Learning Guide · MCAT
Krebs Cycle (TCA / Citric Acid Cycle) for the MCAT — Still Rote Memorizing? Quit it. Become a Krebs King. 👑
Did learning Krebs Cycle right after Glycolysis sound like this?
"I just 'learned' (more like stuffed) my brain with glycolysis… only to learn another set of molecules? Ugh. 😣😭"
I know exactly how you feel. That's why I created this visual breakdown going through Krebs Cycle, step-by-step. This isn't just a cheat sheet, though. I take you on a journey where you get to discover it for yourselves! (Btw, if you stuffed your brain with Glycolysis → try this instead!)
One thing. Information decays fast because our brains are not a storage box. Rather, it is a pattern-recognition machine. And, like with all tools, if we use tools (our brains) the right way, we can maximize our gains. So, if you actually grasp the patterns behind the Krebs Cycle, you won’t need a mnemonic ∵ it’ll automatically stick. In fact, mnemonics just become a tool to increasing efficiency rather than something to memorize. Do it properly once, and prosper forever after. Obviously, revisit it occasionally (maybe while brushing your teeth). After understanding it once, reviewing the pathway takes just minutes (less than 2)!
{Quick Life Update: I got into medical school this cycle (gonna be a doctor! 🥳)! I wanted to give back to you guys, especially because I wrote my Glycolysis God post as one of my activities for my AMCAS application. Thank you all for your numerous support and feedback on that post! 🤣🔥}
Before you start, go ahead and take a sneak peek of the last image 👀. You're going to be building up to that — layer-by-layer. It's going to be SOO easy, so get excited!
Essential MCAT knowledge
Before you memorize anything, know what the cycle is doing.
Where it happens
The Krebs cycle happens in the mitochondrial matrix. Succinate dehydrogenase is the exception: it sits in the inner mitochondrial membrane because it is also Complex II of the electron transport chain.
What is being reduced?
NAD⁺ becomes NADH and FAD becomes FADH₂. That means the cofactors are reduced while the carbon skeleton is being oxidized and gradually released as CO₂.
Why the enzymes matter
Every enzyme is a response to the cell’s energy state. High ATP and NADH say “we are good for now,” so the cycle slows. ADP and Ca²⁺ say “we need energy,” so the cycle moves forward. This is homeostasis, not random memorization.
Commonly tested on the MCAT: location, which cofactors are reduced, the rate-limiting step (isocitrate dehydrogenase), irreversible steps, and the fact that succinate dehydrogenase belongs to both the Krebs cycle and electron transport chain. The energy scorecard comes later—once you know what each “side character” is doing.
Now, let’s start.
You are not trying to learn eight names yet. First, let your eyes see the pattern.
Stage 1 · Carbon skeleton
Start with the dots, not the names.
All 8 molecules of Krebs Cycle are simplified to their carbon skeleton. I've taken the liberty to start our cycle with Citrate, so that we have an easy way to remember the structural changes. Just go ahead and notice the patterns. Furthermore, the functional groups are color-coded to reduce cognitive overload — so enjoy!
First, observe
How many dots are there?
6 → 6 → 5 → 4 → 4 → 4 → 4 → 4
My pattern
For me, I noticed there were five 4's after the 5.
The hook
The '5' holds two meanings: (a) number of carbons in step 3 AND (b) # of 4s after 5. Hence, I've bolded the 5 for that reason.
Stage 2 · Labeling
Time to put a name to the face.
At this stage, we will be labeling (a) the functional groups and (b) the names of these eight molecules.
There are 4 major structural features: carboxylic acids, hydroxyl, carbonyls, and double bonded. First, I'll show you carboxylic acids. Then, I'll show you hydroxyls, carbonyls, and double-bonded carbons together.
This is what are building up to in Stage 2. Go ahead and go down below because we'll build it layer-by-layer.
Where are the red dots? Which one always stays?
