The element present in every amino acid molecule, but not in carbohydrate or fat molecules, is nitrogen (N).

Here's why:

* Amino acids are the building blocks of proteins. Their defining characteristic is the presence of an amino group (-NH2) attached to a central carbon atom. This amino group contains nitrogen.

* Carbohydrates are composed of carbon, hydrogen, and oxygen (CHO).

* Fats (lipids) are primarily composed of carbon, hydrogen, and oxygen, with some containing small amounts of other elements like phosphorus.

Therefore, nitrogen is the unique element that distinguishes amino acids from carbohydrates and fats.