Cellular Respiration: Glycolysis, Link Reaction, and Krebs Cycle

The Process of Glycolysis

Step 1: Glucose Phosphorylation

Glucose is phosphorylated. Two phosphate groups are added to glucose to form hexose biphosphate. These two phosphate groups are provided by two molecules of ATP.

Step 2: Lysis of Hexose Biphosphate

Hexose biphosphate splits into two molecules of triose phosphate.

Step 3: Oxidation of Triose Phosphate

Each triose phosphate molecule is oxidised. Two atoms of hydrogen are removed from each molecule. The energy released by the oxidation is used to add another phosphate group to each molecule. This results in two 3-carbon compounds, each carrying two phosphate groups. NAD+ is the hydrogen carrier that accepts the hydrogen atoms lost from each triose phosphate molecule.

Step 4: Pyruvate Formation

Two pyruvate molecules are formed by removing two phosphate groups from each molecule. These phosphate groups are transferred to ADP molecules to form ATP.

Glycolysis occurs in the cytoplasm of cells. Two ATP molecules are used and 4 ATP molecules are produced, resulting in a net yield of two ATP molecules. Additionally, two NAD+ are converted into NADH + H+ during glycolysis.

The Link Reaction

Mitochondria take up the pyruvate formed during glycolysis. Once inside the mitochondrial matrix, enzymes remove hydrogen and carbon dioxide from the pyruvate:

• Oxidation: Removal of hydrogen.
• Decarboxylation: Removal of carbon dioxide.

This process is called oxidative decarboxylation. The hydrogen removed is accepted by NAD+. The link reaction results in the formation of an acetyl group, which is then accepted by CoA to form acetyl CoA.

The Krebs Cycle

Step 1

The acetyl group from acetyl CoA is transferred to a four-carbon compound to form a six-carbon compound.

Step 2

The six-carbon compound undergoes decarboxylation (CO2 removed) and oxidation (hydrogen removed) to form a five-carbon compound. The hydrogen is accepted by NAD+ to form NADH + H+.

Step 3

The five-carbon compound undergoes decarboxylation and oxidation again to form a four-carbon compound. The hydrogen is accepted by NAD+ to form NADH + H+.

Step 4

The four-carbon compound undergoes substrate-level phosphorylation to produce ATP. Oxidation also occurs twice: one hydrogen is accepted by NAD+ (forming NADH + H+) and the other by FAD (forming FADH2). The four-carbon compound is then ready to accept a new acetyl group to repeat the cycle.

The carbon dioxide removed is a waste product excreted from the body. The oxidations release energy stored by carriers, which is later used by the electron transport chain to produce ATP.