Red Pen Prep

54 lessons. 30 seconds each.

The MCAT, made visible: every lesson in one place. Each one is built on a number you won’t forget and the trap that costs points. Watch one, then check yourself.

Acid series 7

Orgo and Chem/Phys. Seven parts, from one O–H bond to a titration curve.

Acetic acid is about 100 billion times more acidic than ethanol.Part 1Acetic acid is about 100 billion times more acidic than ethanol.Trap: it’s not the O–H bond. It’s the ion left behind.Watch · 30 s Phenol is about a million times more acidic than ethanol (pKa 9.89 vs 16).Part 2Phenol is about a million times more acidic than ethanol (pKa 9.89 vs 16).Trap: phenoxide spreads its charge into the ring (ortho and para, never meta).Watch · 30 s One chlorine makes acetic acid almost 80 times more acidic (4.76 → 2.87).Part 3One chlorine makes acetic acid almost 80 times more acidic (4.76 → 2.87).Trap: the inductive pull fades fast with distance.Watch · 30 s In your blood, charged aspirin outnumbers neutral aspirin about 8,000 to 1.Part 4In your blood, charged aspirin outnumbers neutral aspirin about 8,000 to 1.Trap: only the neutral form crosses membranes.Watch · 30 s Your blood keeps about 20 bicarbonates for every CO₂ (pKa 6.1, pH 7.4).Part 5Your blood keeps about 20 bicarbonates for every CO₂ (pKa 6.1, pH 7.4).Trap: each pH unit above the pKa means 10× more base.Watch · 30 s At blood pH, glycine is +1 and −1 at the same time. Net charge: zero.Part 6At blood pH, glycine is +1 and −1 at the same time. Net charge: zero.Trap: each group follows its own pKa.Watch · 30 s Titrate acetic acid halfway and the pH reads 4.76, exactly its pKa.Part 7Titrate acetic acid halfway and the pH reads 4.76, exactly its pKa.Trap: concentration doesn’t move the half-equivalence point.Watch · 30 s

Practice it in the course

In the course, 10 passages and sets open with one of these lessons.

Play with it: the Buffer Labtitrate a weak acid and watch pH land on pKa halfway, live

Enzyme series 3

Biochem. What an enzyme can and cannot change.

This enzyme turns 78 million years into 18 milliseconds: about 10¹⁷× faster.Part 1NewThis enzyme turns 78 million years into 18 milliseconds: about 10¹⁷× faster.Trap: an enzyme lowers Ea. It does not change ΔG or the equilibrium.Watch · 30 s
Your brain gets glucose first: about 50× the affinity of your liver’s enzyme.Part 2Your brain gets glucose first: about 50× the affinity of your liver’s enzyme.Trap: Km is the [S] that gives half of Vmax, not half of Vmax.Coming this week
Twelve enzymes, random collisions: the Michaelis–Menten curve draws itself.SimulationTwelve enzymes, random collisions: the Michaelis–Menten curve draws itself.Trap: Vmax is a ceiling set by how much enzyme there is, not by substrate.Watch · 30 s Competitive?3D questionMalonate inhibits succinate dehydrogenase. Competitive or noncompetitive?Trap: It mimics succinate: competitive, so Km rises and Vmax stays.Watch · 30 s

Practice it in the course

In the course, 5 passages and sets open with one of these lessons.

Play with it: the Enzyme Labadd an inhibitor and watch Km and Vmax move, live

Speedruns 10

Bio/Biochem. Whole pathways in seconds, in order.

