Now, if the PH changes in either direction the activity of our enzyme will basically decrease symmetrically as shown in the following diagram. Notice in this case we have a sharp drop but in this case we have the same exact slope on both sides. So basically, if this y axis is the rate of activity of the enzyme and the x axis is our PH and a PH of about 8.0 the enzymes in our small intestines, such as Chimetrypsin and Tryptin basically function optimally at this PH of 8.0. But if we decrease or increase our PH the activity of those enzymes will drop as shown by these decreasing slopes. Now, the final factor that we want to discuss is the concentration of the substrate. So let's suppose that we begin with a relatively small quantity of substrate as well as some fixed amount of our enzyme.