Why can’t some food molecules be digested?

The size.

Explanation:

Some molecules are too big to be digested by the gut without being broken down so the stomach must then use the hydrochloric acid that lies within the stomach & energy to break down the food to be able to absorb the energy from the bigger food molecules.

he is very short, and he can only reach halfway up on the elevator buttons. but on a rainy day, he has his umbrella with him, and he uses it to poke the button at the top.

"a.) well–developed vascular system."