Pepsin, a common enzyme found in the stomach, is most active at ph values between 0.5 and 2.5. it works slowly between ph values of 2.6 and 3.4. beyond a ph of about 3.5, it becomes completely inactive. why does this happen?

Ivan is looking at a cross section of a stem taken from a vascular plant. he sees two different types of vascular tissue: the xylem, which is closer to the center of the stem, and the phloem, located around the xylem, closer to the outside of the stem. how do these two structures work together in a living plant?