Study Guide

Biomolecules and the Chemistry of Life - Monomers, Polymers, and Dehydration Synthesis

Monomers, Polymers, and Dehydration Synthesis

Some biological molecules are relatively small and may contain a handful of atoms bound together. Others are large and unwieldy and can contain hundreds or thousands of atoms.

If you were trying to correctly assemble a molecule that big, you would probably want to start by putting together some smaller fragments and then carefully link those fragments together at the end. Picture it like piecing together patches of a quilt. Or if quilts aren't your thing, think of it like trying to find out who among your Facebook friends knows a guy who knows a guy who knows a guy who can introduce you to Matthew Lewis. Longbottom love!

Um...not that we've ever done that. 

In the molecular world, the small subunits that ultimately link together to form larger molecules are called monomers, which literally means "single unit" (mono = one). When a bunch of monomers join together into a much larger molecule, they form a polymer, meaning "many units" (poly = many).

How does this "linking together" happen? Are we talking about one monomer grabbing another monomer’s little molecular hand and never letting go? Not exactly, though that would make for an adorable webcomic. We would call it "Monamours." Clever, no? Anyway...

There is a process by which this joining usually occurs, and it's called dehydration synthesis. The process begins when two monomers line up next to each other. Just when you think they’re going to start line dancing, a hydrogen (H) from one monomer binds with a hydroxyl group (OH) from the other monomer, and voilà! A water molecule is born: H+ + OH- = H2O.

While this is happening, the two monomers are binding to each other where they were bound to their respective hydrogen (–H) or hydroxyl (–OH) groups. (We add a dash to molecular groups, to show that they are attached to something else.) Having bonded, our lonely monomers are now a single polymer. This blissful union is presided over by an enzyme, which is mainly there to help speed things along.

The name of this whole process is dehydration synthesis because monomers are literally coming together and synthesizing a polymer by dehydrating, or removing a water molecule.

Need a picture? We were thinking that, too. On the chemical bond level of things, here is what happens when your body makes a triglyceride.

First, here's what one dehydration reaction looks like:

Great. Let's do it again. And again. This is called dehydration synthesis:

Super. Why is everything so straight and awkward-looking? Fine, we will fix it. Here is the final product. Happy? (Psst. When there are zigzag lines with no atom in sight (/\/\/\), this means that carbons and hydrogens are the only connecting atoms. Yes, chemists are lazy.)

Brain Snack

Some anti-HIV drugs, called nucleotide analogs, work by interfering with the synthesis of DNA from monomers. Check out the structure of this drug and see if you can figure out why.