Notice where the colors are located. Why is Molecule 5 considered the turning point? Take a guess.
You got this. Keep going!
Time to put a name to the face!
Stage 3 · Structural group flow
Watch the handoff.
Notice how the dots related to Hydroxyls, Carbonyls, and Double-bonded Carbons flow from one to the next. It's kind of like a relay race.
Watch the structural group flow. See how one molecule help build the next one?
Stage 4 · Enzymes
Form follows function.
At this point, the enzymes almost name themselves.
General Enzyme Formula = "Substrate" + "Enzyme Subclass"
(not every enzyme will follow this, but it is a good rule of thumb) The best thing you can do for yourself is learn the major class of enzymes. That way you'll understand what the enzyme does (and link it with the pattern you found before!). Now you've gone from behavior → name → function! Learn the chemistry first, and the enzyme names become much easier to reconstruct.
One more connection: the enzyme names tell you the chemistry, but the regulation tells you why the cell cares. Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase are not just “irreversible steps” to memorize. They are the control points that let the cycle match energy production to energy need. High ATP/NADH slows the system down; ADP and Ca²⁺ push it forward. That is the relation back to homeostasis.
Let’s figure out enzymes. Form follows function!
Stage 5 · The side characters
Nothing here is random.
NADH, FADH₂, CO₂, GTP, H₂O, Acetyl-CoA. Notice they aren't random either. Each one appears because the molecule has undergone a structural change. This is a moment to put everything together!
Now add the side characters and put the whole story together.
The energy scorecard
Now the side characters get to speak.
For each acetyl-CoA, the cycle gives you 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂. The cycle directly makes only 1 GTP, but NADH and FADH₂ carry the bigger energy payoff forward to the electron transport chain—roughly 10 ATP equivalents total.
Notice how that is not just a list. CO₂ appears as the carbon skeleton is oxidized. NAD⁺ and FAD are reduced when they collect those electrons. GTP appears when the chemistry gives the cell a direct, usable energy payout. Each side character is there because something structural happened.
Stage 6 · Destination
Put the whole story together.
Now you can see the Krebs cycle as one connected system—not a pile of molecules.
See, this wasn't so hard!
Keep this handy
High-yield information:
Per turn
Each turn of the Krebs cycle produces 3 NADH, 1 FADH2, 1 GTP, and 2 CO2.
Rate-limiting
The rate-limiting step is isocitrate dehydrogenase.
Irreversible steps
The major irreversible steps are catalyzed by citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.
The special one
Also know that succinate dehydrogenase is special because it is part of both the Krebs cycle and the electron transport chain, and that FAD is used in the succinate-to-fumarate step because that oxidation is not favorable enough to reduce NAD+.
Take it one step further
Exercises left to the reader
- 1Where do the products of Krebs cycle go?
- 2What are other entry points for other biochemical processes?
- 3Why is GTP made in the cycle, and why is that basically interchangeable with ATP for MCAT purposes?
- 4How would a lack of oxygen indirectly slow or shut down the Krebs cycle, even though oxygen is not used directly in the cycle itself? How does the cycle compensate in these situations?
- 5Aconitase does not simply “move an -OH around.” What is the actual logic of the citrate-to-isocitrate rearrangement, and why is that rearrangement necessary for the next step?
- 6Malate dehydrogenase is thermodynamically unfavorable under standard conditions. Why does the reaction still proceed forward in the cell?
- 7Krebs is amphibolic, not just catabolic. What does that mean, and how does the cycle support both energy extraction and biosynthesis?
Take the pattern with you
Understanding is the first rep.
This is a first-stage introduction to the Krebs cycle. Come back after a few questions, redraw the pathway from memory, and notice which relationship still feels fuzzy. That is where the real learning starts.
You do not need to hold every molecule in your head at once. Build the pattern first. Then let the names, enzymes, and side characters attach to something that already makes sense.
For more visual, hand-held breakdowns, keep exploring the Library.