The letter “C” shows up across nearly every subject on the MCAT, and its meaning changes depending on the context. In physics, lowercase c is the speed of light. In chemistry, c typically stands for specific heat capacity. In biology, C is the nucleotide base cytosine and also the single-letter code for the amino acid cysteine. And at the exam level, two of the four MCAT sections have abbreviations starting with C. Here’s a breakdown of each meaning you’re likely to encounter while studying.
Lowercase c in Physics: Speed of Light
In physics equations, lowercase c represents the speed of light in a vacuum, approximately 3.0 × 10⁸ meters per second. You’ll see it most often in Einstein’s mass-energy equivalence equation, E = mc², where E is energy, m is mass, and c is the speed of light. While the MCAT rarely tests relativistic physics directly, the constant c appears in discussions of electromagnetic radiation, photon energy, and wavelength-frequency relationships. Knowing that c equals the product of wavelength and frequency (c = λν) is essential for problems involving the electromagnetic spectrum.
Uppercase C in Physics: The Coulomb
When you see an uppercase C as a unit in physics problems, it stands for the Coulomb, the standard unit of electric charge. One Coulomb represents the charge carried by roughly 6.24 × 10¹⁸ electrons. You’ll encounter it in Coulomb’s law, which describes the electrostatic force between two charged objects, and in circuits problems involving current (since one ampere equals one Coulomb per second). The MCAT tests electrostatics and circuits regularly, so recognizing C as a unit of charge is important for dimensional analysis and plugging values into equations correctly.
Lowercase c in Chemistry: Specific Heat
In thermodynamics and calorimetry, lowercase c represents specific heat capacity, which is the amount of heat energy needed to raise the temperature of one gram of a substance by one degree Celsius. The equation you’ll use most is q = mcΔT, where q is the heat gained or lost, m is mass, c is specific heat, and ΔT is the change in temperature.
The specific heat of water, 4.18 joules per gram per degree Celsius, is one of the most commonly tested values on the MCAT. Calorimetry problems typically ask you to calculate how much heat transfers between substances, and you need c to do that math. Some resources use a capital C with a subscript (Cₛ for specific heat, Cₘ for molar heat capacity), so pay attention to notation in your prep materials.
C in Chemistry: Concentration
In solution chemistry, uppercase C (or sometimes lowercase c) can represent concentration, typically molarity, which is moles of solute per liter of solution. You’ll see this in equations for dilution, reaction kinetics, and equilibrium. The more common notation for molarity on the MCAT is a capital M, but some passages and formulas use C to denote concentration, especially in rate law expressions or when discussing Beer’s Law (A = εbc, where c is the concentration of the absorbing species).
C in Biology: Cytosine
In molecular biology, C is one of the four nucleotide bases in DNA. Cytosine always pairs with guanine (G) through three hydrogen bonds in a double-stranded DNA molecule. In RNA, the same pairing holds: cytosine pairs with guanine. Recognizing C-G base pairing matters for MCAT questions about DNA replication, transcription, mutations, and calculating the base composition of a nucleic acid. If a problem tells you a DNA strand is 30% cytosine, you immediately know it’s also 30% guanine, with the remaining 40% split equally between adenine and thymine.
C in Biochemistry: Cysteine
In amino acid notation, the single-letter code C stands for cysteine (three-letter abbreviation: Cys). Cysteine is notable for its thiol (sulfhydryl) side chain, which allows two cysteine residues to form a disulfide bond. Disulfide bonds are covalent links that stabilize the three-dimensional structure of proteins, and MCAT questions on protein folding and tertiary structure frequently involve them. Cysteine is also relevant in discussions of the C-terminus, which is the end of a polypeptide chain that has a free carboxyl group. Translation of a protein proceeds from the N-terminus to the C-terminus, a detail the MCAT expects you to know.
C in MCAT Section Names
Two of the four MCAT sections have abbreviations starting with C. Chemical and Physical Foundations of Biological Systems, often shortened to Chem/Phys or CPBS, tests general chemistry, organic chemistry, physics, and biochemistry. Critical Analysis and Reasoning Skills, commonly called CARS, tests reading comprehension and analytical reasoning using passages from the humanities and social sciences. Each section is scored on a scale from 118 to 132, and the four section scores combine for a total between 472 and 528.
When you encounter “C” in study group conversations or prep forums, context usually makes it clear whether someone means CARS, the Chem/Phys section, or a scientific variable. In equations and passages on test day, the surrounding context, units, and subject matter will tell you which “c” applies.