The Krebs cycle, also called the citric acid cycle, is the central pathway that finishes the breakdown of fuel molecules after glycolysis and pyruvate oxidation. This cheat sheet helps students track where carbon atoms, electrons, and energy carriers go during aerobic cellular respiration. It is especially useful because the cycle has many similar-sounding intermediates, enzymes, and products.
A clear reference makes it easier to connect the cycle to ATP production in the electron transport chain.
The cycle begins when acetyl-CoA combines with oxaloacetate to form citrate in the mitochondrial matrix. Each turn uses one acetyl-CoA and produces 3 NADH, 1 FADH2, 1 ATP or GTP, and 2 CO2. Because one glucose forms two acetyl-CoA molecules, the cycle turns twice per glucose.
The most important idea is that the Krebs cycle captures high-energy electrons in NADH and FADH2, which then fuel oxidative phosphorylation.
Key Facts
- The Krebs cycle occurs in the mitochondrial matrix in eukaryotic cells and in the cytoplasm of prokaryotic cells.
- Before the cycle, pyruvate oxidation converts pyruvate into acetyl-CoA, producing 1 NADH and 1 CO2 per pyruvate.
- One turn of the Krebs cycle begins when acetyl-CoA plus oxaloacetate forms citrate.
- The net yield per acetyl-CoA is 3 NADH, 1 FADH2, 1 ATP or GTP, and 2 CO2.
- The net yield per glucose from the Krebs cycle is 6 NADH, 2 FADH2, 2 ATP or GTP, and 4 CO2.
- The two carbon atoms from acetyl-CoA are released as CO2 during oxidation steps of the cycle.
- NAD+ is reduced to NADH, and FAD is reduced to FADH2 when they accept high-energy electrons and hydrogen.
- Oxaloacetate is regenerated at the end of the cycle, allowing the pathway to continue as long as acetyl-CoA and oxidized electron carriers are available.
Vocabulary
- Krebs cycle
- A cyclic pathway in cellular respiration that oxidizes acetyl-CoA and produces NADH, FADH2, ATP or GTP, and CO2.
- Citric acid cycle
- Another name for the Krebs cycle, named because citrate is the first stable molecule formed.
- Acetyl-CoA
- A two-carbon molecule that delivers an acetyl group into the Krebs cycle after pyruvate is oxidized.
- Oxaloacetate
- A four-carbon molecule that combines with acetyl-CoA to start the cycle and is regenerated at the end.
- NADH
- A reduced electron carrier that transports high-energy electrons from the Krebs cycle to the electron transport chain.
- FADH2
- A reduced electron carrier made in the Krebs cycle that donates electrons to the electron transport chain.
Common Mistakes to Avoid
- Counting one turn per glucose is wrong because one glucose produces two acetyl-CoA molecules, so the Krebs cycle turns twice per glucose.
- Saying the Krebs cycle directly makes most ATP is wrong because it mainly produces NADH and FADH2, which power ATP production in the electron transport chain.
- Placing the Krebs cycle in the cytoplasm of eukaryotic cells is wrong because it occurs in the mitochondrial matrix.
- Forgetting that oxaloacetate is regenerated is wrong because the cycle depends on oxaloacetate being available to combine with the next acetyl-CoA.
- Mixing up NADH and NAD+ is wrong because NAD+ is the empty electron carrier and NADH is the reduced, energy-rich form.
Practice Questions
- 1 If 1 acetyl-CoA enters the Krebs cycle, how many NADH, FADH2, ATP or GTP, and CO2 are produced?
- 2 If 1 glucose molecule produces 2 acetyl-CoA molecules, what is the total Krebs cycle yield of NADH and FADH2 per glucose?
- 3 A cell runs the Krebs cycle 6 times. How many CO2 molecules are released during those 6 turns?
- 4 Explain why the Krebs cycle would slow down if the electron transport chain stopped working.
Understanding Krebs (Citric Acid) Cycle Reference
The cycle is best understood as a controlled series of oxidation reactions. Cells remove electrons from carbon compounds step by step instead of releasing all their energy at once. If fuel were oxidized in one large reaction, much of its energy would escape as heat and could damage the cell.
Enzymes hold each intermediate in the right position and lower the energy needed for each reaction. Several steps rearrange the carbon skeleton before oxidation occurs. These rearrangements may seem unimportant on a diagram, but they prepare the molecule so later enzymes can remove electrons or release carbon dioxide.
Carbon tracking can be confusing because the cycle contains molecules with different numbers of carbon atoms. The incoming acetyl group has two carbons. It joins a four-carbon compound, producing a six-carbon compound.
Later, two carbon dioxide molecules leave. In early turns, those released carbons do not always come directly from the acetyl group that just entered. Carbon atoms can remain in the cycle intermediates and leave during later turns.
This is why a carbon-labeling experiment gives a more detailed picture than a simple count. For school problems, distinguish the net result from the exact path of each individual carbon atom.
NADH and FADH2 are not just product labels to memorize. They are reduced electron carriers. NAD plus and FAD begin in oxidized forms, meaning they can accept electrons.
When an enzyme removes electrons from an intermediate, it transfers them to one of these carriers. The carriers later deliver electrons to proteins in the inner mitochondrial membrane. That transfer supports pumping of hydrogen ions across the membrane.
The resulting difference in hydrogen ion concentration drives ATP synthase. Direct ATP production in the cycle is small. Most usable energy linked to this pathway appears later because of the electron carriers it produces.
The cycle depends on conditions elsewhere in the cell. It slows when NADH builds up because too little NAD plus remains available to accept more electrons. It can speed up when cells use ATP quickly and have more ADP available.
Oxygen matters indirectly. Oxygen is not used by the cycle enzymes themselves, but it is needed at the end of the electron transport chain. Without oxygen, NADH cannot be efficiently converted back to NAD plus, so the cycle soon lacks electron acceptors.
Students often memorize every intermediate first. A stronger approach is to identify the major events in order. Follow carbon entry, rearrangement, two oxidation and carbon dioxide release stages, formation of FADH2, direct ATP formation, then regeneration of the starting four-carbon compound.