Link Reaction & Krebs Cycle (AQA A-Level Biology): Revision Notes

Link Reaction & Krebs Cycle

Following glycolysis in the cytoplasm, pyruvate molecules must be processed further to release their remaining potential energy. This happens through two connected processes: the link reaction and the Krebs cycle. Both processes occur exclusively in the matrix of mitochondria in eukaryotic cells.

The link reaction

The link reaction serves as a bridge between glycolysis and the Krebs cycle. Pyruvate molecules produced during glycolysis are actively transported from the cytoplasm into the mitochondrial matrix, where they undergo specific chemical changes.

During the link reaction, two main transformations occur:

1. Decarboxylation: The 3-carbon pyruvate molecule is oxidised to form acetate, losing one carbon atom as carbon dioxide and releasing two hydrogen atoms.
2. Formation of acetyl CoA: The 2-carbon acetate molecule combines with coenzyme A (CoA) to produce acetylcoenzyme A, which can then enter the Krebs cycle.

Overall equation:

$\text{pyruvate} + \text{NAD} + \text{CoA} \rightarrow \text{acetyl CoA} + \text{reduced NAD} + \text{CO}\_2$

C = number of carbon atoms in the molecule involved

The Krebs cycle

Named after British biochemist Hans Krebs, the Krebs cycle (also known as the citric acid cycle) involves a series of oxidation-reduction reactions that completely break down the acetyl portion of acetyl CoA.

The cycle begins when the 2-carbon acetylcoenzyme A combines with a 4-carbon molecule to form a 6-carbon compound. Through subsequent reactions, this 6-carbon molecule is gradually broken down, losing carbon atoms as carbon dioxide and hydrogen atoms to carrier molecules.

Key events in the cycle:

• The 6-carbon molecule loses carbon dioxide and hydrogen to become a 4-carbon molecule
• One molecule of ATP is produced directly through substrate-level phosphorylation
• The 4-carbon molecule can combine with new acetylcoenzyme A molecules to continue the cycle

The cycle effectively regenerates the 4-carbon acceptor molecule, allowing the process to continue as long as acetyl CoA is available.

Products of the link reaction and Krebs cycle

For each pyruvate molecule processed through both the link reaction and Krebs cycle, the following products are generated:

Coenzymes in respiration

Coenzymes are essential helper molecules required by many enzymes to function properly. In respiration, the most important coenzymes are:

1. NAD (nicotinamide adenine dinucleotide): The primary hydrogen carrier in respiration, working with dehydrogenase enzymes to remove hydrogen atoms from substrates and transfer them to the electron transfer chain.
2. FAD (flavin adenine dinucleotide): Another important hydrogen carrier, particularly active during the Krebs cycle.

Significance of the Krebs cycle

The Krebs cycle performs several vital functions in cellular metabolism:

• Energy production: The cycle produces hydrogen atoms carried by reduced coenzymes, which provide the energy for ATP synthesis through oxidative phosphorylation. This represents the major energy yield from glucose breakdown.
• Waste removal: The cycle completely breaks down pyruvate into carbon dioxide, which can be expelled from the organism as a waste product.
• Metabolic hub: The cycle produces intermediate compounds that serve as starting materials for manufacturing other important biological molecules, including fatty acids, amino acids, and chlorophyll.
• Cycle maintenance: The process regenerates the 4-carbon acceptor molecule, preventing accumulation of acetylcoenzyme A and ensuring the cycle can continue operating.

Remember!