 #### Lesson Goals

• To review the many ways in which chemical concentrations are measured and reported.

• Molarity
• Moles
• Copy Number

#### Script

Hi everybody! Welcome back to Synthetic Biology One. Today we’re going to discuss a super practical everyday lab skill: working with concentrations.

Biologists express concentration in a variety of ways. You will often have to convert between different ways of writing concentration when you are preparing your own recipes and protocols. In fact, you will do it so much that you will wish that scientists could just get their act together and agree to express concentration in just one standard way. But they won’t. So give up on that dream and just learn to convert like the rest of us.

1) Percent volume by volume.

This notation makes the most sense when you are mixing liquids in roughly equal proportions. To make a 10% Glycerol solution, I take 10 mL of Glycerol and then add water to a total volume of 100 mL. A common mistake is to think that means adding 100 mL of water. But 100 mL in this case refers to the total volume, not the volume of water. In fact, you’ll be adding around 90 mL of water to 10 mL of Glycerol to get the final solution. I say around 90 mL because I don’t know the actual exact number. Because of the magic of mixing, when you mix two different liquids, the total volume is not always equal to the sum of the separate volumes.

Only chemists really understand why this is. We are biologists and we won’t worry about it. The important thing is that, when you prepare your solution, you keep an eye on the final volume. Start with the 10 mL glycerol, then add water until the whole thing has a volume of 100 mL.

2) Percent weight by volume

This notation is used when we are adding a dry powder to water. It might seem weird to calculate a percent as a weight divided by a volume. Normally we think of a percentage as a unitless number, meaning we should be dividing two things with the same units. The convention makes a little more sense if you consider that one milliliter of water weighs exactly one gram. In biology, almost all of our solutions are water solutions, so you can think of volume and weight as being interchangeable.

Just like last time, remember that the volume is the total volume not the added volume. So watch the total volume level when you prepare the solution.

3) Grams per liter

Oh good an easy one. You take this many grams and dissolve it in this many liters. Remember that adding many grams of a powder will increase the volume. You want one liter of total volume, not one liter of added water.

4) Molarity

Molarity is a measure of the total number of molecules of a substance. It is useful when we care about the total number of molecules, not how much those molecules weigh. To determine how many moles you are adding, take the weight and divide it by the molar mass. Grams divided by grams per mole equals moles.

5) Not quite molarity (pmol/μl)

One mole is a lot of molecules, and one liter is a lot of water. Often in biology we don’t want to think about quantities that large. So we cut down molarity into tiny bite size units. An example of this that I see a lot is picomoles per microliter. This is just another way of saying micromolar, but it is useful when you are mixing microliters with your tiny tiny pipette tips. These can be directly converted to molarity, but don’t forget to convert.

6) Normality

This one doesn’t come up very often, but I mention it because it is sneaky. Normality is expressed with an N, which looks like a lot like an M, the measure for Molarity. I remember being new in the lab and thinking…well probably they meant to write M but their hand just slipped. No! If molarity measures the number of molecules per volume, then normality measures the number of “active” molecules. This comes up most often in the case of acids. 1 M HCl is equal to 1 N HCl. But 1 M H2SO4 is 2 N H2SO4 because each molecule of H2SO4 produces 2 units of acid. So be careful. But honestly, if you are handling acid, you should be careful anyway.

7) Enzyme activity units

When you buy an enzyme, like a restriction enzyme, they won’t give you the concentration in normal units. It is hard to measure the true concentration of an enzyme and companies are lazy and cheap. Instead they will give you the concentration in units, which are determined experimentally based on measurements of what the enzyme is actually doing. For example, if your enzyme cuts DNA they will go in and measure, how much DNA can this cut per minute and call that however many units. The important thing here is that this “unit” is defined differently for each enzyme. So the correct number of units will completely depend on the enzyme and the source. For most enzymes, the manufacturer will tell you the right amount to use in standard protocols.

8) Copy number

Ahh copy number. The most beautiful and intuitive way of expressing a concentration. For this, we say the exact number of a molecule for a given volume. 200 plasmids per cell. 100 mRNA molecules per cell. Unfortunately, this becomes impractical for large volumes and huge numbers of molecules that we use in a typical recipe. But it works great when we shrink down to the world of a single cell.

OK that is all the ways I can think of for expressing concentration. Keep your measurements strait and your experiments will always work.

Until next time, happy concentrating!