International A Level Biology / Cellular respiration
The link reaction and the Krebs cycle
Edexcel International A Level Biology (YBI11), chapter 20: Cellular respiration, specification point 7.3. Free lesson.
Read the revision notes · International A Level Biology past papers
The link reaction and Krebs cycle take place in the mitochondrial matrix and continue the breakdown of pyruvate. They release carbon dioxide, transfer hydrogen to NAD and FAD, and make a small amount of ATP.
What decarboxylation and dehydrogenation mean
In both terms, de means taking something away. These reactions remove carbon dioxide or hydrogen from a molecule, so the molecule is changed. Decarboxylation removes carbon dioxide. The carbon dioxide diffuses out as waste. Dehydrogenation removes hydrogen atoms. The hydrogen is transferred to NAD or FAD, which becomes reduced. When a molecule loses hydrogen, it is oxidised. Repeated removal of carbon dioxide and hydrogen leads to glucose being completely oxidised.
How the link reaction makes acetyl CoA
The link reaction connects glycolysis in the cytoplasm to the Krebs cycle in the mitochondrial matrix. It happens once for each pyruvate, and does not make ATP. A three-carbon pyruvate moves from the cytoplasm into the matrix. Removing one carbon dioxide leaves a two-carbon acetyl group. Removing hydrogen transfers it to NAD, making one reduced NAD. The acetyl group combines with coenzyme A, which carries it into the Krebs cycle as acetyl CoA.
What happens in one turn of the Krebs cycle
Each acetyl CoA joins a four-carbon compound, forming a six-carbon compound. The reactions then remove two carbon dioxide molecules and regenerate the four-carbon compound, which is why the process is a cycle. Coenzyme A is released after the acetyl group joins the four-carbon compound. As the six-carbon compound becomes a five-carbon compound, decarboxylation and dehydrogenation produce one CO₂ and one reduced NAD. As the five-carbon compound becomes a four-carbon compound, a second CO₂ and reduced NAD are produced. One ATP is made by substrate-level phosphorylation, directly in an…
How to total the products per glucose
One glucose produces two pyruvate, so the link reaction happens twice and the Krebs cycle turns twice. Multiply each per-pyruvate or per-turn yield by two, then add the link and Krebs yields. Per pyruvate, the link reaction produces 1 CO₂ and 1 reduced NAD. Per Krebs turn, the products are 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP. Most ATP is made later, when reduced NAD and reduced FAD take part in oxidative phosphorylation.
What to remember in exam answers
Use the correct coenzyme name and link carbon dioxide release to the stages and their location. Glycolysis takes place in the cytoplasm and releases no carbon dioxide. The link reaction and Krebs cycle happen in the mitochondrial matrix. If pyruvate cannot enter mitochondria, the link reaction and Krebs cycle cannot run, so less CO₂ is released. The Krebs cycle produces reduced NAD, not reduced NADP. NADP is associated with photosynthesis.