The Krebs cycle
Speedrun #1Per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂.
MCAT trap No step uses O₂, yet the cycle stops without it: NADH and FADH₂ must be reoxidized by the electron transport chain.
Every MCAT cycle, on one sheet.
The 11 cycles from the Red Pen Prep speedruns: what goes in, what comes out, and the trap the MCAT sets around each one. Save it, or print it: it fits on one page.
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Every MCAT cycle
Red Pen Prep · cheat sheetThe Krebs cycle enzymes
Speedrun #10Dehydrogenases (DH) make the NADH and FADH₂; succinyl-CoA synthetase makes the GTP.
MCAT trap Succinate dehydrogenase is the only Krebs enzyme bound to the inner membrane: it is also complex II of the chain.
The urea cycle
Speedrun #4Per urea: 3 ATP (4 high-energy bonds); starts in the mitochondrion, ends in the cytosol.
MCAT trap Urea’s two N come from different places: one from NH₄⁺, one from aspartate. Fumarate links it to the Krebs cycle.
The Cori cycle
Speedrun #5Net cost: 4 ATP per lap (glycolysis +2, gluconeogenesis −6).
MCAT trap The Cori cycle loses energy overall: the liver pays 6 ATP for the muscle’s 2. It buys time, not energy.
The cell cycle
Speedrun #6S doubles the DNA; M halves it again into two diploid daughter cells.
MCAT trap After S phase a human cell still has 46 chromosomes, now as 92 chromatids.
Beta-oxidation
Speedrun #7Palmitate (16 C): 7 laps → 8 acetyl-CoA, 7 FADH₂, 7 NADH.
MCAT trap 7 laps give 8 acetyl-CoA, not 7: the last lap splits a 4-carbon piece into two.
The action potential
Speedrun #8Threshold about −55 mV, peak about +35 mV, then a dip below rest.
MCAT trap The Na⁺/K⁺ pump does not repolarize the neuron: K⁺ leaving through its channels does.
The cardiac cycle
Speedrun #9Isovolumetric contraction → ejection → isovolumetric relaxation → filling.
MCAT trap Heart sounds are valves closing, never opening: S1 the AV valves, S2 the semilunar valves.
Muscle contraction
Speedrun #13One ATP per cross-bridge cycle; Ca²⁺ on troponin moves tropomyosin off actin.
MCAT trap ATP is needed to let go, not only to pull: without ATP, myosin stays bound (rigor mortis).
The menstrual cycle
Speedrun #11FSH grows the follicle; the corpus luteum makes progesterone; no hCG, day 1 again.
MCAT trap Estrogen usually feeds back negatively; at its peak it turns positive and triggers the LH surge.
The lytic cycle
Speedrun #12The host’s own enzymes and ribosomes copy the virus; the cell lyses.
MCAT trap Lysogenic is different: the viral DNA joins the host genome as a prophage and is copied with it until induced.
Which trap catches you?
The MCAT rarely asks you to recite a cycle. It asks what happens when one step is blocked, and it hides a trap in the answer choices.
Try 3 real questions, about a minute.
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Original content, not AAMC material. MCAT® is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Red Pen Prep and does not endorse it.
Red Pen Prep · redpenprep.pages.dev · Original content, not AAMC material. MCAT® is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Red Pen Prep and does not endorse it.
One cycle, one course question.
From the coursefrom the course, Bio/Biochem
During a prolonged fast, the liver oxidizes large amounts of fatty acids, yet humans cannot convert the carbons of even-chain fatty acids into a net gain of glucose. Which statement best explains this?
Pick one. The red pen does the rest.
The deciding linePyruvate dehydrogenase catalyzes an irreversible reaction, so acetyl-CoA cannot be converted back to pyruvate.
Why D: Gluconeogenesis needs pyruvate or oxaloacetate. Acetyl-CoA cannot become pyruvate, and feeding it into the citric acid cycle yields no net oxaloacetate, because two CO₂ leave for every two carbons that enter.
Why each choice fails
- ADirection reversed: acetyl-CoA activates pyruvate carboxylase, signaling that oxaloacetate is needed; it does not inhibit it.
- BTrue but beside the point: the fasting liver does turn acetyl-CoA into ketone bodies, but that does not explain why acetyl-CoA cannot yield net glucose.
- CLook-alike terms: the glycerol backbone of triacylglycerols is gluconeogenic (glycerol 3-phosphate is oxidized to DHAP); it is the even-chain fatty acid carbons that cannot become net glucose.
- DHolds: pyruvate dehydrogenase is irreversible, so acetyl-CoA cannot be turned back into pyruvate, and each acetyl group entering the cycle is matched by two CO₂ released, so no net oxaloacetate is left for gluconeogenesis.
This is 1 of 3,122 questions in the Pass, all corrected like this one.
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