But the major difference between gel electrophoresis and gel filtration chromatography is in gel electrophoresis, all the proteins actually move along that region, while in gel filtration chromatography, the very small proteins essentially are trapped inside the beads of that system, as we'll see in a future lecture. Now, in isoelectric focusing, this is where we separate our proteins based on their isoelectric point. And the isoelectric point is that specific PH value at which the net charge on that protein is essentially neutral, and we'll see exactly is essentially zero, and we'll see exactly what that means once again in a future lecture. Now, two dimensional electrophoresis basically combines the isoelectrofocusing method and the gel electrophoresis method. So every time we conduct one of these protein purification processes, as well as following the cell fractionation process, we basically have to determine what the enzyme activity is and what the enzyme concentration is in that sample. Because it's the enzyme activity and the concentration values that allows us to calculate the specific activity and recall it's a specific activity value that allows us to determine whether or not our sample is pure.