Synthesis reactions where molecules in the human body are made or joined together are called anabolism.

Here's a breakdown:

* Anabolism: This is the process of building up complex molecules from simpler ones. It requires energy, which is often supplied by the breakdown of food (catabolism).

* Catabolism: This is the process of breaking down complex molecules into simpler ones. This process releases energy.

Together, anabolism and catabolism make up metabolism, the sum of all chemical reactions that occur in the body.