Why is starch unable to pass through the gut wall

Explanation:

Starch is a large molecule, so it cannot pass through certain structures, like selectively permeable cell membranes or parts of the digestive system.

This is why starch has to be digested multiple times. Digestion breaks down the starch into smaller molecules that can pass through structures. In the mouth, saliva contains an enzyme known as amylase that breaks down starch. Once the food passed to the small intestine, juice from the pancreas continues to break down the carbohydrates with the same enzyme.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.