Trypsin and pepsin are digestive enzymes. They are large tangled chains of amino acids folded into shapes that promote the cleavage of protein molecules. But trypsin and pepsin themselves are proteins. Let me ask you this: why don't they digest themselves? The reason also explains why pepsin won't digest gelatin. Enzymes are specific. Each kind can cleave (or digest) only a certain type of molecule. Trypsin and pepsin do not have the right molecular shape to attack themselves. A gelatin molecule has a particular shape that "fits" trypsin, making it digestible, but it won't fit pepsin, so it doesn't get broken down.