The entire Krebs cycle. Per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂.#1The entire Krebs cycle. Per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂.Trap: one of the eight intermediates still carries CoA.Watch · 30 s All 20 amino acids, named and grouped.#2All 20 amino acids, named and grouped.Trap: “polar” and “charged” are not the same group.Watch · 30 s Glycolysis: all 10 enzymes, in order.Glycolysis: all 10 enzymes, in order.Trap: three steps are irreversible, and gluconeogenesis has to bypass them.Watch · 30 s Every Krebs cycle enzyme, in one run.Every Krebs cycle enzyme, in one run.Trap: succinyl-CoA synthetase makes GTP, not ATP, in most textbooks.Watch · 30 s Beta-oxidation, step by step.#7Beta-oxidation, step by step.Trap: each round cuts off two carbons as acetyl-CoA.Watch · 30 s The action potential.#8The action potential.Trap: the peak stops when Na⁺ channels inactivate, not when they “run out”.Watch · 30 s The cell cycle.#6The cell cycle.Trap: DNA is copied in S phase, not in mitosis.Watch · 30 s The cardiac cycle.The cardiac cycle.Trap: heart sounds come from valves closing.Watch · 30 s The urea cycle, step by step.#4The urea cycle, step by step.Trap: Part of it runs in the mitochondria, part in the cytosol.Watch · 30 s The Cori cycle.#5The Cori cycle.Trap: Lactate goes to the liver; glucose comes back to the muscle.Watch · 30 s

Practice it in the course

In the course, 19 passages and sets open with one of these lessons.

Take the 10-question trap testabout 15 minutes, no account

Physiology & neuro 13

Bio/Biochem and Psych/Soc foundations, in living 3D.

Panic breathing makes your fingers tingle. It’s not a lack of oxygen.Panic breathing makes your fingers tingle. It’s not a lack of oxygen.Trap: fast breathing raises pH, and ionized Ca²⁺ falls.Watch · 30 s What stops the rise at the peak of an action potential?Living Lab #1What stops the rise at the peak of an action potential?Trap: Na⁺ channels inactivate as K⁺ channels open.Watch · 30 s This pufferfish poison stops your nerves from firing.Living Lab #2This pufferfish poison stops your nerves from firing.Trap: it blocks Na⁺ channels, so no action potential at all.Watch · 30 s −70 mVThis is what a thought is made of: one neuron firing.Trap: threshold is all-or-none; bigger stimuli fire more often, not bigger.Watch · 30 s This is what a neuron sounds like.This is what a neuron sounds like.Trap: a stronger stimulus means more clicks (spikes), not bigger ones.Watch · 30 s Steroid or peptide?3D questionCortisol or oxytocin: which crosses a cell membrane on its own?Trap: Steroids cross; peptide hormones need a receptor on the surface.Watch · 30 s Exercise doesn’t make hemoglobin weaker. It makes it smarter.PhysiologyExercise doesn’t make hemoglobin weaker. It makes it smarter.Trap: CO₂, H⁺, heat and 2,3-BPG shift the curve right.Watch · 30 s The life of one red blood cell, in 30 seconds.PhysiologyThe life of one red blood cell, in 30 seconds.Trap: No nucleus, so it can’t repair itself.Watch · 30 s POV: you’re a red blood cell. One lap of your body.POVPOV: you’re a red blood cell. One lap of your body.Trap: Saturation climbs from about 75% to 98% in the lungs.Watch · 30 s Osmosis, simulated: 1,615 molecules, real forces.SimulationOsmosis, simulated: 1,615 molecules, real forces.Trap: Nobody pushes the water: it moves toward more solute.Watch · 30 s FLAT PEGMemoryThe anterior pituitary hormones, in two words.Trap: FLAT are tropic; PEG act directly.Watch · 30 s Low T4, low TSHCaseLow thyroid hormone and low TSH: which patient has the pituitary mass?Trap: The TRH test settles it.Watch · 30 s Stare at the dot: your brain paints a red heart that was never there.PerceptionStare at the dot: your brain paints a red heart that was never there.Trap: Opponent process: tired cyan cells leave red behind.Watch · 30 s

Practice it in the course

In the course, 22 passages and sets open with one of these lessons.

