Why is the changeable nature of models considered a limitation?

The changeable nature of models is considered a limitation because this makes it impossible to use the same model in all situations. Usually, models operate within a set of specific conditions. As long as these conditions remain the same, the model can be trusted. However, if the conditions change, the model might not be as useful anymore, and a new model would have to be developed.

where in your body is there an enzyme that does function at a high or low ph? what is the name of that enzyme and its function?