What you need to know
- 7.3link reactionThe stage in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and joined to coenzyme A to form acetyl coenzyme A. and Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP.: carbon dioxide by decarboxylationRemoval of carbon dioxide from a molecule., a little ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. by substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle., reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain. and reduced FADFAD that has accepted hydrogen in the Krebs cycle. It passes electrons to the electron transport chain. by dehydrogenationRemoval of hydrogen from a molecule, usually accepted by a coenzyme such as NAD.. These steps are in mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place.. Other compound names are not required.
Names of the acids in the cycle are not required. Oxaloacetate and citrate appear only as a labelled extra for the carbon count, not as required recall. Acetyl coenzyme A is the usual label for the two-carbon carrier only. Oxidative phosphorylation is 7A 4. The usual per-turn counts below are a balance for calculations, not a list of intermediates.
By the end, you should be able to…
- Locate the link reaction and Krebs cycle in the mitochondrial matrix.
- Describe decarboxylation and dehydrogenation without naming cycle acids.
- State the usual products of one turn.
Build your understanding
The link reaction, once per pyruvate
pyruvateThe three-carbon product of glycolysis. from glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. enters the mitochondrial matrix. Glycolysis itself stays in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles.. In the link reactionThe stage in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and joined to coenzyme A to form acetyl coenzyme A., pyruvate is decarboxylated: a molecule of carbon dioxide is removed. It is also dehydrogenated: hydrogen is accepted by NAD, so reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain. is formed. The remaining two-carbon acetyl group is carried by coenzyme AA coenzyme that carries acetyl groups from the link reaction into the Krebs cycle. into the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP..
The usual name for that carrier-bound group is acetyl coenzyme AThe two-carbon acetyl group joined to coenzyme A, formed in the link reaction. It enters the Krebs cycle.. The specification says names of the other compounds are not required, so do not spend memory on the acids inside the cycle. Do remember carbon dioxide, reduced NAD, and the two-carbon hand-off.
The cycle completes the oxidation
In the Krebs cycle the two-carbon group is taken in and, during one turn, two molecules of carbon dioxide are released by decarboxylationRemoval of carbon dioxide from a molecule.. That is the rest of the carbon from that fragment of the carbohydrate. dehydrogenationRemoval of hydrogen from a molecule, usually accepted by a coenzyme such as NAD. produces reduced NAD and reduced FADFAD that has accepted hydrogen in the Krebs cycle. It passes electrons to the electron transport chain.. One ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. is formed by substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle.. Coenzyme A is freed to collect another acetyl group.
The usual school balance for one turn is two CO₂, three reduced NAD, one reduced FAD and one ATP. Use it if a question asks for a count. It is still not permission to recite citrate and the rest. Per glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. there are two pyruvates, so the link reaction and the cycle both happen twice. The reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD. have not yet made the large ATP yield; they carry hydrogen to the next lesson.
Carbon dioxide here is a metabolic waste
Every decarboxylation in these two stages is a source of the carbon dioxide in the overall equation. Breathing then removes that gas from the blood. The lung did not perform the Krebs cycle. If you write “the lungs respire glucose”, you have swapped ventilation for this matrix chemistry.
Without a supply of oxidised NAD and FAD the cycle stops. Those oxidised coenzymes return only if the electron transport chainA series of carriers in the inner mitochondrial membrane. Energy from electrons passing along it pumps H⁺ ions into the intermembrane space. is running. That is why the Krebs cycle is an aerobic-stage pathway even though oxygen is not a reactant written inside the cycle itself.
Follow the process
Link reaction, in the matrix, once per pyruvate: pyruvate enters → decarboxylation removes one CO₂ and dehydrogenation reduces NAD → the remaining two-carbon group is carried by coenzyme A as acetyl coenzyme A. No ATP is made in the link reaction. Krebs cycle, one turn of that fragment: the two-carbon group is accepted → two decarboxylations release the rest of its carbon as CO₂ → dehydrogenations produce reduced NAD and reduced FAD → one ATP is made by substrate-level phosphorylation → the acceptor molecule is regenerated, so another fragment can enter. Per glucose, both stages run twice. Names of the acids in the cycle are not required.
Follow the process (continued)
Extra, not required: the four-carbon acceptor is called oxaloacetateThe four-carbon compound in the Krebs cycle that accepts the acetyl group to form a six-carbon compound, and is regenerated.. It joins the two-carbon acetyl group to make citrate, which has six carbons, and oxaloacetate is regenerated at the end of the turn. Use those two names only if a question shows the carbon count. Do not memorise the other acids.
NOT: acid names inside the cycle are not required, and oxygen is not a reactant written in the cycle.
- Both stages are in the matrix
- Decarboxylation releases CO₂
- Dehydrogenation reduces NAD and FAD
- Two turns per glucose
Two traps this diagram blocks
- Do not recite named cycle acids unless the question gives them.
- Do not say the lungs carry out the Krebs cycle.
From the diagram alone
Question
Describe the products of the link reaction and one role of coenzyme A. [3]
Show a model answer
The link reaction produces carbon dioxide and reduced NAD. Coenzyme A carries the remaining two-carbon acetyl group into the Krebs cycle.
| Stage | Carbon | Hydrogen carriers and ATP |
|---|---|---|
| Link, per pyruvate | 1 CO₂ removed | 1 reduced NAD; acetyl group to coenzyme A |
| Krebs, per turn | 2 CO₂ removed | Reduced NAD, reduced FAD, 1 substrate-level ATP |
| Per glucose | Both stages happen twice | Cytoplasmic glycolysis is extra and already done |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP. stops when oxygen is absent. Y: oxygen is not a reactant inside the cycle, but without it the electron transport chainA series of carriers in the inner mitochondrial membrane. Energy from electrons passing along it pumps H⁺ ions into the intermembrane space. cannot reoxidise NAD and FAD. Z: dehydrogenationRemoval of hydrogen from a molecule, usually accepted by a coenzyme such as NAD. has nowhere to put hydrogen, so the cycle stops. Incomplete: “Oxygen is used up inside the Krebs cycle.” Improved: “The cycle needs oxidised NAD and FAD. Those return only if the chain can pass electrons to oxygen.”
