Before we explain what we mean by the terms link reaction and Krebs cycle, let's have a quick recap of where we are in the process of respiration.

Respiration can occur aerobically or anaerobically. During both processes, a reaction called glycolysis occurs. This reaction occurs in the cytoplasm of the cell. Glycolysis involves the breakdown of glucose, split from a 6-carbon molecule into two 3-carbon molecules. This 3-carbon molecule is called pyruvate (C3H4O3).

Fig. 1 - Animal and plant cell. Cytoplasm, the location where glycolysis takes place, labelled

In anaerobic respiration, which you may have already covered, this molecule of pyruvate is converted into ATP via fermentation. Pyruvate stays in the cytoplasm of the cell.

However, aerobic respiration produces far more ATP carbon dioxide and water. Pyruvate will need to undergo a series of further reactions to release all of that energy. Two of these reactions are the link reaction and the Krebs cycle.

The link reaction is a process that oxidises pyruvate to produce a compound called acetyl-coenzyme A (acetyl CoA). The link reaction occurs straight after glycolysis.

The Krebs cycle is used to extract ATP from acetyl CoA through a series of oxidation-reduction reactions. Like the Calvin cycle in photosynthesis, the Krebs cycle is regenerative. It produces a range of intermediate compounds used by cells to create a range of important biomolecules.

Where do the link reaction and Krebs cycle take place?

The link reaction and the Krebs cycle occur in a cell's mitochondria. As you will see in figure 2 below, the mitochondria contain a structure of folds within their inner membrane. This is called the mitochondrial matrix and has a range of compounds such as the mitochondria's DNA, ribosomes, and soluble enzymes. After glycolysis, which occurs before the link reaction, pyruvate molecules are transported into the mitochondrial matrix via active transport (active loading of pyruvate requiring ATP). These pyruvate molecules undergo the link reaction and the Krebs cycle within this matrix structure.

Fig. 2 - A diagram showing the general structure of a cell’s mitochondria. Note the structure of the mitochondrial matrix

What are the different steps of the link reaction?

Following glycolysis, pyruvate is transported from the cell's cytoplasm to the mitochondria via active transport. The following reactions then take place:

1. Oxidation - pyruvate is decarboxylated (carboxyl group removed), during which it loses a carbon dioxide molecule. This process forms a 2-carbon molecule called acetate.

2. Dehydrogenation - decarboxylated pyruvate then loses a hydrogen molecule accepted by NAD + to produce NADH. This NADH is used to produce ATP during oxidative phosphorylation.

3. Formation of acetyl CoA - Acetate combines with coenzyme A to produce acetyl CoA.

Overall, the equation for the link reaction is:

What does the link reaction produce?

Overall, for every glucose molecule broken down during aerobic respiration, the link reaction produces:

• Two molecules of carbon dioxide will be released as a product of respiration.

• Two acetyl CoA molecules and two NADH molecules will stay in the mitochondrial matrix for the Krebs cycle.

Most importantly, it is essential to note that no ATP is produced during the link reaction. Instead, this is produced during the Krebs cycle, discussed below.

Fig. 3 - An overall summary of the link reaction

What are the different steps of the Krebs cycle?

The Krebs cycle occurs in the mitochondrial matrix. This reaction involves acetyl CoA, which has just been produced in the link reaction, being converted through a series of reactions into a 4-carbon molecule. This 4-carbon molecule then combines with another molecule of acetyl CoA; hence this reaction is a cycle. This cycle produces carbon dioxide, NADH, and ATP as a by-product.

It also produces reduced FAD from FAD, a molecule that you may not have come across before. FAD (Flavin Adenine Dinucleotide) is a coenzyme that some enzymes require for catalytic activity. NAD and NADP are also coenzymes.

The steps of the Krebs cycle are as follows:

1. Formation of a 6-carbon molecule: Acetyl CoA, a 2-carbon molecule, combines with oxaloacetate, a 4-carbon molecule. This forms citrate, a 6-carbon molecule. Coenzyme A is also lost and exits the reaction as a by-product when citrate is formed.

2. Formation of a 5-carbon molecule: Citrate is converted into a 5-carbon molecule called alpha-ketoglutarate. NAD + is reduced to NADH. Carbon dioxide is formed as a by-product and exits the reaction.

3. Formation of a 4-carbon molecule: Alpha-ketoglutarate is converted back into the 4-carbon molecule oxaloacetate through a series of different reactions. It loses another carbon, which exits the reaction as carbon dioxide. During these different reactions, two more molecules of NAD + are reduced to NADH, one molecule of FAD is converted to reduced FAD, and one molecule of ATP is formed from ADP and inorganic phosphate.

4. Regeneration: Oxaloacetate, which has been regenerated, combines with acetyl CoA again, and the cycle continues.

Fig. 4 - A diagram that summarizes the Krebs cycle

What does the Krebs cycle produce?

Overall, for every molecule of acetyl CoA, the cancer cycle produces:

• Three molecules of NADH and one molecule of reduced FAD: These reduced coenzymes are vital for the electron transport chain during oxidative phosphorylation.

• One Molecule of ATP is used as an energy source to fuel vital biochemical processes in the cell.

• Two molecules of carbon dioxide. These are released as by-products of respiration.