Glucose is phosphorylated twice (using 2 ATP) to form fructose 1,6-bisphosphate, which is split into two triose phosphate molecules; the 2 ATP destabilise the sugar for splitting and trap it inside the cell.
  • (b) Each triose phosphate is oxidised to pyruvate, producing reduced NAD and generating ATP by substrate-level phosphorylation; 4 ATP are produced in total, but since 2 ATP were used earlier, the net gain is 2 ATP per glucose molecule.