The chemical reaction that joins small organic molecules into macromolecules is called dehydration synthesis (also known as condensation reaction).

Here's how it works:

* Monomers: Small organic molecules called monomers are the building blocks of macromolecules.

* Removal of Water: A molecule of water (H₂O) is removed from the monomers. This removal requires energy.

* Covalent Bond: The monomers then join together through a covalent bond, forming a larger molecule called a polymer.

Example:

* Monomers: Glucose monomers

* Polymer: Starch

* Reaction: Dehydration synthesis removes a water molecule, linking two glucose monomers together.

Other Important Points:

* Dehydration synthesis is an anabolic reaction, meaning it builds larger molecules from smaller ones.

* The opposite reaction, hydrolysis, breaks down polymers into monomers by adding a water molecule.

Let me know if you want to know more about specific macromolecules like carbohydrates, lipids, proteins, or nucleic acids!