How to Memorize the Krebs Cycle (Step by Step)
The best way to memorize the Krebs cycle is to learn it in layers: first the eight molecules in order with one mnemonic, then what each step releases (CO2, NADH, FADH2, GTP), and only then the enzymes if your course needs them. Draw the cycle from memory once a day, and learn it as the middle link between glycolysis and the electron transport chain rather than as a stand-alone circle.
If you keep losing track of where the CO2 comes off or which step makes FADH2, FlashDeck turns your notes into one card per step and brings back the ones you miss: Build my study plan.
What you actually need to know about the citric acid cycle
The Krebs cycle has three names: the Krebs cycle, the citric acid cycle and the TCA (tricarboxylic acid) cycle. Same thing. Before you memorize anything, check your syllabus, because the depth varies a lot.
| Course | Usually expected |
|---|---|
| Intro bio / AP Bio | Where it happens, what goes in, what comes out, how it connects to other stages |
| College intro bio | Above, plus the intermediates in order |
| Biochem / pre-med | Above, plus enzymes, structures and regulation |
AP Biology students, for example, generally need the big picture far more than every molecule. Don't spend three nights on enzyme names if nobody will ask for them.
Understanding the Krebs cycle in one paragraph
It happens in the mitochondrial matrix. A 2-carbon acetyl group (carried by acetyl-CoA) joins a 4-carbon molecule (oxaloacetate) to make a 6-carbon molecule (citrate). Over the next steps, two carbons leave as CO2, energy is captured in NADH, FADH2 and GTP (or ATP), and oxaloacetate is rebuilt so the cycle can go again.
The per-turn totals
For each acetyl-CoA (one turn):
- 3 NADH
- 1 FADH2
- 1 GTP or ATP
- 2 CO2
One glucose makes two acetyl-CoA, so double everything per glucose.
Krebs cycle mnemonic for the intermediates
The eight intermediates in order:
- Citrate (6C)
- Isocitrate (6C)
- Alpha-ketoglutarate (5C)
- Succinyl-CoA (4C)
- Succinate (4C)
- Fumarate (4C)
- Malate (4C)
- Oxaloacetate (4C)
The mnemonic
Citrate Is Krebs' Starting Substrate For Making Oxaloacetate
Citrate, Isocitrate, (alpha-)Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate.
Track the carbons: 6, 6, 5, 4, 4, 4, 4, 4
Carbon count is the easiest pattern to spot. The two drops (6 to 5, then 5 to 4) are exactly where CO2 is released. If you know the carbons, you know the CO2 steps without memorizing them separately.
Card each step: how to memorize Krebs cycle enzymes and products
Once the order is solid, attach what happens at each step. This is where most of the test questions live.
| Step | From → To | Enzyme | Made or released |
|---|---|---|---|
| 1 | Acetyl-CoA + oxaloacetate → citrate | Citrate synthase | (none) |
| 2 | Citrate → isocitrate | Aconitase | (none) |
| 3 | Isocitrate → alpha-ketoglutarate | Isocitrate dehydrogenase | NADH, CO2 |
| 4 | Alpha-ketoglutarate → succinyl-CoA | Alpha-ketoglutarate dehydrogenase | NADH, CO2 |
| 5 | Succinyl-CoA → succinate | Succinyl-CoA synthetase | GTP (or ATP) |
| 6 | Succinate → fumarate | Succinate dehydrogenase | FADH2 |
| 7 | Fumarate → malate | Fumarase | (none) |
| 8 | Malate → oxaloacetate | Malate dehydrogenase | NADH |
Patterns that cut the memorizing
- "Dehydrogenase" means an electron carrier is made. All four dehydrogenases (steps 3, 4, 6, 8) produce NADH or FADH2.
- The one FADH2 is at succinate. Succinate dehydrogenase is also part of the electron transport chain (complex II).
- Enzyme names mostly describe their molecule. Citrate synthase makes citrate. Malate dehydrogenase works on malate.
