Scienceblogs is promoting the writing of “Science 101” general topic posts all through the “back to school” month of September. So, here is the first in a multi-part series on Heat Capacity in Biology:
Heat Capacity in Biology 101: What is it?
Heat capacity is basically a proportionality constant. For any substance, the heat capacity tells you how much the temperature of the substance will change when you add a specific amount of heat.
Here is an absolutely beautiful schematic illustration of the difference between a small heat capacity and a large heat capacity (from a website on the physics of X-ray tube behavior) note the temperatures used in the illustration are a bit outside the temperature range that biologists are normally interested in, but the figure still illustrates the concept quite well).
When you apply heat to a substance with a low heat capacity, it will rise in temperature faster than a substance with a high heat capacity. (Keep reading, it gets to biology eventually…)
When you boil water, the temperature of the water stays at 100 degrees C regardless of how much heat you add, until it all boils away. The heat capacity of the water is approaching infinity at this point: it can absorb monstrous amounts of heat, but its temperature remains the same. This is typical behavior for substances during phase changes.
But before you hit the boiling point, the water will rise in temperature as you add heat. In fact, every milliliter (cubic centimeter) of water will rise 1 degree C for every calorie of heat you input – and this is in fact also the definition of a calorie.
“Heat capacity” is one thing, “heat capacity change” is a slightly different thing:
When substances change in any way (e.g. when they change phase, as in the boiling water example; or if they change via chemical reaction, etc.) their heat capacity will also frequently change – and then you get a “change in heat capacity” or ΔCp. (the “p” here denotes constant pressure – since biological reactions virtually all occur at standard, atmospheric pressure, the thermodynamic state functions used in biology are almost always the constant pressure forms rather than the constant volume forms).
Nomenclature alert: Although “heat capacity” and “heat capacity change” are truly different (one is a constant, the other is a difference between two constants) — the two expressions are often used interchangeably in the literature — in biology at least, if the sentence is talking about heat capacity, it almost always is actually talking about “heat capacity change”.
So almost any reaction you might study in biology will exhibit a heat capacity change: protein folding, protein-protein interactions, protein-DNA binding, etc., all have measurable heat capacity changes (just like one can measure the heat capacity change between liquid water and gaseous water) – i.e., all of these processes and substances have one heat capacity (Cp) before the reaction and a different Cp after the reaction, and the difference between before and after = ΔCp.
Heat capacity and heat capacity changes are “thermodynamic state functions” – fundamental, equilibrium properties of a substance or a reaction system – just like free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), among others.
Conclusion of Part 1: Wow, say you can (for example) measure the difference in heat capacity of proteins and DNA before and after they bind to one another (drum roll…) Who cares? (or as Dr. McCoy might say: “I’m a biologist Jim, why are you talking to me about frigging heat capacity?”) Well, stay tuned for Part 2, coming up soon (very soon, because most of us who work on any aspect of heat capacity know that without connecting it to something biological or molecular, to many people it sounds like so much hot air).
Here is a link to a song called heat capacity, which appears to have nothing to do with heat capacity: