Cellular respiration is a process by which living cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), the energy currency of the cell. It breaks down molecules like glucose to release energy. The process involves several stages, each extracting energy. The “link reaction” is an intermediate step, preparing molecules for energy generation.
Understanding the Link Reaction
The link reaction, also known as pyruvate oxidation or pyruvate decarboxylation, connects glycolysis and the Krebs cycle. Glycolysis, occurring in the cytoplasm, breaks down glucose into two pyruvate molecules. In eukaryotic cells, pyruvate moves into the mitochondria for the link reaction.
In eukaryotic cells, this reaction takes place within the mitochondrial matrix. Prokaryotic cells, lacking mitochondria, perform similar reactions in the cytoplasm and at the plasma membrane. The link reaction converts pyruvate, a three-carbon molecule, into acetyl-CoA, a two-carbon molecule, which enters the Krebs cycle.
The Essential Products
The link reaction yields three products from each molecule of pyruvate: acetyl-CoA, carbon dioxide (CO2), and reduced nicotinamide adenine dinucleotide (NADH). Since one glucose molecule yields two pyruvate molecules, the link reaction occurs twice, doubling product output.
Acetyl-CoA is a two-carbon molecule formed when pyruvate loses a carbon atom and attaches to coenzyme A. It is a high-energy compound for further oxidation. Carbon dioxide is a waste product from pyruvate decarboxylation, removed and later expelled from the body.
NADH is a reduced coenzyme, accepting electrons and a proton during pyruvate oxidation. It functions as an electron carrier, holding energy for later stages of cellular respiration.
What Happens Next to the Products?
Each product has a distinct fate in cellular respiration. Acetyl-CoA proceeds into the Krebs cycle (citric acid cycle). In the Krebs cycle, acetyl-CoA’s acetyl group is oxidized, generating more electron carriers like NADH and FADH2, and some ATP.
Carbon dioxide is a metabolic waste product. CO2 diffuses out of the mitochondrial matrix and cell, transports via the bloodstream to the lungs, and is exhaled. This represents a carbon atom lost from the original glucose molecule.
NADH travels to the electron transport chain with its electrons. This final stage of aerobic cellular respiration uses NADH’s electrons to drive significant ATP production. The energy in these electrons powers ATP synthesis, making NADH a contributor to the cell’s energy supply.
The Link Reaction’s Role in Energy Production
The link reaction serves as a connector in cellular energy production. It ensures glycolysis products, pyruvate, are prepared to enter the Krebs cycle. This step is necessary for continued breakdown of glucose derivatives and electron carrier generation.
By converting pyruvate into acetyl-CoA, the link reaction facilitates chemical energy transfer from glucose breakdown to later ATP-yielding stages. The NADH produced contributes to ATP synthesis in the electron transport chain. Without this step, energy flow from glucose to usable ATP would be hampered, reducing energy yield.