Check yourself: the tingle chain4 questions, 60 seconds

Orgo in 3D 10

Chem/Phys. Pick your answer before the reveal.

pH 2 or 7?Which pH lets aspirin (pKa 3.5) cross a cell membrane?Trap: neutral crosses; charged stays out.Watch · 30 s antiWhich butane conformation is lowest in energy?Trap: gauche is staggered too, but anti keeps the methyls farthest apart.Watch · 30 s tert-Butyl or ethyl cation: which is more stable?tert-Butyl or ethyl cation: which is more stable?Trap: more alkyl groups donate more electron density.Watch · 30 s Pyridine or pyrrole: which is more basic?Pyridine or pyrrole: which is more basic?Trap: pyrrole’s lone pair is part of the aromatic ring.Watch · 30 s Phenol or cyclohexanol: which is more acidic?Phenol or cyclohexanol: which is more acidic?Trap: only phenol’s ring can share the charge.Watch · 30 s Acetic acid or ethanol: which is more acidic?3D questionAcetic acid or ethanol: which is more acidic?Trap: The conjugate base decides it, not the O–H bond.Watch · 30 s Ring flip: which chair wins?3D questionRing flip: which chair wins?Trap: Big groups go equatorial, and the flip moves them.Watch · 30 s Two stereocenters. Chiral or not?3-second questionTwo stereocenters. Chiral or not?Trap: Stereocenters don’t guarantee chirality: look for a mirror plane.Watch · 30 s E or Z?3D questionBoth methyls on the same side of the C=C. E or Z?Trap: Rank by priority: Br outranks CH₃, so it is E.Watch · 30 s (2R,3S)3D question(2R,3S)-2,3-dibromobutane: does it rotate plane-polarized light?Trap: It is meso: no rotation.Watch · 30 s

Practice it in the course

In the course, 12 passages and sets open with one of these lessons.

Check yourself on the acid series60 seconds, answers included

Physics, made visible 11

Chem/Phys. Predict it before the reveal, then see why.

A pipe closed at one end plays only odd harmonics.Sound onA pipe closed at one end plays only odd harmonics.Trap: No even harmonics in a closed pipe.Watch · 30 s Every element glows in its own exact colors: its emission lines.Sound onEvery element glows in its own exact colors: its emission lines.Trap: Each line is one electron jump between levels.Watch · 30 s 60°Predict itA laser in water hits the surface at 60°. Escape or bounce back?Trap: Past the critical angle, about 49°, it reflects totally.Watch · 30 s One ball dropped, one fired sideways. Which lands first?Predict itOne ball dropped, one fired sideways. Which lands first?Trap: A tie: horizontal speed doesn’t change the fall.Watch · 30 s a = 0?Predict itAt the very top of its flight, is the ball’s acceleration zero, or g?Trap: Velocity is zero; acceleration is still g.Watch · 30 s Heavy or light?Predict itHeavy bob or light bob: which pendulum swings faster?Trap: Neither: the period depends on length, not mass.Watch · 30 s 8, 50 or 92%?Predict itHow much of a floating ice cube is underwater?Trap: The density ratio decides it: about 92%.Watch · 30 s Ball or feather?Predict itIn a vacuum, a heavy ball or a feather: which lands first?Trap: A tie: without air, mass doesn’t matter.Watch · 30 s This is how you’re seeing this video.VisionThis is how you’re seeing this video.Trap: Cornea and lens land the image upside down on the retina.Watch · 30 s From one hydrogen atom to the human eye, every size to scale.ScaleFrom one hydrogen atom to the human eye, every size to scale.Watch · 30 s Allergy pill, then 90 km/h: how much longer to stop?Do the mathAllergy pill, then 90 km/h: how much longer to stop?Trap: Reaction distance grows with reaction time; braking distance doesn’t.Watch · 30 s

Practice it in the course

In the course, 15 passages and sets open with one of these lessons.

Take the 10-question trap testabout 15 minutes, no account

When you want the full course

3,122 MCAT questions. Every wrong answer named, and corrected in red.

  • All four sections with full passages and their tables
  • 11 full-length exams in test-day order
  • Every lesson built in: 83 passages open with their 30-second lesson
  • The Trap Bank: 155 drills on the traps that cost the most points
  • Every miss comes back 1, 3 and 7 days later, until it sticks

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