Name the reaction type
decarboxylationRemoval of carbon dioxide from a molecule. for CO₂. Dehydrogenation for reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain. or reduced FADFAD that has accepted hydrogen in the Krebs cycle. It passes electrons to the electron transport chain.. substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle. for the small ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi..
Matrix, not cytoplasm
If the question says “where”, glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. is the cytoplasmic one. These two are mitochondrial.
Avoid this misconception
The claim: The Krebs cycle uses oxygen and produces most of the ATP directly.
The correction: Oxygen is the final acceptor on the electron transport chain, not a reactant written in the cycle. The cycle’s own ATP is the small substrate-level yield. Most ATP comes later from the reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD..
Put the link reaction and Krebs cycle in order
After glycolysis, pyruvate is oxidised in the mitochondrial matrix. Order one turn, from pyruvate to the acceptor that lets the cycle turn again.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
One pyruvate from glycolysis enters a mitochondrion. Put the steps in order.
0 of 7 in the right place
The names of the acids in the cycle are not required. Several of the dehydrogenations are combined into fewer steps here; the hydrogen goes to the electron transport chain (7A 4).
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Where do the link reaction and the Krebs cycle take place?
Show suggested answer
In the matrix of the mitochondrion. Glycolysis is in the cytoplasm.
Practice question
Retrieval and explanation · suggested answer
What happens to the carbon of pyruvate in the link reaction?
Show suggested answer
Decarboxylation removes it as carbon dioxide. The remaining two-carbon group is carried by coenzyme A into the Krebs cycle.
Practice question
Retrieval and explanation · suggested answer
State three products of one turn of the Krebs cycle.
Show suggested answer
Carbon dioxide, reduced NAD, reduced FAD and ATP. Any three. The ATP is substrate-level.
Practice question
Retrieval and explanation · suggested answer
Why does the cycle stop if the electron transport chain stops?
Show suggested answer
NAD and FAD are not regenerated, so dehydrogenation cannot continue.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A question shows glucose labelled only in one carbon atom, and later finds labelled carbon dioxide. Assume that carbon survived glycolysis inside pyruvate. Which two stages could have released it, and which stage could not?
Show a hint
Glycolysis does not decarboxylate.
Explore the reasoning
The link reaction or the Krebs cycle could release it as CO₂, depending on which carbon of pyruvate it was. Glycolysis cannot: it does not remove carbon dioxide. You are not expected to track the exact carbon through named acids.
Recall this lesson
Lesson recall questions
Question 1 of 5: State where the link reaction and the Krebs cycle take place, in contrast to glycolysis.
Link reaction and Krebs
Practice question
Retrieval and explanation · suggested answer
State where the link reaction and the Krebs cycle take place, in contrast to glycolysis.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The carbon dioxide in your breath was chemistry first
7A · The Krebs CycleNHLBI describes the lungs as the place where oxygen enters the blood and carbon dioxide leaves it. That is ventilation and gas exchange. The carbon dioxide itself was…Specification 7.3Read & use the scienceClose article
Why this matters
NHLBI describes the lungs as the place where oxygen enters the blood and carbon dioxide leaves it. That is ventilation and gas exchange. The carbon dioxide itself was produced by decarboxylation in cells. The lung page does not teach the Krebs cycle, and it does not treat breathing as cellular respiration. 7.3: carbon dioxide is formed by decarboxylation in the link reaction and the Krebs cycle, in mitochondria. A measured gas or ATP change cannot, on its own, identify every intermediate enzyme or show which step changed.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.3 Link reaction and Krebs cycle: carbon dioxide, ATP and reduced coenzymes.
Use the science
Why is “I breathed out, so my lungs ran the Krebs cycle” incorrect?
Explore a possible response
Breathing exhales carbon dioxide that cells have already produced. The cycle runs in mitochondrial matrices, not in the lung airway.
Explore the source
- NHLBI: how the lungs workUS National Heart, Lung, and Blood Institute
Carbon can leave before the chain begins
7A · The Krebs CycleThe NCI discussion of tumour metabolism follows carbon from glucose into pathways used for growth. It notes that changing the fate of pyruvate can change how much carbon…Specification 7.3Read & use the scienceClose article
Why this matters
The NCI discussion of tumour metabolism follows carbon from glucose into pathways used for growth. It notes that changing the fate of pyruvate can change how much carbon enters mitochondrial metabolism. This gives a real reason to keep 7.3 distinct from 7.4: the link reaction and Krebs cycle release carbon dioxide and supply reduced coenzymes, while the electron transport chain handles their electrons later. A change in carbon dioxide output might reflect substrate choice or pathway use; it does not locate a mutation by itself. The cancer setting is an application of carbon flow, not an extra Krebs intermediate to memorise.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.3 Link reaction and Krebs cycle: carbon dioxide, ATP and reduced coenzymes.
Use the science
If less pyruvate enters the link reaction, what happens to its direct carbon dioxide output and to the supply of reduced coenzymes?
Explore a possible response
Both would tend to fall from that route: less pyruvate is decarboxylated, and less substrate reaches the reactions that reduce NAD and FAD. Other fuels could complicate a whole-cell measurement.
Explore the source
- New clarity on the Warburg effectUS National Cancer Institute