Cards that match how questions are asked
- "Which Krebs step makes FADH2?" → Succinate → fumarate (succinate dehydrogenase)
- "Which two steps release CO2?" → Isocitrate → alpha-ketoglutarate, and alpha-ketoglutarate → succinyl-CoA
- "What does acetyl-CoA combine with to start the cycle?" → Oxaloacetate
- "Where does the Krebs cycle happen?" → Mitochondrial matrix
- "NADH per turn?" → 3
That's already 40-plus facts across the table, and you'll be sure you know some of them when you don't. Judging that honestly, every day, is the hard part.
FlashDeck takes that judging out of your head. Paste your Krebs notes or upload your biochem slides as a PDF, and it writes one card per fact, only from what you pasted. Each day it shows just the cards that are due. Tap Missed it on "which step makes FADH2?" and it's back tomorrow; tap Knew it and it waits a day, then three, then a week. Build my study plan
Draw it from memory: how to memorize the Krebs cycle in 3 minutes
- Draw a big circle and put acetyl-CoA coming in at the top.
- Write the eight intermediates around the circle clockwise, using the mnemonic.
- Add carbon counts next to each one.
- Draw arrows off the circle for each CO2, NADH, FADH2 and GTP.
- Add enzymes on the arrows (only if your course needs them).
- Check and circle every miss. Tomorrow, draw it again on a blank page.
When you can draw the full cycle with products in about three minutes, you've got it. If you also need structures (how to memorize Krebs cycle structures), add them one at a time after the names are automatic: draw citrate, then ask what changes to make isocitrate.
Link to glycolysis and the ETC
The Krebs cycle makes much more sense as part of a chain:
- Glycolysis (cytoplasm): glucose → 2 pyruvate, with a small ATP and NADH payoff. See how to memorize glycolysis steps.
- Pyruvate oxidation (mitochondria): each pyruvate → acetyl-CoA, releasing CO2 and making NADH.
- Krebs cycle (matrix): acetyl-CoA is broken down, making NADH, FADH2, GTP/ATP and CO2.
- Electron transport chain (inner membrane): NADH and FADH2 hand off electrons, and most of the ATP is made here.
If you're learning how to memorize glycolysis and Krebs cycle together, make a card for each handoff: "What does pyruvate become before entering the Krebs cycle?" → Acetyl-CoA.
For a general approach to diagrams and pathways, how to active recall anatomy has drills that work just as well for biochem.
Own the Krebs cycle before your exam
You have the mnemonic, the carbon pattern, a product table and a drawing drill. Paste your notes into FlashDeck and in the next minute you'll have a card for each step and product, then about 10 minutes a day reviewing only what's due.
FAQ
What's the easiest way to memorize the Krebs cycle?
Use "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate" for the order and track carbons (6, 6, 5, 4, 4, 4, 4, 4) to find the CO2 steps. Add products next, then enzymes only if your course tests them. Draw it from memory daily; that habit is how to memorize the Krebs cycle so it lasts past the exam.
How do I memorize Krebs cycle enzymes?
Learn them after the intermediates, since most enzyme names describe their substrate (citrate synthase, malate dehydrogenase). Remember that the four dehydrogenases are the steps that make NADH or FADH2. Make one card per enzyme in both directions.
What are the Krebs cycle products per glucose?
Per turn you get 3 NADH, 1 FADH2, 1 GTP or ATP and 2 CO2. One glucose gives two acetyl-CoA, so per glucose that's 6 NADH, 2 FADH2, 2 GTP/ATP and 4 CO2 from the cycle itself.
Do I need to know the citric acid cycle steps for AP Bio?
Usually the big picture matters most: where it happens, what goes in and out, and how it connects to glycolysis and the electron transport chain. Check your teacher's study guide. If intermediates aren't on it, focus on inputs, outputs and location.
What's the best way to remember the Krebs cycle long term?
Space your reviews. Draw the cycle a few days in a row, then less often, and keep cards for the facts you miss. Coming back just before you'd forget works better than one long session.