Question 1

What equations do I need to know for MCAT biochemistry?

Accepted Answer

The highest-yield set is small. For acids and buffers: pH equals negative log of hydrogen ion concentration, and the Henderson-Hasselbalch equation, pH equals pKa plus the log of conjugate base over acid. For thermodynamics: delta G equals delta H minus T delta S, and delta G standard equals negative RT natural log of the equilibrium constant. For enzymes: the Michaelis-Menten equation and the fact that velocity is half of Vmax when substrate equals Km. Beyond those, know the ideal gas law, Hardy-Weinberg, and cardiac output equals heart rate times stroke volume. Most exam questions test whether you can rearrange and interpret these rather than plug into obscure formulas.

Question 2

Why does pH equal pKa at the half-equivalence point?

Accepted Answer

Because at half-equivalence exactly half the weak acid has been converted to its conjugate base, so their concentrations are equal. In the Henderson-Hasselbalch equation the ratio of conjugate base to acid becomes one, and the log of one is zero, which leaves pH equal to pKa. This is the standard way to read a pKa directly off a titration curve, and it is also why a buffer is strongest at a pH equal to its pKa.

Question 3

What is the difference between delta G and delta G standard?

Accepted Answer

Delta G standard is the free energy change under standard conditions, meaning one molar concentrations, and it is fixed for a given reaction because it is tied to the equilibrium constant by delta G standard equals negative RT natural log of K. Delta G is the actual free energy change under the concentrations present right now, given by delta G equals delta G standard plus RT natural log of Q. This is why a reaction with a positive delta G standard can still run forward in a cell: if the concentrations keep Q well below K, delta G is negative even though delta G standard is not.

Question 4

How do I know which half-life formula to use?

Accepted Answer

It depends entirely on the reaction order. Zero order gives half-life equals starting concentration divided by 2k, so it shortens as reactant is used up. First order gives half-life equals natural log of 2 divided by k, which is constant no matter the concentration. Second order gives half-life equals one over k times starting concentration, so it doubles with each successive half-life. A quick way to identify the order from data is to check whether successive half-lives stay the same, get shorter, or get longer.

Question 5

What values should I memorize for the constants?

Accepted Answer

The gas constant R is 8.314 joules per mole per kelvin when you are working in joules, or 0.08206 liter-atmospheres per mole per kelvin for gas law problems. Faraday's constant is about 96485 coulombs per mole. The ion product of water, Kw, is 1 times 10 to the negative 14 at 25 degrees Celsius, which is why pH plus pOH equals 14. Body temperature is 310 kelvin and room temperature is about 298 kelvin. Knowing which R to use in which equation saves more points than memorizing extra formulas.

Biochem & MCAT Equation Sheet

Constants worth memorizing

How to use this sheet