Energy and Respiration: Question 3

Worked solution

Part (a): Glycolysis

Glycolysis takes place entirely in the cytoplasm, so it does not require oxygen or the mitochondrion to proceed. It begins with the phosphorylation of glucose (6C), using ATP, which eventually forms fructose 1,6-bisphosphate (6C). This 6-carbon sugar is then split into two 3-carbon triose phosphate molecules, which are further oxidised in several steps to form two molecules of pyruvate (3C).

Overall, from one molecule of glucose, glycolysis produces:

• Two molecules of pyruvate.
• A net gain of 2 ATP molecules. ATP is invested early in the pathway to phosphorylate the sugar, but a larger amount of ATP is generated later by substrate-level phosphorylation as triose phosphate is oxidised to pyruvate, giving an overall net gain of 2 ATP per glucose molecule.
• Two molecules of reduced NAD, formed as triose phosphate is oxidised.

Importantly, no carbon dioxide is released during glycolysis, this only begins at the next stage, the link reaction.

Part (b): The link reaction

Before the link reaction can occur, pyruvate produced by glycolysis must be transported from the cytoplasm into the mitochondrion. The link reaction itself takes place in the mitochondrial matrix.

Each molecule of pyruvate (3C) is:

1. Decarboxylated. A carboxyl group is removed as a molecule of carbon dioxide.
2. Oxidised. Hydrogen is removed and used to reduce a molecule of NAD.
3. Combined with coenzyme A, which carries the remaining 2-carbon acetyl group away as acetyl coenzyme A.

Because glycolysis produces two molecules of pyruvate per glucose molecule, the link reaction occurs twice for every glucose molecule respired. In total, this releases 2 molecules of carbon dioxide, produces 2 molecules of reduced NAD, and forms 2 molecules of acetyl coenzyme A.

Part (c): The Krebs cycle

The Krebs cycle also takes place in the mitochondrial matrix, immediately following the link reaction.

The 2-carbon acetyl group carried by acetyl coenzyme A is transferred onto the 4-carbon acceptor molecule oxaloacetate, forming the 6-carbon compound citrate. Over a series of small steps, citrate is progressively converted back into oxaloacetate, a sequence of reactions that involves:

• Decarboxylation reactions, releasing carbon dioxide.
• Dehydrogenation reactions, in which hydrogen is transferred to the coenzymes NAD and FAD, reducing them.
• One substrate-level phosphorylation, in which ATP is formed directly from ADP and inorganic phosphate.

Regenerating oxaloacetate at the end of the cycle allows it to combine with another acetyl group and the cycle to turn again. Since one glucose molecule yields two molecules of acetyl coenzyme A (from the two link reactions), the Krebs cycle turns twice per glucose molecule respired. Combining both turns, the total yield from one glucose molecule is:

• 4 molecules of carbon dioxide.
• 6 molecules of reduced NAD.
• 2 molecules of reduced FAD.
• 2 ATP molecules (by substrate-level phosphorylation).

(Adding the carbon dioxide released in the link reaction (2) and the Krebs cycle (4) gives 6 molecules of carbon dioxide in total per glucose molecule, matching the six carbon atoms originally present in glucose.)

Final answers

• (a) Glycolysis takes place in the cytoplasm; from one glucose molecule it produces 2 pyruvate, a net gain of 2 ATP, and 2 reduced NAD.
• (b) The link reaction takes place in the mitochondrial matrix; each pyruvate is decarboxylated and oxidised to form acetyl coenzyme A, releasing carbon dioxide and reducing NAD, twice per glucose molecule (2 CO₂, 2 reduced NAD, 2 acetyl CoA in total).
• (c) The Krebs cycle takes place in the mitochondrial matrix; acetyl CoA combines with oxaloacetate to form citrate, and the cycle (turning twice per glucose) produces a total of 4 CO₂, 6 reduced NAD, 2 reduced FAD and 2 ATP.